Subjects -> EDUCATION (Total: 2411 journals)
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    - EDUCATION (2043 journals)
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EDUCATION (2043 journals)                  1 2 3 4 5 6 7 8 | Last

Showing 1 - 200 of 857 Journals sorted alphabetically
#Tear : Revista de Educação, Ciência e Tecnologia     Open Access   (Followers: 1)
(Pensamiento), (palabra) y obra     Open Access   (Followers: 2)
21. Yüzyılda Eğitim Ve Toplum Eğitim Bilimleri Ve Sosyal Araştırmalar Dergisi     Open Access  
21st Century Pedagogy     Open Access   (Followers: 5)
Abant İzzet Baysal Üniversitesi Eğitim Fakültesi Dergisi     Open Access  
ABDIMAS ALTRUIS : Jurnal Pengabdian Kepada Masyarakat     Open Access  
Abdimas Toddopuli : Jurnal Pengabdian Pada Masyarakat     Open Access  
About Campus     Hybrid Journal   (Followers: 7)
Academic Medicine     Hybrid Journal   (Followers: 73)
Academic Psychiatry     Full-text available via subscription   (Followers: 27)
Academy of Educational Leadership Journal     Full-text available via subscription   (Followers: 53)
Academy of Management Learning and Education     Full-text available via subscription   (Followers: 57)
Accounting & Finance     Hybrid Journal   (Followers: 30)
Accounting Education: An International Journal     Hybrid Journal   (Followers: 13)
ACM Transactions on Computing Education (TOCE)     Hybrid Journal   (Followers: 7)
Acta Científica : Ciências Humanas     Open Access  
Acta Didactica Norge     Open Access   (Followers: 2)
Acta Educationis Generalis     Open Access  
Acta Paedagogica Vilnensia     Open Access  
Acta Scientiarum. Education     Open Access  
Action in Teacher Education     Hybrid Journal   (Followers: 61)
Action Learning: Research and Practice     Hybrid Journal   (Followers: 44)
Actualidades Pedagógicas     Open Access  
Adelphi series     Hybrid Journal   (Followers: 13)
Adiyaman University Journal of Educational Sciences     Open Access  
Administração Educacional     Open Access  
Advanced Education     Open Access   (Followers: 11)
Advances in Arts, Social Sciences and Education Research     Open Access   (Followers: 16)
Advances in Building Education     Open Access   (Followers: 4)
Advances in Health Sciences Education     Hybrid Journal   (Followers: 32)
Advances in High Energy Physics     Open Access   (Followers: 22)
Advances in School Mental Health Promotion     Partially Free   (Followers: 7)
Africa Education Review     Hybrid Journal   (Followers: 26)
African Journal of Chemical Education     Open Access   (Followers: 5)
African Journal of Educational Studies in Mathematics and Sciences     Full-text available via subscription   (Followers: 7)
African Journal of Health Professions Education     Open Access   (Followers: 5)
African Journal of Research in Mathematics, Science and Technology Education     Hybrid Journal   (Followers: 11)
African Journal of Teacher Education     Open Access   (Followers: 4)
Agora     Full-text available via subscription   (Followers: 3)
AGORA Magazine     Open Access   (Followers: 1)
Ahmad Dahlan Journal of English Studies     Open Access   (Followers: 2)
AIDS Education and Prevention     Full-text available via subscription   (Followers: 8)
Ainedidaktiikka     Open Access  
Akademos     Open Access   (Followers: 2)
AKSIOMATIK : Jurnal Penelitian Pendidikan dan Pembelajaran Matematika     Open Access  
Aksis : Jurnal Pendidikan Bahasa dan Sastra Indonesia     Open Access  
Al-Athfaal : Jurnal Ilmiah Pendidikan Anak Usia Dini     Open Access  
Al-Bahith Journal     Open Access  
Al-Idarah : Jurnal Kependidikan Islam     Open Access  
Al-Jabar : Jurnal Pendidikan Matematika     Open Access  
Al-Mudarris : Journal of Education     Open Access   (Followers: 2)
Al-Tadris : Jurnal Pendidikan Bahasa Arab     Open Access  
Al-Tadzkiyyah : Jurnal Pendidikan Islam     Open Access  
Al-Tanzim : Jurnal Manajemen Pendidikan Islam     Open Access  
Al.Qadisiya journal for the Sciences of Physical Education     Open Access  
Alberta Journal of Educational Research     Full-text available via subscription   (Followers: 1)
Aldaba     Open Access  
Alexandria : Revista de Educação em Ciência e Tecnologia     Open Access  
Alotrop     Open Access  
Alsic : Apprentissage des Langues et Systèmes d'Information et de Communication     Open Access   (Followers: 12)
Alteridad     Open Access  
Ambiente & Educação : Revista de Educação Ambiental     Open Access  
American Annals of the Deaf     Full-text available via subscription   (Followers: 13)
American Journal of Business Education     Open Access   (Followers: 14)
American Journal of Distance Education     Hybrid Journal   (Followers: 30)
American Journal of Education     Full-text available via subscription   (Followers: 162)
American Journal of Educational Research     Open Access   (Followers: 55)
American Journal of Health Education     Hybrid Journal   (Followers: 35)
American Journal of Physics     Full-text available via subscription   (Followers: 57)
Ana Dili Eğitimi Dergisi / Journal of Mother Tongue Education     Open Access   (Followers: 1)
ANALES de la Universidad Central del Ecuador     Open Access   (Followers: 1)
Anargya : Jurnal Ilmiah Pendidikan Matematika     Open Access  
Annales Universitatis Mariae Curie-Sklodowska, sectio N – Educatio Nova     Open Access  
Annali dell'Universita di Ferrara     Hybrid Journal  
Annals of Dyslexia     Hybrid Journal   (Followers: 9)
Annals of Modern Education     Full-text available via subscription   (Followers: 5)
Antistasis : An Open Educational Journal     Open Access   (Followers: 1)
Anuario Pilquen : Sección Divulgación Científica     Open Access  
Apertura. Revista de innovación educativa‏     Open Access   (Followers: 1)
Ápice : Revista de Educación Científica     Open Access  
Applied Environmental Education & Communication     Hybrid Journal   (Followers: 15)
Applied Measurement in Education     Hybrid Journal   (Followers: 11)
Aprender     Open Access  
AR-RIAYAH : Jurnal Pendidikan Dasar     Open Access  
Arabia     Open Access   (Followers: 1)
Arabiyatuna : Jurnal Bahasa Arab     Open Access   (Followers: 1)
Archivos de Ciencias de la Educación     Open Access   (Followers: 1)
Areté, Revista Digital del Doctorado en Educación de la Universidad Central de Venezuela     Open Access  
Arrancada     Open Access  
Ars Educandi     Open Access  
Art Design & Communication in Higher Education     Hybrid Journal   (Followers: 22)
Art Education     Hybrid Journal  
Arts Education Policy Review     Hybrid Journal   (Followers: 4)
Artseduca : Revista electrónica de educación en las ARTES     Open Access  
ASHE Higher Education Reports     Hybrid Journal   (Followers: 18)
Asia Pacific Education Review     Hybrid Journal   (Followers: 12)
Asia Pacific Journal of Education     Hybrid Journal   (Followers: 20)
Asia-Pacific Education Researcher     Hybrid Journal   (Followers: 15)
Asia-Pacific Journal of Teacher Education     Hybrid Journal   (Followers: 27)
Asia-Pacific Science Education     Open Access   (Followers: 1)
Asian Association of Open Universities Journal     Open Access   (Followers: 1)
Asian Education and Development Studies     Hybrid Journal   (Followers: 5)
Asian Journal of Distance Education     Open Access   (Followers: 2)
Asian Journal of Education and Social Studies     Open Access   (Followers: 1)
Asian Journal of English Language Teaching     Full-text available via subscription   (Followers: 16)
Asian-Pacific Journal of Second and Foreign Language Education     Open Access   (Followers: 5)
ASp     Open Access  
Assessing Writing     Hybrid Journal   (Followers: 16)
Assessment & Evaluation in Higher Education     Hybrid Journal   (Followers: 125)
Assessment in Education: Principles, Policy & Practice     Hybrid Journal   (Followers: 43)
Assessment Update     Hybrid Journal   (Followers: 5)
AStA Wirtschafts- und Sozialstatistisches Archiv     Hybrid Journal   (Followers: 3)
At-Ta'dib Jurnal Kependidikan Islam     Open Access  
At-Taqaddum     Open Access  
At-Turats     Open Access  
ATENA Didaktik     Open Access  
Athenea Digital     Open Access  
ATIKAN : Jurnal Kajian Pendidikan (Journal of Educational Studies)     Open Access  
Aula Abierta     Open Access  
Aula de Encuentro     Open Access  
Australasian Journal of Educational Technology     Open Access   (Followers: 13)
Australasian Journal of Engineering Education     Hybrid Journal   (Followers: 3)
Australasian Journal of Gifted Education     Full-text available via subscription   (Followers: 5)
Australian Art Education     Full-text available via subscription   (Followers: 6)
Australian Educational Researcher     Hybrid Journal   (Followers: 29)
Australian Journal of Adult Learning     Full-text available via subscription   (Followers: 13)
Australian Journal of Dyslexia and Other Learning Difficulties     Full-text available via subscription   (Followers: 7)
Australian Journal of Environmental Education     Full-text available via subscription   (Followers: 9)
Australian Journal of Indigenous Education, The     Full-text available via subscription   (Followers: 13)
Australian Journal of Learning Difficulties     Hybrid Journal   (Followers: 7)
Australian Journal of Music Education     Full-text available via subscription   (Followers: 6)
Australian Journal of Public Administration     Hybrid Journal   (Followers: 230)
Australian Journal of Teacher Education     Open Access   (Followers: 28)
Australian Mathematics Teacher, The     Full-text available via subscription   (Followers: 7)
Australian Primary Mathematics Classroom     Full-text available via subscription   (Followers: 4)
Australian Screen Education     Full-text available via subscription   (Followers: 2)
Australian TAFE Teacher     Full-text available via subscription   (Followers: 4)
Australian Universities' Review, The     Full-text available via subscription   (Followers: 4)
Azalea: Journal of Korean Literature & Culture     Full-text available via subscription   (Followers: 3)
Bahastra     Open Access  
Balkan Region Conference on Engineering and Business Education     Open Access   (Followers: 2)
Baltic Journal of Career Education and Management     Open Access  
Barn : Forskning om barn og barndom i Norden     Open Access  
Basastra : Jurnal Bahasa, Sastra, dan Pengajarannya     Open Access  
BC TEAL Journal     Open Access  
Becoming : Journal of the Georgia Middle School Association     Open Access  
Behavioural Sciences Undergraduate Journal     Open Access   (Followers: 1)
BELAJEA : Jurnal Pendidikan Islam     Open Access  
BELIA : Early Childhood Education Papers     Open Access   (Followers: 8)
Berkeley Review of Education     Open Access   (Followers: 10)
Biblioteca Escolar em Revista     Open Access  
Biblioteka i Edukacja     Open Access   (Followers: 4)
Bio-Lectura     Open Access  
BIODIK : Jurnal Ilmiah Pendidikan Biologi     Open Access  
Bioedukasi : Jurnal Pendidikan Biologi FKIP UM Metro     Open Access   (Followers: 1)
Bioeduscience     Open Access  
Bioma : Jurnal Ilmiah Biologi     Open Access  
Biomedical Engineering Education     Hybrid Journal  
Biosfer : Jurnal Biologi dan Pendidikan Biologi     Open Access  
Biosfer : Jurnal Tadris Biologi     Open Access  
BISE : Jurnal Pendidikan Bisnis dan Ekonomi     Open Access  
Biuletyn Historii Wychowania     Open Access  
BMC Medical Education     Open Access   (Followers: 45)
Boletim Cearense de Educação e História da Matemática     Open Access  
Boletim Técnico do Senac     Open Access  
Bordón : Revista de Pedagogía     Open Access  
British (Jurnal Bahasa dan Sastra Inggris)     Open Access  
British Educational Research Journal     Hybrid Journal   (Followers: 157)
British Journal of Educational Studies     Hybrid Journal   (Followers: 127)
British Journal of Educational Technology     Hybrid Journal   (Followers: 94)
British Journal of Music Education     Hybrid Journal   (Followers: 18)
British Journal of Religious Education     Hybrid Journal   (Followers: 7)
British Journal of Sociology of Education     Hybrid Journal   (Followers: 57)
British Journal of Special Education     Hybrid Journal   (Followers: 43)
Brock Education : A Journal of Educational Research and Practice     Open Access  
Brookings Trade Forum     Full-text available via subscription   (Followers: 4)
Buckingham Journal of Education     Open Access   (Followers: 6)
Bulletin De L' Association Thaïlandaise Des Professeurs de Français     Open Access  
Business, Management and Education     Open Access   (Followers: 17)
Caderno Brasileiro de Ensino de Física     Open Access  
Caderno de Educação     Open Access  
Caderno Intersaberes     Open Access  
Cadernos de Educação     Open Access  
Cadernos de Estudos e Pesquisa na Educação Básica     Open Access  
Cadernos de Pesquisa     Open Access  
Cadernos de Pesquisa em Educação     Open Access  
Cadmo     Full-text available via subscription  
Cahiers de la recherche sur l'éducation et les savoirs     Open Access   (Followers: 3)
Cakrawala Pendidikan     Open Access  
Calidad en la educación     Open Access   (Followers: 1)
Cambridge Journal of Education     Hybrid Journal   (Followers: 69)
Campbell Systematic Reviews     Open Access   (Followers: 4)
Campus Legal Advisor     Hybrid Journal   (Followers: 2)
Campus Security Report     Hybrid Journal   (Followers: 1)
Canadian and International Education     Open Access   (Followers: 9)
Canadian Journal for New Scholars in Education/ Revue canadienne des jeunes chercheures et chercheurs en éducation     Open Access   (Followers: 7)
Canadian Journal for the Scholarship of Teaching and Learning     Open Access   (Followers: 18)
Canadian Journal for the Study of Adult Education     Open Access   (Followers: 1)
Canadian Journal of Education : Revue canadienne de l'éducation     Open Access   (Followers: 11)
Canadian Journal of Educational Administration and Policy     Open Access  

        1 2 3 4 5 6 7 8 | Last

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AKSIOMA Journal of Mathematics Education
Number of Followers: 2  

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ISSN (Print) 2089-8703 - ISSN (Online) 2442-5419
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  • Fluids, Vol. 7, Pages 1: A CFD Comparative Study of Bubbling Fluidized Bed
           Behavior with Thermal Effects Using the Open-Source Platforms MFiX and
           OpenFOAM

    • Authors: Andrés Reyes-Urrutia, Cesar Venier, Néstor Javier Mariani, Norberto Nigro, Rosa Rodriguez, Germán Mazza
      First page: 1
      Abstract: This work studies the performance of two open-source CFD codes, OpenFOAM and MFiX, to address bubbling fluidized bed system at different temperature and heat transfer conditions. Both codes are used to predict two parameters that are relevant for the design of fluidized units: the minimum fluidization velocity as a function of the temperature of the bed and wall-to-bed heat transfer coefficient from a lateral wall and from internal tubes. Although the CFD solvers are structuraly similar, there are some key differences (available models, meshing techniques, and balance formulations) that are often translated into differences in the fields prediction. The computational results are compared between both codes and against the experimental data. The minimum fluidization velocity can be correctly predicted with both codes at different temperatures while, in general, for the heat transfer and the fluidization patterns, MFiX shows slightly more accurate results compared to OpenFOAM but with low versatility for meshing curved geometries which might translate into higher computational costs for the same level of accuracy.
      Citation: Fluids
      PubDate: 2021-12-21
      DOI: 10.3390/fluids7010001
      Issue No: Vol. 7, No. 1 (2021)
       
  • Fluids, Vol. 7, Pages 2: Dynamics of a Laser-Induced Bubble above the Flat
           Top of a Solid Cylinder—Mushroom-Shaped Bubbles and the Fast Jet

    • Authors: Max Koch, Juan Manuel Rosselló, Christiane Lechner, Werner Lauterborn, Robert Mettin
      First page: 2
      Abstract: The dynamics of a laser-induced bubble on top of a solid cylinder is studied both experimentally and numerically. When the bubble is generated close to the flat top along the axis of the cylinder and its maximum radius exceeds the one of the flat top surface, it collapses in the form of a mushroom with a footing on the cylinder, a long stem and a hat-like cap typical for a mushroom head. The head may collapse forming a thin, fast liquid jet into the stem, depending on bubble size and bubble distance to the top of the cylinder. Several experimental and numerical examples are given. The results represent a contribution to understand the behavior of bubbles collapsing close to structured surfaces and in particular, how thin, fast jets are generated.
      Citation: Fluids
      PubDate: 2021-12-21
      DOI: 10.3390/fluids7010002
      Issue No: Vol. 7, No. 1 (2021)
       
  • Fluids, Vol. 7, Pages 3: Velocity Field Measurements of the California Sea
           Lion Propulsive Stroke Using Bubble PIV

    • Authors: Gino Perrotta, Frank E. Fish, Danielle S. Adams, Ariel M. Leahy, Abigal M. Downs, Megan C. Leftwich
      First page: 3
      Abstract: California sea lions are among the most agile of swimming mammals. Most marine mammals swim with their hind appendages—flippers or flukes, depending on the species—whereas sea lions use their foreflippers for propulsion and maneuvering. The sea lion’s propulsive stroke generates thrust by forming a jet between the flippers and the body and by dragging a starting vortex along the suction side of the flipper. Prior experiments using robotic flippers have shown these mechanisms to be possible, but no flow measurements around live sea lions previously existed with which to compare. In this study, the flow structures around swimming sea lions were observed using an adaptation of particle imaging velocimetry. To accommodate the animals, it was necessary to use bubbles as seed particles and sunlight for illumination. Three trained adult California sea lions were guided to swim through an approximately planar sheet of bubbles in a total of 173 repetitions. The captured videos were used to calculate bubble velocities, which were processed to isolate and inspect the flow velocities caused by the swimming sea lion. The methodology will be discussed, and measured flow velocities will be presented.
      Citation: Fluids
      PubDate: 2021-12-22
      DOI: 10.3390/fluids7010003
      Issue No: Vol. 7, No. 1 (2021)
       
  • Fluids, Vol. 7, Pages 4: The Hydrodynamic Behavior of Vortex Shedding
           behind Circular Cylinder in the Presence of Group Focused Waves

    • Authors: Iskander Abroug, Nizar Abcha, Fahd Mejri, Emma Turki, Elena Ojeda
      First page: 4
      Abstract: Vortex shedding behind an elastically mounted circular cylinder in the presence of group focused waves propagating upstream was investigated using a classical approach (time series and FFT) and nonclassical approach (complex 2D Morlet wavelets). Wavelet analysis emerged as a novel solution in this regard. Our results include wave trains with different nonlinearities propagating in different water depths and derived from three types of spectra (Pierson–Moskowitz, JONSWAP (γ = 3.3 or γ = 7)). It was found that the generated wave trains could modify regimes of shedding behind the cylinder, and subharmonic frequency lock-in could arise in particular situations. The occurrence of a lock-in regime in the case of wave trains propagating in intermediate water locations was shown experimentally even for small nonlinearities. Moreover, the application of time-localized wavelet analysis was found to be a powerful approach. In fact, the frequency lock-in regime and its duration could be readily identified from the wavelet-based energy and its corresponding ridges.
      Citation: Fluids
      PubDate: 2021-12-22
      DOI: 10.3390/fluids7010004
      Issue No: Vol. 7, No. 1 (2021)
       
  • Fluids, Vol. 7, Pages 5: Numerical Study of Large-Scale Fire in
           Makkah’s King Abdulaziz Road Tunnel

    • Authors: Kamel Guedri, Abdullah A. Abdoon, Omar S. Bagabar, Mowffaq Oreijah, Abdessattar Bouzid, Shadi M. Munshi
      First page: 5
      Abstract: Tunnel fires are one of the most dangerous catastrophic events that endanger human life. They cause damage to infrastructure because of the limited space in the tunnel, lack of escape facilities, and difficulty that intervention forces have in reaching the fire position, especially in highly crowded areas, such as Makkah in the Hajj season. Unfortunately, performing experimental tests on tunnel fire safety is particularly challenging because of the prohibitive cost, limited possibilities, and losses that these tests can cause. Therefore, large-scale modeling, using fire dynamic simulation, is one of the best techniques used to limit these costs and losses. In the present work, a fire scenario in the Makkah’s King Abdulaziz Road tunnel was analyzed using the Fire Dynamics Simulator (FDS). The effects of the heat released per unit area, soot yield, and CO yield on the gas temperature, radiation, concentrations of the oxygen and combustion products CO and CO2, and air velocity were examined. The results showed that the radiation increased with the heat released per unit area and the soot yield affected all parameters, except the oxygen concentration and air velocity. The CO yield significantly affects CO concentration, and its influence on the other studied parameters is negligible. Moreover, based on the validation part, the results proved that FDS have limitations in tunnel fires, which impact the smoke layer calculation at the upstream zone of the fire. Therefore, the users or researchers should carefully be concerned about these weaknesses when using FDS to simulate tunnel fires. Further comprehensive research is crucial, as tunnel fires have severe impacts on various aspects of people’s lives.
      Citation: Fluids
      PubDate: 2021-12-22
      DOI: 10.3390/fluids7010005
      Issue No: Vol. 7, No. 1 (2021)
       
  • Fluids, Vol. 7, Pages 6: A New Anisotropic Four-Parameter Turbulence Model
           for Low Prandtl Number Fluids

    • Authors: Giacomo Barbi, Valentina Giovacchini, Sandro Manservisi
      First page: 6
      Abstract: Due to their interesting thermal properties, liquid metals are widely studied for heat transfer applications where large heat fluxes occur. In the framework of the Reynolds-Averaged Navier–Stokes (RANS) approach, the Simple Gradient Diffusion Hypothesis (SGDH) and the Reynolds Analogy are almost universally invoked for the closure of the turbulent heat flux. Even though these assumptions can represent a reasonable compromise in a wide range of applications, they are not reliable when considering low Prandtl number fluids and/or buoyant flows. More advanced closure models for the turbulent heat flux are required to improve the accuracy of the RANS models dealing with low Prandtl number fluids. In this work, we propose an anisotropic four-parameter turbulence model. The closure of the Reynolds stress tensor and turbulent heat flux is gained through nonlinear models. Particular attention is given to the modeling of dynamical and thermal time scales. Numerical simulations of low Prandtl number fluids have been performed over the plane channel and backward-facing step configurations.
      Citation: Fluids
      PubDate: 2021-12-22
      DOI: 10.3390/fluids7010006
      Issue No: Vol. 7, No. 1 (2021)
       
  • Fluids, Vol. 6, Pages 421: Predictions of Vortex Flow in a Diesel
           Multi-Hole Injector Using the RANS Modelling Approach

    • Authors: Aishvarya Kumar, Jamshid Nouri, Ali Ghobadian
      First page: 421
      Abstract: The occurrence of vortices in the sac volume of automotive multi-hole fuel injectors plays an important role in the development of vortex cavitation, which directly influences the flow structure and emerging sprays that, in turn, influence the engine performance and emissions. In this study, the RANS-based turbulence modelling approach was used to predict the internal flow in a vertical axis-symmetrical multi-hole (6) diesel fuel injector under non-cavitating conditions. The project aimed to predict the aforementioned vortical structures accurately at two different needle lifts in order to form a correct opinion about their occurrence. The accuracy of the simulations was assessed by comparing the predicted mean axial velocity and RMS velocity of LDV measurements, which showed good agreement. The flow field analysis predicted a complex, 3D, vortical flow structure with the presence of different types of vortices in the sac volume and the nozzle hole. Two main types of vortex were detected: the “hole-to-hole” connecting vortex, and double “counter-rotating” vortices emerging from the needle wall and entering the injector hole facing it. Different flow patterns in the rotational direction of the “hole-to-hole” vortices have been observed at the low needle lift (anticlockwise) and full needle lift (clockwise), due to their different flow passages in the sac, causing a much higher momentum inflow at the lower lift with its much narrower flow passage.
      Citation: Fluids
      PubDate: 2021-11-23
      DOI: 10.3390/fluids6120421
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 422: Turbulent Characteristics and Air Entrainment
           Patterns in Breaking Surge Waves

    • Authors: Zhuoran Li, Akash Venkateshwaran, Shooka Karimpour
      First page: 422
      Abstract: Breaking surge waves are highly turbulent three-dimensional (3D) flows, which occur when the water flow encounters a sudden change in depth or velocity. The 3D turbulent structures across a breaking surge are induced by the velocity gradient across the surge and phase discontinuity at the front. This paper examined the turbulent structures in breaking surge waves with Froude numbers of 1.71 and 2.13 by investigating the air entrainment and perturbation patterns across the surge front. A combination of the Volume Of Fluid (VOF) method and Large Eddy Simulation (LES) was utilized to capture air entrainment and turbulent structures simultaneously. The 3D nature of the vortical structures was simulated by implementing a spanwise periodic boundary. The water surface perturbation and air concentration profiles were extracted, and the averaged air concentration profiles obtained from the numerical simulations were consistent with laboratory observations reported in the literature. The linkage between turbulent kinetic energy distribution and air entrainment was also explored in this paper. Finally, using quadrant analysis and the Q-criterion, this paper examined the role of the spanwise perturbations in the development of turbulent structures in the surge front.
      Citation: Fluids
      PubDate: 2021-11-23
      DOI: 10.3390/fluids6120422
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 423: Stability and Consolidation of Sediment
           Tailings Incorporating Unsaturated Soil Mechanics

    • Authors: Alfrendo Satyanaga, Martin Wijaya, Qian Zhai, Sung-Woo Moon, Jaan Pu, Jong R. Kim
      First page: 423
      Abstract: Tailing dams are commonly used to safely store tailings without damaging the environment. Sand tailings (also called Sediment tailings) usually have a high water content and hence undergo consolidation during their placement. As the sediment tailings are usually placed above the ground water level, the degree of saturation and permeability of the sediment tailing is associated with the unsaturated condition due to the presence of negative pore-water pressure or suction. Current practices normally focus on the analyses saturated conditions. However, this consolidation process requires the flow of water between saturated and unsaturated zones to be considered. The objective of this study is to investigate the stability and consolidation of sediment tailings for the construction of road pillars considering the water flow between saturated and unsaturated zones. The scope of this study includes the unsaturated laboratory testing of sediments and numerical analyses of the road pillar. The results show that the analyses based on saturated conditions overestimate the time required to achieve a 90% degree of consolidation. The incorporation of the unsaturated soil properties is able to optimize the design of slopes for road pillars into steeper slope angles.
      Citation: Fluids
      PubDate: 2021-11-24
      DOI: 10.3390/fluids6120423
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 424: LES of Particle-Laden Flow in Sharp Pipe Bends
           with Data-Driven Predictions of Agglomerate Breakage by Wall Impacts

    • Authors: Ali Khalifa, Jasper Gollwitzer, Michael Breuer
      First page: 424
      Abstract: The breakage of agglomerates due to wall impact within a turbulent two-phase flow is studied based on a recently developed model which relies on two artificial neural networks (ANNs). The breakup model is intended for the application within an Euler-Lagrange approach using the point-particle assumption. The ANNs were trained based on comprehensive DEM simulations. In the present study the entire simulation methodology is applied to the flow through two sharp pipe bends considering two different Reynolds numbers. In a first step, the flow structures of the continuous flow arising in both bend configurations are analyzed in detail. In a second step, the breakage behavior of agglomerates consisting of spherical, dry and cohesive silica particles is predicted based on the newly established simulation methodology taking agglomeration, fluid-induced breakage and breakage due to wall impact into account. The latter is found to be the dominant mechanism determining the resulting size distribution at the bend outlet. Since the setups are generic geometries found in dry powder inhalers, important knowledge concerning the effect of the Reynolds number as well as the design type (one-step vs. two-step deflection) can be gained.
      Citation: Fluids
      PubDate: 2021-11-25
      DOI: 10.3390/fluids6120424
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 425: Two-Dimensional Steady Boussinesq Convection:
           Existence, Computation and Scaling

    • Authors: Jeremiah S. Lane, Benjamin F. Akers
      First page: 425
      Abstract: This research investigates laser-induced convection through a stream function-vorticity formulation. Specifically, this paper considers a solution to the steady Boussinesq Navier–Stokes equations in two dimensions with a slip boundary condition on a finite box. A fixed-point algorithm is introduced in stream function-vorticity variables, followed by a proof of the existence of steady solutions for small laser amplitudes. From this analysis, an asymptotic relationship is demonstrated between the nondimensional fluid parameters and least upper bounds for laser amplitudes that guarantee existence, which accords with numerical results implementing the algorithm in a finite difference scheme. The findings indicate that the upper bound for laser amplitude scales by O(Re−2Pe−1Ri−1) when Re≫Pe, and by O(Re−1Pe−2Ri−1) when Pe≫Re. These results suggest that the existence of steady solutions is heavily dependent on the size of the Reynolds (Re) and Peclet (Pe) numbers, as noted in previous studies. The simulations of steady solutions indicate the presence of symmetric vortex rings, which agrees with experimental results described in the literature. From these results, relevant implications to thermal blooming in laser propagation simulations are discussed.
      Citation: Fluids
      PubDate: 2021-11-25
      DOI: 10.3390/fluids6120425
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 426: Optimization of a Grid-Connected Microgrid
           Using Tidal and Wind Energy in Cook Strait

    • Authors: Navid Majdi Nasab, Md Rabiul Islam, Kashem Muttaqi, Danny Sutanto
      First page: 426
      Abstract: The Cook Strait in New Zealand is an ideal location for wind and tidal renewable sources of energy due to its strong winds and tidal currents. The integration of both technologies can help to avoid the detrimental effects of fossil fuels and to reduce the cost of electricity. Although tidal renewable sources have not been used for electricity generation in New Zealand, a recent investigation, using the MetOcean model, has identified Terawhiti in Cook Strait as a superior location for generating tidal power. This paper investigates three different configurations of wind, tidal, and wind plus tidal sources to evaluate tidal potential. Several simulations have been conducted to design a DC-linked microgrid for electricity generation in Cook Strait using HOMER Pro, RETScreen, and WRPLOT software. The results show that Terawhiti, in Cook Strait, is suitable for an offshore wind farm to supply electricity to the grid, considering the higher renewable fraction and the lower net present cost in comparison with those using only tidal turbines or using both wind and tidal turbines.
      Citation: Fluids
      PubDate: 2021-11-25
      DOI: 10.3390/fluids6120426
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 427: Shock Structures Using the OBurnett Equations
           in Combination with the Holian Conjecture

    • Authors: Ravi Sudam Jadhav, Amit Agrawal
      First page: 427
      Abstract: In the present work, we study the normal shock wave flow problem using a combination of the OBurnett equations and the Holian conjecture. The numerical results of the OBurnett equations for normal shocks established several fundamental aspects of the equations such as the thermodynamic consistency of the equations, and the existence of the heteroclinic trajectory and smooth shock structures at all Mach numbers. The shock profiles for the hydrodynamic field variables were found to be in quantitative agreement with the direct simulation Monte Carlo (DSMC) results in the upstream region, whereas further improvement was desirable in the downstream region of the shock. For the discrepancy in the downstream region, we conjecture that the viscosity–temperature relation (μ∝Tφ) needs to be modified in order to achieve increased dissipation and thereby achieve better agreement with the benchmark results in the downstream region. In this respect, we examine the Holian conjecture (HC), wherein transport coefficients (absolute viscosity and thermal conductivity) are evaluated using the temperature in the direction of shock propagation rather than the average temperature. The results of the modified theory (OBurnett + HC) are compared against the benchmark results and we find that the modified theory improves upon the OBurnett results, especially in the case of the heat flux shock profile. We find that the accuracy gain is marginal at lower Mach numbers, while the shock profiles are described better using the modified theory for the case of strong shocks.
      Citation: Fluids
      PubDate: 2021-11-26
      DOI: 10.3390/fluids6120427
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 428: Flow Dynamics in a Model of a Left Ventricle
           with Different Mitral Valve Orientations

    • Authors: Ghassan Maraouch, Lyes Kadem
      First page: 428
      Abstract: The formation of vortex rings at valve leaflets during ventricular inflow has been a topic of interest for many years. It is generally accepted nowadays that the purpose of vortex rings is to conserve energy, reduce the workload on the heart, and minimize particle residence time. We investigated these claims by testing three different levels of annulus angle for the mitral valve: a healthy case, a slightly angled case (20°), and a highly angled case (46°). Circulation was determined to be reversed in the non-healthy case, with a dominant counterclockwise rotation instead of clockwise. Viscous energy dissipation was highest in the slightly angled case, followed by the healthy case and then the highly angled case. A Lagrangian analysis demonstrated that the healthy case resulted in the least amount of stasis, requiring eight cardiac cycles to evacuate 99% of initial ventricle volume compared to the 16 and 13 cardiac cycles required by the slightly angled and highly angled cases, respectively.
      Citation: Fluids
      PubDate: 2021-11-26
      DOI: 10.3390/fluids6120428
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 429: Tomographic Particle Image Velocimetry and
           Dynamic Mode Decomposition (DMD) in a Rectangular Impinging Jet: Vortex
           Dynamics and Acoustic Generation

    • Authors: Hassan H. Assoum, Jana Hamdi, Marwan Alkheir, Kamel Abed Meraim, Anas Sakout, Bachar Obeid, Mouhammad El Hassan
      First page: 429
      Abstract: Impinging jets are encountered in ventilation systems and many other industrial applications. Their flows are three-dimensional, time-dependent, and turbulent. These jets can generate a high level of noise and often present a source of discomfort in closed areas. In order to reduce and control such mechanisms, one should investigate the flow dynamics that generate the acoustic field. The purpose of this study is to investigate the flow dynamics and, more specifically, the coherent structures involved in the acoustic generation of these jets. Model reduction techniques are commonly used to study the underlying mechanisms by decomposing the flow into coherent structures. The dynamic mode decomposition (DMD) is an equation-free method that relies only on the system’s data taken either through experiments or through numerical simulations. In this paper, the DMD technique is applied, and the spatial modes and their frequencies are presented. The temporal content of the DMD’s modes is then correlated with the acoustic signal. The flow is generated by a rectangular jet impinging on a slotted plate (for a Reynolds number Re = 4458) and its kinematic field is obtained via the tomographic particle image velocimetry technique (TPIV). The findings of this research highlight the coherent structures signature in the DMD’s spectral content and show the cross correlations between the DMD’s modes and the acoustic field.
      Citation: Fluids
      PubDate: 2021-11-27
      DOI: 10.3390/fluids6120429
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 430: Combined Stereoscopic Particle Image
           Velocimetry Measurements in a Single Plane for an Impinging Jet around a
           Thin Control Rod

    • Authors: Marwan Alkheir, Hassan H. Assoum, Nour Eldin Afyouni, Kamel Abed Meraim, Anas Sakout, Mouhammad El Hassan
      First page: 430
      Abstract: Impinging jets are of high interest in many industrial applications and their flow dynamics has a complex three-dimensional behavior. These jets can result in a high noise generation leading to acoustic discomfort. Thus, a passive control mechanism which consists of introducing a thin rod in the flow of the jet is proposed in order to reduce the noise generation. The stereoscopic particle image velocimetry (SPIV) technique is employed to measure the three velocity components in a plane. An experimental difficulty is encountered to acquire images of the flow in the shadow of the rod which block a part of the field of interest. In this paper, an experimental arrangement is proposed in order to overcome this experimental difficulty using a combined SPIV technique denoted by (C-SPIV). This technique consists of using an inclined mirror to illuminate the area under the rod by reflecting the laser light and two independent systems of SPIV synchronized and correlated together in order to obtain the combined field of velocity in the same plane above and below the rod. The C-SPIV measurements allowed to obtain the kinematic field in the whole area of interest. Thus, vortex shedding frequency, Turbulent Kinetic Energy were calculated and analyzed along with the acoustic signal. These results are of high interest when seeking for noise reduction in such jet configuration.
      Citation: Fluids
      PubDate: 2021-11-28
      DOI: 10.3390/fluids6120430
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 431: The Emergence and Identification of Large-Scale
           Coherent Structures in Free Convective Flows of the Rayleigh-Bénard
           Type

    • Authors: Sergei Smirnov, Alexander Smirnovsky, Sergey Bogdanov
      First page: 431
      Abstract: The revealing of the turbulence archetypes is one of the fundamental problems in the study of turbulence, which is important not only from the fundamental point of view but also for practical applications, e.g., in geophysics of ocean and lakes. The paper is devoted to the study of the emergence of coherent structures and the identification of their turbulent archetypes, typical for the free convective flows of the Rayleigh-Bénard type. Using Direct Numerical Simulation, we perform a numerical study of two refined convective flows: convection in a cylinder heated from below and internally heated convection in a layer. The main purpose of the study is identifying coherent structures (CS), investigating its main features and properties, and determining the turbulence archetypes using the anisotropy invariant map (AIM). We show that, in both configurations considered, CS takes place. In a cylinder, CS is a single large-scale vortex that can rotate azimuthally in non-titled container, but is almost “fixed” in the case of slightly tilted cylinder; in a layer, CS is a quasi-2D vortex, which can arise, exist for some time, disrupt, and then re-emerge again in the orthogonal direction. Nevertheless, the turbulence archetypes represented by the AIM are quite similar for both cases, and there are the distinct CS fingerprints on AIM.
      Citation: Fluids
      PubDate: 2021-11-29
      DOI: 10.3390/fluids6120431
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 432: Onset of Thermal Instabilities in the Plane
           Poiseuille Flow of Weakly Elastic Fluids: Viscous Dissipation Effects

    • Authors: Silvia C. Hirata, Mohamed Najib Ouarzazi
      First page: 432
      Abstract: The onset of thermal instabilities in the plane Poiseuille flow of weakly elastic fluids is examined through a linear stability analysis by taking into account the effects of viscous dissipation. The destabilizing thermal gradients may come from the different temperatures imposed on the external boundaries and/or from the volumetric heating induced by viscous dissipation. The rheological properties of the viscoelastic fluid are modeled using the Oldroyd-B constitutive equation. As in the Newtonian fluid case, the most unstable structures are found to be stationary longitudinal rolls (modes with axes aligned along the streamwise direction). For such structures, it is shown that the viscoelastic contribution to viscous dissipation may be reduced to one unique parameter: γ=λ1(1−Γ), where λ1 and Γ represent the relaxation time and the viscosity ratio of the viscoelastic fluid, respectively. It is found that the influence of the elasticity parameter γ on the linear stability characteristics is non-monotonic. The fluid elasticity stabilizes (destabilizes) the basic Poiseuille flow if γ<γ* (γ>γ*) where γ* is a particular value of γ that we have determined. It is also shown that when the temperature gradient imposed on the external boundaries is zero, the critical Reynolds number for the onset of such viscous dissipation/viscoelastic-induced instability may be well below the one needed to trigger the pure hydrodynamic instability in weakly elastic solutions.
      Citation: Fluids
      PubDate: 2021-11-29
      DOI: 10.3390/fluids6120432
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 433: Evaluation of the Thermofluidic Performance of
           Climatic Chambers: Numerical and Experimental Studies

    • Authors: Bahareh Ramezani, António Tadeu, Tiago Jesus, Michael Brett, Joel Mendes
      First page: 433
      Abstract: Climatic chambers are highly important in research and industrial applications and are used to examine manufactured samples, specimens, or components in controlled environment conditions. Despite the growing industrial demand for climatic chambers, only a few published studies have specifically concentrated on performance analysis and functional improvements through numerical and experimental studies. In this study, a 3D computational fluid dynamics (CFD) model of a climatic chamber was developed using Ansys Fluent to simulate the fluid flow, heat, and mass transfer to obtain the velocity, temperature, and relative humidity fields in the interior box of a 1200 L climatic chamber. The results were then validated with experimental data from a prototype. Finally, the heat losses of the surrounding components of the chamber were calculated, and the relationship between the inside temperature and the overall thermal loss was modelled. This validated numerical model provides the possibility of optimising the performance of climate chambers by reducing the thermal loss from the walls and modifying the air flow pattern inside the chamber.
      Citation: Fluids
      PubDate: 2021-11-30
      DOI: 10.3390/fluids6120433
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 434: Equation of State’s Crossover Enhancement
           of Pseudopotential Lattice Boltzmann Modeling of CO2 Flow in Homogeneous
           Porous Media

    • Authors: Assetbek Ashirbekov, Bagdagul Kabdenova, Ernesto Monaco, Luis R. Rojas-Solórzano
      First page: 434
      Abstract: The original Shan-Chen’s pseudopotential Lattice Boltzmann Model (LBM) has continuously evolved during the past two decades. However, despite its capability to simulate multiphase flows, the model still faces challenges when applied to multicomponent-multiphase flows in complex geometries with a moderately high-density ratio. Furthermore, classical cubic equations of state usually incorporated into the model cannot accurately predict fluid thermodynamics in the near-critical region. This paper addresses these issues by incorporating a crossover Peng–Robinson equation of state into LBM and further improving the model to consider the density and the critical temperature differences between the CO2 and water during the injection of the CO2 in a water-saturated 2D homogeneous porous medium. The numerical model is first validated by analyzing the supercritical CO2 penetration into a single narrow channel initially filled with H2O, depicting the fundamental role of the driving pressure gradient to overcome the capillary resistance in near one and higher density ratios. Significant differences are observed by extending the model to the injection of CO2 into a 2D homogeneous porous medium when using a flat versus a curved inlet velocity profile.
      Citation: Fluids
      PubDate: 2021-12-01
      DOI: 10.3390/fluids6120434
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 435: Numerical Modeling of the Wave-Plate-Current
           Interaction by the Boundary Element Method

    • Authors: Hasna Akarni, Laila El Aarabi, Laila Mouakkir, Soumia Mordane
      First page: 435
      Abstract: The aim of this work is to propose a numerical study of the interaction of a wave-horizontal plate fixed and completely immersed in a flat-bottomed tank with a uniform current flowing in the same direction as the incident wave. We investigate in particular the effect of the plate at minimizing the impact of the wave on the coast of beaches by studying the free surface elevation and the reflection coefficient, as well as the influence of the various geometrical parameters on the latter, taking into account the presence of the current. The numerical method used in this study is the boundary element method (BEM), and the results obtained will be confronted with experimental and analytical data existing in the literature.
      Citation: Fluids
      PubDate: 2021-12-01
      DOI: 10.3390/fluids6120435
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 436: Data-Driven Modeling of Geometry-Adaptive
           Steady Heat Convection Based on Convolutional Neural Networks

    • Authors: Jiang-Zhou Peng, Xianglei Liu, Zhen-Dong Xia, Nadine Aubry, Zhihua Chen, Wei-Tao Wu
      First page: 436
      Abstract: Heat convection is one of the main mechanisms of heat transfer, and it involves both heat conduction and heat transportation by fluid flow; as a result, it usually requires numerical simulation for solving heat convection problems. Although the derivation of governing equations is not difficult, the solution process can be complicated and usually requires numerical discretization and iteration of differential equations. In this paper, based on neural networks, we developed a data-driven model for an extremely fast prediction of steady-state heat convection of a hot object with an arbitrary complex geometry in a two-dimensional space. According to the governing equations, the steady-state heat convection is dominated by convection and thermal diffusion terms; thus the distribution of the physical fields would exhibit stronger correlations between adjacent points. Therefore, the proposed neural network model uses convolutional neural network (CNN) layers as the encoder and deconvolutional neural network (DCNN) layers as the decoder. Compared with a fully connected (FC) network model, the CNN-based model is good for capturing and reconstructing the spatial relationships of low-rank feature spaces, such as edge intersections, parallelism, and symmetry. Furthermore, we applied the signed distance function (SDF) as the network input for representing the problem geometry, which contains more information compared with a binary image. For displaying the strong learning and generalization ability of the proposed network model, the training dataset only contains hot objects with simple geometries: triangles, quadrilaterals, pentagons, hexagons, and dodecagons, while the testing cases use arbitrary and complex geometries. According to the study, the trained network model can accurately predict the velocity and temperature field of the problems with complex geometries, which has never been seen by the network model during the model training; and the prediction speed is two orders faster than the CFD. The ability of accurate and extremely fast prediction of the network model suggests the potential of applying reduced-order network models to the applications of real-time control and fast optimization in the future.
      Citation: Fluids
      PubDate: 2021-12-01
      DOI: 10.3390/fluids6120436
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 437: Shapes and Rise Velocities of Single Bubbles in
           a Confined Annular Channel: Experiments and Numerical Simulations

    • Authors: Andrea Cioncolini, Mirco Magnini
      First page: 437
      Abstract: Shapes and rise velocities of single air bubbles rising through stagnant water confined inside an annular channel were investigated by means of experiments and numerical simulations. Fast video imaging and image processing were used for the experiments, whilst the numerical simulations were carried out using the volume of fluid method and the open-source package OpenFOAM. The confinement of the annular channel did not affect the qualitative behavior of the bubbles, which exhibited a wobbling rise dynamic similar to that observed in bubbles rising through unconfined liquids. The effect of the confinement on the shape and rise velocity was evident; the bubbles were less deformed and rose slower in comparison with bubbles rising through unconfined liquids. The present data and numerical simulations, as well as the data collected from the literature for use here, indicate that the size, shape, and rise velocity of single bubbles are closely linked together, and prediction methods that fail to recognize this perform poorly. This study and the limited evidence documented in the literature indicate that the confinement effects observed in non-circular channels of complex shape are more complicated than those observed with circular tubes, and much less well understood.
      Citation: Fluids
      PubDate: 2021-12-02
      DOI: 10.3390/fluids6120437
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 438: Spin-Up from Rest of a Liquid Metal with
           Deformable Free Surface in a Cylinder under the Influence of a Uniform
           Axial Magnetic Field

    • Authors: Toshio Tagawa, Kewei Song
      First page: 438
      Abstract: Spin-up from rest of a liquid metal having deformable free surface in the presence of a uniform axial magnetic field is numerically studied. Both liquid and gas phases in a vertically mounted cylinder are assumed to be an incompressible, immiscible, Newtonian fluid. Since the viscous dissipation and the Joule heating are neglected, thermal convection due to buoyancy and thermocapillary effects is not taken into account. The effects of Ekman number and Hartmann number were computed with fixing the Froude number of 1.5, the density ratio of 800, and the viscosity ratio of 50. The evolutions of the free surface, three-component velocity field, and electric current density are portrayed using the level-set method and HSMAC method. When a uniform axial magnetic field is imposed, the azimuthal momentum is transferred from the rotating bottom wall to the core region directly through the Hartmann layer. This is the most striking difference from spin-up of the nonmagnetic case.
      Citation: Fluids
      PubDate: 2021-12-02
      DOI: 10.3390/fluids6120438
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 439: Stationary Mach Configurations with Pulsed
           Energy Release on the Normal Shock

    • Authors: Mikhail V. Chernyshov, Anna S. Kapralova, Stanislav A. Matveev, Karina E. Savelova
      First page: 439
      Abstract: We obtained a theoretical analysis of stationary Mach configurations of shock waves with a pulsed energy release at the main (normal) shock and a corresponding change in gas thermodynamic properties. As formation of the stationary Mach configuration corresponds to one of two basic, well-known criteria of regular/Mach shock reflection transition, we studied here how the possibility of pulsed energy release at the normal Mach stem shifts the von Neumann criterion, and how it correlates then with another transition criterion (the detachment one). The influence of a decrease in the “equilibrium” gas adiabatic index at the main shock on a shift of the solution domain was also investigated analytically and numerically. Using a standard detonation model for a normal shock in stationary Mach configuration, and ordinary Hugoniot relations for other oblique shocks, we estimated influence of pulsed energy release and real gas effects (expressed by decrease of gas adiabatic index) on shift of von Neumann criterion, and derived some analytical relations that describe those dependencies.
      Citation: Fluids
      PubDate: 2021-12-05
      DOI: 10.3390/fluids6120439
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 440: Pressure Change for Single- and Two-Phase
           Non-Newtonian Flows through Sudden Contraction in Rectangular Microchannel
           

    • Authors: Masaki Toshimitsu, Yukihiro Yonemoto, Akimaro Kawahara
      First page: 440
      Abstract: The flow characteristics of the single-phase liquid and the gas–liquid two-phase flows including the Newtonian and non-Newtonian liquids were experimentally investigated in a horizontal rectangular micro-channel with a sudden contraction—specifically the pressure change across the contraction. The rectangular cross-sectional dimension has Wu × Hu (width × height) = 0.99 × 0.50 mm2 on the upstream side of the contraction and Wd × Hd = 0.49 × 0.50 mm2 on the downstream side. The resulting contraction ratio, σA (=Wd/Wu), was 0.5. Air was used as the test gas (in the case of the gas–liquid two-phase flow experiment), distilled water and three kinds of aqueous solution, i.e., glycerin 25 wt%, xanthangum 0.1 wt% and polyacrylamide 0.11 wt% were used as the test liquid. The pressure distribution in the flow direction upstream and downstream of the channel was measured. The pressure change and loss at the sudden contraction were determined from the pressure distribution. In addition, the pressure change data were compared with the calculation by several correlations proposed by various researchers as well as a newly developed correlation in this study. From the comparisons, it was found that calculations by the newly developed correlations agreed well with the measured values within the error of 30%.
      Citation: Fluids
      PubDate: 2021-12-07
      DOI: 10.3390/fluids6120440
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 441: Thrust Vectoring of a Fixed Axisymmetric
           Supersonic Nozzle Using the Shock-Vector Control Method

    • Authors: Emanuele Resta, Roberto Marsilio, Michele Ferlauto
      First page: 441
      Abstract: The application of the Shock Vector Control (SVC) approach to an axysimmetric supersonic nozzle is studied numerically. SVC is a Fluidic Thrust Vectoring (FTV) strategy that is applied to fixed nozzles in order to realize jet-vectoring effects normally obtained by deflecting movable nozzles. In the SVC method, a secondary air flow injection close to the nozzle exit generates an asymmetry in the wall pressure distribution and side-loads on the nozzle, which are also lateral components of the thrust vector. SVC forcing of the axisymmetric nozzle generates fully three-dimensional flows with very complex structures that interact with the external flow. In the present work, the experimental data on a nozzle designed and tested for a supersonic cruise aircraft are used for validating the numerical tool at different flight Mach numbers and nozzle pressure ratios. Then, an optimal position for the slot is sought and the fully 3D flow at flight Mach number M∞=0.9 is investigated numerically for different values of the SVC forcing.
      Citation: Fluids
      PubDate: 2021-12-07
      DOI: 10.3390/fluids6120441
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 442: Linear Non-Modal Growth of Planar Perturbations
           in a Layered Couette Flow

    • Authors: Emmanouil G. Iliakis, Nikolaos A. Bakas
      First page: 442
      Abstract: Layered flows that are commonly observed in stratified turbulence are susceptible to the Taylor–Caulfield Instability. While the modal stability properties of layered shear flows have been examined, the non-modal growth of perturbations has not been investigated. In this work, the tools of Generalized Stability Theory are utilized to study linear transient growth within a finite time interval of two-dimensional perturbations in an inviscid, three-layer constant shear flow under the Boussinesq approximation. It is found that, for low optimization times, small-scale perturbations utilize the Orr mechanism and achieve growth equal to that in the case of an unstratified flow. For larger optimization times, transient growth is much larger compared to growth for an unstratified flow as the Kelvin–Orr waves comprising the continuous spectrum of the dynamical operator and the gravity edge-waves comprising the discrete spectrum interact synergistically. Maximum growth is obtained for perturbations with scales within the region of instability, but significant growth is maintained for modally stable perturbations as well. For perturbations with scales within the unstable region, the unstable normal modes are excited at high amplitude by their bi-orthogonals. For perturbations with modally stable scales, the Orr mechanism is utilized to excite at high amplitude neutral propagating waves resembling the neutral Taylor–Caulfield modes.
      Citation: Fluids
      PubDate: 2021-12-08
      DOI: 10.3390/fluids6120442
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 443: Visualization and Sound Measurements of
           Vibration Plate in a Boiling Bubble Resonator

    • Authors: Junichiro Ono, Noriyuki Unno, Kazuhisa Yuki, Jun Taniguchi, Shin-ichi Satake
      First page: 443
      Abstract: We developed a boiling bubble resonator (BBR) as a new heat transfer enhancement method aided by boiling bubbles. The BBR is a passive device that operates under its own bubble pressure and therefore does not require an electrical source. In the present study, high-speed visualization of the flow motion of the microbubbles spouted from a vibration plate and the plate motion in the BBR was carried out using high-speed LED lighting and high-speed cameras; the sounds in the boiling chamber were simultaneously captured using a hydrophone. The peak point in the spectrum of the motion of the vibration plate and the peak point in the spectrum of the boiling sound were found to be matched near a critical heat-flux state. Therefore, we found that it is important to match the BBR vibration frequency to the condensation cycle of the boiling bubble as its own design specification for the BBR.
      Citation: Fluids
      PubDate: 2021-12-09
      DOI: 10.3390/fluids6120443
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 444: Reduction of Energy Consumption for Water Wells
           Rehabilitation. Technology Optimization

    • Authors: Maxim Omelyanyuk, Irina Pakhlyan, Nikolay Bukharin, Mouhammad El Hassan
      First page: 444
      Abstract: Groundwater wells are widely used in the energy sector, including for drinking water supplies and as water source wells in the oil and gas industry to increase production of natural gas and petroleum. Water well clogging, which can happen to any well for various reasons, is a serious problem that can lead to increased power costs due to a higher head to the pump, a reduction in the flow rate and various drawdown issues. If rehabilitation procedures do not take place in time, this can result in permanent loss of the well, and a new well must be drilled, which is not a sustainable approach. Rehabilitation methods for water wells usually include mechanical and chemical treatments, and even though these methods are well established and have been used for many years we can still observe many abandoned wells which could be rehabilitated. In this study, sets of cavitation generators are developed and used in combination with common conic hydrodynamic nozzles. This combination reduces the pressure in the system and makes the cleaning setup much lighter and more mobile. The designed nozzles were successfully used in hydrodynamic cleaning of four water wells.
      Citation: Fluids
      PubDate: 2021-12-09
      DOI: 10.3390/fluids6120444
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 445: Extraction of Tangential Momentum and Normal
           Energy Accommodation Coefficients by Comparing Variational Solutions of
           the Boltzmann Equation with Experiments on Thermal Creep Gas Flow in
           Microchannels

    • Authors: Tommaso Missoni, Hiroki Yamaguchi, Irina Graur, Silvia Lorenzani
      First page: 445
      Abstract: In the present paper, we provide an analytical expression for the first- and second-order thermal slip coefficients, σ1,T and σ2,T, by means of a variational technique that applies to the integrodifferential form of the Boltzmann equation based on the true linearized collision operator for hard-sphere molecules. The Cercignani-Lampis scattering kernel of the gas-surface interaction has been considered in order to take into account the influence of the accommodation coefficients (αt, αn) on the slip parameters. Comparing our theoretical results with recent experimental data on the mass flow rate and the slip coefficient for five noble gases (helium, neon, argon, krypton, and xenon), we found out that there is a continuous set of values for the pair (αt, αn) which leads to the same thermal slip parameters. To uniquely determine the accommodation coefficients, we took into account a further series of measurements carried out with the same experimental apparatus, where the thermal molecular pressure exponent γ has been also evaluated. Therefore, the new method proposed in the present work for extracting the accommodation coefficients relies on two steps. First of all, since γ mainly depends on αt, we fix the tangential momentum accommodation coefficient in such a way as to obtain a fair agreement between theoretical and experimental results. Then, among the multiple pairs of variational solutions for (αt, αn), giving the same thermal slip coefficients (chosen to closely approximate the measurements), we select the unique pair with the previously determined value of αt. The analysis carried out in the present work confirms that both accommodation coefficients increase by increasing the molecular weight of the considered gases, as already highlighted in the literature.
      Citation: Fluids
      PubDate: 2021-12-09
      DOI: 10.3390/fluids6120445
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 446: Development of a Methodology and Software
           Package for Predicting the Formation of Organic Deposits Based on the
           Results of Laboratory Studies

    • Authors: Pavel Ilushin, Kirill Vyatkin, Alexander Menshikov
      First page: 446
      Abstract: One of the main problems in the oil industry is the fallout of asphaltene–resin–paraffin deposits (ARPDs) during oil production and transportation. The formation of organic deposits leads to reduced equipment life and reduced production. Currently, there is no single methodology for the numerical simulation of the ARPD dropout process. The aim of our work was to obtain a correlation dependence characterizing the rate of wax growth over time for oils in the Perm Krai, depending on temperature, pressure, and speed conditions. Experimental data for 20 oil samples were obtained using a Wax Flow Loop installation that simulates fluid movement in tubing. The developed correlation was tested in 154 wells. The results of numerical modeling of the paraffin precipitation process made it possible to correct the inter-treatment period of scraping for 109 wells (71%), indicating the high accuracy of the developed approach.
      Citation: Fluids
      PubDate: 2021-12-10
      DOI: 10.3390/fluids6120446
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 447: Topographically Controlled
           Marangoni-Rayleigh-Bénard Convection in Liquid Metals

    • Authors: Marcello Lappa, Aydin Sayar, Wasim Waris
      First page: 447
      Abstract: Convection induced in a layer of liquid with a top free surface by a distribution of heating elements at the bottom can be seen as a variant of standard Marangoni–Rayleigh–Bénard Convection where in place of a flat boundary at constant temperature delimiting the system from below, the underlying thermal inhomogeneity reflects the existence of a topography. In the present work, this problem is investigated numerically through solution of the governing equations for mass, momentum and energy in their complete, three-dimensional time-dependent and non-linear form. Emphasis is given to a class of liquids for which thermal diffusion is expected to dominate over viscous effects (liquid metals). Fixing the Rayleigh and Marangoni number to 104 and 5 × 103, respectively, the sensitivity of the problem to the geometrical, kinematic and thermal boundary conditions is investigated parametrically by changing: the number and spacing of heating elements, their vertical extension, the nature of the lateral boundary (solid walls or periodic boundary) and the thermal behavior of the portions of bottom wall between adjoining elements (assumed to be either adiabatic or at the same temperature of the hot blocks). It is shown that, like the parent phenomena, this type of thermal flow is extremely sensitive to the specific conditions considered. The topography can be used to exert a control on the emerging flow in terms of temporal response and patterning behavior.
      Citation: Fluids
      PubDate: 2021-12-10
      DOI: 10.3390/fluids6120447
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 448: Secondary Flow in Smooth and Rough Turbulent
           Circular Pipes: Turbulence Kinetic Energy Budgets

    • Authors: Paolo Orlandi, Sergio Pirozzoli
      First page: 448
      Abstract: Direct Numerical Simulations have been performed for turbulent flow in circular pipes with smooth and corrugated walls. The numerical method, based on second-order finite discretization together with the immersed boundary technique, was validated and applied to various types of flows. The analysis is focused on the turbulence kinetic energy and its budget. Large differences have been found in the near-wall region at low Reynolds number. The change in the near-wall turbulent structures is responsible for increase of drag and turbulence kinetic energy. To investigatselinae the effects of wall corrugations, the velocity fields have been decomposed so as to isolate coherent and incoherent motions. For corrugated walls, we find that coherent motions are strongest for walls covered with square bars aligned with the flow direction. In particular, the coherent contribution is substantial when the bars are spaced apart by a distance larger than their height. Detailed analysis of the turbulence kinetic energy budget shows for this set-up a very different behavior than for the other types of corrugations.
      Citation: Fluids
      PubDate: 2021-12-10
      DOI: 10.3390/fluids6120448
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 449: Large Eddy Simulation of Hypersonic Turbulent
           Boundary Layers

    • Authors: Nadia Kianvashrad, Doyle Knight
      First page: 449
      Abstract: The recent revival of interest in developing new hypersonic vehicles brings attention to the need for accurate prediction of hypersonic flows by computational methods. One of the challenges is prediction of aerothermodynamic loading over the surface of the vehicles. Reynolds Average Navier-Stokes (RANS) methods have not shown consistent accuracy in prediction of such flows. Therefore, new methods including Large Eddy Simulations (LES) should be investigated. In this paper, the LES method is used for prediction of the boundary layer over a flat plate. A new recycling-rescaling method is tested. The method uses total enthalpy and static pressure along with the velocity components to produce the best results for the Law of the Wall, turbulent statistics and turbulent Prandtl number.
      Citation: Fluids
      PubDate: 2021-12-11
      DOI: 10.3390/fluids6120449
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 450: Rayleigh–Bénard Instability of an
           Ellis Fluid Saturated Porous Channel with an Isoflux Boundary

    • Authors: Pedro Vayssière Brandão, Michele Celli, Antonio Barletta
      First page: 450
      Abstract: The onset of the thermal instability is investigated in a porous channel with plane parallel boundaries saturated by a non–Newtonian shear–thinning fluid and subject to a horizontal throughflow. The Ellis model is adopted to describe the fluid rheology. Both horizontal boundaries are assumed to be impermeable. A uniform heat flux is supplied through the lower boundary, while the upper boundary is kept at a uniform temperature. Such an asymmetric setup of the thermal boundary conditions is analysed via a numerical solution of the linear stability eigenvalue problem. The linear stability analysis is developed for three–dimensional normal modes of perturbation showing that the transverse modes are the most unstable. The destabilising effect of the non–Newtonian shear–thinning character of the fluid is also demonstrated as compared to the behaviour displayed, for the same flow configuration, by a Newtonian fluid.
      Citation: Fluids
      PubDate: 2021-12-11
      DOI: 10.3390/fluids6120450
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 451: Investigation of a Light Boxplane Model Using
           Tuft Flow Visualization and CFD

    • Authors: Elena Karpovich, Djahid Gueraiche, Natalya Sergeeva, Alexander Kuznetsov
      First page: 451
      Abstract: In this paper, we addressed the flow patterns over a light boxplane scale model to explain the previously discovered disagreement between its predicted and experimental aerodynamic characteristics. By tuft flow and CFD visualization, we explored the causes yielding a large zero lift pitching moment coefficient, lateral divergence, difference in fore and aft elevator lift, and poor high lift performance of the aircraft. The investigation revealed that the discrepancy in the pitching moment coefficient and lateral stability derivatives can be attributed to insufficient accuracy of the used predictive methods. The difference in fore and aft elevator lift and poor high lift performance of the aircraft may occur due to the low local Reynolds number, which causes the early flow separation over the elevators and flaperons when deflected downward at angles exceeding 10°. Additionally, some airframe changes are suggested to alleviate the lateral divergence of the model.
      Citation: Fluids
      PubDate: 2021-12-11
      DOI: 10.3390/fluids6120451
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 452: Estimation of Turbulent Triplet Covariances for
           Bora Flows

    • Authors: Željko Večenaj, Barbara Malečić, Branko Grisogono
      First page: 452
      Abstract: Bora is a strong or severe, relatively cold, gusty wind that usually blows from the northastern quadrant at the east coast of the Adriatic Sea. In this study bora’s turbulence triplet covariances were analysed, for the first time, for bora flows. The measurements used were obtained from the measuring tower on Pometeno brdo (“Swept-Away Hill”), in the hinterland of the city of Split, Croatia. From April 2010 until June 2011 three components of wind speed and sonic temperature were measured. The measurements were performed on three heights, 10, 22 and 40 m above the ground with the sampling frequency of 5 Hz. During the observed period, total of 60 bora episodes were isolated. We analyse the terms in prognostic equations for turbulence variances. In that respect, the viscous dissipation term was calculated using two approaches: (i) inertial dissipation method (εIDM) and (ii) direct approach from the prognostic equations for variances of turbulence (εEQ). We determine that the direct approach can successfully reproduce the shape of the curve, but the values are for several orders of magnitudes smaller compared to the real data. Further, linear relationship between εIDM and εEQ was obtained. Using the results for εEQ, viscous dissipation rate in longitudinal, transversal and vertical direction was determined. It is shown that viscous dissipation has the greatest impact on bora’s longitudinal direction. The focus is on the turbulence transport term, i.e., the triplet covariance term. For the first time, it is found that turbulence transport is very significant for the intensity of near−surface bora flows. Furthermore, turbulence transport can be both positive and negative, yet intensive. It is mostly negative at the upper levels and positive at the lower levels. Therefore, turbulence transport, in most cases, takes away turbulence variance from the upper levels and brings it down to the lower ones. This is one of the main findings of this study; it adds to the understanding of peculiarities of bora wind, and perhaps some other severe winds.
      Citation: Fluids
      PubDate: 2021-12-13
      DOI: 10.3390/fluids6120452
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 453: Self-Preservation of Turbulence Statistics in
           the Wall-Wake Flow of a Bed-Mounted Horizontal Pipe

    • Authors: Kalpana Devi, Prashanth Reddy Hanmaiahgari, Ram Balachandar, Jaan H. Pu
      First page: 453
      Abstract: This research article analyzed the self-preserving behaviour of wall-wake region of a circular pipe mounted horizontally over a flat rigid sand bed in a shallow flow in terms of mean velocity, RSS, and turbulence intensities. The study aims to investigate self-preservation using appropriate length and velocity scales.in addition to that wall-normal distributions of the third-order correlations along the streamwise direction in the wake region are analyzed. An ADV probe was used to record the three-dimensional instantaneous velocities for four different hydraulic and physical conditions corresponding to four cylinder Reynolds numbers. The results revealed that the streamwise velocity deficits, RSS deficits, and turbulence intensities deficits distributions displayed good collapse on a narrow band when they were non-dimensionalized by their respective maximum deficits. The wall-normal distance was non-dimensionalized by the half velocity profile width for velocity distributions, while the half RSS profile width was used in the case of the RSS deficits and turbulence intensities deficits distributions. The results indicate the self-preserving nature of streamwise velocity, RSS, and turbulence intensities in the wall-wake region of the pipe. The third-order correlations distributions indicate that sweep is the dominant bursting event in the near-bed zone. At the same time, ejection is the dominant bursting event in the region above the cylinder height.
      Citation: Fluids
      PubDate: 2021-12-14
      DOI: 10.3390/fluids6120453
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 454: Generation of Localised Vertical Streams in
           Unstable Stratified Atmosphere

    • Authors: Oleg Onishchenko, Viktor Fedun, Istvan Ballai, Aleksandr Kryshtal, Gary Verth
      First page: 454
      Abstract: A new model of axially symmetric concentrated vortex generation was developed herein. In this work, the solution of a nonlinear equation for internal gravity waves in an unstable stratified atmosphere was obtained and analysed in the framework of ideal hydrodynamics. The related expressions for the velocities in the inner and outer regions of the vortex were described by Bessel functions and modified zeroth-order Bessel functions. The proposed new nonlinear analytical model allows the study of the structure and dynamics of vortices in the radial region. The formation of jets (i.e., structures elongated in the vertical direction with finite components of the poloidal (radial and vertical) velocities that grow exponentially in time in an unstable stratified atmosphere) was also analysed. The characteristic growth time was determined by the inverse growth rate of instability. It is shown that a seed vertical vorticity component may be responsible for the formation of vortices with a finite azimuthal velocity.
      Citation: Fluids
      PubDate: 2021-12-15
      DOI: 10.3390/fluids6120454
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 455: Similarities and Contrasts in Time-Mean
           Striated Surface Tracers in Pacific Eastern Boundary Upwelling Systems:
           The Role of Ocean Currents in Their Generation

    • Authors: Ali Belmadani, Pierre-Amaël Auger, Nikolai Maximenko, Katherine Gomez, Sophie Cravatte
      First page: 455
      Abstract: Eastern boundary upwelling systems feature strong zonal gradients of physical and biological properties between cool, productive coastal oceans and warm, oligotrophic subtropical gyres. Zonal currents and jets (striations) are therefore likely to contribute to the transport of water properties between coastal and open oceanic regions. For the first time, multi-sensor satellite data are used to characterize the time-mean signatures of striations in sea surface temperature (SST), salinity (SSS), and chlorophyll-a (Chl-a) in subtropical eastern North/South Pacific (ENP/ESP) upwelling systems. In the ENP, tracers exhibit striated patterns extending up to ~2500 km offshore. Striated signals in SST and SSS are highly correlated with quasi-zonal jets, suggesting that these jets contribute to SST/SSS mesoscale patterns via zonal advection. Striated Chl-a anomalies are collocated with sea surface height (SSH) bands, a possible result of mesoscale eddy trains trapping nutrients and forming striated signals. In the ESP, the signature of striations is only found in SST and coincides with the SSH bands, consistently with quasi-zonal jets located outside major zonal tracer gradients. An interplay between large-scale SST/SSS advection by the quasi-zonal jets, mesoscale SST/SSS advection by the large-scale meridional flow, and eddy advection may explain the persistent ENP hydrographic signature of striations. These results underline the importance of quasi-zonal jets for surface tracer structuring at the mesoscale.
      Citation: Fluids
      PubDate: 2021-12-15
      DOI: 10.3390/fluids6120455
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 456: Features of the Hydrocarbon Distribution in the
           Bottom Sediments of the Norwegian and Barents Seas

    • Authors: Inna A. Nemirovskaya, Anastasia V. Khramtsova
      First page: 456
      Abstract: The results of the study of hydrocarbons (HCs): aliphatic (AHCs) and polycyclic aromatic hydrocarbons (PAHs) in bottom sediments (2019 and 2020, cruises 75 and 80 of the R/V Akademik Mstislav Keldysh) in the Norwegian-Barents Sea basin: Mohns Ridge, shelf Svalbard archipelago, Sturfiord, Medvezhinsky trench, central part of the Barents Sea, Novaya Zemlya shelf, Franz Victoria trough are presented. It has been established that the organo-geochemical background of the Holocene sediments was formed due to the flow of sedimentary material in the coastal regions of the Barents Sea on shipping routes. The anthropogenic input of HCs into bottom sediments leads to an increase in their content in the composition of Corg (in the sandy sediments of the Kaninsky Bank at an AHC concentration up to 64 μg/g, when its proportion in the composition of Corg reaches 11.7%). The endogenous influence on the of the Svalbard archipelago shelf in Sturfiord and in the Medvezhinsky Trench determines the specificity of local anomalies in the content and composition of HCs. This is reflected in the absence of a correlation between HCs and the grain size composition of sediments and Corg content, as well as a change in hydrocarbon molecular markers. At the same time, the sedimentary section is enriched in light alkanes and naphthalene’s that may be due to emission during point discharge of gas fluid from sedimentary rocks of the lower stratigraphic horizons and/or sipping migration.
      Citation: Fluids
      PubDate: 2021-12-15
      DOI: 10.3390/fluids6120456
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 457: Dynamic Stall Characteristics of Pitching Swept
           Finite-Aspect-Ratio Wings

    • Authors: Al Habib Ullah, Kristopher L. Tomek, Charles Fabijanic, Jordi Estevadeordal
      First page: 457
      Abstract: An experimental investigation regarding the dynamic stall of various swept wing models with pitching motion was performed to analyze the effect of sweep on the dynamic stall. The experiments were performed on a wing with a NACA0012 airfoil section with an aspect ratio of AR = 4. The experimental study was conducted for chord-based Reynolds number Rec =2×105 and freestream Mach number Ma=0.1. First, a ‘particle image velocimetry’ (PIV) experiment was performed on the wing with three sweep angles, Λ=0o, 15o, and 30o, to obtain the flow structure at several wing spans. The results obtained at a reduced frequency showed that a laminar separation bubble forms at the leading edge of the wing during upward motion. As the upward pitching motion continues, a separation burst occurs and shifts towards the wing trailing edge. As the wing starts to pitch downward, the growing dynamic stall vortex (DSV) vortex sheds from the wing’s trailing edge. With the increasing sweep angle of the wing, the stall angle is delayed during the dynamic motion of the wing, and the presence of DSV shifts toward the wingtip. During the second stage, a ‘turbo pressure-sensitive paint’ (PSP) technique was deployed to obtain the phase average of the surface pressure patterns of the DSV at a reduced frequency, k=0.1. The phase average of pressure shows a distinct pressure map for two sweep angles, Λ=0o, 30o, and demonstrates a similar trend to that presented in the published computational studies and the experimental data obtained from the current PIV campaign.
      Citation: Fluids
      PubDate: 2021-12-16
      DOI: 10.3390/fluids6120457
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 458: Modulation Instability of Hydro-Elastic Waves
           Blown by a Wind with a Uniform Vertical Profile

    • Authors: Susam Boral, Trilochan Sahoo, Yury Stepanyants
      First page: 458
      Abstract: An interesting physical phenomenon was recently observed when a fresh-water basin is covered by a thin ice film that has properties similar to the property of a rubber membrane. Surface waves can be generated under the action of wind on the air–water interface that contains an ice film. The modulation property of hydro-elastic waves (HEWs) in deep water covered by thin ice film blown by the wind with a uniform vertical profile is studied here in terms of the airflow velocity versus wavenumber. The modulation instability of HEWs is studied through the analysis of coefficients of the nonlinear Schrödinger (NLS) equation with the help of the Lighthill criterion. The NLS equation is derived using the multiple scale method in the presence of airflow. It is demonstrated that the potentially unstable hydro-elastic waves with negative energy appear for relatively small wind speeds, whereas the Kelvin–Helmholtz instability arises when the wind speed becomes fairly strong. Estimates of parameters of modulated waves for the typical conditions are given.
      Citation: Fluids
      PubDate: 2021-12-16
      DOI: 10.3390/fluids6120458
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 459: Turbulent Superstructures in Inert Jets and
           Diffusion Jet Flames

    • Authors: Vadim Lemanov, Vladimir Lukashov, Konstantin Sharov
      First page: 459
      Abstract: An experimental study of spatially localized very large-scale motion superstructures, propagating in a jet of carbon dioxide at low Reynolds numbers, was carried out. A hot-wire anemometer and a high-speed 2D PIV with a frequency of 7 kHz were used as measuring instruments. Such a puff-type superstructure in a jet with a longitudinal dimension of up to 20–30 nozzle diameters are initially formed in the jet source—a long tube in a laminar-turbulent transition mode (without artificial disturbances). It is shown that this regime with intermittency in time, when part of the time flow is laminar and the other part of time is turbulent, exists both at the exit from the nozzle and in the near field of the jet. Thus, the structural stability of such turbulent superstructures in the near field of the jet was found. Despite the large longitudinal scale, these formations have a transverse dimension of the order of several nozzle diameters. These structures have a complex internal topology, that is, superstructures are a conglomeration of vortices of different sizes from macroscale to microscale. Using the example of diffusion combustion of methane in air, it is demonstrated that in reacting jets, the existence of such large localized perturbations is a powerful physical mechanism for a global change in the flame topology. At the same time, the presence of a cascade of vortices of different sizes in the puff composition can lead to fractal deformation of the flame front.
      Citation: Fluids
      PubDate: 2021-12-16
      DOI: 10.3390/fluids6120459
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 460: Lagrangian vs. Eulerian: An Analysis of Two
           Solution Methods for Free-Surface Flows and Fluid Solid Interaction
           Problems

    • Authors: Milad Rakhsha, Christopher E. Kees, Dan Negrut
      First page: 460
      Abstract: As a step towards addressing a scarcity of references on this topic, we compared the Eulerian and Lagrangian Computational Fluid Dynamics (CFD) approaches for the solution of free-surface and Fluid–Solid Interaction (FSI) problems. The Eulerian approach uses the Finite Element Method (FEM) to spatially discretize the Navier–Stokes equations. The free surface is handled via the volume-of-fluid (VOF) and the level-set (LS) equations; an Immersed Boundary Method (IBM) in conjunction with the Nitsche’s technique were applied to resolve the fluid–solid coupling. For the Lagrangian approach, the smoothed particle hydrodynamics (SPH) method is the meshless discretization technique of choice; no additional equations are needed to handle free-surface or FSI coupling. We compared the two approaches for a flow around cylinder. The dam break test was used to gauge the performance for free-surface flows. Lastly, the two approaches were compared on two FSI problems—one with a floating rigid body dropped into the fluid and one with an elastic gate interacting with the flow. We conclude with a discussion of the robustness, ease of model setup, and versatility of the two approaches. The Eulerian and Lagrangian solvers used in this study are open-source and available in the public domain.
      Citation: Fluids
      PubDate: 2021-12-16
      DOI: 10.3390/fluids6120460
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 461: Development of an Algorithm for Prediction of
           the Wind Speed in Renewable Energy Environments

    • Authors: Efthimiou, Barmpas, Tsegas, Moussiopoulos
      First page: 461
      Abstract: The aim of this work is to develop an algorithm that is able to provide predictions of wind speed statistics (WSS) in renewable energy environments. The subject is clearly interesting, as predictions of storms and extreme winds are important for decision makers and emergency response teams in renewable energy environments, e.g., in places where wind turbines could be located, including cities. The goal of the work is achieved through two phases: (a) During the preparation phase, the construction of a big WSS database based on computational fluid dynamics (CFD) is carried out, which includes flow fields of different wind directions in all grid numerical points; (b) In the second phase, the algorithm is used to find the records in the WSS database with the closest meteorological conditions to the meteorological conditions of interest. The evaluation of the CFD model (including both RANS and LES turbulence methodologies) is performed using the experimental data of the MUST (Mock Urban Setting Test) wind tunnel experiment.
      Citation: Fluids
      PubDate: 2021-12-16
      DOI: 10.3390/fluids6120461
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 462: Turbulent Premixed Flame Modeling Using the
           Algebraic Flame Surface Wrinkling Model: A Comparative Study between
           OpenFOAM and Ansys Fluent

    • Authors: Halit Kutkan, Joel Guerrero
      First page: 462
      Abstract: Hereafter, we used the Algebraic Flame Surface Wrinkling (AFSW) model to conduct numerical simulations of the Paul Scherrer Institute (PSI) high-pressure, turbulent premixed Bunsen flame experiments. We implemented the AFSW model in OpenFOAM and in Ansys Fluent, and we compared the outcome of both solvers against the experimental results. We also highlight the differences between both solvers. All the simulations were performed using a two-dimensional axisymmetric model with the standard k−ϵ turbulence model with wall functions. Two different fuel/air mixtures were studied, namely, a 100%CH4 volumetric ratio and a 60%CH4+ 40%H2 volumetric ratio. The thermophysical and transport properties of the mixture were calculated as a function of temperature using the library Cantera (open-source suite of tools for problems involving chemical kinetics, thermodynamics, and transport processes), together with the GRI-Mech 3.0 chemical mechanism. It was found that the outcome of the AFSW model implemented in both solvers was in good agreement with the experimental results, quantitatively and qualitatively speaking. Further assessment of the results showed that, as much as the chemistry, the turbulence model and turbulent boundary/initial conditions significantly impact the flame shape and height.
      Citation: Fluids
      PubDate: 2021-12-17
      DOI: 10.3390/fluids6120462
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 463: Functional Acrylic Surfaces Obtained by
           Scratching

    • Authors: Abraham Medina, Abel López-Villa, Carlos A. Vargas
      First page: 463
      Abstract: By using sandpaper of different grit, we have scratched up smooth sheets of acrylic to cover their surfaces with disordered but near parallel micro-grooves. This procedure allowed us to transform the acrylic surface into a functional surface; measuring the capillary rise of silicone oil up to an average height h¯, we found that h¯ evolves as a power law of the form h¯∼tn, where t is the elapsed time from the start of the flow and n takes the values 0.40 or 0.50, depending on the different inclinations of the sheets. Such behavior can be understood alluding to the theoretical predictions for the capillary rise in very tight, open capillary wedges. We also explore other functionalities of such surfaces, as the loss of mass of water sessile droplets on them and the generic role of worn surfaces, in the short survival time of SARS-CoV-2, the virus that causes COVID-19.
      Citation: Fluids
      PubDate: 2021-12-18
      DOI: 10.3390/fluids6120463
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 464: Using Computational Modelling to Study
           Extensional Rheometry Tests for Inelastic Fluids

    • Authors: Mohammadreza Aali, Célio Fernandes, Olga Sousa Carneiro, João Miguel Nóbrega
      First page: 464
      Abstract: The present work focuses on the extensional rheometry test, performed with the Sentmanat extensional rheometer (SER) device, and its main objectives are: (i) to establish the modelling requirements, such as the geometry of the computational domain, initial and boundary conditions, appropriate case setup, and (ii) to investigate the effect of self-induced errors, namely on the sample dimensions and test temperature, on the extensional viscosity obtained through the extensional rheometry tests. The definition of the modelling setup also comprised the selection of the appropriate mesh refinement level to model the process and the conclusion that gravity can be neglected without affecting the numerical predictions. The subsequent study allowed us to conclude that the errors on the sample dimensions have similar effects, originating differences on the extensional viscosity proportional to the induced variations. On the other hand, errors of a similar order of magnitude on the test temperature promote a significant difference in the predicted extensional viscosity.
      Citation: Fluids
      PubDate: 2021-12-19
      DOI: 10.3390/fluids6120464
      Issue No: Vol. 6, No. 12 (2021)
       
  • Fluids, Vol. 6, Pages 371: Numerical Analysis of an Electroless Plating
           Problem in Gas–Liquid Two-Phase Flow

    • Authors: Po-Yi Wu, Olivier Pironneau, Po-Shao Shih, ChengHeng Robert Kao
      First page: 371
      Abstract: Electroless plating in micro-channels is a rising technology in industry. In many electroless plating systems, hydrogen gas is generated during the process. A numerical simulation method is proposed and analyzed. At a micrometer scale, the motion of the gaseous phase must be addressed so that the plating works smoothly. Since the bubbles are generated randomly and everywhere, a volume-averaged, two-phase, two-velocity, one pressure-flow model is applied. This fluid system is coupled with a set of convection–diffusion equations for the chemicals subject to flux boundary conditions for electron balance. The moving boundary due to plating is considered. The Galerkin-characteristic finite element method is used for temporal and spatial discretizations; the well-posedness of the numerical scheme is proved. Numerical studies in two dimensions are performed to validate the model against earlier one-dimensional models and a dedicated experiment that has been set up to visualize the distribution of bubbles.
      Citation: Fluids
      PubDate: 2021-10-20
      DOI: 10.3390/fluids6110371
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 372: Comparison of Techniques for the Estimation of
           Flow Parameters of Fan Inflow Turbulence from Noisy Hot-Wire Data

    • Authors: Luciano Caldas, Carolin Kissner, Maximilian Behn, Ulf Tapken, Robert Meyer
      First page: 372
      Abstract: Turbulence parameters, in particular integral length scale (ILS) and turbulence intensity (Tu), are key input parameters for various applications in aerodynamics and aeroacoustics. The estimation of these parameters is typically performed using data obtained via hot-wire measurements. On the one hand, hot-wire measurements are affected by external disturbances resulting in increased measurement noise. On the other hand, commonly applied turbulence parameter estimators lack in robustness. If not addressed correctly, both issues may impede the accuracy of the turbulence parameter estimation. In this article, a procedure consisting of several signal processing steps is presented to filter non-turbulence related disturbances from the unsteady velocity data. The signal processing techniques comprise time- and frequency-domain approaches. For the turbulence parameter estimation, two different models of the turbulence spectra—the von Kármán model and the Bullen model—are fitted to match the spectrum of the measured data. The results of several parameter estimation techniques are compared. Computational Fluid Dynamics (CFD) data are used to validate the estimation techniques and also to assess the influence of the variation in window size on the estimated parameters. Additionally, hot-wire data from a high-speed fan rig are analyzed. ILS and Tu are assessed at several radial positions for two fan speeds. It is found that most techniques yield similar values for ILS and Tu. The comparison of the fitted spectra with the spectra of the measured data shows a good agreement in most cases provided that a sufficiently fine frequency resolution is applied. The ratio of ILS and Tu of the velocity components in longitudinal and transverse direction allows the assessment of flow-isotropy. Results indicate that the turbulence is anisotropic for the investigated flow fields.
      Citation: Fluids
      PubDate: 2021-10-20
      DOI: 10.3390/fluids6110372
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 373: Pipeline Condition Assessment by Instantaneous
           Frequency Response over Hydroinformatics Based Technique—An Experimental
           and Field Analysis

    • Authors: Muhammad Hanafi Yusop, Mohd Fairusham Ghazali, Mohd Fadhlan Mohd Yusof, Muhammad Aminuddin Pi Remli
      First page: 373
      Abstract: A common issue in water infrastructure is that it suffers from leakage. The hydroinformatics technique for recognizing the presence of leaks in the pipeline system by means of pressure transient analysis was briefly explored in this study. Various studies have been done of improvised leak detection methods, and Hilbert Huang Transform has the potential to overcome the concern. The HHT processing algorithm has been successfully proven through simulation and experimentally tested to evaluate the ability of pressure transient analysis to predict and locate the leakage in the pipeline system. However, HHT relies on the selection of the suitable IMF in the pre-processing phase which will determine the precision of the estimated leak location. This paper introduces a NIKAZ filter technique for automatic selector of Intrinsic Mode Function (IMF). A laboratory-scale experimental test platform was constructed with a 68-metre long Medium Polyethylene (MDPE) pipe with 63 mm in diameter used for this study and equipped with a circular orifice as an artificial leak in varying sizes with a system of 2 bar to 4 bar water pressure. The results showed that, although with a low ratio of signal-to-noise, the proposed method could be used as an automatic selector for Intrinsic Mode Function (IMF). Experimental tests showed the efficiency, and the work method was successful as an automatic selector of IMF. The proposed mathematical algorithm was then finally evaluated on field measurement tested on-site of a real pipeline system. The results recommended NIKAZ as an automatic selector of IMF to increase the degree of automation of HHT technique, subsequently enhancing the detection and identification of water pipeline leakage.
      Citation: Fluids
      PubDate: 2021-10-21
      DOI: 10.3390/fluids6110373
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 374: Influence of Particle Mass Fraction over the
           Turbulent Behaviour of an Incompressible Particle-Laden Flow

    • Authors: Carlos Alberto Duque-Daza, Jesus Ramirez-Pastran, Santiago Lain
      First page: 374
      Abstract: The presence of spherical solid particles immersed in an incompressible turbulent flow was numerically investigated from the perspective of the particle mass fraction (PMF or ϕm), a measure of the particle-to-fluid mass ratio. Although a number of different changes have been reported to be obtained by the presence of solid particles in incompressible turbulent flows, the present study reports the findings of varying ϕm in the the turbulent behaviour of the flow, including aspects such as: turbulent statistics, skin-friction coefficient, and the general dynamics of a particle-laden flow. For this purpose, a particle-laden turbulent channel flow transporting solid particles at three different friction Reynolds numbers, namely Reτ=180, 365, and 950, with a fixed particle volume fraction of ϕv=10−3, was employed as conceptual flow model and simulated using large eddy simulations. The value adopted for ϕv allowed the use of a two-way coupling approach between the particles and the flow or carrier phase. Three different values of ϕm were explored in this work ϕm≈1,2.96, and 12.4. Assessment of the effect of ϕm was performed by examining changes of mean velocity profiles, velocity fluctuation profiles, and a number of other relevant turbulence statistics. Our results show that attenuation of turbulence activity of the carrier phase is attained, and that such attenuation increases with ϕm at fixed Reynolds numbers and ϕv. For the smallest Reynolds number case considered, flows carrying particles with higher ϕm exhibited lower energy requirements to sustain constant fluid mass flow rate conditions. By examining the flow velocity field, as well as instantaneous velocity components contours, it is shown that the attenuation acts even on the largest scales of the flow dynamics, and not only at the smaller levels. These findings reinforce the concept of a selective stabilising effect induced by the solid particles, particularly enhanced by high values of ϕm, which could eventually be exploited for improvement of energetic efficiency of piping or equivalent particles transport systems.
      Citation: Fluids
      PubDate: 2021-10-21
      DOI: 10.3390/fluids6110374
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 375: Stability of a Buoyant Oldroyd-B Flow
           Saturating a Vertical Porous Layer with Open Boundaries

    • Authors: Stefano Lazzari, Michele Celli, Antonio Barletta
      First page: 375
      Abstract: The performance of several engineering applications are strictly connected to the rheology of the working fluids and the Oldroyd-B model is widely employed to describe a linear viscoelastic behaviour. In the present paper, a buoyant Oldroyd-B flow in a vertical porous layer with permeable and isothermal boundaries is investigated. Seepage flow is modelled through an extended version of Darcy’s law which accounts for the Oldroyd-B rheology. The basic stationary flow is parallel to the vertical axis and describes a single-cell pattern where the cell has an infinite height. A linear stability analysis of such a basic flow is carried out to determine the onset conditions for a multicellular pattern. This analysis is performed numerically by employing the shooting method. The neutral stability curves and the values of the critical Rayleigh number are evaluated for different retardation time and relaxation time characteristics of the fluid. The study highlights the extent to which the viscoelasticity has a destabilising effect on the buoyant flow. For the limiting case of a Newtonian fluid, the known results available in the literature are recovered, namely a critical value of the Darcy–Rayleigh number equal to 197.081 and a corresponding critical wavenumber of 1.05950.
      Citation: Fluids
      PubDate: 2021-10-21
      DOI: 10.3390/fluids6110375
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 376: Parameter Variability in Viscous Convection

    • Authors: Ekkehard Holzbecher
      First page: 376
      Abstract: For the optimal design of cooling and heating devices, the properties of the included fluids are crucial. The temperature dependence of viscosity deserves attention, as changes can be one order of magnitude or more. Here we examine the influence on convective motions by simulating a heating and cooling experiment with a vertical cylinder by finite element computational fluid dynamics (CFD) models. Such an experimental setup in which flow patterns are determined by transient viscous convection has not been simulated before. Evaluating the general behavior of the experiment in 2D, we find a dynamic phase after and before phases with moderate changes. Flow patterns in the dynamic phase change significantly with the temperature range of the experiment. We compare the outcome of the numerical models with results from laboratory experiments, finding major discrepancies concerning the flow patterns in the dynamic phase. 3D modeling shows weaker dynamics but does not show good timing with the experiment. The study depicts the importance of parameter dependencies for convective motions and demonstrates the capabilities and limitations of models to reproduce details of viscous convection.
      Citation: Fluids
      PubDate: 2021-10-22
      DOI: 10.3390/fluids6110376
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 377: Computational Fluid Dynamics Using the Adaptive
           Wavelet-Collocation Method

    • Authors: Yash Mehta, Ari Nejadmalayeri, Jonathan David Regele
      First page: 377
      Abstract: Advancements to the adaptive wavelet-collocation method over the last decade have opened up a number of new possible areas for active research. Volume penalization techniques allow complex immersed boundary conditions to be used with high efficiency for both internal and external flows. Anisotropic methods make it possible to use body-fitted meshes while still taking advantage of the dynamic adaptability properties wavelet-based methods provide. The parallelization of the approach has made it possible to perform large high-resolution simulations of detonation initiation and fluid instabilities to uncover new physical insights that would otherwise be difficult to discover. Other developments include space-time adaptive methods and nonreflecting boundary conditions. This article summarizes the work performed using the adaptive wavelet-collocation method developed by Vasilyev and coworkers over the past decade.
      Citation: Fluids
      PubDate: 2021-10-22
      DOI: 10.3390/fluids6110377
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 378: Geometry and Flow Properties Affect the Phase
           Shift between Pressure and Shear Stress Waves in Blood Vessels

    • Authors: Haifeng Wang, Timm Krüger, Fathollah Varnik
      First page: 378
      Abstract: The phase shift between pressure and wall shear stress (WSS) has been associated with vascular diseases such as atherosclerosis and aneurysms. The present study aims to understand the effects of geometry and flow properties on the phase shift under the stiff wall assumption, using an immersed-boundary-lattice-Boltzmann method. For pulsatile flow in a straight pipe, the phase shift is known to increase with the Womersley number, but is independent of the flow speed (or the Reynolds number). For a complex geometry, such as a curved pipe, however, we find that the phase shift develops a strong dependence on the geometry and Reynolds number. We observed that the phase shift at the inner bend of the curved vessel and in the aneurysm dome is larger than that in a straight pipe. Moreover, the geometry affects the connection between the phase shift and other WSS-related metrics, such as time-averaged WSS (TAWSS). For straight and curved blood vessels, the phase shift behaves qualitatively similarly to and can thus be represented by the TAWSS, which is a widely used hemodynamic index. However, these observables significantly differ in other geometries, such as in aneurysms. In such cases, one needs to consider the phase shift as an independent quantity that may carry additional valuable information compared to well-established metrics.
      Citation: Fluids
      PubDate: 2021-10-23
      DOI: 10.3390/fluids6110378
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 379: Diffusive Mass Transfer and Gaussian Pressure
           Transient Solutions for Porous Media

    • Authors: Ruud Weijermars
      First page: 379
      Abstract: This study revisits the mathematical equations for diffusive mass transport in 1D, 2D and 3D space and highlights a widespread misconception about the meaning of the regular and cumulative probability of random-walk solutions for diffusive mass transport. Next, the regular probability solution for molecular diffusion is applied to pressure diffusion in porous media. The pressure drop (by fluid extraction) or increase (by fluid injection) due to the production system may start with a simple pressure step function. The pressure perturbation imposed by the step function (representing the engineering intervention) will instantaneously diffuse into the reservoir at a rate that is controlled by the hydraulic diffusivity. Traditionally, the advance of the pressure transient in porous media such as geological reservoirs is modeled by two distinct approaches: (1) scalar equations for well performance testing that do not attempt to solve for the spatial change or the position of the pressure transient without reference to a well rate; (2) advanced reservoir models based on numerical solution methods. The Gaussian pressure transient solution method presented in this study can compute the spatial pressure depletion in the reservoir at arbitrary times and is based on analytical expressions that give spatial resolution without gridding-meaning solutions that have infinite resolution. The Gaussian solution is efficient for quantifying the advance of the pressure transient and associated pressure depletion around single wells, multiple wells and hydraulic fractures. This work lays the basis for the development of advanced reservoir simulations based on the superposition of analytical pressure transient solutions.
      Citation: Fluids
      PubDate: 2021-10-23
      DOI: 10.3390/fluids6110379
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 380: Instability of Lenticular Vortices: Results
           from Laboratory Experiments, Linear Stability Analysis and Numerical
           Simulations

    • Authors: Noé Lahaye, Alexandre Paci, Stefan G. Llewellyn Smith
      First page: 380
      Abstract: The instability of surface lenticular vortices is investigated using a comprehensive suite of laboratory experiments combined with numerical linear stability analysis as well as nonlinear numerical simulations in a two-layer Rotating Shallow Water model. The development of instabilities is discussed and compared between the different methods. The linear stability analysis allows for a clear description of the origin of the instability observed in both the laboratory experiments and numerical simulations. While global qualitative agreement is found, some discrepancies are observed and discussed. Our study highlights that the sensitivity of the instability outcome is related to the initial condition and the lower-layer flow. The inhibition or even suppression of some unstable modes may be explained in terms of the lower-layer potential vorticity profile.
      Citation: Fluids
      PubDate: 2021-10-23
      DOI: 10.3390/fluids6110380
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 381: A Model for Stokes Flow in Domains with
           Permeable Boundaries

    • Authors: Ricardo Cortez, Marian Hernandez-Viera, Owen Richfield
      First page: 381
      Abstract: We derive a new computational model for the simulation of viscous incompressible flows bounded by a thin, flexible, porous membrane. Our approach is grid-free and models the boundary forces with regularized Stokeslets. The flow across the porous membranes is modeled with regularized source doublets based on the notion that the flux velocity across the boundary can be viewed as the flow induced by a fluid source/sink pair with the sink on the high-pressure side of the boundary and magnitude proportional to the pressure difference across the membrane. Several validation examples are presented that illustrate how to calibrate the parameters in the model. We present an example consisting of flow in a closed domain that loses volume due to the fluid flux across the permeable boundary. We also present applications of the method to flow inside a channel of fixed geometry where sections of the boundary are permeable. The final example is a biological application of flow in a capillary with porous walls and a protein concentration advected and diffused in the fluid. In this case, the protein concentration modifies the pressure in the flow, producing dynamic changes to the flux across the walls. For this example, the proposed method is combined with finite differences for the concentration field.
      Citation: Fluids
      PubDate: 2021-10-23
      DOI: 10.3390/fluids6110381
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 382: A Comparison of Ansys Fluent and MFiX in
           Performing CFD-DEM Simulations of a Spouted Bed

    • Authors: Filippo Marchelli, Renzo Di Felice
      First page: 382
      Abstract: The CFD-DEM methodology is a popular tool for the study of fluid–particle systems, and there are several programs that permit using it. In this study, we employed it to simulate a pseudo-2D spouted bed, comparing the performance of the programs Ansys Fluent and MFiX. The results are analysed and commented on in terms of both accuracy and computational efforts. Despite the similarity of the setup, MFiX seems to perform significantly better. The similarities and differences between the two programs are discussed in detail, offering useful insights to researchers regarding the selection of one over the other, depending on the application. The better suitability of the Di Felice drag model is confirmed for the device, while it is shown that the effect of the Magnus lift force may be more limited than was shown in a previous study.
      Citation: Fluids
      PubDate: 2021-10-25
      DOI: 10.3390/fluids6110382
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 383: Testing Basic Gradient Turbulent Transport
           Models for Swirl Burners Using PIV and PLIF

    • Authors: Alexey Savitskii, Aleksei Lobasov, Dmitriy Sharaborin, Vladimir Dulin
      First page: 383
      Abstract: The present paper reports on the combined stereoscopic particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) measurements of turbulent transport for model swirl burners without combustion. Two flow types were considered, namely the mixing of a free jet with surrounding air for different swirl rates of the jet (Re = 5 × 103) and the mixing of a pilot jet (Re = 2 × 104) with a high-swirl co-flow of a generic gas turbine burner (Re = 3 × 104). The measured spatial distributions of the turbulent Reynolds stresses and fluxes were compared with their predictions by gradient turbulent transport models. The local values of the turbulent viscosity and turbulent diffusivity coefficients were evaluated based on Boussinesq’s and gradient diffusion hypotheses. The studied flows with high swirl were characterized by a vortex core breakdown and intensive coherent flow fluctuations associated with large-scale vortex structures. Therefore, the contribution of the coherent flow fluctuations to the turbulent transport was evaluated based on proper orthogonal decomposition (POD). The turbulent viscosity and diffusion coefficients were also evaluated for the stochastic (residual) component of the velocity fluctuations. The high-swirl flows with vortex breakdown for the free jet and for the combustion chamber were characterized by intensive turbulent fluctuations, which contributed substantially to the local turbulent transport of mass and momentum. Moreover, the high-swirl flows were characterized by counter-gradient transport for one Reynolds shear stress component near the jet axis and in the outer region of the mixing layer.
      Citation: Fluids
      PubDate: 2021-10-25
      DOI: 10.3390/fluids6110383
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 384: Flow Structures Identification through Proper
           Orthogonal Decomposition: The Flow around Two Distinct Cylinders

    • Authors: Ângela M. Ribau, Nelson D. Gonçalves, Luís L. Ferrás, Alexandre M. Afonso
      First page: 384
      Abstract: Numerical simulations of fluid flows can produce a huge amount of data and inadvertently important flow structures can be ignored, if a thorough analysis is not performed. The identification of these flow structures, mainly in transient situations, is a complex task, since such structures change in time and can move along the domain. With the decomposition of the entire data set into smaller sets, important structures present in the main flow and structures with periodic behaviour, like vortices, can be identified. Therefore, through the analysis of the frequency of each of these components and using a smaller number of components, we show that the Proper Orthogonal Decomposition can be used not only to reduce the amount of significant data, but also to obtain a better and global understanding of the flow (through the analysis of specific modes). In this work, the von Kármán vortex street is decomposed into a generator base and analysed through the Proper Orthogonal Decomposition for the 2D flow around a cylinder and the 2D flow around two cylinders with different radii. We consider a Newtonian fluid and two non-Newtonian power-law fluids, with n=0.7 and n=1.3. Grouping specific modes, a reconstruction is made, allowing the identification of complex structures that otherwise would be impossible to identify using simple post-processing of the fluid flow.
      Citation: Fluids
      PubDate: 2021-10-25
      DOI: 10.3390/fluids6110384
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 385: Numerical Simulation of a Periodic
           Quasi-Switching Mode of Flow around a Conical Dimple with a Slope Angle of
           10 Degrees on the Wall of a Narrow Channel Using URANS

    • Authors: Sergey Isaev, Dmitry Nikushchenko, Alexandr Sudakov, Nikita Tryaskin, Ann Egorova, Leonid Iunakov, Alexandr Usachov, Valery Kharchenko
      First page: 385
      Abstract: The applicability of the solution of the unsteady Reynolds-averaged Navier–Stokes equations (URANS) for the numerical simulation of the periodic quasi-switching regime of vortex generation and heat transfer in a deep conical dimple with a slope angle of 10∘ on the wall of a narrow channel is substantiated. To calculate the turbulent regime, the model of shear stress transfer by Menter 2003, modified taking into account the influence of the curvature of streamlines within the framework of the Rodi-Leshziner-Isaev approach, is used. At Reynolds number Re=104, the oscillation period of the transverse Rz and longitudinal forces Rx, as well as the total heat transfer Numm to the control section of the heated channel wall with a dimple, is set equal to 60, which corresponds to the Strouhal number St=0.0167. Computer visualization of swirling jet-vortex flows demonstrates focus-type sources on the side faces of the dimple. In the self-oscillating mode, a two-cell vortex system is formed with different intensities at the oscillation period Rz. Periodic changes in friction, Nusselt numbers and temperature are recorded in the longitudinal and transverse median sections of the dimple and reflect the oscillations of the vortex structure from left to right and from right to left. The formation of a fan jet is shown, which oscillates relative to the plane of longitudinal symmetry, causing a redistribution of power and thermal loads.
      Citation: Fluids
      PubDate: 2021-10-26
      DOI: 10.3390/fluids6110385
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 386: On Determining the Critical Velocity in the
           Shot Sleeve of a High-Pressure Die Casting Machine Using Open Source CFD

    • Authors: Sebastian Kohlstädt, Michael Vynnycky, Stephan Goeke, Andreas Gebauer-Teichmann
      First page: 386
      Abstract: This paper investigates the critical plunger velocity in high-pressure die casting during the slow phase of the piston motion and how it can be determined with computational fluid dynamics (CFD) in open source software. The melt-air system is modelled via an Eulerian volume-of-fluid approach, treating the air as a compressible perfect gas. The turbulence is treated via a Reynolds-averaged Navier Stokes (RANS) approach that uses the Menter SST k-ω model. Two different strategies for mesh motion are presented and compared against each other. The solver is validated via analytical models and empirical data. A method is then presented to determine the optimal velocity using a two-dimensional (2D) mesh. As a second step, it is then discussed how the results are in line with those obtained for an actual, industrially relevant, three-dimensional (3D) geometry that also includes the ingate system of the die.
      Citation: Fluids
      PubDate: 2021-10-28
      DOI: 10.3390/fluids6110386
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 387: Using Experimentally Calibrated Regularized
           Stokeslets to Assess Bacterial Flagellar Motility Near a Surface

    • Authors: Orrin Shindell, Hoa Nguyen, Nicholas Coltharp, Frank Healy, Bruce Rodenborn
      First page: 387
      Abstract: The presence of a nearby boundary is likely to be important in the life cycle and evolution of motile flagellate bacteria. This has led many authors to employ numerical simulations to model near-surface bacterial motion and compute hydrodynamic boundary effects. A common choice has been the method of images for regularized Stokeslets (MIRS); however, the method requires discretization sizes and regularization parameters that are not specified by any theory. To determine appropriate regularization parameters for given discretization choices in MIRS, we conducted dynamically similar macroscopic experiments and fit the simulations to the data. In the experiments, we measured the torque on cylinders and helices of different wavelengths as they rotated in a viscous fluid at various distances to a boundary. We found that differences between experiments and optimized simulations were less than 5% when using surface discretizations for cylinders and centerline discretizations for helices. Having determined optimal regularization parameters, we used MIRS to simulate an idealized free-swimming bacterium constructed of a cylindrical cell body and a helical flagellum moving near a boundary. We assessed the swimming performance of many bacterial morphologies by computing swimming speed, motor rotation rate, Purcell’s propulsive efficiency, energy cost per swimming distance, and a new metabolic energy cost defined to be the energy cost per body mass per swimming distance. All five measures predicted that the optimal flagellar wavelength is eight times the helical radius independently of body size and surface proximity. Although the measures disagreed on the optimal body size, they all predicted that body size is an important factor in the energy cost of bacterial motility near and far from a surface.
      Citation: Fluids
      PubDate: 2021-10-29
      DOI: 10.3390/fluids6110387
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 388: Computational Analysis of Lung and Isolated
           Airway Bifurcations under Mechanical Ventilation and Normal Breathing

    • Authors: Jongwon Kim, Ramana M. Pidaparti
      First page: 388
      Abstract: Mechanical ventilation is required for many patients who cannot breathe normally as a result of lung disease and other factors that result in reduced lung function. In this study, we investigated the effects of mechanical ventilation and normal breathing on whole lung geometry as well as isolated bifurcations through computational fluid dynamic (CFD) simulations. Results of flow characteristics (airflow velocity, wall pressure, and wall shear stress) obtained from the CFD simulations are presented. Similar flow patterns and pressure drops were obtained between the whole lung geometry and isolated bifurcations under both normal breathing and mechanical ventilation, respectively. Results obtained from simulations suggest that analyzing specific local bifurcations may be a more feasible alternative as it may reduce the computational time and numerical errors resulting from computations as compared to simulating a complex whole lung geometry. The approach presented in this study also demonstrated that analyses of isolated bifurcations gave similar flow characteristics to that of whole lung geometry. Therefore, this approach may be useful for quickly obtaining results that will assist in making clinical predictions and other applications.
      Citation: Fluids
      PubDate: 2021-10-29
      DOI: 10.3390/fluids6110388
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 389: Computational Study of the Dynamics of the
           Taylor Bubble

    • Authors: Evgenii L. Sharaborin, Oleg A. Rogozin, Aslan R. Kasimov
      First page: 389
      Abstract: We perform high-resolution numerical simulations of three-dimensional dynamics of an elongated bubble in a microchannel at moderate Reynolds numbers up to 1800. For this purpose, we use the coupled Brinkman penalization and volume of fluid methods implemented in the open-source framework Basilisk. The new results are validated with available experimental data and compared with previous numerical and theoretical predictions. We extend existing results to regimes with significant inertia, which are characterized by intense deformations of the bubble, including cases with azimuthal symmetry breaking. Various dynamical features are analyzed in terms of their spatiotemporal characteristics, such as frequencies and wavelengths of the bubble surface undulations and vortical structures in the flow.
      Citation: Fluids
      PubDate: 2021-10-29
      DOI: 10.3390/fluids6110389
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 390: Flow Structure and Deformation of Two Bubbles
           Rising Side by Side in a Quiescent Liquid

    • Authors: Hiroaki Kusuno, Toshiyuki Sanada
      First page: 390
      Abstract: In the motion of two spherical bubbles rising side by side, the bubbles are known to attract each other at a high Reynolds number (Re = ρUd/μ). Furthermore, spherical bubbles kiss and bounce under certain conditions; however, deformable bubbles repel each other without kissing. This paper experimentally and numerically presents the flow structures and shape of the nonkissing repulsion of deformable bubbles. For the experimental analysis, we organized bubble behaviors by Galilei number (Ga = ρg1/2d3/2/μ) and Bond number (Bo = ρgd2/σ). The bubbles repelled each other without kissing near the unstable critical curve of a single bubble. The curvature inside the gap, which is similar to the shape of a zigzag behavior bubble, was large. For the numerical analysis, the velocity of the equatorial plane inside the gap was larger due to the potential interaction, although the velocity behind was the opposite due to the strengthened vorticity generated at the surface. Furthermore, the double-threaded wake emerged behind the interacting bubbles, and it showed that the rotation direction was repulsion regardless of whether the bubbles attracted or repelled each other. The streamline behind the bubbles in the 2D plane was from the outside to the inside.
      Citation: Fluids
      PubDate: 2021-11-01
      DOI: 10.3390/fluids6110390
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 391: CFD Simulation Study on the Performance of a
           Modified Ram Air Turbine (RAT) for Power Generation in Aircrafts

    • Authors: Magedi Moh M. Saad, Sofian Mohd, Mohd Fadhli Zulkafli, Nor Afzanizam Samiran, Djamal Hissein Didane
      First page: 391
      Abstract: The present paper aims to study the possibility of dispensing an auxiliary power unit (APU) in an aircraft powered by fossil fuels to reduce air pollution. It particularly seeks to evaluate the amount of power generated by the ram air turbine (RAT) using the novel counter-rotating technique while characterizing its optimum axial distance. The ram air turbine (RAT), which is already equipped in aircrafts, was enhanced to generate the amount of energy produced by the APU. The approach was implemented by a CRRAT system. Six airfoil profiles were tested based on 2D models and the best airfoil was chosen for implantation on the RAT and CRRAT systems. The performance of the conventional single-rotor RAT and CRRAT were analyzed using FLUENT software based on 3D models. The adopted numerical scheme was the Navier–Stokes equation with k–ω SST turbulence modeling. The dynamic mesh and user-defined function (UDF) were used to revolve the rotor turbine via wind. The results indicated that the FX63-137 airfoil profile showed a higher performance in terms of the lift-to-drag ratio compared to the other airfoils. The optimum axial distance between the two rotors was 0.087 m of the rotor diameter and the efficiency of the new CRRAT increased to almost 45% compared to the single-rotor RAT.
      Citation: Fluids
      PubDate: 2021-11-01
      DOI: 10.3390/fluids6110391
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 392: Parametric Study of Unsteady Flow and Heat
           Transfer of Compressible Helium–Xenon Binary Gas through a Porous
           Channel Subjected to a Magnetic Field

    • Authors: Pornthep Pattanavanitkul, Watit Pakdee
      First page: 392
      Abstract: A numerical analysis of unsteady fluid and heat transport of compressible Helium–Xenon binary gas through a rectangular porous channel subjected to a transverse magnetic field is herein presented. The binary gas mixture consists of Helium (He) and Xenon (Xe). In addition, the compressible gas properties are temperature-dependent. The set of governing equations are nondimensionalized via appropriate dimensionless parameters. The dimensionless equations involve a number of dimensionless groups employed for detailed parametric study. Consequently, the set of equations is discretized using a compact finite difference scheme and solved by using the 3rd-order Runge–Kutta method. The model’s computed results are compared with data from past literature, and very favorable agreement is achieved. The results show that the magnetic field, compressibility and variable fluid properties profoundly affect heat and fluid transport. Variations of density with temperature as well as pressure result in an asymmetric mass flow profile. Furthermore, the friction coefficient is greater for the upper wall than for the lower wall due to larger velocity gradients along the top wall.
      Citation: Fluids
      PubDate: 2021-11-01
      DOI: 10.3390/fluids6110392
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 393: A Review on BGK Models for Gas Mixtures of Mono
           and Polyatomic Molecules

    • Authors: Marlies Pirner
      First page: 393
      Abstract: We consider the Bathnagar–Gross–Krook (BGK) model, an approximation of the Boltzmann equation, describing the time evolution of a single momoatomic rarefied gas and satisfying the same two main properties (conservation properties and entropy inequality). However, in practical applications, one often has to deal with two additional physical issues. First, a gas often does not consist of only one species, but it consists of a mixture of different species. Second, the particles can store energy not only in translational degrees of freedom but also in internal degrees of freedom such as rotations or vibrations (polyatomic molecules). Therefore, here, we will present recent BGK models for gas mixtures for mono- and polyatomic particles and the existing mathematical theory for these models.
      Citation: Fluids
      PubDate: 2021-11-01
      DOI: 10.3390/fluids6110393
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 394: Aerodynamic Study of a NACA 64418 Rectangular
           Wing under Forced Pitching Motions

    • Authors: Dimitris Gkiolas, Dimitrios Mathioulakis
      First page: 394
      Abstract: The aerodynamic behavior of a pitching NACA 64418 rectangular wing was experimentally studied in a subsonic wind tunnel. The wing had a chord c = 0.5 m, a span which covered the distance between the two parallel tunnel walls and an axis of rotation 0.35 c far from the leading edge. Based on pressure distribution and flow visualization, intermittent flow separation (double stall) was revealed near the leading edge suction side when the wing was stationary, at angles higher than 17° and Re = 0.5 × 106. Under pitching oscillations, aerodynamic loads were calculated by integrating the output data of fast responding surface pressure transducers for various mean angles of attack (αm (max) = 15°), reduced frequencies (kmax = 0.2) and angle amplitudes Δα in the interval [2°, 8°]. The impact of the above parameters up to Re = 0.75 × 106 on the cycle-averaged lift and pitching moment loops is discussed and the cycle aerodynamic damping coefficient is calculated. Moreover, the boundaries of the above parameters are defined for the case that energy is transferred from the flow to the wing (negative aerodynamic damping coefficient), indicating the conditions under which aeroelastic instabilities are probable to occur.
      Citation: Fluids
      PubDate: 2021-11-02
      DOI: 10.3390/fluids6110394
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 395: Development of a Scalable Thermal Reservoir
           Simulator on Distributed-Memory Parallel Computers

    • Authors: Hui Liu, Zhangxin Chen, Xiaohu Guo, Lihua Shen
      First page: 395
      Abstract: Reservoir simulation is to solve a set of fluid flow equations through porous media, which are partial differential equations from the petroleum engineering industry and described by Darcy’s law. This paper introduces the model, numerical methods, algorithms and parallel implementation of a thermal reservoir simulator that is designed for numerical simulations of a thermal reservoir with multiple components in three-dimensional domain using distributed-memory parallel computers. Its full mathematical model is introduced with correlations for important properties and well modeling. Efficient numerical methods (discretization scheme, matrix decoupling methods, and preconditioners), parallel computing technologies, and implementation details are presented. The numerical methods applied in this paper are suitable for large-scale thermal reservoir simulations with dozens of thousands of CPU cores (MPI processes), which are efficient and scalable. The simulator is designed for giant models with billions or even trillions of grid blocks using hundreds of thousands of CPUs, which is our main focus. The validation part is compared with CMG STARS, which is one of the most popular and mature commercial thermal simulators. Numerical experiments show that our results match commercial simulators, which confirms the correctness of our methods and implementations. SAGD simulation with 7406 well pairs is also presented to study the effectiveness of our numerical methods. Scalability testings demonstrate that our simulator can handle giant models with billions of grid blocks using 100,800 CPU cores and the simulator has good scalability.
      Citation: Fluids
      PubDate: 2021-11-02
      DOI: 10.3390/fluids6110395
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 396: On the Characteristics of the Super-Critical
           Wake behind a Circular Cylinder

    • Authors: Ivette Rodriguez, Oriol Lehmkuhl
      First page: 396
      Abstract: The flow topology of the wake behind a circular cylinder at the super-critical Reynolds number of Re=7.2×105 is investigated by means of large eddy simulations. In spite of the many research works on circular cylinders, there are no studies concerning the main characteristics and topology of the near wake in the super-critical regime. Thus, the present work attempts to fill the gap in the literature and contribute to the analysis of both the unsteady wake and the turbulent statistics of the flow. It is found that although the wake is symmetric and preserves similar traits to those observed in the sub-critical regime, such as the typical two-lobed configuration in the vortex formation zone, important differences are also observed. Owing to the delayed separation of the flow and the transition to turbulence in the attached boundary layer, Reynolds stresses peak in the detached shear layers close to the separation point. The unsteady mean flow is also investigated, and topological critical points are identified in the vortex formation zone and the near wake. Finally, time-frequency analysis is performed by means of wavelets. The study shows that in addition to the vortex shedding frequency, the inception of instabilities that trigger transition to turbulence occurs intermittently in the attached boundary layer and is registered as a phenomenon of variable intensity in time.
      Citation: Fluids
      PubDate: 2021-11-03
      DOI: 10.3390/fluids6110396
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 397: Toward a Better Understanding of Sediment
           Dynamics as a Basis for Maintenance Dredging in Nagan Raya Port, Indonesia
           

    • Authors: Muhammad Zikra, Shaskya Salsabila, Kriyo Sambodho
      First page: 397
      Abstract: The Port of 2 × 110 MW Nagan Raya Coal Fired Steam Power Plant is one of the facilities constructed by the State Electricity Company in Aceh Province, Indonesia. During its operation, which began in 2013, the port has dealt with large amounts of sedimentation within the port and ship entrances. The goal of this study is to mitigate the sedimentation problem in the Nagan Raya port by evaluating the effect of maintenance dredging. Field measurements, and hydrodynamic and sediment transport modeling analysis, were conducted during this study. Evaluation of the wind data showed that the dominant wind direction is from south to west. Based on the analysis of the wave data, the dominant wave direction is from the south to the west. Therefore, the wave-induced currents in the surf zone were from south to north. Based on the analysis of longshore sediment transport, the supply of sediments to Nagan Raya port was estimated to be around 40,000–60,000 m3 per year. Results from the sediment model showed that sedimentation of up to 1 m was captured in areas of the inlet channel of Nagan Raya port. The use of a passing system for sand is one of the sedimentation management solutions proposed in this study. The dredged sediment material around the navigation channel was dumped in a dumping area in the middle of the sea at a depth of 11 m, with a distance of 1.5 km from the shoreline. To obtain a greater maximum result, the material disposal distance should be dumped further away, at least at a depth of 20 m or a distance of 20 miles from the coastline.
      Citation: Fluids
      PubDate: 2021-11-03
      DOI: 10.3390/fluids6110397
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 398: Investigation of the Effects of Nanoparticle
           Concentration and Cutting Parameters on Surface Roughness in MQL Hard
           Turning Using MoS2 Nanofluid

    • Authors: Ngo Minh Tuan, Tran Bao Ngoc, Tran Le Thu, Tran The Long
      First page: 398
      Abstract: Minimum quantity lubrication (MQL) has gained significant attention in various research fields and industrial applications for its advantages of being environmentally friendly and suitable for sustainable production. The effectiveness of MQL is increasing significantly by using nano cutting fluid, which can be produced by suspending nanoparticles in the based cutting fluid. This study aims to investigate the effects of MoS2 nanoparticle concentration, cutting speed, and feed rate on MQL hard turning of 90CrSi steel in terms of surface roughness and surface microstructure. The Box–Behnken experimental design was used to analyze the influence of input parameters and their interaction effects as well as to find the optimal set of variables. The obtained results prove the improvement of the machinability of carbide tools due to higher cooling and lubricating performance created by MoS2 nanofluid MQL, which contributes to improve the surface quality and reduce the manufacturing cost. There is an interaction effect between nanoparticle concentration and feed rate which has a strong influence on surface roughness.
      Citation: Fluids
      PubDate: 2021-11-04
      DOI: 10.3390/fluids6110398
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 399: Dynamics of Shock Structure and Frontal Drag
           Force in a Supersonic Flow Past a Blunt Cone under the Action of Plasma
           Formation

    • Authors: Irina Znamenskaya, Vladimir Chernikov, Olga Azarova
      First page: 399
      Abstract: The paper is devoted to the experimental and CFD investigation of a plasma formation impact on the supersonic flow over a body “blunt cone-cylinder”. In the experiments, a series of schlieren pictures of bow shock wave–blast waves non-stationary interaction was obtained with the use of high speed shadowgraphy. The accompanying calculations are based on the system of Euler equations. The freestream Mach number is 3.1. The plasmoid is modeled by the instantaneous release of energy into a bounded volume of gas, increasing the pressure in the volume. The research of the dynamics of a shock wave structure caused by the bow shock wave and blast flow interaction has been conducted. The significant value of energy released to a supersonic flow (500J) allowed constructing a diagram of the generation and dynamics of the resulting shock waves and contact discontinuities, as well as obtaining a significant drop in the drag force and stagnation pressure (up to 80%). The dynamics of a low density and high gas temperature zone, which becomes the main factor reducing the frontal body drag force, was researched. The dynamics of the front surface drag forces have been studied for different values of the plasmoid energy as well. Qualitative agreement of the numerical flow patterns with the experiment ones has been obtained.
      Citation: Fluids
      PubDate: 2021-11-04
      DOI: 10.3390/fluids6110399
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 400: A CFD Tutorial in Julia: Introduction to
           Compressible Laminar Boundary-Layer Flows

    • Authors: Furkan Oz, Kursat Kara
      First page: 400
      Abstract: A boundary-layer is a thin fluid layer near a solid surface, and viscous effects dominate it. The laminar boundary-layer calculations appear in many aerodynamics problems, including skin friction drag, flow separation, and aerodynamic heating. A student must understand the flow physics and the numerical implementation to conduct successful simulations in advanced undergraduate- and graduate-level fluid dynamics/aerodynamics courses. Numerical simulations require writing computer codes. Therefore, choosing a fast and user-friendly programming language is essential to reduce code development and simulation times. Julia is a new programming language that combines performance and productivity. The present study derived the compressible Blasius equations from Navier–Stokes equations and numerically solved the resulting equations using the Julia programming language. The fourth-order Runge–Kutta method is used for the numerical discretization, and Newton’s iteration method is employed to calculate the missing boundary condition. In addition, Burgers’, heat, and compressible Blasius equations are solved both in Julia and MATLAB. The runtime comparison showed that Julia with for loops is 2.5 to 120 times faster than MATLAB. We also released the Julia codes on our GitHub page to shorten the learning curve for interested readers.
      Citation: Fluids
      PubDate: 2021-11-05
      DOI: 10.3390/fluids6110400
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 401: In-Vitro Validation of Self-Powered Fontan
           Circulation for Treatment of Single Ventricle Anomaly

    • Authors: Arka Das, Ray Prather, Eduardo Divo, Michael Farias, Alain Kassab, William DeCampli
      First page: 401
      Abstract: Around 8% of all newborns with a Congenital Heart Defect (CHD) have only a single functioning ventricle. The Fontan operation has served as palliation for this anomaly for decades, but the surgery entails multiple complications, and the survival rate is less than 50% by adulthood. A rapidly testable novel alternative is proposed by creating a bifurcating graft, or Injection Jet Shunt (IJS), used to “entrain” the pulmonary flow and thus provide assistance while reducing the caval pressure. A dynamically scaled Mock Flow Loop (MFL) has been configured to validate this hypothesis. Three IJS nozzles of varying diameters 2, 3, and 4 mm with three aortic anastomosis angles and pulmonary vascular resistance (PVR) reduction have been tested to validate the hypothesis and optimize the caval pressure reduction. The MFL is based on a Lumped-Parameter Model (LPM) of a non-fenestrated Fontan circulation. The best outcome was achieved with the experimental testing of a 3 mm IJS by producing an average caval pressure reduction of more than 5 mmHg while maintaining the clinically acceptable pulmonary flow rate (Qp) to systemic flow rate (Qs) ratio of ~1.5. Furthermore, alteration of the PVR helped in achieving higher caval pressure reduction with the 3 mm IJS at the expense of an increase in Qp/Qs ratio.
      Citation: Fluids
      PubDate: 2021-11-06
      DOI: 10.3390/fluids6110401
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 402: A Cartesian Method with Second-Order Pressure
           Resolution for Incompressible Flows with Large Density Ratios

    • Authors: Michel Bergmann, Lisl Weynans
      First page: 402
      Abstract: An Eulerian method to numerically solve incompressible bifluid problems with high density ratio is presented. This method can be considered as an improvement of the Ghost Fluid method, with the specificity of a sharp second-order numerical scheme for the spatial resolution of the discontinuous elliptic problem for the pressure. The Navier–Stokes equations are integrated in time with a fractional step method based on the Chorin scheme and discretized in space on a Cartesian mesh. The bifluid interface is implicitly represented using a level-set function. The advantage of this method is its simplicity to implement in a standard monofluid Navier–Stokes solver while being more accurate and conservative than other simple classical bifluid methods. The numerical tests highlight the improvements obtained with this sharp method compared to the reference standard first-order methods.
      Citation: Fluids
      PubDate: 2021-11-06
      DOI: 10.3390/fluids6110402
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 403: Mantle Electrical Conductivity and the Magnetic
           Field at the Core–Mantle Boundary

    • Authors: John V. Shebalin
      First page: 403
      Abstract: The Earth’s magnetic field is measured on and above the crust, while the turbulent dynamo in the outer core produces magnetic field values at the core–mantle boundary (CMB). The connection between the two sets of values is usually assumed to be independent of the electrical conductivity in the mantle. However, the turbulent magnetofluid in the Earth’s outer core produces a time-varying magnetic field that must induce currents in the lower mantle as it emerges, since the mantle is observed to be electrically conductive. Here, we develop a model to assess the possible effects of mantle electrical conductivity on the magnetic field values at the CMB. This model uses a new method for mapping the geomagnetic field from the Earth’s surface to the CMB. Since numerical and theoretical results suggest that the turbulent magnetic field in the outer core as it approaches the CMB is mostly parallel to this boundary, we assume that this property exists and set the normal component of the model magnetic field to zero at the CMB. This leads to a modification of the Mauersberger–Lowes spectrum at the CMB so that it is no longer flat, i.e., the modified spectrum depends on mantle conductance. We examined several cases in which mantle conductance ranges from low to high in order to gauge how CMB magnetic field strength and mantle ohmic heat generation may vary.
      Citation: Fluids
      PubDate: 2021-11-08
      DOI: 10.3390/fluids6110403
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 404: Development of a Numerical Investigation
           Framework for Ground Vehicle Platooning

    • Authors: Charles Patrick Bounds, Sudhan Rajasekar, Mesbah Uddin
      First page: 404
      Abstract: This paper presents a study on the flow dynamics involving vehicle interactions. In order to do so, this study first explores aerodynamic prediction capabilities of popular turbulence models used in computational fluid dynamics simulations involving tandem objects and thus, ultimately presents a framework for CFD simulations of ground vehicle platooning using a realistic vehicle model, DrivAer. Considering the availability of experimental data, the simulation methodology is first developed using a tandem arrangement of surface-mounted cubes which requires an understanding on the role of turbulence models and the impacts of the associated turbulence model closure coefficients on the prediction veracity. It was observed that the prediction accuracy of the SST k−ω turbulence model can be significantly improved through the use of a combination of modified values for the closure coefficients. Additionally, the initial validation studies reveal the inability of the Unsteady Reynolds-Averaged Navier-Stokes (URANS) approach to resolve the far wake, and its frailty in simulating tandem body interactions. The Improved Delayed Detached Eddy Simulations (IDDES) approach can resolve the wakes with a reasonable accuracy. The validated simulation methodology is then applied to the fastback DrivAer model at different longitudinal spacing. The results show that, as the longitudinal spacing is reduced, the trailing car’s drag is increased while the leading car’s drag is decreased which supports prior explanations of vortex impingement as the reason for drag changes. Additionally, unlike the case of platooning involving Ahmed bodies, the trailing model drag does not return to an isolated state value at a two car-length separation. However, the impact of the resolution of the far wake of a detailed DrivAer model, and its implication on the CFD characterization of vehicle interaction aerodynamics need further investigations.
      Citation: Fluids
      PubDate: 2021-11-09
      DOI: 10.3390/fluids6110404
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 405: Numerical Bifurcation Analysis of a Film
           Flowing over a Patterned Surface through Enhanced Lubrication Theory

    • Authors: Nicola Suzzi, Giulio Croce
      First page: 405
      Abstract: The bifurcation analysis of a film falling down an hybrid surface is conducted via the numerical solution of the governing lubrication equation. Instability phenomena, that lead to film breakage and growth of fingers, are induced by multiple contamination spots. Contact angles up to 75∘ are investigated due to the full implementation of the free surface curvature, which replaces the small slope approximation, accurate for film slope lower than 30∘. The dynamic contact angle is first verified with the Hoffman–Voinov–Tanner law in case of a stable film down an inclined plate with uniform surface wettability. Then, contamination spots, characterized by an increased value of the static contact angle, are considered in order to induce film instability and several parametric computations are run, with different film patterns observed. The effects of the flow characteristics and of the hybrid pattern geometry are investigated and the corresponding bifurcation diagram with the number of observed rivulets is built. The long term evolution of induced film instabilities shows a complex behavior: different flow regimes can be observed at the same flow characteristics under slightly different hybrid configurations. This suggest the possibility of controlling the rivulet/film transition via a proper design of the surfaces, thus opening the way for relevant practical application.
      Citation: Fluids
      PubDate: 2021-11-09
      DOI: 10.3390/fluids6110405
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 406: Impact of High Inertia Particles on the Shock
           Layer and Heat Transfer in a Heterogeneous Supersonic Flow around a Blunt
           Body

    • Authors: Andrey Sposobin, Dmitry Reviznikov
      First page: 406
      Abstract: One of the most important and complex effects associated with the presence of particles in the flow is the gas-dynamic interaction of particles with the shock layer. Of particular interest is the intensification of heat transfer by high inertia particles rebounding from the surface or by the products of erosion destruction, which reach the front of the bow shock wave and violate the gas-dynamic structure of the flow. In this case, according to experimental data, the increase in heat fluxes is much greater than it could be predicted based on the combined action of the kinetic energy of particles and a high-speed flow. The problem is related to the destruction of the flow structure. In this paper, the problem is studied with numerical simulation. We show that the key role in the intensification of heat transfer is played by the formation of an impact jet flowing onto the surface. An area of increased pressure and heat flux is formed in the zone of action of the impact jet. This effect is maintained over time by the successive action of particles.
      Citation: Fluids
      PubDate: 2021-11-09
      DOI: 10.3390/fluids6110406
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 407: An Arbitrary Hybrid Turbulence Modeling
           Approach for Efficient and Accurate Automotive Aerodynamic Analysis and
           Design Optimization

    • Authors: Saule Maulenkul, Kaiyrbek Yerzhanov, Azamat Kabidollayev, Bagdaulet Kamalov, Sagidolla Batay, Yong Zhao, Dongming Wei
      First page: 407
      Abstract: The demand in solving complex turbulent fluid flows has been growing rapidly in the automotive industry for the last decade as engineers strive to design better vehicles to improve drag coefficients, noise levels and drivability. This paper presents the implementation of an arbitrary hybrid turbulence modeling (AHTM) approach in OpenFOAM for the efficient simulation of common automotive aerodynamics with unsteady turbulent separated flows such as the Kelvin–Helmholtz effect, which can also be used as an efficient part of aerodynamic design optimization (ADO) tools. This AHTM approach is based on the concept of Very Large Eddy Simulation (VLES), which can arbitrarily combine RANS, URANS, LES and DNS turbulence models in a single flow field depending on the local mesh refinement. As a result, the design engineer can take advantage of this unique and highly flexible approach to tailor his grid according to his design and resolution requirements in different areas of the flow field over the car body without sacrificing accuracy and efficiency at the same time. This paper presents the details of the implementation and careful validation of the AHTM method using the standard benchmark case of the Ahmed body, in comparison with some other existing models, such as RANS, URANS, DES and LES, which shows VLES to be the most accurate among the five examined. Furthermore, the results of this study demonstrate that the AHTM approach has the flexibility, efficiency and accuracy to be integrated with ADO tools for engineering design in the automotive industry. The approach can also be used for the detailed study of highly complex turbulent phenomena such as the Kelvin–Helmholtz instability commonly found in automotive aerodynamics. Currently, the AHTM implementation is being integrated with the DAFoam for gradient-based multi-point ADO using an efficient adjoint solver based on a Sparse Nonlinear optimizer (SNOPT).
      Citation: Fluids
      PubDate: 2021-11-10
      DOI: 10.3390/fluids6110407
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 408: Proposal of a Mask and Its Performance Analysis
           with CFD for an Enhanced Aerodynamic Geometry That Facilitates Filtering
           and Breathing against COVID-19

    • Authors: Boris Miguel López-Rebollar, Abad Posadas-Bejarano, Daury García-Pulido, Adrián Torres-Maya, Carlos Díaz-Delgado
      First page: 408
      Abstract: As a result of the recent events associated with the SARS-CoV-2 around the world, there has been a need for research to strengthen health care. The use of masks or respirators has been an effective measure, reducing the risk of contagion caused by the spread of the virus in public places. Currently, there are masks that retain up to 99% of particles >0.3 microns; however, they lack an airtight seal with the face, leading to discomfort and poor protection in conditions without social distancing and areas without ventilation. The device proposed in this study includes a geometric design of static valves with convergent spirals and interior baffles that promotes enhanced aerodynamics with bidirectional flow. According to the analysis and CFD simulation of the proposed reusable, washable, and economic mask and valve system for breathing, coughing, and sneezing events, enhanced air exchange could be maintained, facilitating a higher inhalation flow through the side of the mask (62%) and a higher exhalation through the front of the mask (74%), thereby avoiding the recirculation of the flow to the interior of the mask. The inclusion of filters with KN95 characteristics in the inlets and outlets maintains velocities below 10 cm/s, reducing the probability of infection.
      Citation: Fluids
      PubDate: 2021-11-10
      DOI: 10.3390/fluids6110408
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 409: A Review on Process and Practices in Operation
           and Design Modification of Ejectors

    • Authors: Ravi Koirala, Quoc Linh Ve, Baoshan Zhu, Kiao Inthavong, Abhijit Date
      First page: 409
      Abstract: This work reviews the current operational condition and activities on design modification for different applications of ejectors. Ejectors being a simple mechanical system capable of performing multiple fluid related functions (vacuum generation, pumping, mixing, condensing and heat exchanging), have been an essential part of several industrial processes. Two areas have been emphasized; internal flow and application-based modifications in components of ejectors. The geometry and inlet flow conditions were found to be the prime influencing factor of its performance. The objective and application-based modifications were performed on the primary nozzle, secondary nozzle, mixing chamber, throat and diffuser. The resultant performance was found to be dependent on operational condition and fluid type. This emphasizes the requirement of application-based design selection of the technology. In addition, the flow dynamics of condensing, non-condensing, particle and slurry flow has been studied based on available literatures. The one-point final objective is to identify the usability of primary water jet ejectors for active vapor transport and condensation, to replace vacuum pump and condenser in compact domestic water desalination system.
      Citation: Fluids
      PubDate: 2021-11-11
      DOI: 10.3390/fluids6110409
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 410: Effect of Surfactant Concentration on the
           Long-Term Properties of a Colloidal Chemical, Biological and Radiological
           (CBR) Decontamination Gel

    • Authors: Alban Gossard, Fabien Frances, Camille Aloin, Clara Penavayre, Nicolas Fabrègue, Célia Lepeytre
      First page: 410
      Abstract: Chemically, biologically, or radiologically contaminated surfaces can be treated using colloidal “vacuumable” gels containing alumina particles as a thickening agent, decontaminating solutions to inhibit/eliminate biological and chemical contaminants, and Pluronic PE 6200 as a surfactant to adjust the gel’s physicochemical properties. These gels have been shown to remain efficient even after prolonged storage. In the present study, the properties of gels with different surfactant concentrations were monitored over several months using rheological analyses, contact angle measurements, and ion chromatography. Results show that the surfactant reacts with the hypochlorite ions in the decontaminating solution. This leads to sedimentation, which modifies the rheological properties of the gel. Increasing the surfactant concentration ensures the physicochemical properties of the gel are preserved for longer, but because the surfactant reacts with the hypochlorite ions, the concentration of the latter decreases drastically and thus so do the decontamination properties of the gel. There is therefore a trade-off between the efficiency of the gel against chemical and biological contamination at a given time and how long its physicochemical properties are preserved, with the optimal balance depending on its intended use.
      Citation: Fluids
      PubDate: 2021-11-12
      DOI: 10.3390/fluids6110410
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 411: The Role of the Double-Layer Potential in
           Regularised Stokeslet Models of Self-Propulsion

    • Authors: David J. Smith, Meurig T. Gallagher, Rudi Schuech, Thomas D. Montenegro-Johnson
      First page: 411
      Abstract: The method of regularised stokeslets is widely used to model microscale biological propulsion. The method is usually implemented with only the single-layer potential, the double-layer potential being neglected, despite this formulation often not being justified a priori due to nonrigid surface deformation. We describe a meshless approach enabling the inclusion of the double layer which is applied to several Stokes flow problems in which neglect of the double layer is not strictly valid: the drag on a spherical droplet with partial-slip boundary condition, swimming velocity and rate of working of a force-free spherical squirmer, and trajectory, swimmer-generated flow and rate of working of undulatory swimmers of varying slenderness. The resistance problem is solved accurately with modest discretisation on a notebook computer with the inclusion of the double layer ranging from no-slip to free-slip limits; the neglect of the double-layer potential results in up to 24% error, confirming the importance of the double layer in applications such as nanofluidics, in which partial slip may occur. The squirming swimmer problem is also solved for both velocity and rate of working to within a small percent error when the double-layer potential is included, but the error in the rate of working is above 250% when the double layer is neglected. The undulating swimmer problem by contrast produces a very similar value of the velocity and rate of working for both slender and nonslender swimmers, whether or not the double layer is included, which may be due to the deformation’s ‘locally rigid body’ nature, providing empirical evidence that its neglect may be reasonable in many problems of interest. The inclusion of the double layer enables us to confirm robustly that slenderness provides major advantages in efficient motility despite minimal qualitative changes to the flow field and force distribution.
      Citation: Fluids
      PubDate: 2021-11-13
      DOI: 10.3390/fluids6110411
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 412: Vortex Formation Times in the Glottal Jet,
           Measured in a Scaled-Up Model

    • Authors: Michael Krane
      First page: 412
      Abstract: In this paper, the timing of vortex formation on the glottal jet is studied using previously published velocity measurements of flow through a scaled-up model of the human vocal folds. The relative timing of the pulsatile glottal jet and the instability vortices are acoustically important since they determine the harmonic and broadband content of the voice signal. Glottis exit jet velocity time series were extracted from time-resolved planar DPIV measurements. These measurements were acquired at four glottal flow speeds (uSS = 16.1–38 cm/s) and four glottis open times (To = 5.67–23.7 s), providing a Reynolds number range Re = 4100–9700 and reduced vibration frequency f* = 0.01−0.06. Exit velocity waveforms showed temporal behavior on two time scales, one that correlates to the period of vibration and another characterized by short, sharp velocity peaks (which correlate to the passage of instability vortices through the glottis exit plane). The vortex formation time, estimated by computing the time difference between subsequent peaks, was shown to be not well-correlated from one vibration cycle to the next. The principal finding is that vortex formation time depends not only on cycle phase, but varies strongly with reduced frequency of vibration. In all cases, a strong high-frequency burst of vortex motion occurs near the end of the cycle, consistent with perceptual studies using synthesized speech.
      Citation: Fluids
      PubDate: 2021-11-15
      DOI: 10.3390/fluids6110412
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 413: Schlieren Flow Visualization and Analysis of
           Synthetic Jets

    • Authors: John E. Pellessier, Heather E. Dillon, Wyatt Stoltzfus
      First page: 413
      Abstract: This work explores several low-cost methods for the visualization and analysis of pulsed synthetic jets for cooling applications. The visualization methods tested include smoke, Schlieren imaging, and thermography. The images were analyzed using Proper Orthogonal Decomposition (POD) and numerical methods for videos. The results indicated that for the specific nozzle studied, the optimal cooling occurred at a frequency of 80 Hz, which also corresponded to the highest energy in the POD analysis. The combination of Schlieren photography and POD is a unique contribution as a method for the optimization of synthetic jets.
      Citation: Fluids
      PubDate: 2021-11-15
      DOI: 10.3390/fluids6110413
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 414: A Constitutive Equation of Turbulence

    • Authors: Peter W. Egolf, Kolumban Hutter
      First page: 414
      Abstract: Even though applications of direct numerical simulations are on the rise, today the most usual method to solve turbulence problems is still to apply a closure scheme of a defined order. It is not the case that a rising order of a turbulence model is always related to a quality improvement. Even more, a conceptual advantage of applying a lowest order turbulence model is that it represents the analogous method to the procedure of introducing a constitutive equation which has brought success to many other areas of physics. First order turbulence models were developed in the 1920s and today seem to be outdated by newer and more sophisticated mathematical-physical closure schemes. However, with the new knowledge of fractal geometry and fractional dynamics, it is worthwhile to step back and reinvestigate these lowest order models. As a result of this and simultaneously introducing generalizations by multiscale analysis, the first order, nonlinear, nonlocal, and fractional Difference-Quotient Turbulence Model (DQTM) was developed. In this partial review article of work performed by the authors, by theoretical considerations and its applications to turbulent flow problems, evidence is given that the DQTM is the missing (apparent) constitutive equation of turbulent shear flows.
      Citation: Fluids
      PubDate: 2021-11-15
      DOI: 10.3390/fluids6110414
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 415: Wind Effects on a Permeable Double Skin
           Façade, the ENI Head Office Case Study

    • Authors: Giulia Pomaranzi, Ombretta Bistoni, Paolo Schito, Lorenzo Rosa, Alberto Zasso
      First page: 415
      Abstract: Currently, the energy and environmental efficiency of buildings has led to the development of cladding systems that may help to reduce the structure’s energy demand, using techniques such as the Permeable Double Skin Façade (PDSF). Given complex aerodynamic interactions, the presence of an external porous screen in addition to an inner skin may play a crucial role in the fluid-dynamic characterization of such buildings, making the definition of wind effects very complex. A new methodology for the quantitative assessment of the impact of wind-loading conditions on this particular type of cladding is presented. It is based on a combined experimental–numerical approach, essentially based on wind-tunnel tests on a rigid scale model and computational fluid dynamic simulations. A case study is proposed as an application of this methodology. Results include the design pressure values for the inner glazed façade and the permeable facade. An estimation of the flow rate across the porous skin is quantified using the numerical model.
      Citation: Fluids
      PubDate: 2021-11-16
      DOI: 10.3390/fluids6110415
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 416: Irreversibility Analysis for Eyring–Powell
           Nanoliquid Flow Past Magnetized Riga Device with Nonlinear Thermal
           Radiation

    • Authors: Ephesus Olusoji Fatunmbi, Adeshina Taofeeq Adeosun, Sulyman Olakunle Salawu
      First page: 416
      Abstract: The report contained in this article is based on entropy generation for a reactive Eyring–Powell nanoliquid transfer past a porous vertical Riga device. In the developed model, the impacts of viscous dissipation, thermophoresis alongside nonlinear heat radiation and varying heat conductivity are modelled into the heat equation. The dimensionless transport equations are analytically tackled via Homotopy analysis method while the computational values of chosen parameters are compared with the Galerkin weighted residual method. Graphical information of the various parameters that emerged from the model are obtained and deliberated effectively. The consequences of this study are that the temperature field expands with thermophoresis, Brownian motion and temperature ratio parameters as the modified Hartmann number compels a rise in the velocity profile. The entropy generation rises with an uplift in fluid material term as well as Biot and Eckert numbers whereas Bejan number lessens with Darcy and Eckert parameters.
      Citation: Fluids
      PubDate: 2021-11-16
      DOI: 10.3390/fluids6110416
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 417: Comparison between R134a and R1234ze(E) during
           Flow Boiling in Microfin Tubes

    • Authors: Andrea Lucchini, Igor M. Carraretto, Thanh N. Phan, Paola G. Pittoni, Luigi P. M. Colombo
      First page: 417
      Abstract: Environmental concerns are forcing the replacement of commonly used refrigerants, and finding new fluids is a top priority. Soon the R134a will be banned, and the hydro-fluoro-olefin (HFO) R1234ze(E) has been indicated as an alternative due to its smaller global warming potential (GWP) and shorter atmospheric lifetime. Nevertheless, for an optimal replacement, its thermo-fluid-dynamic characteristics have to be assessed. Flow boiling experiments (saturation temperature Tsat = 5 °C, mass flux G = 65 ÷ 222 kg·m−2·s−1, mean quality xm = 0.15 ÷ 0.95, quality changes ∆x = 0.06 ÷ 0.6) inside a microfin tube were performed to compare the pressure drop per unit length and the heat transfer coefficient provided by the two fluids. The results were benchmarked for some correlations. In commonly adopted operating conditions, the two fluids show a very similar behavior, while benchmark showed that some correlations are available to properly predict the pressure drop for both fluids. However, only one is satisfactory for the heat transfer coefficient. In conclusion, R1234ze(E) proved to be a suitable drop-in replacement for the R134a, whereas further efforts are recommended to refine and adapt the available predictive models.
      Citation: Fluids
      PubDate: 2021-11-18
      DOI: 10.3390/fluids6110417
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 418: Characterization and Modeling of the
           Viscoelastic Behavior of Hydrocolloid-Based Films Using Classical and
           Fractional Rheological Models

    • Authors: David Ramirez-Brewer, Oscar Danilo Montoya, Jairo Useche Vivero, Luis García-Zapateiro
      First page: 418
      Abstract: Hydrocolloid-based films are a good alternative in the development of biodegradable films due to their properties, such as non-toxicity, functionality, and biodegradability, among others. In this work, films based on hydrocolloids (gellan gum, carrageenan, and guar gum) were formulated, evaluating their dynamic rheological behavior and creep and recovery. Maxwell’s classical and fractional rheological models were implemented to describe its viscoelastic behavior, using the Vortex Search Algorithm for the estimation of the parameters. The hydrocolloid-based films showed a viscoelastic behavior, where the behavior of the storage modulus (G′) and loss modulus (G″) indicated a greater elastic behavior (G′>G″). The Maxwell fractional model with two spring-pots showed an optimal fit of the experimental data of storage modulus (G′) and loss modulus (G″) and a creep compliance (J) (Fmin<0.1 and R2>0.98). This shows that fractional models are an excellent alternative for describing the dynamic rheological behavior and creep recovery of films. These results show the importance of estimating parameters that allow for the dynamic rheological and creep behaviors of hydrocolloid-based films for applications in the design of active films because they allow us to understand their behavior from a rheological point of view, which can contribute to the design and improvement of products such as food coatings, food packaging, or other applications containing biopolymers.
      Citation: Fluids
      PubDate: 2021-11-18
      DOI: 10.3390/fluids6110418
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 419: Rheological Characterization of Non-Newtonian
           Mixtures by Pressure Pipe Tests

    • Authors: Armando Carravetta, Oreste Fecarotta, Riccardo Martino, Maria Cristina Morani
      First page: 419
      Abstract: The rheological behavior of non-Newtonian fluids in turbulent conditions is an important topic in several fields of engineering. Nevertheless, this topic was not deeply investigated in the past due to the complexity of the experimental tests for the assessment of the constitutive parameters. Pressure pipe tests on Herschel-Bulkley mixtures were proven to be suitable for exploring turbulent conditions, but discrepancies with the results of tests performed in laminar flow were detected. These contradictions could be attributed to the inconsistencies of the Herschel-Bulkley model (HB) for high shear rate flows, proven by Hallbom and Klein, who suggested a more general “yield plastic” model (HK). Hence, in this study, a procedure for the estimation of the rheological parameters of both HB and HK models in pressure pipe tests is defined and rated on a complete set of experiments. The HK model performed much better than HB model in the turbulent range and slightly better than the HB model in the laminar range, confirming the consistency of the “yield plastic” model. The rheological parameters obtained by the proposed procedure were used to numerically model a dam-break propagation of a non-Newtonian fluid, showing significant differences in terms of process evolution depending on the constitutive model.
      Citation: Fluids
      PubDate: 2021-11-20
      DOI: 10.3390/fluids6110419
      Issue No: Vol. 6, No. 11 (2021)
       
  • Fluids, Vol. 6, Pages 420: A Numerical Study of Spray Strips Analysis on
           Fridsma Hull Form

    • Authors: Samuel, Andi Trimulyono, Parlindungan Manik, Deddy Chrismianto
      First page: 420
      Abstract: Spray strips are deflectors added to the hull to reduce the Wetted Surface Area (WSA). The reduced WSA will decrease the total ship drag caused by the deflection of the spray strip installation. The research aimed to predict the function of the spray strip to improve ship performance using Computational Fluid Dynamics (CFD). The numerical approach in this study used the Finite Volume Method (FVM) with the RANS (Reynolds-averaged Navier–Stokes) equation to solve fluid dynamics problems. VOF (Volume of Fluid) was used to model the water and air phases. The results of this study indicated that the number of spray strips would have a significant effect compared to without using a spray strip. Spray strips with three strips could reduce the total resistance by 4.9% at Fr 1.78. Spray strips would increase the total resistance value by 2.1% at low speeds. Spray strips were effective for reducing total resistance at Fr > 1 or the planing mode conditions. The total resistance prediction used three suggestion profiles with the best performance to reduce total resistance by 6.0% at Fr 1.78.
      Citation: Fluids
      PubDate: 2021-11-22
      DOI: 10.3390/fluids6110420
      Issue No: Vol. 6, No. 11 (2021)
       
 
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