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Publisher: Springer-Verlag   (Total: 2209 journals)

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e & i Elektrotechnik und Informationstechnik     Hybrid Journal   (Followers: 8, SJR: 0.146, h-index: 8)
e-Neuroforum     Hybrid Journal  
Early Childhood Education J.     Hybrid Journal   (Followers: 12, SJR: 0.367, h-index: 12)
Earth Science Informatics     Hybrid Journal   (Followers: 3, SJR: 0.245, h-index: 5)
Earth, Moon, and Planets     Hybrid Journal   (Followers: 5, SJR: 0.436, h-index: 28)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 7, SJR: 0.433, h-index: 17)
Earthquake Science     Hybrid Journal   (Followers: 9, SJR: 0.486, h-index: 7)
East Asia     Hybrid Journal   (Followers: 7, SJR: 0.165, h-index: 9)
Eating and Weight Disorders - Studies on Anorexia, Bulimia and Obesity     Hybrid Journal   (Followers: 7, SJR: 0.289, h-index: 23)
EcoHealth     Hybrid Journal   (Followers: 1, SJR: 0.651, h-index: 22)
Ecological Research     Hybrid Journal   (Followers: 8, SJR: 0.698, h-index: 38)
Economic Botany     Hybrid Journal   (Followers: 8, SJR: 0.666, h-index: 40)
Economic Bulletin     Hybrid Journal   (Followers: 4)
Economic Change and Restructuring     Hybrid Journal   (Followers: 1, SJR: 0.263, h-index: 6)
Economic Theory     Hybrid Journal   (Followers: 6, SJR: 1.857, h-index: 31)
Economic Theory Bulletin     Hybrid Journal   (Followers: 1)
Economics of Governance     Hybrid Journal   (Followers: 2, SJR: 0.367, h-index: 12)
Ecosystems     Hybrid Journal   (Followers: 19, SJR: 1.793, h-index: 83)
Ecotoxicology     Hybrid Journal   (Followers: 10, SJR: 1.041, h-index: 53)
Education and Information Technologies     Hybrid Journal   (Followers: 159, SJR: 0.207, h-index: 15)
Educational Assessment, Evaluation and Accountability     Hybrid Journal   (Followers: 13, SJR: 0.519, h-index: 14)
Educational Psychology Review     Hybrid Journal   (Followers: 14, SJR: 1.781, h-index: 52)
Educational Research for Policy and Practice     Hybrid Journal   (Followers: 6, SJR: 0.211, h-index: 8)
Educational Studies in Mathematics     Hybrid Journal   (Followers: 9, SJR: 0.946, h-index: 27)
Educational Technology Research and Development     Partially Free   (Followers: 173, SJR: 1.124, h-index: 45)
Electrical Engineering     Hybrid Journal   (Followers: 11, SJR: 0.352, h-index: 17)
Electrocatalysis     Hybrid Journal   (SJR: 0.542, h-index: 7)
Electronic Commerce Research     Hybrid Journal   (Followers: 3, SJR: 0.636, h-index: 14)
Electronic Markets     Hybrid Journal   (Followers: 5, SJR: 0.326, h-index: 5)
Electronic Materials Letters     Hybrid Journal   (Followers: 3, SJR: 0.566, h-index: 11)
Elemente der Mathematik     Hybrid Journal  
Emergency Radiology     Hybrid Journal   (Followers: 4, SJR: 0.446, h-index: 22)
Empirica     Hybrid Journal   (Followers: 3, SJR: 0.185, h-index: 12)
Empirical Economics     Hybrid Journal   (Followers: 8, SJR: 0.5, h-index: 29)
Empirical Software Engineering     Hybrid Journal   (Followers: 4, SJR: 2.319, h-index: 33)
Employee Responsibilities and Rights J.     Hybrid Journal   (Followers: 2, SJR: 0.21, h-index: 13)
Endocrine     Hybrid Journal   (Followers: 4, SJR: 0.659, h-index: 55)
Endocrine Pathology     Hybrid Journal   (Followers: 2, SJR: 0.555, h-index: 27)
Energy Efficiency     Hybrid Journal   (Followers: 11, SJR: 1.056, h-index: 10)
Energy Systems     Hybrid Journal   (Followers: 9, SJR: 0.589, h-index: 5)
Engineering With Computers     Hybrid Journal   (Followers: 5, SJR: 0.497, h-index: 26)
Entomological Review     Hybrid Journal   (Followers: 3, SJR: 0.128, h-index: 5)
Environment Systems & Decisions     Hybrid Journal   (Followers: 2)
Environment, Development and Sustainability     Hybrid Journal   (Followers: 28, SJR: 0.319, h-index: 26)
Environmental and Ecological Statistics     Hybrid Journal   (Followers: 5, SJR: 0.389, h-index: 29)
Environmental and Resource Economics     Hybrid Journal   (Followers: 17, SJR: 1.651, h-index: 46)
Environmental Biology of Fishes     Hybrid Journal   (Followers: 3, SJR: 0.486, h-index: 53)
Environmental Chemistry Letters     Hybrid Journal   (Followers: 3, SJR: 0.664, h-index: 22)
Environmental Earth Sciences     Hybrid Journal   (Followers: 10, SJR: 0.601, h-index: 55)
Environmental Economics and Policy Studies     Hybrid Journal   (Followers: 6, SJR: 0.35, h-index: 3)
Environmental Evidence     Open Access  
Environmental Fluid Mechanics     Hybrid Journal   (Followers: 2, SJR: 0.732, h-index: 23)
Environmental Geochemistry and Health     Hybrid Journal   (Followers: 2, SJR: 0.909, h-index: 32)
Environmental Geology     Hybrid Journal   (Followers: 11)
Environmental Health and Preventive Medicine     Hybrid Journal   (Followers: 2, SJR: 0.388, h-index: 14)
Environmental Management     Hybrid Journal   (Followers: 32, SJR: 0.773, h-index: 60)
Environmental Modeling & Assessment     Hybrid Journal   (Followers: 11, SJR: 0.413, h-index: 27)
Environmental Monitoring and Assessment     Hybrid Journal   (Followers: 9, SJR: 0.671, h-index: 46)
Environmental Science and Pollution Research     Hybrid Journal   (Followers: 11, SJR: 0.878, h-index: 42)
Epidemiologic Perspectives & Innovations     Open Access   (Followers: 1, SJR: 1.002, h-index: 14)
Epileptic Disorders     Hybrid Journal   (Followers: 1, SJR: 0.669, h-index: 34)
EPJ A - Hadrons and Nuclei     Hybrid Journal   (Followers: 1, SJR: 1.435, h-index: 58)
EPJ B - Condensed Matter and Complex Systems     Hybrid Journal   (Followers: 3, SJR: 0.749, h-index: 85)
EPJ direct     Hybrid Journal  
EPJ E - Soft Matter and Biological Physics     Hybrid Journal   (Followers: 1, SJR: 0.661, h-index: 57)
EPMA J.     Open Access   (SJR: 0.161, h-index: 4)
ERA-Forum     Hybrid Journal   (Followers: 2, SJR: 0.13, h-index: 2)
Erkenntnis     Hybrid Journal   (Followers: 11, SJR: 0.62, h-index: 14)
Erwerbs-Obstbau     Hybrid Journal   (SJR: 0.173, h-index: 8)
Esophagus     Hybrid Journal   (SJR: 0.268, h-index: 9)
Estuaries and Coasts     Hybrid Journal   (Followers: 3, SJR: 1.111, h-index: 61)
Ethical Theory and Moral Practice     Hybrid Journal   (Followers: 7, SJR: 0.278, h-index: 8)
Ethics and Information Technology     Hybrid Journal   (Followers: 178, SJR: 0.363, h-index: 20)
Ethik in der Medizin     Hybrid Journal   (SJR: 0.204, h-index: 6)
Euphytica     Hybrid Journal   (Followers: 7, SJR: 0.709, h-index: 57)
Eurasian Soil Science     Hybrid Journal   (Followers: 2, SJR: 0.271, h-index: 10)
EURO J. of Transportation and Logistics     Hybrid Journal   (Followers: 4)
EURO J. on Computational Optimization     Hybrid Journal  
EURO J. on Decision Processes     Hybrid Journal  
Europaisches J. fur Minderheitenfragen     Hybrid Journal  
European Actuarial J.     Hybrid Journal   (Followers: 3)
European Archives of Oto-Rhino-Laryngology     Hybrid Journal   (Followers: 4, SJR: 0.737, h-index: 37)
European Archives of Paediatric Dentistry     Hybrid Journal   (Followers: 1, SJR: 0.446, h-index: 12)
European Archives of Psychiatry and Clinical Neuroscience     Hybrid Journal   (Followers: 2, SJR: 1.334, h-index: 62)
European Biophysics J.     Hybrid Journal   (Followers: 4, SJR: 0.979, h-index: 53)
European Child & Adolescent Psychiatry     Hybrid Journal   (Followers: 4, SJR: 1.269, h-index: 51)
European Clinics in Obstetrics and Gynaecology     Hybrid Journal   (Followers: 4)
European Food Research and Technology     Hybrid Journal   (Followers: 8, SJR: 0.773, h-index: 49)
European J. for Education Law and Policy     Hybrid Journal   (Followers: 5)
European J. for Philosophy of Science     Partially Free   (Followers: 4, SJR: 0.165, h-index: 2)
European J. of Ageing     Hybrid Journal   (Followers: 8, SJR: 0.49, h-index: 17)
European J. of Applied Physiology     Hybrid Journal   (Followers: 7, SJR: 1.044, h-index: 74)
European J. of Clinical Microbiology & Infectious Diseases     Hybrid Journal   (Followers: 10, SJR: 0.958, h-index: 74)
European J. of Clinical Pharmacology     Hybrid Journal   (Followers: 9, SJR: 0.916, h-index: 69)
European J. of Dermatology     Hybrid Journal   (Followers: 7)
European J. of Drug Metabolism and Pharmacokinetics     Hybrid Journal   (Followers: 6, SJR: 0.24, h-index: 25)
European J. of Epidemiology     Hybrid Journal   (Followers: 17, SJR: 1.946, h-index: 60)
European J. of Forest Research     Hybrid Journal   (Followers: 3, SJR: 0.864, h-index: 25)
European J. of Health Economics     Hybrid Journal   (Followers: 11, SJR: 0.67, h-index: 25)
European J. of Law and Economics     Hybrid Journal   (Followers: 175, SJR: 0.242, h-index: 13)

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Journal Cover Experiments in Fluids
   Journal TOC RSS feeds Export to Zotero [8 followers]  Follow    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
     ISSN (Print) 1432-1114 - ISSN (Online) 0723-4864
     Published by Springer-Verlag Homepage  [2209 journals]   [SJR: 1.033]   [H-I: 62]
  • Coupled measurements of PIV and photoelasticimetry on a thixotropic yield
           stress fluid flow
    • Abstract: Abstract To study the behavior of thixotropic yield stress fluids, information at the local scale is required in order to determine precisely the yield point value, and the shear rate and stresses can be obtained all over the flow. This study focuses on the flow in a large shear cell of a Laponite suspension. In order to be able to construct a local rheogram for this suspension, two different methods issued from fluid mechanics and solid mechanics are used. Local velocities are determined with a PIV technique, and local stresses are determined with the photoelasticimetry technique.
      PubDate: 2014-10-22
       
  • Velocity field and parametric analysis of a subsonic, medium-Reynolds
           number cavity flow
    • Abstract: Abstract Cavity flows are a class of flows bounded by material structures, where a recirculation region is present, and they are found in many practical applications. In the present study, the interaction between a boundary layer and an open parallelepipedic cavity develops a Kelvin–Helmholtz-like instability coupled with the cavity recirculation. PIV measurements of the flow are carried out in two orthogonal planes inside the cavity, for different aspect ratios, incompressible flow conditions, and Reynolds numbers in the range 1,900–12,000. Mean velocity and second-order moments of velocity fluctuations reveal the flow morphology. For particular conditions, centrifugal instabilities appear that are induced by flow curvature due to wall confinement. The use of an identification criterion indicates the presence of pairs of counter-rotating vortices winded around the recirculation. A parametric analysis is conducted, and the inviscid Rayleigh discriminant provides the potentially unstable flow regions inside the cavity. Finally, a stability parameter considering the ratio between centrifugal destabilizing effects and stabilizing viscous effects is carried out and gives thresholds for the emergence of the centrifugal instability. The study draws to an end with a comparison with a well-documented lid-driven cavity flow.
      PubDate: 2014-10-22
       
  • Three-dimensional effects in quasi two-dimensional free surface scalar
           experiments
    • Abstract: Abstract The disagreement between free surface scalar experiments and the two-dimensional (2D) transport equation is discussed. An effective diffusivity coefficient, \(\kappa _{{\rm eff}}\) , is introduced and defined as the quotient between variance decay and mean gradient square. In all the experiments performed, \(\kappa _{{\rm eff}}\) is significantly larger than the scalar diffusivity, \(\kappa \) . Three mechanisms are identified as responsible for the differences between the quasi two-dimensional (Q2D) experiments and the 2D behaviour of a diffusive scalar. These are the vertical velocity gradients, the free surface divergence and the gravity currents induced by the scalar. These mechanisms, which affect the diffusive term in the 2D transport equation for large Péclet number ( \(Pe\gg 1\) ), are evaluated for steady and time-dependant laminar flows driven by electromagnetic body forces.
      PubDate: 2014-10-21
       
  • Dead time effects in laser Doppler anemometry measurements
    • Abstract: Abstract We present velocity power spectra computed by the so-called direct method from burst-type laser Doppler anemometer (LDA) data, both measured in a turbulent round jet and generated in a computer. Using today’s powerful computers, we have been able to study more properties of the computed spectra than was previously possible, and we noted some unexpected features of the spectra that we now attribute to the unavoidable influence of a finite measurement volume (MV). The most prominent effect, which initially triggered these studies, was the appearance of damped oscillations in the higher frequency range, starting around the cutoff frequency due to the finite size of the MV. Using computer-generated data mimicking the LDA data, these effects have previously been shown to appear due to the effect of dead time, i.e., the finite time during which the system is not able to acquire new measurements. These dead times can be traced back to the fact that the burst-mode LDA cannot measure more than one signal burst at a time. Since the dead time is approximately equal to the residence time for a particle traversing a measurement volume, we are dealing with widely varying dead times, which, however, are assumed to be measured for each data point. In addition, the detector and processor used in the current study introduce a certain amount of fixed processing and data transfer times, which further contribute to the distortion of the computed spectrum. However, we show an excellent agreement between a measured spectrum and our modeled LDA data, thereby confirming the validity of our model for the LDA burst processor.
      PubDate: 2014-10-21
       
  • Comparison of magnetic resonance concentration measurements in water to
           temperature measurements in compressible air flows
    • Abstract: Abstract Magnetic resonance imaging (MRI) measurements in liquid flows provide highly detailed 3D mean velocity and concentration data in complex turbulent mixing flow applications. The scalar transport analogy is applied to infer the mean temperature distribution in high speed gas flows directly from the MRI concentration measurements in liquid. Compressibility effects on turbulent mixing are known to be weak for simple flows at high subsonic Mach number, and it was not known if this would hold in more complex flows characteristic of practical applications. Furthermore, the MRI measurements are often done at lower Reynolds number than the compressible application, although both are generally done in fully turbulent flows. The hypothesis is that the conclusions from MRI measurements performed in water are transferable to high subsonic Mach number applications. The present experiment is designed to compare stagnation temperature measurements in high speed airflow (M = 0.7) to concentration measurements in an identical water flow apparatus. The flow configuration was a low aspect ratio wall jet with a thick splitter plate producing a 3D complex downstream flow mixing the wall-jet fluid with the mainstream flow. The three-dimensional velocity field is documented using magnetic resonance velocimetry in the water experiment, and the mixing is quantified by measuring the mean concentration distribution of wall-jet fluid marked with dissolved copper sulfate. The airflow experiments are operated with a temperature difference between the main stream and the wall jet. Profiles of the stagnation temperature are measured with a shielded thermocouple probe. The results show excellent agreement between normalized temperature and concentration profiles after correction of the temperature measurements for the effects of energy separation. The agreement is within 1 % near the edges of the mixing layer, which suggests that the mixing characteristics of the large scale turbulence structures are the same in the two flows.
      PubDate: 2014-10-18
       
  • Three-dimensional flow measurements on flapping wings using synthetic
           aperture PIV
    • Abstract: Abstract We present the results of 3D velocity measurements of the flow fields around a free-flying painted lady butterfly (Vanessa cardui) and a tethered mechanical flapper using Synthetic Aperture PIV (SAPIV). The velocity fields presented for the free-flying butterfly have limited spatial resolution; however, leading edge vortices (LEV) and trailing edge vortices (TEV) can be seen during the downstroke of the butterfly. The results show that SAPIV has potential as a flow analysis tool to obtain whole-field, time-resolved velocities surrounding freely flying insects. The results of a tethered mechanical flapper focus mainly on the LEV and TEV through an entire flapping cycle. The results are compared to velocity measurements taken using traditional PIV techniques. Additionally, force measurements of the lift and thrust generated by the mechanical flapper are compared with the calculated forces from the measured velocity data and circulation in the flow field. The reconstructed visual hull of the butterfly and mechanical flapper is also discussed.
      PubDate: 2014-10-15
       
  • Marangoni convection in evaporating meniscus with changing contact angle
    • Abstract: Abstract In this work, the Marangoni convection in the liquid phase of an evaporating meniscus interface in open air has been studied for varying contact angles. Ethanol undergoes self-evaporation inside a capillary tube of borosilicate glass with internal diameter of 1 mm. The evaporation is not uniform along the meniscus interface pinned at the capillary tube mouth, and this creates a gradient of temperature between the wedge and the centre of the meniscus. It is this temperature difference and the scale (1 mm) that generate a gradient of surface tension that is acknowledged to drive the vigorous Marangoni convection in the meniscus liquid phase. In previous studies of this configuration, the meniscus has mainly been concave and for this reason, other researchers attributed the differential temperature along the meniscus to the fact that the meniscus wedge is closer to the tube mouth and also further away from the warmer liquid bulk than the meniscus centre. The present study investigates concave, flat and convex meniscus by using a syringe pump that forces the meniscus to the wanted shape. With the present investigation, we want to further demonstrate that it is instead the larger evaporation at the meniscus triple line near the wedge that controls the phenomenon. Flow visualization and infrared temperature measurements have been performed. For concave and convex meniscus, the temperature measurements are in line with the predicted trend; the Marangoni vortices for these two menisci shapes spin in the same direction according to the temperature differences along the meniscus. For a flat meniscus, an intriguing experimental evidence has been found: the temperature difference is inverted with respect to concave and convex menisci, but surprisingly, the Marangoni vortices spin in the same direction than for concave and convex menisci.
      PubDate: 2014-10-11
       
  • The impact of twine/mesh ratio on the flow dynamics through a porous
           cylinder
    • Abstract: Abstract The impact of the twine/mesh ratio on the flow through a porous hollow cylinder of diameter D has been experimentally investigated at Reynolds number Re = 800 with a surface porosity varying from 0.67 to 0.90. Our porous cylinder models are inspired by aquaculture pens in that they have similar geometries, and porosities, to those nets commonly used within the aquaculture industry. We show that the surface porosity alone is not the key parameter determining the flow topology of the model, but rather a non-dimensional parameter \(\alpha =t^{0.5}D^{0.5}/m\) (based upon twine thickness t, mesh void m and cylinder diameter D) effectively collapses first-order moments. Three different wake regimes are observed in the flow for different twine/mesh ratios: a laminar flow regime where streamlines pass through the model without significant deformation; a partially occluded flow, where the mean flow is decelerated, and a flow with a fully developed recirculation zone exhibiting a von Kármán vortex street similar to that produced in the wake of a solid cylinder. Our observation that the flow structure depends upon the parameter \(\alpha \) , rather than simply the surface porosity, is supported by calculated dispersion times of virtual particles released both inside the model and within the wake. The particle distributions display three distinct dispersion behaviours, from nearly linear to a logarithmic decay slower than that of a solid cylinder, thus emphasising the existence of multiple flow regimes and the importance of the relative twine/mesh ratio.
      PubDate: 2014-10-07
       
  • Use of PIV to highlight possible errors in hot-wire Reynolds stress data
           over a 2D rough wall
    • Abstract: Abstract Particle image velocimetry (PIV) measurements are carried out in a turbulent boundary layer over a 2D rough surface consisting of transverse square bars. The aim of this work is to investigate a possible cause for the near-wall X-wire measurement errors observed on similar rough surfaces. The PIV measurements do not show the anomalous near-wall deficit of Reynolds stresses as measured with X-wires over the same surface. An extensive flow visualization analysis of the PIV data for a spacing between the roughness elements of p = 7k (k is the roughness element height) shows the occurrence of large-scale inward (sweeps) and outward (ejections) motions with a period of about 10.6δ/U 0 (δ and U 0 are the boundary layer thickness and the free-stream velocity). While these motions dominate the near-wall region and contribute almost equally to the Reynolds shear stress −‹uv›, the mean outward deviation from the mean flow direction is stronger than the inward deviation. Also, when the roughness spacing is reduced to p = 3k, the outward deviation reduces significantly more than the inward deviation. The results support the argument that the outward motions, which can have an instantaneous deviation angle of more than 50° in the case p = 7k, make the X-wire probe inefficient for detecting the ejection events (associated with the outward motions), particularly if the apex angle of the X-wire is not optimized for capturing the strong flow ejections with large deviations. The results explain in part the disparate information on the effect of the roughness on the Reynolds stresses in the outer region of the turbulent boundary layer over rough walls.
      PubDate: 2014-10-07
       
  • Experimental study on drag-reduction effect due to sinusoidal riblets in
           turbulent channel flow
    • Abstract: Abstract The drag-reduction effect of a three-dimensional sinusoidal riblet surface is experimentally evaluated in a fully developed turbulent channel flow. The lateral spacing of the adjacent walls of the riblet is varied sinusoidally in the streamwise direction. The obtained maximum total drag-reduction rate is approximately 12 % at a bulk Reynolds number of 3,400. The flow structure over the sinusoidal riblet surface is also analyzed in the velocity field by using two-dimensional particle image velocimetry. The velocity field is compared with the corresponding flow over a flat surface. It is found through pathlines and Reynolds shear stress analyses that the drag-reduction mechanism is similar to those of two-dimensional riblets. A different point is that the present riblet respectively induces a downward and upward flows in the expanded and contracted regions, which prevent vortices from hitting the bottom wall with wider lateral spacing of the riblet. In consequence, the wetted area of the present sinusoidal riblet is smaller than those of two-dimensional riblets, resulting in the high drag-reduction effect.
      PubDate: 2014-10-04
       
  • Interference-based overlapping particle tracking velocimetry for fluidized
           beds
    • Abstract: Abstract Fluidized beds are used in a wide number of applications, including power plant boilers and chemical facilities. This study proposes a novel particle tracking velocimetry algorithm for semi-dilute suspensions present in fluidized beds. The proposed algorithm is based on thresholding and profile matching algorithms. Image intensity thresholding is used to find regions which need additional image processing. These regions are then processed using interference-based profile matching algorithm to refine the solution quality. The key idea is to limit heavy profile matching computations only to identified clusters to save as much computation time as possible. The method was tested with simulated data and experimental data. The simulations showed that the proposed method was better than the previous boundary arc detection-based method in noisy conditions and cluster sizes ranging from 2 to 6 particles. The difference between the previous approach and the proposed method was small in cluster sizes larger than six particles, even though the proposed method was still slightly better. In low noise conditions with only two particles, the boundary arc detection could outperform the proposed method, but the difference was small. The method was also tested with experimental data from a small cold model fluidized bed. The velocity distributions obtained from the bed are shown for qualitative evaluation. The velocity distributions were realistic, which suggests the usability of the method. The proposed method was also compared to particle image velocimetry to see if both methods produce similar results. The results were divided into histogram intervals according to image intensity that was proportional to local solids volume fraction. The comparison showed that both methods produced similar results, in particular in the low-intensity range, which supports the ability of the method to produce realistic results also in the semi-dilute range where particles form small clusters.
      PubDate: 2014-09-27
       
  • PIV measurements of the unsteady flow structures in a volute centrifugal
           pump at a high flow rate
    • Abstract: Abstract An experimental investigation based on PIV measurements is presented on the unsteady flow in a centrifugal pump with vaneless volute. The study has focused on the fluid–dynamic interaction between impeller and volute associated with the blade passage period when the pump operates well above design conditions (150 % of nominal flow rate). The test pump, which is fully transparent, has a 2D-shaped geometry and the impeller has 6 arc-shaped backward-curved blades. The flow in the mid-plane perpendicular to the pump axis was captured using fluorescent seeding particles and two fields of view (FOV), each with different magnification: a large FOV (low magnification) that covers a large portion of the impeller, volute and discharge duct, and a small FOV (high magnification) to observe the details of flow evolution at the tongue region. Results are presented on the in-plane phase-averaged relative and absolute velocities, turbulent kinetic energy, its production and vorticity fields. Their evolution over time during one blade passage is described, including a frequency analysis based on 32 blade positions. The spatial distribution of the spectral components at the blade-passing frequency and some harmonics reveals that the second harmonic is dominant in the narrow region of the volute at the tongue tip. The data obtained show that the fluid–dynamic blade–tongue interaction is dominated by high-vorticity sheets (positive and negative) being shed from the impeller channels, especially from the blade trailing edges, and their impingement on the tongue tip with subsequent cutting and distortion. The turbulence production is seen to be mainly concentrated in the wake regions from the blade leading and trailing edges and from the tongue tip. In particular, it gets maximum behind the blade trailing edge when the blade aligns with the tongue tip.
      PubDate: 2014-09-25
       
  • Algorithm for soot sheet quantification in a piloted turbulent jet
           non-premixed natural gas flame
    • Abstract: Abstract A novel method to quantify soot sheets from planar images of soot volume fraction is presented and demonstrated in a well-characterised turbulent, non-premixed flame, known as the ‘Delft-Adelaide Flame’. The image processing algorithm presented is based on the adaption and combination of two existing computational methodologies that were presented in the literature. The algorithm starts by identifying the longest line spanning the object. The line is subsequently segmented repetitively to generate ‘anchor points’ that are forced to lie along the centreline of the object. The length of the soot sheet is obtained by fitting straight lines to the anchor points, whilst the average characteristic width of the sheet is determined from the mean thickness of the ellipses that are fitted to the segmented soot sheet. The algorithm employed in this method, which has an uncertainty of \(\sim \) 11 %, is found to be well suited to extract the characteristic dimension and position information of soot sheet with bend, irregular shape and random orientation. Statistical assessments of these dimensions in this flame reveal that: (1) the characteristic width and length of the soot sheets range from \(\sim \) 6 to \(\sim \) 10 mm, and \(\sim \) 30 to \(\sim \) 50 mm, respectively, (2) a strong correlation exists between the soot characteristic width and length, and (3) the soot sheets display high sensitivity to local flow dynamics, with measured normalised interaction lengths ranging from 3.4 to 3.9, for the present flame.
      PubDate: 2014-09-25
       
  • Effect of linear image processing on the depth of correlation in micro PIV
    • Abstract: Abstract This work investigates the effect of selected linear image processing methods on the depth of correlation (DoC) in micro particle image velocimetry using a single camera. In practice, band-pass and high-pass filters (background subtraction) are commonly applied to micro particle image velocimetry images. This work provides analytical models describing the effect of the parameters of low-, high-, and band-pass filters on the DoC and verifies the models by experiments. Furthermore, we propose a scheme that allows computing the weighting function and DoC for more complicated cases numerically.
      PubDate: 2014-09-19
       
  • Flow around two tandem square cylinders near a plane wall
    • Abstract: Abstract An experimental study has been conducted to investigate the flow around two identical square cylinders in tandem arrangement and placed near a plane wall at a Reynolds number of 6,300. The inter-cylinder spacing ratio was varied from S * = 0.5 to 6, and the cylinder-to-wall gap ratio from G * = 0.25 to 2. Totally, 42 cases were considered to systematically examine the effects of wall proximity and the mutual interference between the two cylinders in the normalized gap–spacing (G *–S *) plane. The flow fields were captured using digital particle image velocimetry, in conjunction with measurements of the fluid forces (drag and lift) acting on the downstream cylinder using a piezoelectric load cell. The results show that the flow is highly dependent on the combined values of G * and S *. Categories relating to G * could be broadly classified as small-gap regime (G * < 0.5) at which periodic vortex shedding from the cylinders is suppressed, intermediate-gap regime (0.5 < G * < 1) where vortex shedding occurs but is under the influence of the wall proximity, and large-gap regime (G * > 1) where the wall effects become negligible. Similarly, the flow interference between the two cylinders can be divided into three basic categories as a function of S *, namely, shielding regime at S * < 1, reattachment regime at 1 < S * < 3, and impinging regime at S * > 3. Variations of force coefficients, amplitude spectra, Strouhal numbers, and Reynolds shear stress with G * and S * are presented to characterize the different flow regimes.
      PubDate: 2014-09-19
       
  • Experimental determination of three-dimensional finite-time Lyapunov
           exponents in multi-component flows
    • Abstract: Abstract We present an experimental approach for estimating finite-time Lyapunov exponent fields (FTLEs) in three-dimensional multi-component or multi-phase flows. From time-resolved sequences of particle images, we directly compute the flow map and coherent structures, while avoiding and outperforming the computationally costly numerical integration. Performing this operation independently on each flow component enables the determination of three-dimensional Lagrangian coherent structures (LCSs) without any bias from the other components. The locations of concurrent LCSs for different flow elements (e.g., passive tracers, inertial particles, bubbles, or active particles) can provide new insight into the interpenetrating FTLE structure in complex flows.
      PubDate: 2014-09-17
       
  • Simultaneous concentration and velocity field measurements in a
           shock-accelerated mixing layer
    • Abstract: Abstract A novel technique to obtain simultaneous velocity and concentration measurements is applied to the Richtmyer–Meshkov instability. After acceleration by a Mach 2.2 shock wave, the interface between the two gases develops into a turbulent mixing layer. A time-separated pair of acetone planar laser-induced fluorescence images are processed to yield concentration and, through application of the Advection-Corrected Correlation Image Velocimetry technique, velocity fields. This is the first application of this technique to shock-accelerated flows. We show that when applied to numerical simulations, this technique reproduces the velocity field to a similar quality as particle image velocimetry. When applied to the turbulent mixing layer of the experiments, information about the Reynolds number and anisotropy of the flow is obtained.
      PubDate: 2014-09-16
       
  • In vitro post-stenotic flow quantification and validation using echo
           particle image velocimetry (Echo PIV)
    • Abstract: Abstract Echo particle image velocimetry (Echo PIV) presents itself as an attractive in vivo flow quantification technique to traditional approaches. Promising results have been acquired; however, limited quantification and validation is available for post-stenotic flows. We focus here on the comprehensive evaluation of in vitro downstream stenotic flow quantified by Echo PIV and validated in relation to digital particle image velocimetry (DPIV). A Newtonian blood analog was circulated through a closed flow loop and quantified immediately downstream of a 50 % axisymmetric blockage at two Reynolds numbers (Re) using time-averaged Echo PIV and DPIV. Centerline velocities were in good agreement at all Re; however, Echo PIV measurements presented with elevated standard deviation (SD) at all measurements points. SD was improved using increased line density (LD); however, frame rate or field of view (FOV) is compromised. Radial velocity profiles showed close agreement with DPIV with the largest disparity in the shear layer and near-wall recirculation. Downstream recirculation zones were resolved by Echo PIV at both Re; however, magnitude and spatial coverage was reduced compared to DPIV that coincided with reduced contrast agent penetration beyond the shear layer. Our findings support the use of increased LD at a cost to FOV and highlight reduced microbubble penetration beyond the shear layer. High local SD at near-wall measurements suggests that further refinement is required before proceeding to in vivo quantification studies of wall shear stress in complex flow environments.
      PubDate: 2014-09-16
       
  • Experimental investigation of the fluid–structure interaction in an
           elastic 180° curved vessel at laminar oscillating flow
    • Abstract: Abstract Fluid–structure interaction phenomena are extremely important when laminar flows through elastic vessels such as in biomedical flow problems are considered. In general, such elastic vessels are curved which is why an elastic 180° bend at a curvature ratio \(\delta = D/D_{\rm C} = 0.\bar{2}\) defines the reference geometry in this study. It is the purpose of this study to compare the results with the steady flow through a 180° rigid pipe bend and to quantify the impact of the fluid–structure interaction on the overall flow pattern and the vessel deformation at oscillating fully developed entrance flow. The findings comprise velocity, pressure, and structure deformation measurements. The vessel dilatation amplitude was varied between 3.75 % and 7 % of the vessel diameter at Dean De and Womersley number Wo ranges of \(327\,\le\,De\,\le\,350\) and \(7\,\le\,Wo\,\le\,8.\) The flow is investigated by time-resolved stereoscopic particle-image velocimetry in five radial cross sections located in the elastic 180° bend and in the inlet pipes. The unsteady static vessel pressure is measured synchronously at these cross sections. The comparison of the steady with the unsteady flow field shows a strong change in the axial and secondary velocity distributions at periods of transition between the centrifugal forces and the unsteady inertia forces dominated regimes. These changes are characterized by asymmetric fluctuations of the centers of the counter-rotating vortex pair. The investigation of the impact of the structure deformation amplitude on these fluctuations reveals a significant attenuation at high deformation amplitudes. The spatial motion of the elastic vessel due to the forces applied by the flow exhibits amplitudes up to 15 % of the vessel diameter. Considering the fluid–structure interaction, an amplification of the volume flux amplitude by a factor of 2.1 at the vessel outlet and phase lags up to 30° occur. The static pressure distribution is characterized by a pronounced asymmetry between forward and backward flow with a 40 % higher peak magnitude at backward flow and phase lags of 35°. The results evidence that a strong distortion of the velocity distribution in the bend, which is caused by the oscillating nature of the flow, is reduced as a result of the fluid–structure interaction.
      PubDate: 2014-09-16
       
  • Development and application of a point Doppler velocimeter featuring
           two-beam multiplexing for time-resolved measurements of high-speed flow
    • Abstract: Abstract A novel point Doppler velocimeter (pDV) based upon the Doppler global velocimetry principle is presented, which is capable of three-component velocity vector measurements at 100 kHz mean rates over extended time periods. In this implementation, two laser beams are multiplexed to illuminate the flow over alternating time windows, providing for a reduction in the number of sensors required. The implications of this multiplexing paradigm coupled with the fundamental limits set by the optical absorption filter are examined in detail, and uncertainties are predicted via instrumentation modeling and representative synthetic flow data. The results indicate that the multiplexing pDV instrument provides the required temporal and velocity resolution for turbulent shear flows at velocities of nominally 500 m/s. As a demonstration and validation of this time-resolved technique, statistics of three-velocity component measurements in a cold, supersonic, over-expanded jet at jet exit Mach number M j  = 1.4 (design Mach number M d  = 1.65) are presented. Time resolution up to 250 kHz and instantaneous velocity uncertainties between 6.6 and 11.1 m/s were obtained. Comparisons of mean pDV data with laser Doppler velocimetry data are consistent with uncertainty predictions for the technique. The ultimate value of the instrument is exhibited in the analysis of Reynolds stress spectra in the screeching jet, exposing the spatial development of motions at the harmonics of the screech tone, variable phase-coordinated shock motions, and growth of turbulent fluctuations in the developing shear layer of the jet. From the data presented, the screech tone phenomenon is suspected to be linked to the production of radial–azimuthal shear stresses in extended regions beyond the potential core.
      PubDate: 2014-09-13
       
 
 
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