bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical and Experimental Study on Metal Organic Vapor Phase Epitaxy of InGaN/GaN Multi Quantum Wells
Authors: Kim Changsung Sean , Hong Jongpa , Shim Jihye , Kim Bum Joon , Kim Hak-Hwan , Yoo Sang Duk , Lee Won Shin
Published: March 2009

A numerical and experimental study has been performed to characterize the metal organic vapor phase epitaxy (MOVPE) growth of InGaN/GaN multi-quantum wells (MQW). One of the major objectives of the present study is to predict the optimal operating conditions that would be suitable for the fabrication of GaN-based light-emitting diodes (LED) using three different reactors, vertical, horizontal and planetary. Computational fluids dynamic (CFD) simulations considering gas phase chemical reactions and surface chemistry were carried out and compared with experimental measurements. Through a lot of CFD simulations, the database for the multiparametric dependency of indium incorporation and growth rate in InGaN/GaN layers has been established in a wide range of growth conditions. Also, a heating system using radio frequency (RF) power was verified to obtain the uniform temperature distribution by simulating the electro-magnetic field as well as thermodynamic flow fields. The present multidisciplinary approach has been applied to the development of a novel-concept MOVPE system as well as performance enhancement of existing commercial reactors.
CFD Development for Macro Particle Simulations
Authors: Zhang Sijun , Gentela Madhaveswer
Published: March 2009

In this paper, a macro particle method is developed to model the fluid-particle flows. The macro particle is formed by a collection of micro-sized particle so that the number of macro particle to be tracked is much less than the number of smaller particle. Unlike the calculations of instantaneous point variables of fluid phase with moving discrete boundaries of the smaller particles with direct numerical simulation, the boundary of each macro particle is just dealt with the blocked-off approach. On the other hand, the flow fields based on the present method is solved by original Navier-Stokes, rather than the modified ones based on the locally averaged theorem. The flow fields are solved on the length scale of computational cell, while the resolutions of solid particles are the size of macro particle, which is determined as needed in specific applications. The macro particle method is validated by several cases, which demonstrate the macro particle method could accurately resolve fluid-particle systems in an efficient, robust and flexible fashion.
Influence of Concentration and Number of Image Pairs in μ-PIV Experiments
Authors: Chiavaroli Stefano , Newport David , Morini Gian Luca , Barrot Lattes Christine , Baldas Lucien , Colin Ste´phane
Published: March 2009

Micro Particle Image Velocimetry (μ-PIV) is a non-intrusive technique widely used nowadays to experimentally obtain the velocity field of a micro flow. The main goal of this research was to examine the influence of particle concentration and the number of images acquired, on the accuracy of the μ-PIV velocity measurement. For this reason, a comparison between experimental and analytical values was made. It has been demonstrated that the influence of the seeding concentration on the accuracy of the velocity measurements, into the investigated range, can be considered insignificant. On the other hand, the number of images selected for the cross-correlation is more important for the accuracy of the measurements. By increasing the quantity of images processed it is possible to artificially increase the seeding concentration and reduce the scatter. However, this considerably increases the processing time for the experiment. A trade-off is required between obtaining a highly accurate result without losing precious experimental down time. When the range of the concentration is fixed, it is possible to set the maximum inaccuracy allowance tolerated for the experiment. There is a compromise between a better precision and adequate time to process the data.
Aerodynamic Noise Simulation of Propeller Fan by Large Eddy Simulation
Authors: Hamada Shingo , Nakashima Seiji , Kato Chisachi , Yamade Yoshinobu
Published: March 2009

In this paper, unsteady flow and aerodynamic noise are numerically investigated for a half-open type propeller fan used for outdoor air conditioner components. The flow field is calculated by Front Flow/Blue, which is based on Large Eddy Simulation (LES). The Standard Smagorinsky Model (SSM) and Dynamic Smagorinsky Model (DSM) were used as sub-grid scale models. Aerodynamic noise was calculated by Curle’s equation based on the pressure fluctuation on the blade surface computed by LES. The computed static pressure rise of the fan showed reasonable agreement with the measured equivalent. The time-averaged distributions of the three velocity components downstream of the blades were also compared with those measured by hotwire anemometry, which showed satisfactory agreement between the computed and measured velocity profiles. But the tip vortex passage which was detached from the blade surface predicted by LES was not stable as measured by the experiment. Finally, the predicted far-field sound spectrum agrees reasonably well with measurements in a frequency range of 100 to 1000 Hz although the sound pressure level was underpredicted in the lower frequency range.
Sedimentation of a Stratified Suspension in a Quiescent Fluid
Authors: Mitsui Takashi , Harada Shusaku , Asakura Kuniomi
Published: March 2009

The sedimentation of fine particles in a stratified suspension, which has both the upper and lower interfaces, is studied experimentally. We measure the settling velocity of particles and observe the diffusion behavior near the interfaces. Especially we examine whether the macroscopic or the microscopic natures of suspension is dominant during sedimentation, i.e., the particles settle as an particle assembly relative to surrounding fluid or as a continuous suspension. The experimental observation shows that the gravity-induced instability of suspension-fluid interface, which has been already reported, governs the particle motion for high concentration and small particle size. However, the microscopic nature of suspension comes out in case of larger particles. In our study, the dependency of the sedimentation behavior on the particle size and the concentration is discussed quantitatively.
Multieffects on Mixing Quality of an Active Micromixer
Authors: Dinh Thien X. , Ogami Yoshifumi
Published: March 2009

The convective mixing performance of an active micromixer is analyzed by using computational fluid dynamics (CFD). The mixer consists of a Y-shaped channel and an N-paddle (3, 4, and 5) rotor with radius R suspended in the junction of the channel. Numerical simulations are performed for a wide range of rotation speed of the rotor, ω, and mean velocity in the mixer, U. The asymptotic mixing performance is investigated by means of Lagrangian particle tracking simulation, stretching of a material line, dispersive and distributive mixing efficiencies. The results show that the mixing performance depends on the combined variable ωR/U, whereas paddle number has ignorable effects. Physically, the convective ratio of rotation speed to mean velocity governs the mixing process in the mixer. Contrastively, paddle number affects significantly to pressure loss and fluid torque exercising on the rotor. The time-averaged fluid torque depends linearly on rotation speed regardless of flow rate. Pressure loss relates linearly to flow rate, negligibly to rotation speed. It shows that a smaller paddle number produces lesser pressure loss and fluid torque for the same mixing efficiency.
Turbulence Model Upgrades for Swirling Flows
Authors: Chenoweth J. D. , York B. , Hosangadi A.
Published: March 2009

The ability to accurately model axisymmetric, turbulent swirling jet flows over a variety of inflow conditions is evaluated. The deficiency of the standard k-ε turbulence model in predicting mixing rates in flows with streamline curvature is well known. A relatively straightforward modification to this model is made based on a local value of the flux Richardson number which accounts for the azimuthal velocity and its variation. To evaluate the effectiveness of this modification two different experimental data sets are used to compare the computational results against. All calculations were performed using the structured, density based, CRAFT CFD® code utilizing a preconditioning methodology. Both cases have initial swirl distributions that are equivalent to a solid-body rotation profile, and have swirl numbers that are low enough to remain below the vortex breakdown regime. They also have non-swirling jet data available for the same geometries and operating conditions which allows the increased jet mixing rate of swirling jets over purely axial jets to be confirmed. All calculations showed a significant improvement of centerline velocity decay as well as downstream radial velocity profiles when the Richardson number correction was activated. For the case with turbulence data, the centerline decay of turbulent kinetic energy was also much improved. An important result that was discovered was the extreme sensitivity of the downstream evolution of the jet to the specification of the initial k and ε profiles, highlighting the critical need for a comprehensive experimental characterization of all flow properties at the jet exit.
Assessment of Reacting and Non-Reacting Supersonic Mixing
Authors: Campioli Theresa L. , Schetz Joseph A.
Published: March 2009

A computational study of sonic jet injection into a supersonic crossflow with and without combustion was performed. The gaseous injector was 3.226 mm in diameter and was injected 30 degrees to the horizontal. Simulated conditions involved sonic injection of hydrogen into a Mach 4 air cross-stream with a jet to freestream momentum flux ratio of 2.1. The numerical flow solver used was GASP v. 4.2. The inviscid fluxes were computed in three dimensions using third-order Roe Flux in the streamwise and lateral directions and third-order Van Leer flux in the vertical direction. The algorithms were chosen because of their robustness, shock resolution capabilities and efficiency. The Mentor Supersonic Transport (SST) turbulence model was used since the algorithm has good capability of solving both wall-bounded and free-shear flows. The reaction model used for hydrogen and air was a 9 species, 18 reactions model created by Drummond. The main results of this work can be summarized as: 1) modeling of combustion does not significantly alter the mixing behavior of the solution, 2) the size of the fuel plume is larger for the analysis which includes reacting flow, 3) the difference in size and shape of the plume between the reacting and non-reacting cases increases with downstream distance from injection.
Effect of Shear Layer Interaction on Acoustic Response of Coaxial Side Branch Resonators
Authors: Oshkai P. , Velikorodny A. , Yan T.
Published: March 2009

Fully turbulent inflow past a coaxial side branch resonator mounted in a duct can give rise to pronounced flow oscillations due to coupling between separated shear layers and standing acoustic waves. Experimental investigation of acoustically-coupled shear layers is conducted using digital particle image velocimetry in conjunction with unsteady pressure measurements. Global instantaneous flow images, as well as phase-averaged images, are evaluated to provide insight into the flow physics during tone generation. The emphasis is on the effect of shear layer interaction on the acoustic response of the resonator during the first and second hydrodynamic modes of the shear layer oscillation. Onset of the locked-on resonant states is characterized in terms of the acoustic pressure amplitudes and the quality factors of the corresponding spectral peaks. Moreover, patterns of generated acoustic power are calculated using a semi-empirical approach. As the level of interaction between the separated shear layers is increased, spatial structure of the acoustic source undergoes a substantial transformation.
Effect of Viscoelasticity on the Turbulence Kinetic Energy Budget of the Non-Newtonian Fluid Turbulent Flow Accompanied by Drag Reduction
Authors: Kagawa Y. , Yu B. , Kawaguchi Y.
Published: March 2009

For the purpose of elucidating the mechanism of drag reduction by additives and finding a way to judge optimum drag-reducing additives through a simple rheological test, we performed DNS analysis of viscoelastic fluid turbulent flow in a two-dimensional channel. In this calculation, we employed the Giesekus constitutive equation to model the interaction between water-soluble polymer, or the elastic micellar network structure, and solvent. We calculated the fluid flow by varying the rheological parameters of the model. We examined the turbulent kinetic energy budget and studied the “viscoelastic contribution” term in the budget equation for turbulent intensity, which is not apparent in normal Newtonian fluid turbulence. Viscoelastic contribution has a characteristic effect on viscoelastic fluid turbulence. We concluded that the viscoelastic contribution plays a major role in turbulent frictional drag reduction. Dissipation and viscoelastic contribution serve as a key factor of turbulent kinetic energy loss in most areas of the channel. From the visualization of local and instantaneous eddy behavior, we discussed the relationship between viscoelastic contribution, elastic energy and turbulent production. We found that viscoelastic contribution serves as a direct local source of turbulent production, and that energy is stored in the elasticity.
Fine Scale Structure of High Reynolds Number Taylor-Couette Flow
Authors: He W. , Tanahashi M. , Miyauchi T.
Published: March 2009

Direct numerical simulation (DNS) has been conducted to investigate turbulence transition process and fine scale structures in Taylor-Couette flow. Fourier-Chebyshev spectral methods have been used for spatial discretization and DNS are conducted up to Re = 12000. With the increase of Reynolds number, fine scale eddies are formed in a stepwise fashion. In relatively weak turbulent Taylor-Couette flow, fine scale eddies elongated in the azimuthal direction appear near the outflow and inflow boundaries between Taylor vortices. These fine scale eddies in the outflow and inflow boundaries are inclined at about −45/135 degree with respect to the azimuthal direction. With the increase of Reynolds number, the number of fine scale eddies increases and fine scale eddies appear in whole flow fields. The Taylor vortices in high Reynolds number organize lots of fine scale eddies. In high Reynolds number Taylor-Couette flow, fine scale eddies parallel to the axial direction are formed in sweep regions between large scale Taylor vortices. The most expected diameter and maximum azimuthal velocity of coherent fine scale eddies are 8 times of Kolmogorov scale and 1.7 times of Kolmogorov velocity respectively for high Reynolds Taylor-Couette flow. This scaling law coincides with that in other turbulent flow fields.
Investigation of Boundary Layer Flows Over a Flat Plate With Different-Size Transverse Square Grooves at s/w = 10
Authors: Wahidi Redha , Chakroun Walid , Al-Fahad Sami
Published: March 2009

Turbulent boundary layer flows over a flat plate with multiple transverse square grooves spaced 10 element widths apart were investigated. Mean velocity profiles, turbulence intensity profiles, and the distributions of the skin-friction coefficients (Cf) and the integral parameters are presented for two grooved walls. The two transverse square groove sizes investigated are 5mm and 2.5mm. Laser-Doppler Anemometer (LDA) was used for the mean velocity and turbulence intensity measurements. The skin-friction coefficient was determined from the gradient of the mean velocity profiles in the viscous sublayer. Distribution of Cf in the first grooved-wall case (5mm) shows that Cf overshoots downstream of the groove and then oscillates within the uncertainty range and never shows the expected undershoot in Cf. The same overshoot is seen in the second grooved-wall case (2.5mm), however, Cf continues to oscillate above the uncertainty range and never returns to the smooth-wall value. The mean velocity profiles clearly represent the behavior of Cf where a downward shift is seen in the Cf overshoot region and no upward shift is seen in these profiles. The results show that the smaller grooves exhibit larger effects on Cf, however, the boundary layer responses to these effects in a slower rate than to those of the larger grooves.
Effects of the Fraction of Evaporator Bypass Air on Heat Pump Dryer Performance
Authors: Achariyaviriya Siva , Chotswasd Methawut
Published: March 2009

The purposes of this research were to study the effects of the fraction of evaporator bypass (BP) on heat pump dryer performance and the energy efficiency of two systems of heat pump dryer: the Vary Speed Drive Heat Pump Dryer (VSD-HPD) and the Bypass Working Fluid Heat Pump Dryer (BWF-HPD). For the VSD-HPD, the speed of the compressor varied by using the inverter to control the heating load inside the drying chamber and for the BWF-HPD, the external condenser was used to control the heating load inside the drying chamber. The laboratory scale heat pump dryer was constructed by combining these two systems into one. These experiments used the sliced bananas as the testing materials. The diameter of each sliced banana was 30–35 mm. and the thickness was 5 mm. the initial moisture content of sliced banana was approximately 215–229% dry basis. The experiments were carried out by using the fraction of evaporator bypass as 60%, 70%, and 80% consequently. The working system of this heat pump dryer is the closed loop system which has the constant temperature as 60 °C and the air velocity is 1.27 m/s. This present study is conducted to find out the drying rate (DR) and the specific moisture extraction rate (SMER) of both heat pump dryers. It can be concluded that the BP had slight effect on the performance of the VSD-HPD whereas it mostly affected the performance of the BWF-HPD. The comparison of the average DR and SMER of these two systems of heat pump dryers from all experiments presented that the obtained rate of the VSD-HPD were better than the BWF-HPD. The average SMER of VSD-HPD with 60% BP was 0.55 kg water evaporated/kW h.
Solidification in Stretching Flow of Photopolymer and Orientation of Rod-Like Dispersed Particles
Authors: Kiriyama Ayumi , Mizunuma Hiroshi
Published: March 2009

Orientation of rod-like particles was investigated in stretching flow of photopolymers. The photopolymer was stretched between two moving disks, and the following capillary thinning and solidification by UV light were observed. Orientation of particle was quantitatively evaluated as a function of a stretching rate and a particle concentration. It was found that the stretching rate and a start time of the solidification did not influence the particle orientation remarkably. The orientation of the rod-like particles was dependent on the concentration of dispersed particles. The transient diameter profile agreed with the diameter obtained from FEM analysis, assuming the Oldroyd-B model.
Estimation of Dynamic Surface Tension of Surfactant Solutions
Authors: Karasawa Masahiro , Hasegawa Tomiichi , Narumi Takatsune
Published: March 2009

Surfactants have been used in many fields for improving efficiency in processing but often cause some problems complicated and unsolved. We observed liquids dripping out of an aperture at relatively low Reynolds numbers using a CCD camera, measured mass and velocity of falling drops, and estimated the dynamic surface tension of several aqueous solutions of surfactants: polyoxyethylene (23) lauryl ether, polyoxyethylene (20) stearyl ether, etc. It was concluded that the molecular weight of hydrophilic group in surfactants greatly affects the dynamic surface tension of the solutions at the same mol concentration. We provided the normalized dynamic surface tension and it was shown to be correlated with the surface age multiplied by the molecular weight of the hydrophilic group of the surfactant. A model was presented by considering the dynamic behavior of surfactant molecules approaching a static state to predict the normalized dynamic surface tension of surfactants. The predictions of this model agreed well with the experimental results.
Weakly Nonlinear Stability Analysis of a Non-Uniformly Heated Non-Newtonian Falling Film
Authors: Usha R. , Sadiq I. Mohammed Rizwan
Published: March 2009

A thin liquid layer of a non-Newtonian film falling down an inclined plane that is subjected to non-uniform heating has been considered. The temperature of the inclined plane is assumed to be linearly distributed and the case when the temperature gradient is positive or negative is investigated. The film flow is influenced by gravity, mean surface-tension and thermocapillary force acting along the free surface. The coupling of thermocapillary instability and surface-wave instabilities is studied for two-dimensional disturbances. A non-linear evolution equation is derived by applying the long-wave theory and the equation governs the evolution of a power-law film flowing down an inclined plane. The linear stability analysis shows that the film flow system is stable when the plate temperature is decreasing in the downstream direction while it is less stable for increasing temperature along the plate. Weakly non-linear stability analysis using the method of multiple scales has been investigated and this leads to a secular equation of the Ginzburg-Landau type. The analysis shows that both supercritical stability and subcritical instability are possible for the film flow system. The results indicate the existence of finite-amplitude waves and the threshold amplitude and non-linear speed of these waves are influenced by thermocapillarity. The results for the dilatant as well as pseudoplastic fluids are obtained and it is observed that the result for the Newtonian model agrees with the available literature report. The influence of non-uniform heating of the film flow system on the stability of the system is compared with the stability of the corresponding uniformly heated film flow system.
Behavior of Underexpanded Jet Impinging on Sphere
Authors: Endo Masaki , Sakakibara Yoko , Iwamoto Junjiro
Published: March 2009

An underexpanded jet is utilized in industries as well as aviation field, e.g. to cool a body by the jet impingement, to remove molten metal in laser cutting, etc. One of the biggest problems is noise radiating from the jet which has high frequency, or screech tone. It is pointed out that the noise is closely related to the structure of the jet. In this paper, the underexpanded jets on a plate and hemispheres of different radii are visualized using the shadowgraph and Schlieren methods so as to analyze the jet structure, especially the flow field above the object, or the shock region. As a result, the radius of the hemisphere is found to have an effect not on the shape greatly, but on the location of the plate shock, and furthermore on the formation of separation bubble on the surface.
Experiments on Backward-Facing Step Flows Preceding a Filter
Authors: Yao S. , Krishnamoorthy C. , Chambers F. W.
Published: March 2009

The resistance of automotive air filters alters upstream pressure gradients and thereby affects flow separation, the velocity distributions over the filter, and the performance of the filter. Air filters provide a resistance sufficient to alter flows, but not enough to make face velocities uniform. The backward-facing step flow is an archetype with a separation that resembles those found in automotive air filter housings. To gain insight to the problem of separation and filters, experiments were conducted measuring velocity fields for air flows in a 10:1 aspect ratio rectangular duct with a backward-facing step with and without the resistance of an air filter mounted downstream. The expansion ratio for the step was 1:2. The filter was mounted 4.25 and 6.75 step heights downstream of the step; locations both upstream and downstream of the nominal 6 step-height no-filter reattachment point. Experiments were performed at four Reynolds numbers between 2000 and 10,000. The Reynolds numbers were based on step height and inlet maximum velocity. The inlet velocity profiles at the step were developed. A Laser Doppler Anemometer (LDA) was used to measure velocity profiles and map separated regions between the step and the filter. The results indicate that the filter tends to decrease the streamwise velocity on the non-separated side of the channel and increase it on the separated, step, side compared to the no-filter flow. Non-separated flow tends to separate due to the deceleration and separated flow reattaches before the filter, whether the filter is placed at 4.25 or 6.75 step heights. The literature shows that without a filter the reattachment location depends on the Reynolds number in the laminar and transitional regimes, but is constant for turbulent flow. However, the area of the reversed flow may vary with Reynolds number for turbulent flow. With the filter at 4.25 step heights, the area of reversing flow is reduced significantly, and the Reynolds number has little effect on the main properties of the flow. With the filter at 6.75 step heights, the reversing flow area decreases as the Reynolds number increases though the reattachment point is fixed just upstream of the filter.
Flow Around a Suddenly Start Rotating Circular Cylinder and Introduce a New Paradox
Authors: Nagai Baku M. , Rana Muhammed Sohel , Ameku Kazumasa , Chinen Junji
Published: March 2009

There have been many misunderstanding about the flow around vortices for example a stationary and/or moving vortex pair. The authors have pointed out that no fluid dynamics textbooks have accepted the existence of stationary or arbitral speed moving vortices. About the vortex flow, recently the authors have found a new analytic solution of the Navier-Stokes equations for two-dimensional flow around a suddenly start rotating circular cylinder. This analytic solution explains the velocity distribution, vorticity distribution with change in time, and boundary layer thickness close to a vortex filament because of the action of viscosity. The resulting solutions are involved simple exponential function. Authors present a new construction for the solution of the Navier-Stokes equations for suddenly start rotating circular cylinder. New solution is based on the concept of the similarity solution approach using similarity variable, dimensional analysis, initial, & boundary conditions. A brief theoretical discussion is presented about the suddenly start rotating circular cylinder. The second part of the paper deals with the analytic solution being compared with experimental results in various Reynolds number. A typical measurement is that of relaxation of rotational velocities when the cylinder is subjected only to the viscous resistance. To measure the velocity distribution of the flow the experiments were made with the help of tracer particle (aluminum powder and 150-grain diameter meshes) for water and oil (Super Mulpus 68). The effects of the Reynolds number on the laminar asymmetric flow structure in the flow region are studied. The induced speed distribution in the rotation of cylinder (diameter 10 mm) circumference has examined about the Reynolds number from 26 to 522 for water consequent cylinder rpm 10, 25, 50, 75, 100 and 0.12 to 2.32 for Super Mulpus 68 Oil consequent cylinder rpm 5, 10, 25, 50, 75, 100. The relation between the induced speeds after the time had passed enough and the various cylinder rotational speeds for both analytical and experimental results are shown. At lower Reynolds number experimental results are closer to theoretical results for a finite time condition, at that time there is exist vorticity around the cylinder. We can also establish that more difference between experimental and theoretical results with higher Reynolds number. An interesting phenomenon has been observed in the flow patterns at various Reynolds number and is discussed. Finally, authors have explained the significant difference between experimental and theoretical results and a new paradox has been introduced.
Measurement of the Interaction Between a Single Particle and a Single Burgers Vortex
Authors: Tanaka Yohsuke , Otsu Kazuaki , Tsuji Takuya , Tanaka Toshitsugu
Published: March 2009

In the present study, as an elementary physics for modeling a particle-laden turbulent flow, interactions between a single particle and a single Burgers vortex are observed by using 2D-PIV measurement. We used three kinds of particles which have particle Reynolds numbers ranged from 1258 to 2854. Their diameters are larger than the Kolmogorov scale. The vortex Reynolds number used is 79. Especially, we focus on the vorticity field and the superficial fluid divergence field induced by the particle motion on a cross-section perpendicular to the vortex axis. It is found that the maximum vorticity is increased when the particle pass through the forced vortex area, and the increase does not occur when the particle pass through the free vortex area.
Two Dimensional Jet at Low Reynolds Numbers
Authors: Muramatsu Akinori , Motohashi Tatsuo
Published: March 2009

A numerical simulation of two-dimensional jets was carried out using a SOLA method. The two-dimensional jets were discharged from a slit in a wall at Reynolds numbers below 50. The difference between the calculated flow fields and those of the Bickley jet is due to the non-uniformity of the pressure field near the jet exit at the wall. The jet spreads faster than the Bickley jet. The decay of the streamwise velocity on the center line is more rapid than that of the Bickley jet. The streamwise velocity profile is different from that of the Bickley jet, and a reversed flow is generated in the outer part of the jet. The jet develops instability through two processes. First, small fluctuations grow exponentially. Second, vortical motion such as so-called ‘flapping motion’ of the jet develops in the downstream region. The critical Reynolds number, as determined by the growth of an integral of kinetic energy, is approximately 16.5. Integrals of momentum and pressure are calculated on a control surface in order to confirm the momentum conservation law. When the Reynolds number exceeds 20, the generation of fluctuations contributes to streamwise variations in the integrals of momentum and pressure.
Characteristics of Dynamic Thrust on an Unsteady Airfoil in Pitching and Heaving Motions
Authors: Fuchiwaki Masaki , Tanaka Kazuhiro , Nakashima Masahiro
Published: March 2009

The unsteady flow at low Reynolds number regions has been attracted significant attentions in recent years. However, detailed vortex flow structures and characteristics of dynamic forces acting on an unsteady airfoil at a low Reynolds number region have not been clarified sufficiently. In this study, the detailed vortex flow structures behind an unsteady airfoil in pitching and heaving motions at low Reynolds number regions have been measured by PIV measurement. Moreover, the dynamic thrust measurements on an airfoil have been performed by a six-axes sensor in a water tunnel. An unsteady airfoil in pitching and heaving motions forms thrust producing vortex streets with vortices accompanied with large vorticities, without disturbing flow around an airfoil, comparing to airfoils with independent pitching and heaving motions. In the wake of an unsteady airfoil in pitching and heaving motions, jet flow is generated at φ = π/3, π/2, π and 3π/2. In particular, jet velocity becomes over 2.0 at φ = π/3 and π and the values are greater than those of the heaving airfoil. For φ = π/3 and π/2, large dynamic thrusts are produced. The thrust efficiency increases drastically with increase of Strouhal numbers and reaches its maximum value at around St = 0.2, then decreases gradually. At φ = π/3, 2π/3, 5π/6, π, 7π/6 and 4π/3, significantly large thrusts are produced on an unsteady airfoil however at the same time large lifts and moments are generated therefore thrust efficiency becomes small. The maximum thrust efficiency is φ = π/2 and is approximately 0.40.
A Study of Radial Underexpanded Jet by Shadow and Schlieren Photography
Authors: Toriumi Shinta , Endo Masaki , Sakakibara Yoko , Iwamoto Junjiro
Published: March 2009

When an underexpanded jet impinges normally on a flat plate, the so-called plate shock forms in front of the plate and downstream of the shock the flow spreads radially on the wall. The flow field of underexpanded impinging jet has been studied elsewhere and the shock wave patterns, different modes of oscillation etc, were examined so far. Compared to these studies, the flow on the wall, i.e. the wall jet, are investigated less because of the complicated nature of the flow field where it includes the oblique shock waves with various modes of shock oscillation, asymmetric flow patterns under certain conditions, etc. In the present paper a study on the flow pattern of the wall jet has been carried out. Mainly the optical techniques such as schlieren and shadow photography are applied in the experiment to visualize the flow field.
Spectrum Analysis of the Taylor Vortex Flow With a Short Annulus Using the Ultra Sonic Velocity Profiler
Authors: Kawai Hideki , Kikura Hiroshige , Takahashi Hiroshi
Published: March 2009

Boundary effects with a short annulus of Taylor Vortex Flow (TVF) called Ekman boundary, are greatly related to the generation of the various bifurcation modes in vortex structure, which are obtained even in the same Reynolds numbers. Parameters such as the aspect ratio and the radius ratio (Γ and η) are important factors for determination of the flow modes, which have been studied by many researchers and now known as the multiple solutions of the non-linear equations. One of these modes is expected to have mild mixing effects in solid-liquid phase flow, which could be useful for developing new bioreactor equipment. In the present, the spatiotemporal velocity field in the modulated or chaotic flow of Taylor vortex with a small aspect ratio is visualized and measured precisely by using ultrasonic velocity profiler (UVP). Although the flow is dilute solid-liquid flow in the present, this measuring system can be extended to somewhat more concentrated flow regions.
Application of Genetic Algorithm to Prediction of Artery Geometry
Authors: Himeno M. , Noda S. , Himeno R. , Fukasaku K.
Published: March 2009

We used the genetic algorithm (GA) and two-dimensional blood flow analysis to examine the fluid dynamic and engineering factors in multi-objective optimization of blood tubes (vessels). We supposed two factors from fluid dynamics: the wall shear stress (WSS) and the pressure loss, and one materials saving factor: the artery length. As a result, we could get the optimum shapes for each factor. In the case of WSS and artery length, both smoothly curved artery shapes and shapes with bulges were obtained as lower WSS cases. The shapes with bulges were similar to those of aneurysms. In this case, it was found that the WSS increases after the bulge is removed. This means bulges in artery vessel areas with higher WSSs effectively reduce the WSS value. Only the case of WSS and artery length produced a shape like an aneurysm. These results indicate that WSS and artery length are significant factors in determining artery shape.
Bubble Breakup Phenomena in a Venturi Tube
Authors: Fujiwara A. , Okamoto K. , Hashiguchi K. , Peixinho J. , Takagi S. , Matsumoto Y.
Published: March 2009

Microbubble generation techniques have been proposed in former investigations. Here, we study an effective technique using air bubbly flow into a convergent-divergent nozzle (venturi tube). Pressure change in the diverging section induces bubble breakup. The purpose of this study is to clarify the effect of flow velocity at the throat with respect to the bubble breakup process and the bubble behavior in a venturi tube. Relations between generated bubble diameter and bubble breakup process are also described. Using high speed camera for detailed observation of bubble behavior, the following features were obtained. The velocity at the throat is expected to be of the order of the magnitude of the speed of sound of bubbly flow and a drastic bubble expansion and a shrink is induced. Besides, a liquid column appeared after the bubble flowing into the throat, and it grew up to stick to the bubble like in the form of a jet. This jet induced both unstable surface waves and the breakup of a single large bubble into several pieces.
Shear-Induced Lift Force on a Single Bubble in Surfactant Solutions
Authors: Fukuta Masato , Takagi Shu , Matsumoto Yoichiro
Published: March 2009

In this paper, single bubble motion in surfactant solutions is discussed. We focus on the change of the shear-induced lift force acting on a bubble when the bubble surface is contaminated by surfactant adsorption which leads the Marangoni effect. With the increase of Langmuir number corresponding to the decrease of desorption rate constant of surfactant, the lift force on a spherical bubble decreases from that on a clean bubble to near zero value. This reduction is related significantly to the asymmetry of pressure distribution on surface. Comparing the present result with our previous simulation using the stagnant cap model, the lift force of this study is larger than that of the stagnant cap model. This is because in a shear flow, the surface concentration distributes asymmetrically, and the asymmetry of the surface pressure produced by the shear appears stronger than that of the stagnant cap model.
Separated and Reattached Flow Over Square, Rectangular and Semi-Circular Blocks
Authors: Agelinchaab M. , Tachie M. F.
Published: March 2009

A particle image velocimetry is used to study the characteristics of separated and reattached turbulent flow over two-dimensional transverse blocks of square, rectangular and semi-circular cross-sections fixed to the bottom wall of an open channel. The ratio of upstream boundary layer thickness to block height is considerably higher than in prior studies. The results show that the mean and turbulent statistics in the recirculation region and downstream of reattachment are significantly different from the upstream boundary layer. The variation of the Reynolds stresses along the separating streamlines is discussed within the context of vortex stretching, longitudinal strain rate and wall damping. It appears wall damping is a more dominant mechanism in the vicinity of reattachment. The levels of turbulence diffusion and production by the normal stresses are significantly higher than in classical turbulent boundary layers. The bulk of turbulence production occurs in mid-layer and transported into the inner and outer layers. The results also reveal that the curvature of separating streamline, separating bubble beneath it as well as the mean velocity and turbulent quantities depend strongly on block geometry.
PIV Study of Three-Dimensional Wall Jet Over Smooth and Rough Surfaces
Authors: Agelinchaab Martin , Tachie Mark F.
Published: March 2009

This paper reports experimental study of three-dimensional turbulent wall jet over smooth and rough surfaces. The wall jet was created using a square nozzle of size 6 mm and flow into an open channel. The experiments were performed at a Reynolds number based on the nozzle size and jet exit velocity of 4800. A particle image velocimetry was used to conduct detailed measurements over the smooth and rough surfaces at various streamwise-transverse and streamwise-spanwise planes. From these measurements, mean velocities and turbulent quantities were extracted at selected locations. The distributions of the mean velocities, turbulent intensities and Reynolds shear stress were used to provide insight into the characteristics of three-dimensional wall jet flows over smooth and rough surface.
Characteristic Spatial Scales of Particle Clusters Formed in Gas-Solid Flow
Authors: Tsuji T. , Ito A. , Tanaka T.
Published: March 2009

Spatial scale characteristics of particle clusters are investigated by directly performing Fourier transform of spatial particle concentration distributions. Flow field data are obtained by large-scale Eulerian / Lagrangian simulations. All calculations are performed in three-dimensions and more than sixteen million particles are tracked in the maximum case. The inter-particle collision plays an important role for the development of particle clusters. In this study, results obtained by using the stochastic model based on direct simulation Monte Carlo (DSMC) are compared with that by the deterministic model. The results obtained by DSMC method agree quantitatively with deterministic model. Particle clusters consist of multiple-spatial scale components and the low wave-number, hence large-scale structure, is dominant. Dependency on the domain size and resolution is also investigated in detail.
Dielectrophoresis Force Calculation for Application in Digital Microfluidics
Authors: Young Patrick M. , Mohseni Kamran
Published: March 2009

This paper presents numerically obtained results for the net force on a fluid slug transported by dielectrophoresis (DEP) and compares it with previously obtained data for the electrowetting on dielectric (EWOD) effect. The net DEP force is shown to be strongly peaked when a droplet interface is located near the edge of a charged capacitor, and reduces to the well-known lumped parameter result in the appropriate limits. The effect of electrode spacing is seen to have an inversely proportional effect upon the force experienced by the droplet, and the effect of increasing droplet curvature is observed to increase the net force on the droplet.
Analysis of the Parallel Blade Vortex Interaction With Leading Edge Blowing Flow Control Using the Proper Orthogonal Decomposition
Authors: Weiland Chris , Vlachos Pavlos
Published: March 2009

Interactions of vortical unsteady flows with structures are often encountered in several engineering applications. Such flow structure interactions (FSI) can be responsible for generating significant loads and can have many detrimental structural and acoustic side effects, such as structural fatigue, radiated noise and even catastrophic results. Amongst the different types of FSI, the parallel Blade-Vortex Interaction (BVI) is one of the most prominent. The authors in a previous work (Weiland and Vlachos, 2006) reported an active flow control technique that successfully minimizes the parallel BVI. This technique is based on disrupting the incident vortex using a jet issued via Leading Edge Blowing (LEB), hence, hereon we term the method LEB. The effectiveness of the method was experimentally analyzed using Time-Resolved Digital Particle Image Velocimetry (TRDPIV) recorded at a rate sufficient to fully resolve the spatio-temporal dynamics of the flow field combined with simultaneous accelerometer measurements of the structure. These measurements quantitatively document the FSI dynamics. While our results demonstrated that for the range of our experimental parameters the LEB is successful in dramatically modifying the BVI, the question still remains as to which physical processes are responsible for this reduction. This paper represents a continuation of our effort to further understand the dynamics of using active flow control to mitigate BVI. We present Proper Orthogonal Decomposition (POD) analysis of the temporally resolved planar flow fields for two extreme cases that were reported in the previous work. The two cases correspond to a large wake generator and a small wake generator. The POD technique was chosen specifically for its ability to reduce a complicated flow field into its optimal fundamental modes with a description of the energy contained in each mode, thereby simplifying the dynamics of a flow-field system for analysis. Results of the POD analysis for the small wake generator indicate that for no LEB, the fundamental (i.e. most energetic) mode is given by the vortex shedding of the circular cylinder upstream. The addition of LEB reduces the energy contained in this fundamental mode. Thus the LEB jet has the effect of reducing the flow field coherency; the structure of the exciting vortices is broken up into smaller vortices which have less or little effect on the transfer of energy from the wake to the airfoil. For the case of the large wake generator, the LEB jet has the opposite effect: the jet organizes the circular cylinder wake into flow structures that maintain their form, and thus the wake retains its ability to excite the airfoil into vibrations.
Non-Darcy Flow Pore Network Simulation: Development and Validation of a 3D Model
Authors: Lemley Evan C. , Papavassiliou Dimitrios V. , Neeman Henry J.
Published: March 2009

This validation study is part of a larger ongoing study to improve flow simulation in three dimensions in porous materials. Obtaining porous flow parameters such as permeability and Forchheimer’s coefficient is time consuming and expensive, and may be very sample dependent. This study is aimed at verifying a simulation technique that predicts flow parameters, including permeability and Forchheimer’s coefficient, by comparing simulation results to empirical results. The simulation technique used performs Monte Carlo trials by using statistical information about pore size distributions to generate random pore networks consisting of pipes and junctions, then simulating flow through the network. The validations have been performed for a packed bed of glass beads and for sandstone. For glass beads the relationship between permeability and Forchheimer’s coefficient follows an empirical relationship known as Ergun’s equation, while sandstone has been compared to measured data for the permeability versus Forchheimer coefficient relationship. The glass bead simulations resulted in a very close match to Ergun’s equation for the permeability versus Forchheimer coefficient. The sandstone simulations showed similar trends to the empirical data for permeability versus Forchheimer coefficient.
Dense Particulate Flow in a Cold Gas Dynamic Spray System
Authors: Samareh B. , Dolatabadi A.
Published: March 2009

The effect of particle-gas and particle-particle interactions in a cold spray process is studied when the particle loading is high. To examine the effect of the presence of a dense particulate flow on the supersonic gas, an Eulerian-Eulerian approach is used. It is found that when the volume fraction of the injected particles is increased, the turbulence of the gas phase will be augmented by the motion of particles and consequently, the shape, strength and location of the compression and expansion waves will be altered. Shock-particle interactions are demonstrated for various volume fractions. Another important parameter which will affect the spraying deposition efficiency is the substrate stand-off distance. It is found that the stagnation pressure alternates for different stand-off distances because of the formation of compression and expansion waves outside the nozzle exit. Particle normal velocity upon impact is a strong function of the stagnation pressure on the substrate as particles must pierce through the bow shock formed on that region. The substrate stand-off distance can be optimized to provide minimal pressure build-up on the substrate. This will result in a higher normal velocity upon impact and thus a higher deposition efficiency.