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e & i Elektrotechnik und Informationstechnik     Hybrid Journal   (Followers: 7, SJR: 0.146, h-index: 8)
e-Neuroforum     Hybrid Journal  
Early Childhood Education J.     Hybrid Journal   (Followers: 11, 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: 3, 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: 6, SJR: 0.165, h-index: 9)
Eating and Weight Disorders - Studies on Anorexia, Bulimia and Obesity     Hybrid Journal   (Followers: 6, SJR: 0.289, h-index: 23)
EcoHealth     Hybrid Journal   (Followers: 1, SJR: 0.651, h-index: 22)
Ecological Research     Hybrid Journal   (Followers: 7, SJR: 0.698, h-index: 38)
Economic Botany     Hybrid Journal   (Followers: 8, SJR: 0.666, h-index: 40)
Economic Bulletin     Hybrid Journal   (Followers: 3)
Economic Change and Restructuring     Hybrid Journal   (Followers: 1, SJR: 0.263, h-index: 6)
Economic Theory     Hybrid Journal   (Followers: 5, SJR: 1.857, h-index: 31)
Economic Theory Bulletin     Hybrid Journal  
Economics of Governance     Hybrid Journal   (Followers: 2, SJR: 0.367, h-index: 12)
Ecosystems     Hybrid Journal   (Followers: 18, SJR: 1.793, h-index: 83)
Ecotoxicology     Hybrid Journal   (Followers: 10, SJR: 1.041, h-index: 53)
Education and Information Technologies     Hybrid Journal   (Followers: 132, SJR: 0.207, h-index: 15)
Educational Assessment, Evaluation and Accountability     Hybrid Journal   (Followers: 11, SJR: 0.519, h-index: 14)
Educational Psychology Review     Hybrid Journal   (Followers: 13, 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: 7, SJR: 0.946, h-index: 27)
Educational Technology Research and Development     Partially Free   (Followers: 133, SJR: 1.124, h-index: 45)
Electrical Engineering     Hybrid Journal   (Followers: 8, 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: 2, 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: 7, 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: 9, SJR: 1.056, h-index: 10)
Energy Systems     Hybrid Journal   (Followers: 8, 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: 25, 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: 16, 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: 5, 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: 29, 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: 1, 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: 151, 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: 2)
European Archives of Oto-Rhino-Laryngology     Hybrid Journal   (Followers: 3, 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: 9, 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: 9, SJR: 0.67, h-index: 25)
European J. of Law and Economics     Hybrid Journal   (Followers: 147, SJR: 0.242, h-index: 13)

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Journal Cover Experiments in Fluids
   [7 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  [2208 journals]   [SJR: 1.033]   [H-I: 62]
  • Drag reduction using superhydrophobic sanded Teflon surfaces
    • Abstract: Abstract In this paper, a series of experiments are presented which demonstrate drag reduction for the laminar flow of water through microchannels using superhydrophobic surfaces with random surface microstructure. These superhydrophobic surfaces were fabricated with a simple, inexpensive technique of sanding polytetrafluoroethylene (PTFE) with sandpaper having grit sizes between 120- and 600-grit. A microfluidic device was used to measure the pressure drop as a function of the flow rate to determine the drag reduction and slip length of each surface. A maximum pressure drop reduction of 27 % and a maximum apparent slip length of b = 20 μm were obtained for the superhydrophobic surfaces created by sanding PTFE with a 240-grit sandpaper. The pressure drop reduction and slip length were found to increase with increasing mean particle size of the sandpaper up to 240-grit. Beyond that grit size, increasing the pitch of the surface roughness was found to cause the interface to transition from the Cassie–Baxter state to the Wenzel state. This transition was observed both as an increase in the contact angle hysteresis and simultaneously as a reduction in the pressure drop reduction. For these randomly rough surfaces, a correlation between the slip length and the contact angle hysteresis was found. The surfaces with the smallest contact angle hysteresis were found to also have the largest slip length. Finally, a number of sanding protocols were tested by sanding preferentially along the flow direction, across the flow direction and with a random circular pattern. In all cases, sanding in the flow direction was found to produce the largest pressure drop reduction.
      PubDate: 2014-07-27
  • On the feasibility of tomographic-PIV with low pulse energy illumination
           in a lifted turbulent jet flame
    • Abstract: Abstract Tomographic particle image velocimetry (tomographic-PIV) is a recently developed measurement technique used to acquire volumetric velocity field data in liquid and gaseous flows. The technique relies on line-of-sight reconstruction of the rays between a 3D particle distribution and a multi-camera imaging system. In a turbulent flame, however, index-of-refraction variations resulting from local heat-release may inhibit reconstruction and thereby render the technique infeasible. The objective of this study was to test the efficacy of tomographic-PIV in a turbulent flame. An additional goal was to determine the feasibility of acquiring usable tomographic-PIV measurements in a turbulent flame at multi-kHz acquisition rates with current-generation laser and camera technology. To this end, a setup consisting of four complementary metal oxide semiconductor cameras and a dual-cavity Nd:YAG laser was implemented to test the technique in a lifted turbulent jet flame. While the cameras were capable of kHz-rate image acquisition, the laser operated at a pulse repetition rate of only 10 Hz. However, use of this laser allowed exploration of the required pulse energy and thus power for a kHz-rate system. The imaged region was 29 × 28 × 2.7 mm in size. The tomographic reconstruction of the 3D particle distributions was accomplished using the multiplicative algebraic reconstruction technique. The results indicate that volumetric velocimetry via tomographic-PIV is feasible with pulse energies of 25 mJ, which is within the capability of current-generation kHz-rate diode-pumped solid-state lasers.
      PubDate: 2014-07-26
  • Near-field development of a row of round jets at low Reynolds numbers
    • Abstract: Abstract This article reports on an experimental investigation of the near-field behavior of interacting jets at low Reynolds numbers (Re = 2125, 3290 and 4555). Two measurement techniques, particle image velocimetry (PIV) and laser Doppler anemometry (LDA), were employed to measure mean velocity and turbulence statistics in the near field of a row of six parallel coplanar round jets with equidistant spacing. The overall results from PIV and LDA measurements show good agreement, although LDA enabled more accurate measurements in the thin shear layers very close to the nozzle exit. The evolution of all six coplanar jets showed initial, merging, and combined regions. While the length of the potential core and the maximum velocity in the merging region are Reynolds number-dependent, the location of the merging points and the minimum velocity between jets were found to be independent of Reynolds number. Side jets at the edges of the coplanar row showed a constant decay rate of maximum velocity after their core region, which is comparable to a single round jet. Jets closer to the center of the row showed reducing velocity decay in the merging region, which led to a higher maximum velocity compared to a single round jet. A comparison with the flow for an in-line array of 6 × 6 round jets showed that the inward bending of streamwise velocity, which exists in the near field of the 6 × 6 jet array, does not occur in the single row of coplanar jets, although both setups have identical nozzle shape, spacing, and Reynolds number.
      PubDate: 2014-07-25
  • Outer ligament-mediated spray formation of annular liquid sheet by an
           inner round air stream
    • Abstract: Abstract The interfacial jetting phenomena of coaxial air-assisted water jets are studied using high-speed digital camera. Here, an inner round air jet is injected into annular water sheet spray. The experimental photographs show that the outside interface of liquid sheet shoots out large numbers of violent ligaments at high air velocity. The ligament velocity, ligament angle, ligament diameter, fragment size, and distribution are measured and analyzed. There are two kinds of ligament evolution that are breakup and contraction. An empirical model is also proposed for the ligament evolution process. At last, we obtain the criterion of critical Weber number on the ligament breakup based on the experimental results. This suggestion agrees well with the experimental data.
      PubDate: 2014-07-23
  • Schlieren-based techniques for investigating instability development and
           transition in a hypersonic boundary layer
    • Abstract: Abstract Three variants of schlieren techniques are employed to investigate the development of second-mode instability waves in the hypersonic boundary layer of a slender cone in a reflected shock tunnel. First, a previously proposed technique using high frame rate (i.e., at least as high as the dominant instability frequency) schlieren visualization with a continuous light source is shown to provide repeatable measurements of the instability propagation speed and frequency. A modified version of the technique is then introduced whereby a pulsed light source allows the use of a higher-resolution camera with a lower frame rate: this provides significant benefits in terms of spatial resolution and total recording time. A detailed picture of the surface-normal intensity distribution for individual wave packets is obtained, and the images provide comprehensive insight into the unsteady flow structures within the boundary layer. Finally, two-point schlieren deflectometry is implemented and shown to be capable of providing second-mode growth information in the challenging shock tunnel environment.
      PubDate: 2014-07-23
  • A benchmark experiment on gas cavitation
    • Abstract: Abstract Cavitation research is often a matter of experiments conducted in complex machinery. There, it is extremely difficult to look into one of the most important issues of cavitation which is nucleation. This work investigates gas cavitation under well-defined flow conditions. Nuclei are placed in wall bound cavities and are exposed to a radial gap flow featuring independent pressure and shear stress. A reciprocating bubble generation is achieved. Bubble frequency and size are evaluated which turn out to depend on pressure and wall shear stress. The experiment lends itself to systematic research in cavitation.
      PubDate: 2014-07-20
  • Obstacle-induced spiral vortex breakdown
    • Abstract: Abstract An experimental investigation on vortex breakdown dynamics is performed. An adverse pressure gradient is created along the axis of a wing-tip vortex by introducing a sphere downstream of an elliptical hydrofoil. The instrumentation involves high-speed visualizations with air bubbles used as tracers and 2D Laser Doppler Velocimeter (LDV). Two key parameters are identified and varied to control the onset of vortex breakdown: the swirl number, defined as the maximum azimuthal velocity divided by the free-stream velocity, and the adverse pressure gradient. They were controlled through the incidence angle of the elliptical hydrofoil, the free-stream velocity and the sphere diameter. A single helical breakdown of the vortex was systematically observed over a wide range of experimental parameters. The helical breakdown coiled around the sphere in the direction opposite to the vortex but rotated along the vortex direction. We have observed that the location of vortex breakdown moved upstream as the swirl number or the sphere diameter was increased. LDV measurements were corrected using a reconstruction procedure taking into account the so-called vortex wandering and the size of the LDV measurement volume. This allows us to investigate the spatio-temporal linear stability properties of the flow and demonstrate that the flow transition from columnar to single helical shape is due to a transition from convective to absolute instability.
      PubDate: 2014-07-18
  • POD analysis of a finite-length cylinder near wake
    • Abstract: Abstract The near wake of a wall-mounted finite-length square cylinder with an aspect ratio of 7 is investigated based on the proper orthogonal decomposition (POD) of the PIV data measured in three spanwise planes, i.e., z/d = 6, 3.5 and 1.0, near the cylinder free end, mid-span and fixed end (wall), respectively. The Reynolds number based on free-stream velocity (U ∞) and cylinder width (d) is 9,300. A two-dimensional (2D) square cylinder wake is also measured and analyzed at the same Reynolds number for the purpose of comparison. The structures of various POD modes show marked differences between the two flows. While the coefficients, a 1 and a 2, of the POD modes 1 and 2 occur within an annular area centered at a 1 = a 2 = 0 in the 2D wake, their counterparts are scattered all over the entire circular plane at z/d = 1.0 and 3.5 of the finite-length cylinder wake. Flow at z/d = 6 is dominated by POD mode 1, which corresponds to symmetrical vortex shedding and accounts for 54.0 % of the total turbulent kinetic energy (TKE). On the other hand, the POD modes 1 and 2, corresponding to anti-symmetrical vortex shedding, are predominant, accounting for about 45.0 % of the total TKE, at z/d = 3.5 and 1. It has been found that the flow structure may be qualitatively and quantitatively characterized by the POD coefficients. For example, at z/d = 6, a larger a 1 corresponds to a smaller length of flow reversal zone and a stronger downwash flow. At z/d = 3.5 and 1, two typical flow modes can be identified from a 1 and a 2. While large a 1 and/or a 2 correspond to anti-symmetrical vortex shedding, as in a 2D cylinder wake, small a 1 and a 2 lead to symmetrical vortex shedding. Any values between the large and small a 1 and/or a 2 correspond to the flow structure between these two typical flow modes. As such, the probability of occurrence of a flow structure may be determined from the distribution of the POD coefficients.
      PubDate: 2014-07-18
  • Wake instabilities of a blunt trailing edge profiled body at intermediate
           Reynolds numbers
    • Abstract: Abstract Experiments have been conducted to identify and characterize the instabilities in the wake of a blunt trailing edge profiled body, comprised of an elliptical leading edge and a rectangular trailing edge, for a broad range of Reynolds numbers ( \(2{,}000\le Re(d)\le 50{,}000\) based on the thickness of the body). These experiments, which include measurements of the wake velocity field using hot-wire anemometry and particle image velocimetry, complement previous studies of the wake flow for the same geometry at lower and higher Reynolds numbers. The spatial characteristics of the primary wake instability (the von Kármán vortex street) are found to have relatively little variation in the range of Reynolds numbers investigated, in spite of the transition of the boundary layer upstream of the trailing edge from a laminar to a turbulent state. The dominant secondary instability, identified based on the structure of velocity and vorticity fields in the wake extracted using proper orthogonal decomposition, is found to have features similar to the ones described numerically and experimentally by Ryan et al. (J Fluid Mech 538:1–29, 2005), and Naghib-Lahouti et al. (Exp Fluids 52:1547–1566, 2012) at lower Reynolds numbers. The findings suggest that the spatial characteristics of the dominant primary and secondary wake flow instabilities have little dependence on the state of the flow upstream of the separation points, in spite of the distinct change in the normalized vortex shedding frequency upon the transition of the boundary layer.
      PubDate: 2014-07-18
  • Slope effects on the fluid dynamics of a fire spreading across a fuel bed:
           PIV measurements and OH* chemiluminescence imaging
    • Abstract: Abstract Slope is among the most influencing factor affecting the spread of wildfires. A contribution to the understanding of the fluid dynamics of a fire spreading in these terrain conditions is provided in the present paper. Coupled optical diagnostics are used to study the slope effects on the flow induced by a fire at laboratory scale. Optical diagnostics consist of particle image velocimetry, for investigating the 2D (vertical) velocity field of the reacting flow and chemiluminescence imaging, for visualizing the region of spontaneous emission of OH radical occurring during gaseous combustion processes. The coupling of these two techniques allows locating accurately the contour of the reaction zone within the computed velocity field. The series of experiments are performed across a bed of vegetative fuel, under both no-slope and 30° upslope conditions. The increase in the rate of fire spread with increasing slope is attributed to a significant change in fluid dynamics surrounding the flame. For horizontal fire spread, flame fronts exhibit quasi-vertical plume resulting in the buoyancy forces generated by the fire. These buoyancy effects induce an influx of ambient fresh air which is entrained laterally into the fire, equitably from both sides. For upward flame spread, the induced flow is strongly influenced by air entrainment on the burnt side of the fire and fire plume is tilted toward unburned vegetation. A particular attention is paid to the induced air flow ahead of the spreading flame. With increasing the slope angle beyond a threshold, highly dangerous conditions arise because this configuration induces wind blows away from the fire rather than toward it, suggesting the presence of convective heat transfers ahead of the fire front.
      PubDate: 2014-07-18
  • Multi-photon molecular tagging velocimetry with femtosecond excitation
    • Abstract: Abstract We present results for first molecular tagging velocimetry (MTV) measurements in water under resonant femtosecond excitation/emission process of a phosphorescent supramolecule. Both two-photon and three-photon absorption processes are examined, and the feasibility of measurements is demonstrated by single component velocimetry in a simple jet flow. The new capabilities enabled by FemtoMTV include elimination of the need for short wavelength UV excitation source and UV optical access in flow facilities, and potential for high rep-rate flow imaging.
      PubDate: 2014-07-17
  • Ground effect on the aerodynamics of a two-dimensional oscillating airfoil
    • Abstract: Abstract This paper reports results of an experimental investigation into ground effect on the aerodynamics of a two-dimensional elliptic airfoil undergoing simple harmonic translation and rotational motion. Ground clearance (D) ranging from 1c to 5c (where c is the airfoil chord length) was investigated for three rotational amplitudes (α m) of 30°, 45° and 60° (which respectively translate to mid-stroke angle of attack of 60°, 45° and 30°). For the lowest rotational amplitude of 30°, results show that an airfoil approaching a ground plane experiences a gradual decrease in cycle-averaged lift and drag coefficients until it reaches D ≈ 2.0c, below which they increase rapidly. Corresponding DPIV measurement indicates that the initial force reduction is associated with the formation of a weaker leading edge vortex and the subsequent force increase below D ≈ 2.0c may be attributed to stronger wake capture effect. Furthermore, an airfoil oscillating at higher amplitude lessens the initial force reduction when approaching the ground and this subsequently leads to lift distribution that bears striking resemblance to the ground effect on a conventional fixed wing in steady translation.
      PubDate: 2014-07-16
  • Magnetic resonance measurement of fluid dynamics and transport in tube
           flow of a near-critical fluid
    • Abstract: Abstract An ability to predict fluid dynamics and transport in supercritical fluids is essential for optimization of applications such as carbon sequestration, enhanced oil recovery, “green” solvents, and supercritical coolant systems. While much has been done to model supercritical velocity distributions, experimental characterization is sparse, owing in part to a high sensitivity to perturbation by measurement probes. Magnetic resonance (MR) techniques, however, detect signal noninvasively from the fluid molecules and thereby overcome this obstacle to measurement. MR velocity maps and propagators (i.e., probability density functions of displacement) were acquired of a flowing fluid in several regimes about the critical point, providing quantitative data on the transport and fluid dynamics in the system. Hexafluoroethane (C2F6) was pumped at 0.5 ml/min in a cylindrical tube through an MR system, and propagators as well as velocity maps were measured at temperatures and pressures below, near, and above the critical values. It was observed that flow of C2F6 with thermodynamic properties far above or below the critical point had the Poiseuille flow distribution of an incompressible Newtonian fluid. Flows with thermodynamic properties near the critical point exhibit complex flow distributions impacted by buoyancy and viscous forces. The approach to steady state was also observed and found to take the longest near the critical point, but once it was reached, the dynamics were stable and reproducible. These data provide insight into the interplay between the critical phase transition thermodynamics and the fluid dynamics, which control transport processes.
      PubDate: 2014-07-16
  • On the growth of homogeneously nucleated water droplets in nitrogen: an
           experimental study
    • Abstract: Abstract A pulse-expansion wave tube method to determine homogeneous nucleation rates of water droplets has been improved. In particular, by accounting for background scattering, the experimental light scattering can be fitted extremely well with the Mie scattering theory. This results in an accurate determination of the droplet growth curve, which is well defined owing to the sharp monodispersity of the droplet cloud generated by the nucleation pulse method. With this method, water condensation is effectively decoupled in birth (nucleation) and growth of droplets. Droplet growth curves yield information on the diffusion coefficient, which only depends on pressure and temperature and on the supersaturation of the individual experiments. Here, we propose to use this information in the interpretation of nucleation rate data. Experimental results are given for homogeneous nucleation rates of supercooled water droplets at nucleation temperature 240 K and pressure 1.0 MPa and for growth of supercooled water droplets at temperature 247 K and pressure 1.1 MPa. The supersaturation was varied between 10 and 14, resulting in nucleation rates varying between 10 \(^{14}\)  m \(^{-3}\)  s \(^{-1}\) and 10 \(^{17}\)  m \(^{-3}\)  s \(^{-1}\) . For the diffusion coefficient, a value of 1.51  \(\pm\)  0.03 mm \(^2\) s \(^{-1}\) was found (247 K, 1.1 MPa) in agreement with previously reported results. It is discussed how the information from droplet growth data can be used to assess the quality of the individual water nucleation experiments.
      PubDate: 2014-07-15
  • Frictional drag reduction by bubble injection
    • Abstract: Abstract The injection of gas bubbles into a turbulent boundary layer of a liquid phase has multiple different impacts on the original flow structure. Frictional drag reduction is a phenomenon resulting from their combined effects. This explains why a number of different void–drag reduction relationships have been reported to date, while early works pursued a simple universal mechanism. In the last 15 years, a series of precisely designed experimentations has led to the conclusion that the frictional drag reduction by bubble injection has multiple manifestations dependent on bubble size and flow speed. The phenomena are classified into several regimes of two-phase interaction mechanisms. Each regime has inherent physics of bubbly liquid, highlighted by keywords such as bubbly mixture rheology, the spectral response of bubbles in turbulence, buoyancy-dominated bubble behavior, and gas cavity breakup. Among the regimes, bubbles in some selected situations lose the drag reduction effect owing to extra momentum transfer promoted by their active motions. This separates engineers into two communities: those studying small bubbles for high-speed flow applications and those studying large bubbles for low-speed flow applications. This article reviews the roles of bubbles in drag reduction, which have been revealed from fundamental studies of simplified flow geometries and from development of measurement techniques that resolve the inner layer structure of bubble-mixed turbulent boundary layers.
      PubDate: 2014-07-15
  • Cavitation by spall fracture of solid walls in liquids
    • Abstract: Abstract Experiments are carried out to investigate the cavitation process induced by the spill-off from material from a surface in a liquid environment. Therefore, a simplified physical model was designed which allows the optical observation of the process next to a transparent glass rod submerged in a liquid where the rod is forced to fracture at a pre-defined groove. High-speed shadow-imaging and refractive index matching allow observation of the dynamics of the cavitation generation and cavitation bubble breakdown together with the flow. The results show that the initial phase of spill-off is a vertical lift-off of the rod from the surface that is normal to the direction of pendulum impact. A cavitation bubble is immediately formed during spill-off process and grows in size until lateral motion of the rod sets in. While the rod is transported away, the bubble shrinks into hyperbolic shape and finally collapses. This process is regarded as one contributing factor to the high efficiency of hydro-abrasive wear.
      PubDate: 2014-07-15
  • An experimental study of turbulent two-phase flow in hydraulic jumps and
           application of a triple decomposition technique
    • Abstract: Abstract Intense turbulence develops in the two-phase flow region of hydraulic jump, with a broad range of turbulent length and time scales. Detailed air–water flow measurements using intrusive phase-detection probes enabled turbulence characterisation of the bubbly flow, although the phenomenon is not a truly random process because of the existence of low-frequency, pseudo-periodic fluctuating motion in the jump roller. This paper presents new measurements of turbulent properties in hydraulic jumps, including turbulence intensity, longitudinal and transverse integral length and time scales. The results characterised very high turbulent levels and reflected a combination of both fast and slow turbulent components. The respective contributions of the fast and slow motions were quantified using a triple decomposition technique. The decomposition of air–water detection signal revealed “true” turbulent characteristics linked with the fast, microscopic velocity turbulence of hydraulic jumps. The high-frequency turbulence intensities were between 0.5 and 1.5 close to the jump toe, and maximum integral turbulent length scales were found next to the bottom. Both decreased in the flow direction with longitudinal turbulence dissipation. The results highlighted the considerable influence of hydrodynamic instabilities of the flow on the turbulence characterisation. The successful application of triple decomposition technique provided the means for the true turbulence properties of hydraulic jumps.
      PubDate: 2014-07-11
  • The influence of temperature fluctuations on hot-wire measurements in
           wall-bounded turbulence
    • Abstract: Abstract There are no measurement techniques for turbulent flows capable of reaching the versatility of hot-wire probes and their frequency response. Nevertheless, the issue of their spatial resolution is still a matter of debate when it comes to high Reynolds number near-wall turbulence. Another, so far unattended, issue is the effect of temperature fluctuations—as they are, e.g. encountered in non-isothermal flows—on the low and higher-order moments in wall-bounded turbulent flows obtained through hot-wire anemometry. The present investigation is dedicated to document, understand, and ultimately correct these effects. For this purpose, the response of a hot-wire is simulated through the use of velocity and temperature data from a turbulent channel flow generated by means of direct numerical simulations. Results show that ignoring the effect of temperature fluctuations, caused by temperature gradients along the wall-normal direction, introduces—despite a local mean temperature compensation of the velocity reading—significant errors. The results serve as a note of caution for hot-wire measurements in wall-bounded turbulence, and also where temperature gradients are more prevalent, such as heat transfer measurements or high Mach number flows. A simple correction scheme involving only mean temperature quantities (besides the streamwise velocity information) is finally proposed that leads to a substantial bias error reduction.
      PubDate: 2014-07-06
  • Narrowband versus broadband excitation for CH       class="a-plus-plus">2O PLIF imaging in flames
           using a frequency-tripled Nd:YAG laser
    • Abstract: Abstract Spectrally-narrow- (~0.003 cm−1) and broadband (>1 cm−1) fluorescence excitation of the \(\tilde{A}^{1} A_{2} - \tilde{X}^{1} A_{1} ,4_{0}^{1}\) electronic transition of formaldehyde (CH2O) in laminar premixed and non-premixed flames is investigated using the third-harmonic output from a tunable, injection-seeded Nd:YAG laser. Spectrally-resolved, CH2O fluorescence excitation spectra are examined over a broad range of conditions including room-temperature vapor cells and lean-to-rich premixed methane/air and dimethyl ether/air flames in order to understand the origin of the fluorescence using both narrowband and broadband excitation strategies. The measured CH2O excitation spectra are nearly identical in all conditions considered which cover a broad range of composition and temperature conditions. These results imply that the predominant emission signature is CH2O and suggest the potential for quantitative in-flame CH2O LIF measurements using room-temperature calibration and existing fluorescence models. A specific emphasis of this study is on CH2O isolation and potential fluorescence interference in the context of single-shot planar laser-induced fluorescence (PLIF) imaging in flames. The PLIF results indicate that for the premixed flames investigated, both narrowband (Nd:YAG laser operating in single mode) and broadband (no injection seeding) excitation yield a reliable marker of the CH2O distribution, with no indication of major interference from additional species. However, frequency-tuned narrowband excitation resulted in a collected fluorescence emission signal that increased by a factor of two as compared to broadband excitation. In the methane-based non-premixed flames, evidence of the excitation of additional species (such as PAH) was noted; however, the impact of this interference is reduced when using narrowband excitation. Similar to the premixed flames, the CH2O fluorescence emission signal increased by approximately a factor of two when using spectrally tuned, narrowband excitation from the third-harmonic output of an injection-seeded Nd:YAG laser. The current results indicate that narrowband excitation of CH2O near 355 nm using the third-harmonic output of an injection-seeded Nd:YAG laser results in increased fluorescence emission signal and hence a reduced effect of interference from additional flame-generated species as compared to conventional broadband excitation using a frequency-tripled Nd:YAG laser.
      PubDate: 2014-07-04
  • Microscale schlieren visualization of near-bubble mass transport during
           boiling of 2-propanol/water mixtures in a square capillary
    • Abstract: Abstract In this study, we successfully utilize the microscale schlieren method to visualize the microscale mass transport near the vapor–liquid interface during boiling of 2-propanol/water mixtures in a square capillary. Because the variation in the refractive index with composition is much greater than that with temperature, the microscale schlieren method proves to be a powerful tool for investigating the solutocapillary convection without the interference of thermocapillarity. When the difference between the equilibrium vapor and liquid mole fractions is large, we observe high concentration gradients near the vapor–liquid interface due to both mass diffusion and the solutocapillary effects. Although the solutocapillary convection is decidedly affected by the eruptive nature of the boiling process, the near-bubble mass transport still plays a vital role in boiling heat transfer. In a square capillary of d = 900 μm, mass diffusion dominates and the depletion of 2-propanol near the vapor–liquid interface increases. This leads to an increase in the local bubble point causing the deterioration of heat transfer for 2-propanol/water mixtures. However, in the smaller square capillary of d = 500 μm, the solutocapillary effect becomes more important. The induced convection near the contact line helps to augment the boiling heat transfer at x = 0.015, despite the fact that mass diffusion tends to cause a higher concentration gradient normal to the bubble front during the boiling process. Herein, we prove that the microscale schlieren method is able to provide valuable insight into the leverage between different mechanisms in heat transfer during the vaporization process of 2-propanol/water mixtures in a square capillary.
      PubDate: 2014-07-03
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Heriot-Watt University
Edinburgh, EH14 4AS, UK
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