bluuebunny/crossref_metadata_2025 huggingface dataset:
Immersed Boundary and Finite Element Methods Approach for Interaction of an Elastic Body and Fluid by Two-Stage Correction of Velocity and Pressure
Authors: Tuan Ya Tuan Mohammad Yusoff Shah , Takeuchi Shintaro , Kajishima Takeo
Published: March 2009

A new method for solving fluid-structure interaction problem is proposed by coupling immersed boundary method (IBM) and finite element method (FEM). This new method enables efficient simulation of fluid-structure interaction. Pressure and viscous forces are used for fluid and elastic body coupling. Pressure field is solved using fractional step method (FSM). An efficient pressure retrieval method is developed to allocate pressure information from the nearest fluid cell to Lagrangian points on the object’s surface. At the cells occupied by solid region, velocity field is subjected to momentum exchange procedure. The second correction of pressure is undertaken in the occupied region. Simplified marker and cell (SMAC) method is used to find scalar values for velocity and pressure correction. The present method is applied to 2-D flow fields with an elastic body to show the applicability of the method. A NACA 0012 airfoil is used as the neutral shape of the elastic object within the flow. The airfoils are initially placed in a fluid at rest with attack angle of −5° and −10°. Then, a uniform flow is given at the inlet. The coupling of IBM and FEM to study fluid and elastic body interaction problem have shown promising results. The method also shows good performance in obtaining solution in a feasible computation time.
Study of Buoyancy Induced Suppression of Cylinder Wake Instability Using Schlieren-Interferometry
Authors: Singh S. K. , Panigrahi P. K.
Published: March 2009

The control of horizontal square cylinder wake using thermal buoyancy has been experimentally investigated at low Reynolds numbers. The cylinder with an aspect ratio of 60 is mounted in a vertical test cell. The cylinder is electrically heated such that the buoyancy aids to the inertia of the mean flow. The operating parameters i.e. Reynolds number (87–118) and Richardson number (0.065–0.171) are varied to examine the flow behaviour over a range of experimental conditions. Laser schlieren-interferometry has been used for visualization and analysis of flow structures. The complete vortex shedding sequence has been recorded using a highspeed camera. The suppression of vortex shedding by heat input has been demonstrated by schlieren image visualization, time traces of light intensity, corresponding power spectra and Strouhal number. The study provides new experimental information on processes and mechanisms involved in the heat-induced changes of the vortex structures under the influence of buoyancy. The formation length of the vortex structures increases with increase in Richardson number i.e. heating level. The sequence of instantaneous schlieren images show that shape of vortex structures becomes slender at a sufficiently high Richardson number and the vortices from opposite shear layers rub with each other without increasing the circulation level and the two shear layers combine to form a single plume. The plume becomes steady at critical value of heat input leading to suppression of vortex shedding. The corresponding spectra evolve from having a clear peak at the vortex shedding frequency to broadband spectra when vortex shedding is suppressed.
Multi-Time-Delay LSE-POD Complementary Approach Applied to Wake Flow Behind a Bluff Body
Authors: Durgesh Vibhav , Naughton Jonathan W.
Published: March 2009

An understanding of the near wake dynamics of a bluff body is desired to better link base drag reduction observed on these bodies with the coherent structures in the wake. This investigation explores different Linear Stochastic Estimation-Proper Orthogonal Decomposition (LSE-POD) methods that can be employed to estimate the dynamics of the energy containing structure. Statistically independent two-dimensional PIV measurements and time-resolved surface pressure measurements are used to determine spatial POD modes and LSE coefficients for estimating the time-varying POD coefficients using measured surface pressures. These results are used with the time-resolved surface pressure measurements to estimate the time-varying POD coefficients that may be used for a low-order, time-resolved reconstruction of the flow field. The multi-time LSE approach formulated in the time domain (multi-time-delay LSE) is found to be successful in capturing the important near wake dynamics.
Evaluation of Planar Elongation Viscosity for Low Viscous Fluids Using Two-Dimensional Converging Channel Piston Technique
Authors: Takahashi Tsutomu , Shirakashi Masataka
Published: March 2009

The planar elongation viscosity is one of the important properties on the processes for film formation and blow formation. In this study, a new technique to evaluate planar elongation viscosity for low viscous fluids is investigated. To generate the planar elongation flow, a rectangular cylinder piston with two-dimensional converging channel is pushed into a reservoir. The sample fluid is filled in the reservoir. The rectangular piston is inserted into the reservoir at a constant velocity by an electrical actuator. The fluid is pushed out through the two-dimensional converging channel in the piston. The elongation rate in the converging channel is a function of the velocity of the piston. The planar elongation stress is measured by the flow-birefringence method. The evaluated planar elongation viscosity of a Maxwell fluid is almost four times larger than shear viscosity and it shows this technique is appreciable accurately to evaluate the planar elongation viscosity.
An Educational Software Package for CFD Simulation and Visualization
Authors: Manesh H. F. , Hashemipour M.
Published: March 2009

The development of the Virtual Reality (VR) techniques for visualizaton of the computational simulations of complex problem has opened some new avenues for heat transfer and fluid flow research. The importance of data visualization is clearly recognized for to better understanding of the 3-D nature of the flow fields. This work introduces the educational user friendly “VRJET” package designed for teaching fluid mechanics and heat transfer. A Software is developed with C++ standard Programming language using an object-oriented approach to visualize the flow field with high performance computing including advanced support for data presentation and navigation techniques through 3D virtual environment. This work deals with 3-D visualization of the data of impinging laminar single square jet on a heated flat surface, obtained from numerical simulation. This package can be used for research, educational, and engineering.
Drag Reduction in a Small-Scale Circulation System of Surfactant Solutions
Authors: Tominaga Shinzo , Mizunuma Hiroshi
Published: March 2009

Turbulent drag reduction due to surfactant solutions was investigated for a small-scale pipeline system composed complex elements. The effects of the counter-ion concentration were discussed on the drag reduction, the viscoelastic characteristics, and the vortex inhibition. When the molar concentration of the counter-ion was ten times higher than that of the surfactant, the drag reduction appeared over the wide range of the surfactant concentration. This combination of counter-ion and surfactant exhibited a modest increase in viscosity, and the vortex inhibition appeared at a high stretching rate.
Analysis of Solid Structures and Stresses in a Gas Fluidized Bed
Authors: Sun Jin , Battaglia Francine
Published: March 2009

Structures and stresses for the solid phase in a gas-solid fluidized bed are analyzed using results from hybrid simulations. The hybrid method couples the discrete element method (DEM) for particle dynamics with the averaged two-fluid (TF) equations for the gas phase. The coupling between the two phases is modeled using an interphase momentum transfer term. Structure information is characterized using force network size distribution, which shows no large force network existing in the fluidized bed. The normal contact forces have an exponentially decaying distribution. Solid phase continuum fields (local volume fraction, strain rate, stress tensor, and granular temperature) are computed using a coarse-graining process. The results show that the stress has difference in normal stress components. The collisional contribution is larger than the kinetic contribution and spatially correlated to force networks. Stresses are also computed using a kinetic theory stress model. It is demonstrated that the kinetic theory model predicts no difference in normal stress components and larger normal stresses than those computed from the coarse-graining process.
Study on Nonlinear Deformation Behaviors of Electrostatic Levitating Liquid Drop
Authors: Awazu Shigeru , Matsumoto Satoshi , Abe Yutaka
Published: March 2009

Under the micro-gravity environment in space, it is possible to hold a liquid drop without using container. The electrostatic levitation is one of the containerless processing techniques. Using this technique, thermo-physical properties of extreme high temperature molten metals have been measured. For example, the surface tension and viscosity have been estimated from the resonance frequency and the damping constant respectively with an oscillating drop method. This method is based on theoretical equations derived under linear approximations. However, an actual levitating drop includes nonlinear effects. Nonlinear effects should be taken into account for highly precise measurements of physical properties, but it is unknown experimentally. The purpose of the present study is to investigate nonlinear effects of a levitating liquid drop from fluid-dynamic point of view. In the present study, at first, axisymmetric oscillation was applied to a liquid drop levitated by electrostatic force and the oscillation frequency was related to the oscillation amplitude. Next, the rotation in the horizontal direction was applied. The drop deformation and the dependence of resonance frequency on rotational angular velocity were investigated. These experimental results agreed with theoretical predictions.
Anomalous Behavior in Interference Fringes of a Laser Through a Slot Flow
Authors: Okawara Makoto , Hasegawa Tomiichi , Iino Yoshihiko , Narumi Takatsune
Published: March 2009

Flows through a slot were optically examined by Mach-Zehnder interferometer in a range of low Reynolds numbers. Fluids examined in this study are the so-called Newtonian fluids: ion exchange water, silicone oils and liquid paraffin. It was found that interference fringes of laser lights are anomalously deformed around the slot. The deformation was small but non-zero even for ion exchange water. It was getting larger and changing with time for silicone oils and liquid paraffin. Moreover, some bright lines appeared around the slot. Also it was found that there is some region around the slot where the refractive index is higher than that in the surrounding region. The flow field was visualized using reflex powders and a laser light sheet, and found to be a normal converging and diverging flows expected for the low Reynolds number realized in the present experiment. These facts mean that the region around the slot is apparently in lower temperature and it is very hard to explain it from the heat generation by the flow around the slot.
Thrusts and Pressure Losses of the Flow of Water Passing Through Micro Orifices
Authors: Takahashi Masanao , Hasegawa Tomiichi , Narumi Takatsune
Published: March 2009

Jet thrusts and pressure losses of water through micro orifices and slots were measured by using an electric balance and a pressure transducer respectively. It was found that the thrusts for the orifice of 1mm order agree with the result of numerical simulation using Navier-Stokes equation, but the thrusts for the fine apertures of the order 10 μm are well below those predicted. Similarly, the experimental result indicated that the pressure losses for the orifice of 1mm order agree with the theoretical one, but those for the fine apertures of the order 10 μm significantly decrease.
Effects of Drying Conditions of Fixed Bed Longan Drying on Optimal Bed Thickness
Authors: Achariyaviriya Aree , Nuthong Paradorn
Published: March 2009

In this work, it is presented a study of the effects of drying conditions on the optimal bed thickness of the whole longan. The criteria for evaluation of the drying process are specific energy consumption and drying time which the difference of moisture between top and bottom of drying chamber is less than 10%dry basis. The mathematical model is developed for finding the effects of the drying conditions on the optimal bed thickness. The drying conditions are drying air temperature, specific air flow rate, and fraction of recycled air. Experimental data were compared with the simulated results to verify the model. Furthermore, the sensitivity analysis of the fraction of air recycled, drying air temperature, specific airflow rate, initial moisture content, and bed thickness of longan are study. The results showed that there was good agreement between the simulated drying rate and those experimentally observed. In addition, there was a well agreement with respect to the shapes of the drying air temperature and product temperature profiles. From the simulated results, the optimal bed thickness of 40 cm, the specific energy consumption of 10.56 MJ/kg-water and drying time of 64.2 h were found. The responsive conditions were drying air temperature of 75°C, the fraction of recycled air of 90%, and the specific air flow rate of 73 kg-dry air/h-kg dry longan.
Nonlinear Stability Analysis of a Falling Film in the Presence of Gas Flow
Authors: Uma B. , Usha R.
Published: March 2009

A viscous liquid film flows down along the interior of an annular region under gravity with a countercurrent/cocurrent stream of gas phase adjoining the free surface. The interfacial shear stress effects on the stability of the film flow system in the presence of gas flow has been analyzed for the model that describes the motion for the annular countercurrent/cocurrent gas-liquid two-dimensional falling film. A nonlinear evolution of Benney type describing the film thickness in the presence of gasflow has been derived using long wave theory and lubrication approximation. Linear and weakly nonlinear stability analysis of the evolution equation show that both supercritical stability and subcritical instability are possible for the film flow system in the presence of gas flow. The nonlinear equation has been solved numerically in a periodic domain and the results show that the shape and amplitude of the permanent wave are greatly influenced by the countercurrent/cocurrent gas flow.
Surface Tension Model Using Inter-Particle Force in Particle Method
Authors: Kondo M. , Koshizuka S. , Suzuki K. , Takimoto M.
Published: March 2009

Moving particle semi-implicit (MPS) method is widely used for the free surface flow analysis. In small scale problems, the surface tension is dominant. There are two approaches to calculate surface tension in particle methods. One is based on continuum mechanics governing equation which needs the normal vector and the curvature calculation. However, it requires complex formulation. The other is introducing inter-particle potential force. It is known that surface tension can be deduced from inter-molecular force. We propose inter-particle force whose potential is similar to the molecular potential. We reveal the inter-relation between this potential force and the surface tension coefficient used in continuum mechanics. The surface tension can be calculated using this inter-particle potential force. The oscillation cycle roughly agrees with the theoretical value. It is also important to introduce the contact angle to the solid wall, as well as the surface tension. The inter-particle force is also available to model the contact angle. The potential force ratio between fluid-fluid and fluid-wall can be derived from Young’s equation. Using this theoretical ratio of potential force, a larger contact angle appears, if we give a smaller potential force ratio.
Experimental Investigation of Turbulent Boundary Layer Over Smooth Flat Plate With Towing Tank: Attempt of Measurement of Frictional Stress
Authors: Mori Kiyoto , Imanishi Hiroki , Tsuji Yoshiyuki , Kashiwagi Masashi , Inada Masaru , Mochizuki Shinsuke , Masaharu Matsubara
Published: March 2009

The purpose of this study is to evaluate the frictional resistance with sufficient accuracy and to evaluate the drag coefficient at high Reynolds numbers. We have measured the resistance of flat plate with using a towing tank. Correcting the wave-making resistance, pressure resistance, and drag on turbulence simulator, it is found that the measured frictional resistance is smaller than the Karman-Schoenherr formula. But it agrees with the values suggested by Osaka et. al and Osterlund et. al.
Internal Flow in Vacuum Ejector Using Water Table
Authors: Kurahashi Naoya , Endo Masaki , Iwamoto Junjiro
Published: March 2009

The purpose of this study elucidates a structure of internal flow in vacuum ejector. The vacuum ejector is used as a part of automatic conveyance systems, in which precision parts or foods are handled utilizing vacuum pressure from the ejector. So far, the vacuum ejector has been designed mostly by experience and the internal flow in vacuum ejector is not well analyzed. The flow field of ejector is so small that conventional measurements can not be employed and it is difficult to obtain the detailed information. Thus, the flow in the ejector is simulated on water table using hydraulic analogy. In this paper, to obtain optimum performance of the ejector, we examined the different ejector models with parameters, such as the nozzle width and the distance between nozzle and inlet of mixing chamber. As a result, it is shown that maximum performance is obtained for the ejector model with the nozzle width of 40mm, the mixing chamber width of 100mm and the port size of 40mm.
The Generation and Development of the Go¨rtler Vortices in Boundary Layer on Rotating Curved Wall
Authors: Hasegawa Y. , Nishikawa M. , Tani M. , Kikuyama K.
Published: March 2009

The effects of system rotation and wall curvature on the instability of the laminar boundary layer development are studied experimentally using two channels of different centerline curvature, R = 1,000 and 2,000mm. These channels of high aspect ratio are mounted on a rotating table to change the magnitude and direction of the Coriolis force exerting on the flow. Detailed measurements of the time-mean and turbulent velocity components are made using hot-wire probes and the generation, development and mixing processes of the Go¨rtler vortices in the concave side of the curved wall are clarified. Moreover, to examine the developing process of the vortices due to the system rotation in more detail, vortex generators are installed near the leading edge of the plate. The main results are summarized as follows: With the increase in the resultant force acting normal to the boundary layer, the transverse interval of the Go¨rtler vortices generation tends to be smaller in the laminar condition but the vortices tend to be looser in the downstream sections due to the secondary instability and the merge of the vortices. By decreasing the effects of the resultant forces, the behavior of the generated vortices tends to be more unstable, showing a various type of secondary instability modes. When the Coriolis force acts opposite to the centrifugal force due to the channel curvature, the occurrence of the Go¨rtler vortices is suppressed. The three-dimensional configuration of the vortices is enlarged in the transition state of the boundary layer, leading to the early transition to the turbulent state of the boundary layer. When the initial interval of the vortices is fixed by using vortex generators, the configuration of the vortices is kept almost unchanged under the condition that the resultant force of Coriolis and centrifugal ones acts to the wall. When the resultant force acts away from the wall the interval is increased and the development of the vortices is suppressed.
A Study on Flow Field and Behavior of Underexpanded Impinging Jet on a Cone
Authors: Nakayama Isamu , Iwamoto Junjiro
Published: March 2009

In order to investigate the flow field and behavior of underexpanded impinging jet on a cone, the flow field is visualized by schlieren and shadowgraph method, and the pressure distributions on the cone are measured. A cone used for the experiment has a cone angle of 120 deg. The pressure ratio across the nozzle NPR, the ratio of stagnation to ambient pressure, is varied from 2.0 to 4.0 at the nozzle-cone distance non-dimensionalized by the nozzle diameter from 1.0 to 3.5. The shape of the pressure distribution on the wall has different types at different nozzle-cone distances. The visualized flow patterns show that the expansion regions and compression regions are present in the wall jet which can also be seen from the wavy distribution of the pressure. The plate shock, which forms in front of the cone, varies its geometry between flat type and bow type dependent upon nozzle-cone distance. The flat type shock wave is obtained when it is in the expansion region of the jet and the bow type shock when in the compression region. This is qualitatively valid regardless of NPR. When the nozzle-cone distance is short, the jet is very stable without any flow oscillation, while the oscillation occurs more violently as the nozzle-cone distance becomes larger. It is probably due to the more unstable nature of the jet boundary at distance further from the nozzle.
Self-Sustained Oscillations With Tonal Sound Around a Backward-Facing Step With a Small Upstream Step
Authors: Yokoyama H. , Tsukamoto Y. , Kato C. , Iida A.
Published: March 2009

Self-sustained oscillations with acoustic feedback take place in a flow over a two-dimensional two-step configuration: a small forward-backward facing step, which we hereafter call “a bump”, and a relatively large backward-facing step (backstep). These oscillations can radiate intense tonal sound and fatigue nearby components of industrial products. We clarify the mechanism of these oscillations by directly solving the compressible Navier-Stokes equations. The results show that vortices are shed from the leading edge of the bump and acoustic waves are radiated when these vortices pass the trailing edge of the backstep. The propagated acoustic waves shed new vortices by stretching the vortex formed by the flow separation at the leading edge of the bump, and a feedback loop is formed. Moreover, we propose a formula for predicting the frequencies of the tonal sound based on the detailed investigation of the phase relationship between the vortices and the acoustic waves. Also, we investigate the flow conditions for these oscillations by changing the bump configurations. The results show that the oscillations strongly occur when the bump is sufficiently high and the trailing edge of the bump is sufficiently distant from the backstep.
Heat Transfer Estimation of Turbulent Flow With Surfactant Base on the Coherent Fine Scale Eddy
Authors: Okada H. , Tanahashi M. , Miyauchi T.
Published: March 2009

Experiments for heat transfer and friction drag of turbulent water flow with surfactant have been conducted to investigate the relations between heat transfer and drag reduction and coherent fine scale eddies in near-wall turbulence. Experiments were conducted for several different concentrations of surfactant solution and for several pipes (4m long) with different diameters. In the case of high concentration conditions (500 and 750ppm), the range of drag reduction without heat exchange, increases with the increase of temperature for low temperature region (< 313K), which is irrespective of the concentration. On the other hand, in high temperature region (> 313K), the drag reduction range depend on the concentration, and higher concentration leads to larger drag reduction range. The time scale of coherent fine scale eddies well characterizes the range of heat transfer and drag reduction, simultaneously. As the fluid near the wall is cooled in the heat exchanging process, the heat transfer reduction range is suppressed for the high temperature and high concentration conditions.
Cinematic Angiography for Measurements of Blood Flow in Cerebral Aneurysms With Stents
Authors: Ohta Makoto , Fujimura Naoko , Augsburger Luca , Yilmaz Hasan , Ru¨fenacht Daniel A.
Published: March 2009

Background and Purpose: The assessment of blood flow speed by imaging modalities is important for endovascular treatments, such as stent implantation, of cerebral aneurysms. The subtracted vortex centers path line method (SVC method) is one of the ways of determining flow speed quantitatively using the image sequence. And a cinematic angiography (CA) is a high speed image acquisition system using X-ray and contrast media integrated in Digital Subtraction Angiography (DSA) for endovascular therapy. The combination of SVC and CA may useful for determining the blood flow speed during the operation using DSA. In this study, we applied this combination to analyze hemodynamic changes before and after stenting. Methods: A transparent tubular model was constructed of silicone which included an aneurysm 10 mm in diameter and having a 5 mm neck on a straight parent artery with a diameter of 3.5 mm. The model was integrated into a pulsatile circulation system. A double layer stent was placed in the parent artery on the aneurysm. By CA, successive images at 25 frames per second with injection of contrast were obtained. Results and conclusion: Rotating vortexes of contrast, which advanced along the wall of the aneurysm, were observed in successive images of the aneurysm cavity. The movement distance of the vortex center was measured and the results show that the vortex speed decrease after stenting. This indicates the possibility of applying the SVC method to medical imaging equipment for analysis of the flow in aneurysms containing stent.
Identifying Sources of Stereoscopic PIV Measurements Errors on Turbulent Round Jets
Authors: Wanstrom Maja , George William K. , Meyer Knud Erik , Westergaard Carsten
Published: March 2009

This paper addresses some hurdles in experimental data acquisition and analysis when sampling an axisymmetric flow in Cartesian coordinates. The problems are illustrated using recent SPIV data taken in the far field of a turbulent round jet in air. Discrepancies became apparent when trying to reconstruct the polar coordinate moments from Cartesian ones, since both components of the in-plane moments of the latter must be measured to within the same relative accuracy in order to recover the axisymmetry. Various sources for the observed discrepancies are considered including differential resolution of the Cartesian components, peak-locking and misregistration in the stereoscopic recombination. The last seems to be the most important one.
Time-Resolved PIV and Pressure Measurements of Oscillating and Pulsating Flow in a Rapid Expansion
Authors: Smith Barton L. , King Cameron V.
Published: March 2009

Separating oscillating and pulsating flows in an internal adverse pressure gradient geometry are studied experimentally. Time-resolved PIV measurements and simultaneous pressure measurements reveal that, in oscillating flow, during the accelerating portion of the cycle, the flow remains attached in spite of a very large adverse pressure gradient. During the decelerating portion of the cycle, the flow is more prone to separation. The duration and extent of the separation depend strongly on the oscillation displacement amplitude relative to the cross-stream dimension. In some cases, the flow separates but reattaches as the separated shear layer is accelerated temporally. The time-varying pressure measurements are used to determine the resultant minor losses for the flow in each direction. These are found to be an increasing function of displacement amplitude and a decreasing function of the Reynolds number and can be greater than or less than those for steady flow. Pressure and velocity measurements are presented for pulsating flow with various DC components.
Different Modes of Gas Entrainment Through Bottom Discharge Branch by Using Three Dimensional Particle Image Velocimetry System
Authors: Saleh Wael Fairouz , Hassan Ibrahim Galal
Published: March 2009

The discharge of two-phase flow from a stratified region through single or multiple branches is an important process in many industrial applications including the pumping of fluid from storage tanks, shell-and-tube heat exchangers, and the fluid flow through small breaks in cooling channels of nuclear reactors during loss-of-coolant accidents (LOCA). Knowledge of the flow phenomena involved along with the quality and mass flow rate of the discharging stream(s) is necessary to adequately predict the different phenomena associated with the process. Particle Image Velocimetry (PIV) in three dimension was used to provide detailed measurements of the flow patterns involving distributions of mean velocity, vorticity field, and flow structure. The experimental investigation was carried out to simulate two phase discharge from a stratified region through branches located on a semi-circular wall configuration during LOCA scenarios. The semi-circular test section is in close dimensional resemblance with that of a CANDU header-feeder system, with branches mounted at orientation angles of zero, 45 and 90 degrees from the horizontal. The experimental data for the phase development (mean velocity, flow structure, etc.) was done during single discharge through the bottom branch from an air/water stratified region over a three selected Froude numbers. These measurements were used to describe the effect of outlet flow conditions on phase redistribution in headers and understand the entrainment phenomena.
Multilayer Nano-Particle Image Velocimetry in Microscale Poiseuille Flows
Authors: Li Haifeng , Yoda Minami
Published: March 2009

Nano-particle image velocimetry (nPIV) uses evanescent waves generated by total internal reflection at a glass-water interface to illuminate fluorescent colloidal tracers and measure the two velocity components parallel to the wall. For blue light at 488 nm, the exponential decay in the intensity of the illumination with distance normal to the wall z ensures that only about the first 300 nm next to the wall are imaged. The exponential decay also suggests that illuminated tracers in nPIV that are closer to the wall have images that are brighter than those farther from the wall. This variation in tracer intensity is exploited in the “multilayer nPIV” technique, which determines a velocity “profile” at a few different z-locations within the region illuminated by the evanescent wave—and hence velocity gradients within several hundred nanometers of the wall. The feasibility of this technique has already been demonstrated using artificial images of plane Couette flow [1]. We describe here the application of multilayer nPIV to experimental images of incompressible Poiseuille flow through rectangular microchannels with cross-sectional dimensions of 40 μm × 312 μm. In all cases, the flow Reynolds number is O(1) or less, and the velocity profile over the first 400 nm next to the wall is essentially linear. Calibration experiments that incorporate the effects of tracer polydispersity are used to determine the intensity of the tracers at a given distance from the wall. These calibration data are then used to classify and divide the tracers in a given nPIV image into three different layers. The results show that velocities are overestimated in the layer nearest the wall, most likely because of the asymmetry of the Brownian diffusion in this region. The results also show that velocities are underestimated in the layer farthest from the wall because of the nonuniform illumination inherent to evanescent wave-based velocimetry. The extent of this effect is estimated using artificial images. This estimate is then used to correct the experimental result. The mnPIV results in the two layers farther away from the wall are in good agreement with the classic analytical solution for two-dimensional fully-developed laminar Poiseuille flow after this correction, giving a velocity gradient within 7% of the expected value.
Phase Correlation Processing for DPIV Measurements: Part I — Spatial Domain Analysis
Authors: Eckstein Adric C. , Charonko John J. , Vlachos Pavlos P.
Published: March 2009

A novel Digital Particle Image Velocimetry (DPIV) correlation method is presented using nonlinear filtering techniques coupled with a phase-only filter (POF) generalized cross-correlation (GCC). Error analysis demonstrates the deficiency of the GCC alone to successfully improve measurement accuracy, as shown from a decrease in valid vector detection as well as an increase in bias and RMS errors. The use of spatial filters is commonly argued to be disadvantageous for standard Fourier based cross-correlation analysis [1]. However, here we demonstrate that appropriate spatial filters applied to the GCC provide vastly superior performance. The zeropadding method as well as a new spatial filter derived from the Blackman window is introduced, both of which are able to enhance the vector detection capabilities of the GCC processor. In addition, a Gaussian transform filtering technique is introduced which dramatically increases the subpixel resolution of the GCC processor, alleviating apparent effects of peak-locking. The use of these filters with the GCC is able to provide striking improvements in DPIV image correlation.
Noise Characteristics and Shock Structures of Under-Expanded Jet Array Configurations
Authors: Sherikar Sanjay V.
Published: March 2009

Results on noise characteristics and flow-visualization shock structures of 8 × 8 arrays of jets for stagnation pressure ratios in the range 2.4 – 22, and spacing-to-diameter ratios of 1.4 – 4, are presented. Three regimes of noise generation were observed for the arrays tested: regime of weak interactions at low pressure ratios, where the noise spectrum is essentially similar to that for individual jets in the array; regime of combined jet at high pressure ratios, where noise spectrum is similar to that for a single large jet; and, regime of strong interactions, which appears between these two regimes and where individual jets as well as the combined jet significantly contribute to noise generation. The domain of each of these regimes is determined by pressure ratio and spacing-todiameter ratio for the array. The influence of neighboring jets on shock structures of individual jets in the array appears to be insignificant in flow-visualization images at low pressure ratios; however, with increasing pressure ratio, the shock structures increase in size and increasingly influence their neighbors. Large shocks, which span the width of the array and similar to that for a single jet, were observed in the combined jet regime. The combined jet, which is formed by merger of individual jets from the array, dominates noise generation when Mach number in the mixed stream is greater than 0.75. The noise characteristics of the jet arrays correlate well with their corresponding form of shock structures in the three regions described.
Time and Spatial Pressure and Velocity Correlation in a Microbubble Laden Boundary Layer
Authors: Dominguez-Ontiveros Elvis E. , Estrada-Perez Carlos , Hassan Yassin A.
Published: March 2009

Measurements of the velocity fields and wall pressure have been performed in a microbubble laden boundary layer in order to have a better understanding of the degree of correlation between these two parameters. Cross-correlation coefficients have been obtained from synchronized measurements of pressure and velocity at different distances from the wall in a channel flow. The results show a high correlation between pressure and both the streamwise and normal components of the velocity vector for the two-phase flow case. In contrast, the correlation coefficient between pressure and velocity is high only for the streamwise component of the velocity vector for single phase flow (no microbubbles in the flow). A practical application of these measurements is obtaining data and information to better describe the mechanism responsible for the microbubble drag reduction phenomenon, which has great potential for energy savings on different transport means.
2D CFD Studies Using Different Turbulence Models of a Circulation Control Inlet Guide Vane
Authors: Hill H. E. , Ng W. F. , Vlachos P. P. , Guillot S. A. , Bailie S. T.
Published: March 2009

The IGV in this study is an uncambered airfoil, designed to use the Coanda effect to achieve flow vectoring. For this internal flow problem, two isotropic turbulence models are compared to an anisotropic model. Good trend comparison was seen for turning angle and pressure loss performance characteristics given a nominal fixed trailing edge geometry. However, as the trailing edge geometry is modified in an attempt to increase turning, discrepancies become evident, possibly due to the effects of streamline curvature. As the trailing edge radius decreases, significant variations in the jet separation location are predicted, which translates directly into flow turning predictions. Further, one turbulence model was examined using a second CFD code to ensure software independence of the solutions.
Hypersonic Nonequilibrium Aerothermodynamics in CFD-FASTRAN
Authors: Zhang Sijun , Blosch Ed
Published: March 2009

This paper presents the implementation of a state of the art Navier-Stokes code CFD-FASTRAN capable of simulating flow in thermochemical non-equilibrium. CFD-FASTRAN models the fluid as a reacting gas in chemical and thermal non-equilibrium using standard finite rate chemistry models and a two-temperature model for the gas. To validate, a code-to-code comparison between LAURA code and CFD-FASTRAN was carried out.
Numerical Study of Gas and Cavitation Bubble Dynamics in Liquid Mercury Under Negative Pressure
Authors: Ida Masato , Naoe Takashi , Futakawa Masatoshi
Published: March 2009

Gas and cavitation bubble dynamics have been studied numerically to evaluate the effect of gas bubble injection on the suppression of cavitation inception. In our previous studies it has been demonstrated by direct observation that cavitation occurs in liquid mercury when mechanical impacts are imposed and it must seriously shorten the lifetime of nuclear facilities using liquid mercury, such as the mercury spallation target of the J-PARC (Japan Proton Accelerator Research Complex). In this paper, using single-bubble and multibubble models we have performed numerical studies on the dynamics of cavitation bubbles in liquid mercury with and without preexisting gas bubbles, and have clarified that if the mercury involves gas bubbles much larger than the cavitation nuclei, cavitation inception is effectively suppressed due to the positive pressure radiated by the gas bubbles. Our recent experimental results (not shown in the present paper) have confirmed the effectiveness of the bubble injection.
Assessment of DES Models for Separated Flow From a Hump in a Turbulent Boundary Layer
Authors: Lyons Daniel C. , Peltier Leonard J. , Zajaczkowski Frank J. , Paterson Eric G.
Published: March 2009

Turbulent flow past the Glauert-Goldschmied body, a flow-control hump in a turbulent boundary layer, is studied numerically using detached-eddy simulation (DES), zonal detached-eddy simulation (ZDES), delayed detached-eddy simulation (DDES), and Reynolds-Averaged Navier-Stokes (RANS) modeling. The geometry is smooth so the downstream separation point is not set by facets of the geometry but is a function of the pressure gradient, a challenging condition for turbulence models. Comparisons to experimental data show that RANS with the Spalart-Allmaras turbulence model predicts the mean-field statistics well. The ZDES and DDES methods perform better than the DES formulation and are comparable to RANS in most statistics. An analysis of model behavior indicates that modeled stress depletion in the detached shear layer shortly after separation leads to loss of accuracy in the DES variants.
Numerical Evaluation of Fluid Mixing Phenomena in Boiling Water Reactor Using Advanced Interface-Tracking Method
Authors: Yoshida Hiroyuki , Nagayoshi Takuji , Takase Kazuyuki , Akimoto Hajime
Published: March 2009

Thermal-hydraulic design of the current boiling water reactor (BWR) is performed by correlations with empirical results of actual-size tests. Then, for the Innovative Water Reactor for Flexible Fuel Cycle (FLWR) core, an actual size test that simulates its design is required to confirm or modify the correlations. Development of a method that enables the thermal-hydraulic design of nuclear rectors without these actual size tests is desired, because these tests take a long time and entail great cost. For this reason we developed an advanced thermal-hydraulic design method for FLWRs using innovative two-phase flow simulation technology. In this study, detailed two-phase flow simulation code using advanced interface tracking method: TPFIT is developed to get the detailed information of the two-phase flow. In this paper, firstly, we tried to verify the TPFIT code comparing with the existing 2-channel air-water mixing experimental results. Secondary, the TPFIT code was applied to simulation of steamwater two-phase flow in modeled two subchannels of current BWRs rod bundle. The fluid mixing was observed at a gap between the subchannels. The existing two-phase flow correlation for fluid mixing is evaluated using detailed numerical simulation data. From the data, pressure difference between fluid channels is responsible for the fluid mixing, and effects of the time averaged and fluctuating pressure difference must be incorporated in the two-phase flow correlation for fluid mixing.
One-Dimensional Analysis of Full Load Draft Tube Surge
Authors: Chen Changkun , Nicolet Christophe , Yonezawa Koichi , Farhat Mohamed , Avellan Francois , Tsujimoto Yoshinobu
Published: March 2009

One-dimensional stability analysis of a hydraulic system composed of a penstock, a runner and a draft tube was carried out to determine the cause of the full load draft tube surge. It is assumed that the cavity volume at the runner exit is a function of the pressure at the vortex core evaluated from the instantaneous local pressure at the runner exit and an additional pressure decrease due to the centrifugal force on the swirling flow. It was found that the diffuser effect of the draft tube has a destabilizing effect over all flow rates while the swirl effects stabilize/destabilize the system at larger/smaller flow rates than the swirl free flow rate. Explanations of the destabilizing mechanism are given for the diffuser and swirl flow effects.
An Inverse Method for Material Characterization of Magnetorheological Dampers
Authors: Letelier Mario F. , Siginer Dennis A. , Rouliez Jean-Paul , Corral Omar F.
Published: March 2009

Flow of magnetorheological (MR) fluids in dampers is investigated. The MR fluid flows through narrow passages in the damper subject to a magnetic field applied across the passages. The inverse problem of the determination of the required constitutive properties of the MR fluid together with the corresponding flow pattern for the efficient damping of a given load is solved. The fluid is modeled as a Bingham plastic with time-varying yield-stress. Flow is governed by the continuously adjustable constitutive parameters of the MR fluid which are determined to generate variable resistance to flow to dampen the selected load efficiently. The method developed leads to the determination of the magnetic field variation necessary to achieve a specific displacement of the piston in the damper. The governing equations are solved for any time history of the dimensionless yield stress of the fluid. Relationships that correlate damping load and magnetic field time variations are obtained. The analytical tools developed are helpful in damper design.
Parametric Study of Turbulent Separated Convection Flow Over a Backward-Facing Step in a Duct
Authors: Nie Jianhu , Chen Yitung , Boehm Robert F. , Hsieh Hsuan-Tsung
Published: March 2009

Simulations of turbulent convection flow adjacent to a two dimensional backward-facing step are presented to explore the effects of step height, step inclination angle, a mounted rib and Prandtl number on velocity field and heat transfer. Reynolds number and duct’s height downstream from the step are kept constant at Re0 = 28000 and H = 0.19m, respectively. Uniform and constant heat flux of qw = 270W/m2 is specified at the stepped wall downstream from the step, while other walls are treated as adiabatic. The selection of the values for these parameters is motivated by the fact that measurements are available for this geometry and they can be used to validate the flow and heat transfer simulation code. The simulated results compare very well the measurements. The primary and secondary recirculation regions increase in size as the step height increases. The friction coefficient becomes smaller in magnitude with the increase of the step height. The peak Stanton number becomes smaller as the step height increases. The reattachment location becomes longer as the step inclination angle increases. With increase of the step inclination angle, the secondary recirculation region disappears. The peak friction coefficient inside the primary recirculation region becomes smaller as the step inclination angle decreases. Installation of a baffle on the upper wall causes the primary recirculation region to become smaller. The Stanton number decreases as the Prandtl number increases.
Passive Scalar Turbulent Channel Flow at Pr=25: DNS-LES Approach
Authors: Tiselj Iztok , Sˇtrubelj Luka
Published: March 2009

DNS-LES numerical simulations of a passive scalar field in the turbulent channel flow were performed at friction Reynolds number Re_Tau = 180 and Prandtl number Pr = 25. Direct numerical simulation is used for description of the velocity field. Temperature field is described with LES-like approach with the smallest resolved temperature scales equal to the smallest scales of the velocity field. The consistency of the applied physical modelling and pseudo-spectral scheme is tested with the grid refinement study (grid refine ∼3 times in each direction) and with comparison of the results with the existing DNS simulations of Schwertfirm and Manhart (2006) at the same conditions. The comparison shows that the proposed approach produces very accurate mean temperature profiles, heat transfer coefficients and other low-order moments of the turbulent thermal field. It is shown that the mean temperature profiles near the wall can be accurately predicted even when the temperature scales between the Batchelor and Kolmogorov scale are not resolved. The key to the success of the proposed approach lies in the fact that the large-scale structures govern the turbulent heat transfer at high Prandtl numbers, while the role of the sub-Kolmogorov temperature scales in the diffusive sublayer and the thermal buffer layer (y+<5) is practically negligible. The contribution of the sub-Kolmogorov thermal scales becomes relevant above the thermal buffer layer (y+>5), where the unresolved temperature scales affect spectra and RMS temperature fluctuations, but not the log-law shape of the mean temperature profile and the mean heat transfer coefficient.