bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental Study of Uniform-Shear Flow Past a Rotating Cylinder
Authors: Sung Hyung Jin , Chun Chong Kuk , Hyun Jae Min
Published: February 2008

An experimental study is made of two-dimensional uniform-shear flow (U = Uc + Gy) past a rotating cylinder of diameter D. A water-tunnel, equipped with a shear generator, was constructed. Laser-Doppler velocity measurements were undertaken to describe the wake characteristics. Data are compiled over the ranges of 600≤Re≤1,200, the shear parameter K[≡GD/Uc] up to 0.15, and the value of the cylinder rotation parameter α[≡ωD/2Uc], – 2.0≤α≤2.0. The power spectra of velocity measurements at downstream locations were analyzed to examine the vortex shedding patterns. In general, the dominant shedding frequency is shifted to a higher value as |α| and K increase. When |α| increases beyond a certain threshold value, the dominant frequency becomes less distinct. If |α| takes a value larger than around 1.5, the velocity field becomes randomized and diffuse, and the organized Karman vortex street activity weakens. The variations of the Strouhal number with K and α are described. The evolution of mean velocity profiles in the wake field is depicted. Characterizations of the velocity profiles, as K and α vary, are made based on the measurement data.
Experimental and Numerical Investigation of Flow Oscillations in a Rectangular Cavity
Authors: Pereira J. C. F. , Sousa J. M. M.
Published: February 2008

The unsteady, incompressible, moderate Reynolds number flow past a rectangular cavity is experimentally and numerically investigated. Laser-Doppler anemometry, flow visualization and unsteady numerical simulation using fully second-order accuracy in time and space, were the tools employed to meet this purpose. Large-amplitude organized oscillations are reported to occur in the investigated geometry due to fluid-dynamic instability. Detailed flow visualization and unsteady predictions clearly show that the instability process involves a complex coupling of shear layer and recirculating flowfield dynamics. The paper also demonstrates the accuracy of the present calculations.
Numerical Analysis of a Vortex Controlled Diffuser
Authors: Spall R. E.
Published: February 2008

A numerical study of a prototypical vortex controlled diffuser is performed. The basic diffuser geometry consists of a step expansion in a pipe of area ratio 2.25:1. The incompressible Reynolds averaged Navier-Stokes equations, employing the RNG based κ − ∈ turbulence model, are solved. Results are presented for bleed rates ranging from 1 to 7 percent. Diffuser efficiencies in excess of 80 percent were obtained. These results are in good qualitative agreement with previous experimental work. The results do not confirm previous suggestions that the increases in effectiveness of the VCD over a step expansion result from an inhibition of flow separation due to the generation and downstream convection of extremely high levels of turbulence generated in the region of the bleed gap. The results do indicate that the effectiveness of the diffuser is a consequence of the turning of the flow toward the outer wall due to the influence of the low pressure vortex chamber. Calculations employing the RNG based turbulence model were able to capture the abrupt increase in diffuser effectiveness that has been shown experimentally to occur at low bleed rates. Calculations employing the standard κ − ∈ model were unable to predict this occurrence.
Unsteady 2-D Compressible Flow in Unbounded Domain
Authors: Wang Deguan , Wylie E. Benjamin
Published: February 2008

An unsteady isentropic flow model is presented to calculate the two-dimensional flow field in an arbitrarily closed region or in an open fluid domain. In the open domain, a unique boundary condition is implemented to simulate the infinite character of the open space. The characteristics-like method presented herein is shown to be robust over the entire subsonic flow range and, with the implementation of the infinite boundary, provides numerical results in agreement with analytic solutions and experimental data.
Computation of Vortex Lock-In in the Laminar Wake of a Circular Cylinder Using Unsteady Monopole Sources
Authors: Pan Dartzi , Chin Yu-Chi , Chang Chih-Hao
Published: February 2008

The vortex lock-in in the laminar wake behind a circular cylinder induced by the unsteady monopole source is numerically simulated in this paper. The artificial compressibility method is employed to solve the incompressible Navier-Stokes equations. A high-order accurate upwind flux-difference finite-volume scheme is used to discretize the flow field. The unsteady monopole source is simulated by a pulsating volume flux through the cylinder surface at a prescribed forcing frequency and amplitude. The forcing amplitude is set to a fixed value while the frequency is varied to search for the lock-in region. The flow field of the periodic lock-in state is examined in detail. Finally, the effects of a higher amplitude and a different source location are briefly investigated.
Application of Turbulence Models to Separated Flow Over Rough Surfaces
Authors: Patel V. C. , Yoon J. Y.
Published: February 2008

Principal results of classical experiments on the effects of sandgrain roughness are briefly reviewed, along with various models that have been proposed to account for these effects in numerical solutions of the fluid-flow equations. Two models that resolve the near-wall flow are applied to the flow in a two-dimensional, rough-wall channel. Comparisons with analytical results embodied in the well-known Moody diagram show that the k–ω model of Wilcox performs remarkably well over a wide range of roughness values, while a modified two-layer k–ε based model requires further refinement. The k–ω model is applied to water flow over a fixed sand dune for which extensive experimental data are available. The solutions are found to be in agreement with data, including the flow in the separation eddy and its recovery after reattachment. The results suggest that this modeling approach may be extended to other types of surface roughness, and to more complex flows.
Multidimensional In-Cylinder Flow Calculations and Flow Visualization in a Motored Engine
Authors: Khalighi Bahram
Published: February 2008

Multidimensional simulations of coupled intake port/valve and in-cylinder flow structures in a pancake-shape combustion chamber engine are reported. The engine calculations include moving piston, moving intake valve, and valve stem. In order to verify the calculated results, qualitative flow visualization experiments were carried out for the same intake geometry during the induction process using a transient water analog. During the intake process the results of the multidimensional simulation agreed very well with the qualitative flow visualization experiments. An important finding in this study is the generation of a well-defined tumbling flow structure at BDC in the engine. In addition, this tumbling flow is sustained and amplified by the compression process and in turn causes generation of a high turbulence level before TDC. Many interesting features of the in-cylinder flow structures such as tumble, swirl, and global turbulent kinetic energy are discussed.
Analysis of Throughflow Velocity in Two-Dimensional Fluidized Bed Bubbles
Authors: Gera Dinesh , Gautam Mridul
Published: February 2008

The formation of gas bubbles is one of the most characteristic phenomena of fluidized beds. Many unique properties of fluidized beds can be related directly to the presence of bubbles and are dominated by their behavior. Therefore, accurate prediction of parameters such as bubble shape and size, voidage variation and throughflow are practically important. In the present analysis, an approximate model, based on a strongly idealized picture of the bubble formation has been presented. The bubbling gas fluidized bed has regions of low solids density comprised of gas pockets or voids. The observed voids exhibited a variety of shapes (Halow and Nicoletti, 1992), depending upon the material and fluidization velocity. In the low-velocity experiments with the finer materials, rounded voids are observed. However, with coarser materials, voids were typically large and bluntnosed. In the image analyses work, reported by Gautam (1989), in a bed operating slightly above the incipient fluidization, elongated bubbles (a > b, as shown in Figure 1) were observed for glass beads (sp. gravity = 2.5) of mean diameter 500 μm and flattened bubbles (a < b) were seen for mean particle diameter of 350 μm. Also, he noticed the dependence of throughflow velocity on the elongation of the bubble as it traverses up the bed. Additionally, throughflow velocity was found to be independent of the excess gas flow rate through the bed. The digitized image of a typical bubble (refer Gautam et al., 1994) which shows that the bubble were elongated in the vertical direction and were more elliptical than circular. Therefore, description of a bubble on the basis of just one diameter, either the horizontal or the vertical or an equivalent diameter, as has been done by many researchers in the past, is rather incomplete. It is inferred from the present work that the bubble aspect ratio plays an important role in predicting an accurate gas flow through the bubble.
Measuring Unsteady Velocity Profiles and Integral Parameters Using Digital Image Processing of Hydrogen Bubble Timelines
Authors: Bruneau S. D. , Pauley W. R.
Published: February 2008

To aid in the investigation of artificially generated turbulent spots a technique was developed to make instantaneous measurements of full velocity profiles in an unsteady water boundary layer. These profiles were used to document the unsteady behavior of scalar descriptors such as the displacement thickness. The classical hydrogen bubble flow visualization technique was augmented to yield quantitative information by incorporating digital image processing of videotaped boundary layer flow. Digital images were converted into velocity fields using a corrected time-of-flight technique. The capability and limitations of this method in both steady and unsteady flows have been verified using an analytical uncertainty analysis and through comparison with laser-Doppler anemometry measurements. Recommendations are made for effective use of this technique.
Theoretical Study of a Laminar Jet in a Double-Diffusion Environment
Authors: Priven M. , Atkinson J. F. , Bemporad G. A. , Rubin H.
Published: February 2008

The flow field development associated with the injection of negatively, neutrally, and positively buoyant fluid layers into a stratified environment was analyzed in this study. The analysis considered two-dimensional (horizontal slot) fluid injection under laminar conditions in either temperature-stratified, salinity-stratified or double-diffusive stratified environments. The major features characterizing the buoyant layer development were identified. A numerical model, which integrates in a local reference frame the equations governing mass, momentum, heat and salinity fluxes, was developed. The model was used to examine the characteristic flow patterns for the two-dimensional buoyant discharges of interest in this study. It was also possible to show that double-diffusion effects may significantly influence the development of initially neutral fluid layers and cause a certain vertical deviation.
Turbulent Vortex Ring/Free Surface Interaction
Authors: Weigand A. , Gharib M.
Published: February 2008

The interaction of turbulent vortex rings that approach a clean water surface under various angles is experimentally investigated. The temporal evolution of the vortex rings with an initial Reynolds number of Re0 = 7500 is characterized by the laminar/turbulent transition and asymptotic relaminarization of the flow. Using the shadowgraph technique, two major flow cases were identified as a result of the vortex-ring/free-surface interaction: a trifurcation case that results from the interaction during the transition stage, and a bifurcation case that evolves during the fully-developed turbulent stage. In contrast to the laminar interaction, the turbulent bifurcation pattern is characterized by the reconnection and mutual interaction of many small-scale structures. Simultaneous digital particle image velocimetry (DPIV) and shadowgraph measurements reveal that the evolution of the small-scale structures at the free surface is strongly dominated by the bifurcation pattern, which in turn is a consequence of the persisting laminar sublayer in the core regions of the reconnected turbulent vortex loops.
Interaction of Transient Waves With a Circular Surface-Piercing Body
Authors: Yu Xing , Yeung Ronald W.
Published: February 2008

A pseudo-spectral formulation for solving unsteady, three-dimensional fluid motion with a free surface in cylindrical coordinates is presented. An effective method for treating the Laplace equation, as a special application of a generalized Poisson solver, is developed. This approach is demonstrated by studying the evolution of transient surface waves near a vertical circular cylinder enclosed in open or closed domains. Results are observed to have a high degree of precision and spatial resolution even at large time. Potential applications of this method to other problems are discussed.
The Effect of Surfactants on the Wave Growth and Transition to Slug Flow
Authors: Kordyban Eugene , Okleh Abdul Hakim
Published: February 2008

In this work the effect of the surfactants on the wave growth is investigated analytically and experimentally. It is found that short waves are damped by surfactants quite effectively and since the natural wave growth begins with short waves, the transition to slug is shifted to higher gas velocities by a substantial amount. Slugs, in fact, form on level liquid surface. However, when long waves are produced by a wave maker, the slugs form in the usual manner.
A Modified Low-Reynolds-Number Turbulence Model Applicable to Recirculating Flow in Pipe Expansion
Authors: Chang K. C. , Hsieh W. D. , Chen C. S.
Published: February 2008

A modified low-Reynolds-number k-ε turbulence model is developed in this work. The performance of the proposed model is assessed through testing with fully developed pipe flows and recirculating flow in pipe expansion. Attention is specifically focused on the flow region around the reattachment point. It is shown that the proposed model is capable of correctly predicting the near-wall limiting flow behavior while avoiding occurrence of the singular difficulty near the reattachment point as applying to the recirculating flow in sudden-expansion pipe.
Parallel Pipelining
Authors: Joseph D. D. , Bai R. , Liao T. Y. , Huang A. , Hu H. H.
Published: February 2008

In this paper we introduce the idea of parallel pipelining for water lubricated transportation of oil (or other viscous material). A parallel system can have major advantages over a single pipe with respect to the cost of maintenance and continuous operation of the system, to the pressure gradients required to restart a stopped system and to the reduction and even elimination of the fouling of pipe walls in continuous operation. We show that the action of capillarity in small pipes is more favorable for restart than in large pipes. In a parallel pipeline system, we estimate the number of small pipes needed to deliver the same oil flux as in one larger pipe as N = (R/r)α, where r and R are the radii of the small and large pipes, respectively, and α = 4 or 19/7 when the lubricating water flow is laminar or turbulent.
Experimental Investigation of the Flow Inside a Water Model of a Gas Turbine Combustor: Part 1—Mean and Turbulent Flowfield
Authors: McGuirk J. J. , Palma J. M. L. M.
Published: February 2008

The present study examines the flow inside the water model of a gas turbine combustor, with the two main objectives of increasing the understanding of this type of flow and providing experimental data to assist the development of mathematical models. The main features of the geometry are the interaction between two rows of radially opposed jets penetrating a cross-flowing axial stream with and without swirl, providing a set of data of relevance to all flows containing these features. The results, obtained by laser Doppler velocimetry, showed that under the present flow conditions, the first row of jets penetrate almost radially into the combustor and split after impingement, giving rise to a region of high turbulence intensity and a toroidal recirculation zone in the head of the combustor. Part 1 discusses the mean and turbulent flowfield, and the detailed study of the region near the impingement of the first row of jets is presented in Part 2 of this paper.
Experimental Investigation of the Flow Inside a Water Model of a Gas Turbine Combustor: Part 2—Higher Order Moments and Flow Visualization
Authors: McGuirk J. J. , Palma J. M. L. M.
Published: February 2008

The measurements of mean and turbulent quantities presented in Part 1 showed a strong influence of the primary jet system and evidence of the existence of bimodal distributions of the azimuthal velocity was also noted. Due to the importance of this phenomenon for combustor operation, a further study was carried out and measurements of higher order moments (skewness and flatness) were taken, followed by spectral analysis and high-speed flow visualization. These showed that, under the present flow conditions, the time behavior of the six radially impinging primary jets is similar to that encountered in single jet instability studies, with a dominant frequency corresponding to a Strouhal number of 0.27, when correlated with the primary jet characteristics. The unsteady nature of the flow around the primary jets and the high turbulence anisotropies observed suggest that accurate calculations of gas turbine combustor flows are likely to be impossible with models based on time-averaged version of the governing equations, even with closure at the second moment level.
Unsteady Flowfield and Torque Predictions During the Rotation of the Guidevanes of Hydraulic Turbine
Authors: Papantonis D. E. , Pothou K. P.
Published: February 2008

In the present work, the numerical prediction of the steady as well as of the unsteady flowfield through the cascade of guidevanes of a reaction turbine is examined. The unsteadiness of the flow results from the rotation of the guidevanes around their pivot. The applied numerical procedure is based on the coupling of the boundary element and the vortex particle method and is supplemented by two different procedures for the evaluation of the viscous-inviscid interaction and the boundary layers developed on the vanes. The cinematic conditions at every point of the flowfield are calculated first; however, special attention is paid to the prediction of the torque acting on the cascade and, namely, the variation of the torque during the rotation of the guidevanes, as compared with the torque corresponding to the steady state.
The Response of a Centrifugal Pump to Fluctuating Rotational Speed
Authors: Tsukamoto H. , Yoneda H. , Sagara K.
Published: February 2008

A theoretical and experimental study has been made on the dynamic characteristics of a centrifugal pump subject to sinusoidal changes in rotational speed. Time-dependent rotational speed, flow-rate, and total pressure rise are measured for a variety of amplitude and frequency of the fluctuating rotational speed. Measured flow-rate as well as total pressure rise is compared with the quasi-steady ones. Unsteady flow analysis is made for a two-dimensional circular cascade by use of the singularity method. The calculated frequency characteristics are compared with the corresponding experimental ones. The deviation of unsteady characteristics from quasi-steady ones is evident, and the numerical results agree qualitatively with the measured ones. It was found that with the increased frequency of rotational speed fluctuations the dynamic characteristics deviate remarkably from quasi-steady ones. Moreover, a criterion for the assumption of quasi-steady change is presented.
Preliminary Investigation of the Use of Air Injection to Mitigate Cavitation Erosion
Authors: Arndt R. E. A. , Ellis C. R. , Paul S.
Published: February 2008

This project was initiated as part of a new research and development focus to improve hydropower generation. One aspect of the problem is severe cavitation erosion which is experienced when hydroturbines are operated at best power or in spinning reserve. Air injection has been used successfully to minimize or eliminate cavitation erosion in other applications. Thus, an investigation was initiated to determine whether or not air injection would be an effective solution for turbine erosion problems. A specially instrumented hydrofoil of elliptic planform and a NACA 0015 cross section was tested at flow velocities up to 20 m s–1, at various values of cavitation index. Although pit sizes were measured on a soft aluminum insert, pitting rate was not measured directly but was inferred from direct measurement of impulsive pressures on the surface of the hydrofoil and by monitoring accelerometers mounted at the base of the hydrofoil. Cavitation noise was also measured by a hydrophone positioned in the water tunnel test section. Air was injected through small holes in the leading edge of the foil. Air injection was found to be very effective in minimizing erosion as inferred from all three cavitation erosion detection techniques.
Particle Size and Velocity Discrimination in a Sediment-Laden Turbulent Flow Using Phase Doppler Anemometry
Authors: Bennett S. J. , Best J. L.
Published: February 2008

A series of experiments were performed in a mixing box in order: (1) to investigate the applicability of phase Doppler anemometry (PDA) to discriminate fluid and sediment particle sizes and velocities in sediment-laden turbulent flows; and (2) to relate the size and amount of sediment in suspension to the grid-generated turbulence. Natural impurities within the water provide excellent “seeding” to represent the fluid and can be easily discriminated from spherical glass beads (75-355 μm) used as sediment. Slight asphericity in the glass beads results in larger grain size ranges determined by PDA compared to the nominal sieved sizes. The mean, root-mean-square and skewness of the vertical fluid velocities increase at higher grid oscillation frequencies but decrease with distance from the grid. Similarly, the size and amount of suspended sediment increase with grid oscillation frequency and decrease with distance from the grid. The suspension of sediment is shown to be dependent on the magnitude and anisotropy of the fluctuating vertical component of velocity. Phase Doppler anemometry offers a unique methodology to investigate the complex links between the transport of sediment and the turbulent flow field.
Developing Lengths in Horizontal Two-Phase Bubbly Flow
Authors: Warren B. A. , Klausner J. F.
Published: February 2008

An air-water two-phase flow facility with a 19.1 mm i.d. test section has been fabricated. Local measurements of pressure drop for two-phase horizontal bubbly flow and single-phase flow downstream of various orifices have been obtained over a range of flow conditions. The wall shear stress developing length is obtained from the pressure drop profile. A developing length correlation is presented in which the relative deviation is 6 percent. The fully developed vapor volume fraction has also been measured up- and downstream of the orifice. A simple correlation for vapor volume fraction is presented in which the relative deviation is 7 percent. Photographs of the two-phase flow pattern in the developing region reveal that the flow structure is extremely complex and continuously evolves until approximately fully developed flow conditions are achieved.
On the Effect of Surface Roughness on the Vapor Flow Under Leidenfrost-Levitated Droplets
Authors: Prat M. , Schmitz P. , Poulikakos D.
Published: February 2008

In this paper a theoretical investigation is reported on the effect of surface roughness on the phenomenon of Leidenfrost-levitation of droplets above a hot surface. The problem is solved first approximately using a macroscopic approach in which the roughness is replaced by a semi-empirical slip condition of the Beavers-Joseph type. Next, a microscopic model which determines the vapor flow in the close vicinity of the rough surface is solved numerically. Three basic periodic roughnesses are examined: triangular, rectangular and semi-cylindrical. The effect of the relative size of the droplet and the roughness elements on the vapor flow is investigated in the course of the study.
A Note on the Baldwin-Lomax Turbulence Model
Authors: He J. , Walker J. D. A.
Published: February 2008

An analytic expression is obtained for the constant Ccp in the Baldwin-Lomax model by comparing self-similar solutions for velocity in a constant pressure incompressible flow that are based on the Cebeci-Smith and Baldwin-Lomax models; both models are shown to give identical results when the model Ccp has a specific analytical form.
Stability of Computational Algorithms Used in Molecular Dynamics Simulations
Authors: Satoh Akira
Published: February 2008

The present study focuses on a three-dimensional Lennard-Jones system in a thermodynamic equilibrium in order to discuss divergence processes, the relationship between time intervals and divergence times, and the influence of time intervals on thermodynamic quantities and transport coefficients under various number density and temperature. It is found that the velocities of molecules in a system gradually increase with time until the system suddenly diverges exponentially. The time interval-divergence time relationship can be expressed in approximate terms as linear functions if the data are plotted on logarithmic scales, and the system diverges more easily as temperature or number density increases. Thermodynamic quantities show the influence of large time intervals more clearly than do transport coefficients.
A Simple, Accurate Integral Solution for Accelerating Turbulent Boundary Layers With Transpiration
Authors: Sucec James
Published: February 2008

The inner law for transpired turbulent boundary layers is used as the velocity profile in the integral form of the x momentum equation. The resulting ordinary differential equation is solved numerically for the skin friction coefficient, as well as boundary layer thicknesses, as a function of position along the surface. Predicted skin friction coefficients are compared to experimental data and exhibit reasonably good agreement with the data for a variety of different cases. These include blowing and suction, with constant blowing fractions F for both mild and severe acceleration. Results are also presented for more complicated cases where F varies with x along the surface.
Assessment of a Reynolds Stress Closure Model for Appendage-Hull Junction Flows
Authors: Chen Hamn-Ching
Published: February 2008

A multiblock numerical method, for the solution of the Reynolds-Averaged Navier-Stokes equations, has been used in conjunction with a near-wall Reynolds stress closure and a two-layer isotropic eddy viscosity model for the study of turbulent flow around a simple appendage-hull junction. Comparisons of calculations with experimental data clearly demonstrate the superior performance of the present second-order Reynolds stress (second-moment) closure over simpler isotropic eddy viscosity models. The second-moment solutions are shown to capture the most important features of appendage-hull juncture flows, including the formation and evolution of the primary and secondary horseshoe vortices, the complex three-dimensional separations, and interaction among the hull boundary layer, the appendage wake and the root vortex system.
An Experimental Study of Pulsatile Flow in Canine Larynges
Authors: Alipour F. , Scherer R. C. , Patel V. C.
Published: February 2008

Pulsatile flow in an excised canine larynx was investigated with simultaneous recordings of air velocity, subglottal pressure, volume flow rate, and the signal from an electro-glottograph (EGG) for various conditions of phonation. Canine larynges were mounted on a pseudotrachea and sustained oscillations were established and maintained with sutures attached to cartilages to mimic the function of laryngeal muscles. The pitch and amplitude of the oscillations were controlled by varying the airflow, and by adjusting glottal adduction and vocal-fold elongation. Measurements with hot-wire probes suggest that subglottal inlet flow to the larynx is pulsatile but mostly laminar, while the exiting jet is non-uniform and turbulent. In the typical ranges of flow rate, subglottal pressure, and oscillation frequencies, the Reynolds number based on the mean glottal velocity and glottal hydraulic diameter varied between 1600 to 7000, the Strouhal number based on the same parameters varied between 0.002 and 0.032, and the Womersley number ranged from 2.6 to 15.9. These results help define the conditions required for computational models of laryngeal flow.
Three-Dimensional Chaotic Mixing of Fluids in a Cylindrical Cavity
Authors: Miles K. C. , Nagarajan B. , Zumbrunen D. A.
Published: February 2008

Three-dimensional chaotic mixing of Newtonian fluids in a previously uninvestigated cylindrical cavity was studied both experimentally and numerically for creeping flow conditions. Such mixing processes have practical application to the blending of viscous fluids, biological suspensions, or can be used as test beds to study waterborne pollutant formation. A mixing chamber was fabricated which consisted of a cylindrical glass cavity with independently rotating upper and lower circular disks. Fluid motion was revealed by digitizing successive video images of a small neutrally buoyant sphere placed into the mixing cavity and also by photographing dyed blobs. Experimental particle tracking studies were supplemented by numerical simulations. Phase-space trajectories, return maps, and Lyapunov exponents were used to characterize the mixing process and to confirm chaotic behavior.
Calculation of Laminar Separated Flow in Symmetric Two-Dimensional Diffusers
Authors: Tsui Yeng-Yung , Wang Chia-Kang
Published: February 2008

This study is concerned with numerical analysis of laminar separated flow in symmetric, two-dimensional, straight-walled diffusers. With Reynolds numbers Re = 56 and 114 and expansion ratios ER = 3 and 4, totally, there are four cases considered. At the low Reynolds number and the low expansion ratio the flow in the diffuser is nearly symmetric to the center line, irrespective of the diffusion angle. As Reynolds number or expansion ratio increases, a large recirculation region forms at one side wall and a small one at the other side. For the case with Re = 114 and ER = 4 the small recirculating flow disappears at small diffusion angles and a third recirculating flow appear in the same side of the small main recirculation region for large diffusion angles. The pressure recovery reaches its peak value somewhere downstream of the reattachment point of the large recirculating flow. The effectiveness of the diffuser deteriorates as the diffusion angle increases, apart from that at Re = 56 the effectiveness increases from θ = 15 to 30 deg. Symmetric flow solutions can be obtained by incorporating a symmetric relaxation method. The pressure recovery is higher for the symmetric flow than that for the asymmetric flow owing to the weaker recirculating strength in the former.
The Interaction Between a Jet and a Flat Plate—An Inviscid Analysis
Authors: Chow W. L. , Ke Z. P. , Lu J. Q.
Published: February 2008

The problem of jet-plate interaction has been examined. It is shown that the problem of this type is governed by the mechanisms of inviscid interaction. The method of hodograph transformation has been employed to formulate the problem, and the solution is obtained from numerical computations in the hodograph plane. The flow pattern in the physical plane is produced from additional integrations. Extensions to the compressible flow regime with practical applications have also been mentioned.
Velocity and Turbulence Characteristics of Isothermal Lobed Mixer Flows
Authors: Koutmos P. , McGuirk J. J.
Published: February 2008

This work describes an experimental and computational study of flows in model multilobed mixers. Laser Velocimetry was used to obtain the velocity and turbulence fields in the downstream mixing duct. Flow development was quantified by examination of the large cross-plane velocities whose direction implied the formation of two streamwise vortices per lobe. A change from coplanar to scarfed geometry increased vortex strength by 25 percent. Vortex cell formation, roll-up and breakdown to fine scale mixing was attained within a distance of 5 lobe heights. The computational investigation of the coplanar configuration adopted a non-aligned mesh to solve the 3-D Reynolds averaged Navier-Stokes equations. The calculations of the lobe and mixing duct flows were coupled to predict the complete mixer. Comparisons between measurements and calculations using a standard k-ε model suggested good qualitative agreement with maximum disagreement of about 20 percent in peak radial velocities.
Numerical Simulation of the Effects of Rotor-Stator Spacing and Wake/Blade Count Ratio on Turbomachinery Unsteady Flows
Authors: Yu W.-S. , Lakshminarayana B.
Published: February 2008

A two-dimensional time-accurate Navier-Stokes solver for incompressible flows is used to simulate the effects of the axial spacing between an upstream rotor and a stator, and the wake/blade count ratio on turbomachinery unsteady flows. The code uses a pressure-based method. A low-Reynolds number two-equation turbulence model is incorporated to account for the turbulence effect. By computing cases with different spacing between an upstream rotor wake and a stator, the effect of the spacing is simulated. Wake/blade count ratio effect is simulated by varying the number of rotor wakes in one stator passage at the computational inlet plane. Results on surface pressure, unsteady velocity vectors, blade boundary layer profiles, rotor wake decay and loss coefficient for all the cases are interpreted. It is found that the unsteadiness in the stator blade passage increases with a decrease in the blade row spacing and a decrease in the wake/blade count ratio. The reduced frequency effect is dominant in the wake/blade count ratio simulation. The time averaged loss coefficient increases with a decrease in the axial blade row spacing and an increase in the wake/blade count ratio.
Multigrid Computations of Unsteady Rotor-Stator Interaction Using the Navier-Stokes Equations
Authors: Arnone A. , Pacciani R. , Sestini A.
Published: February 2008

A Navier-Stokes time-accurate solver has been extended to the analysis of unsteady rotor-stator interaction. In the proposed method, a fully-implicit time discretization is used to remove stability limitations. A four-stage Runge-Kutta scheme is used in conjunction with several accelerating techniques typical of steady-state solvers, instead of traditional time-expensive factorizations. Those accelerating strategies include local time stepping, residual smoothing, and multigrid. Direct interpolation of the conservative variables is used to handle the interfaces between blade rows. Two-dimensional viscous calculations of unsteady rotor-stator interaction in a modern gas turbine stage are presented to check for the capability of the procedure.
High-Frequency Heat Flux Sensor Calibration and Modeling
Authors: Holmberg D. G. , Diller T. E.
Published: February 2008

A new method of in-situ heat flux gage calibration is evaluated for use in convective facilities with high heat transfer and fast time response. A Heat Flux Microsensor (HFM) was used in a shock tunnel to simultaneously measure time-resolved surface heat flux and temperature from two sensors fabricated on the same substrate. A method is demonstrated for estimating gage sensitivity and frequency response from the data generated during normal transient test runs. To verify heat flux sensitivity, shock tunnel data are processed according to a one-dimensional semi-infinite conduction model based on measured thermal properties for the gage substrate. Heat flux signals are converted to temperature, and vice versa. Comparing measured and calculated temperatures allows an independent calibration of sensitivity for each data set. The results match gage calibrations performed in convection at the stagnation point of a free jet and done by the manufacturer using radiation. In addition, a finite-difference model of the transient behavior of the heat flux sensor is presented to demonstrate the first-order response to a step input in heat flux. Results are compared with shock passing data from the shock tunnel. The Heat Flux Microsensor recorded the heat flux response with an estimated time constant of 6 μs, which demonstrates a frequency response covering DC to above 100 kHz.
Free Surface Flow Profile and Fluctuations of a Circular Hydraulic Jump Formed by an Impinging Jet
Authors: Stevens J. W.
Published: February 2008

A fine wire probe was used to make quantitative measurements of the free surface profile and surface fluctuations around the hydraulic jump formed by a normally impinging free liquid jet. Representative magnitudes of both radial and axial fluctuations were presented for two nozzle sizes and several jet Reynolds numbers and subcritical flow depths. The results were compared to previous measurements of the supercritical flow depth and to theoretical predictions of the circular hydraulic jump size. The agreement appeared reasonable for the supercritical flow depth while the analytical expressions predicted a shorter hydraulic jump than that found by the measurements for the same supercritical flow conditions.
The Introduction of Micro Cells to Treat Pressure in Free Surface Fluid Flow Problems
Authors: Raad Peter E. , Chen Shea , Johnson David B.
Published: February 2008

A new method of calculating the pressure field in the simulation of two-dimensional, unsteady, incompressible, free surface fluid flow by use of a marker and cell method is presented. A critical feature of the new method is the introduction of a finer mesh of cells in addition to the regular mesh of finite volume cells. The smaller (micro) cells are used only near the free surface, while the regular (macro) cells are used throughout the computational domain. The movement of the free surface is accomplished by the use of massless surface markers, while the discrete representation of the free surface for the purpose of the application of pressure boundary conditions is accomplished by the use of micro cells. In order to exploit the advantages offered by micro cells, a new general equation governing the pressure field is derived. Micro cells also enable the identification and treatment of multiple points on the free surface in a single surface macro cell as well as of points on the free surface that are located in a macro cell that has no empty neighbors. Both of these situations are likely to occur repeatedly in a free surface fluid flow simulation, but neither situation has been explicitly taken into account in previous marker and cell methods. Numerical simulation results obtained both with and without the use of micro cells are compared with each other and with theoretical solutions to demonstrate the capabilities and validity of the new method.
Numerical Simulation of Vertical Forced Plume in a Crossflow of Stably Stratified Fluid
Authors: Hwang Robert R. , Chiang T. P.
Published: February 2008

In this study, an investigation using a three-dimensional numerical model, which treats conservation of mass, momentum, and salinity simultaneously, was carried out to study the character of a vertical forced plume in a uniform cross-stream of stably linear stratified environment. A k-ε turbulence model was used to simulate the turbulent phenomena and close the solving problem. The performance of the three-dimensional model is evaluated by comparison of the numerical results with some available experimental measurements. Results indicate that the numerical computation simulates satisfactorily the plume behavior in a stratified crossflow. The secondary vortex pairs in the cross section induced by the primary one change as the plume flows downstream. This denotes the transformation of entrainment mechanism in stratified crossflow.
On the Mechanism of Fluidelastic Instability of a Tube Placed in an Array Subjected to Two-Phase Crossflow
Authors: Joo Y. , Dhir V. K.
Published: February 2008

In this work, the fluidelastic instability of a tube placed in an array subjected to two-phase crossflow has been studied. For the determination of fluidelastic instability, acrylic tubes 2.2 cm or 2.37 cm in diameter and 20 cm in length were arranged in a triangular array with pitch to diameter ratio of 1.4 and 1.3. The test tube was flexibly supported with two cantilever beams. By installing cantilever beams horizontally and vertically, drag and lift direction tube vibration were studied. The total damping ratio is found to increase as the liquid gap velocity or void fraction increases. It has been found that the effective spring constant decreases rapidly with increase of liquid gap velocity or void fraction just prior to the onset of instability. For the range of parameters studied in this work, the onset of fluidelastic instability is associated with a rapid decrease of the effective spring constant because for a given applied force the amplitude of the tube vibration is inversely proportional to the spring constant.
Flow Patterns and Pressure Drop in Air/Water Two-Phase Flow in Horizontal Helicoidal Pipes
Authors: Awwad A. , Xin R. C. , Dong Z. F. , Ebadian M. A. , Soliman H. M.
Published: February 2008

An experimental investigation is conducted for air/water two-phase flow in horizontal helicoidal pipes. The helicoidal pipes are constructed of 25.4 mm I.D. Tygon tubing wrapped around cylindrical concrete forms with outside diameters of 62 cm and 124 cm. The helix angles of the helicoidal pipes vary from 1 to 20 deg. The experiments are performed for superficial water velocity in a range of UL = 0.008 ~ 2.2 m/s and for superficial air velocity in a range of UG = 0.2 ~ 50 m/s. The flow patterns are discerned and recorded photographically. The pressure drop of the air/water two-phase flow in the coils is measured and the Lockhart-Martinelli approach is used to analyze the data. The results are presented in the form of frictional pressure drop multipliers versus the Lockhart-Martinelli parameter. It was found that the flow patterns differ greatly from those of the straight pipe, and that the frictional pressure drop multipliers depend on both the Lockhart-Martinelli parameter and the flow rates. The correlation of the frictional pressure drop has been provided based on the current data. Furthermore, it was also found that the helix angle of the helicoidal pipe had almost no effect on the air/water two-phase flow pressure drop in the present experimental ranges.
Collaborative Testing of Turbulence Models
Authors: Bradshaw P. , Launder B. E. , Lumley J. L.
Published: February 2008

This is a brief report on a special project. Data from experiments and simulations were collected and distributed to over 100 turbulence modelers and other participants in 16 different countries. The modelers mailed the results of their “predictions” back to the organizers for distribution to all the participants for comment. The data covered most types of turbulent flow, compressible and incompressible, but the emphasis was on complex (strongly nonequilibrium) flows. No model was capable of predicting the full range of flows to good engineering accuracy, but stress-transport models seemed to perform best. The exchange of views and results alone has been of real benefit to modelers.
Large-Eddy Simulation of Turbulent Flow in a Curved Pipe
Authors: Boersma B. J. , Nieuwstadt F. T. M.
Published: February 2008

In this paper, we use Large-Eddy Simulation (LES) to compute a fully-developed turbulent flow in a curved pipe. The results allow us to study how the curvature influences the mean velocity profile and also various turbulent statistics. We find reasonable agreement with the few experiments that are available. Our simulation also allows a detailed study of secondary motion in the cross section of the pipe which are caused by the centrifugal acceleration due to the pipe curvature. It is known that this secondary motion may consist of one, two, or three circulation cells. In our simulation results we find one circulation cell.
Modeling Reynolds-Number Effects in Wall-Bounded Turbulent Flows
Authors: So R. M. C. , Aksoy H. , Yuan S. P. , Sommer T. P.
Published: February 2008

Recent experimental and direct numerical simulation data of two-dimensional, isothermal wall-bounded incompressible turbulent flows indicate that Reynolds-number effects are not only present in the outer layer but are also quite noticeable in the inner layer. The effects are most apparent when the turbulence statistics are plotted in terms of inner variables. With recent advances made in Reynolds-stress and near-wall modeling, a near-wall Reynolds-stress closure based on a recently proposed quasi-linear model for the pressure strain tensor is used to analyse wall-bounded flows over a wide range of Reynolds numbers. The Reynolds number varies from a low of 180, based on the friction velocity and pipe radius/channel half-width, to 15406, based on momentum thickness and free stream velocity. In all the flow cases examined, the model replicates the turbulence statistics, including the Reynolds-number effects observed in the inner and outer layers, quite well. Furthermore, the model reproduces the correlation proposed for the location of the peak shear stress and an appropriately defined Reynolds number, and the variations of the near-wall asymptotes with Reynolds numbers. It is conjectured that the ability of the model to replicate the asymptotic behavior of the near-wall flow is most responsible for the correct prediction of the Reynolds-number effects.
High Freestream Turbulence Effects on Turbulent Boundary Layers
Authors: Thole K. A. , Bogard D. G.
Published: February 2008

High freestream turbulence levels significantly alter the characteristics of turbulent boundary layers. Numerous studies have been conducted with freestreams having turbulence levels of 7 percent or less, but studies using turbulence levels greater than 10 percent have been essentially limited to the effects on wall shear stress and heat transfer. This paper presents measurements of the boundary layer statistics for the interaction between a turbulent boundary layer and a freestream with turbulence levels ranging from 10 to 20 percent. The boundary layer statistics reported in this paper include mean and rms velocities, velocity correlation coefficients, length scales, and power spectra. Although the freestream turbulent eddies penetrate into the boundary layer at high freestream turbulence levels, as shown through spectra and length scale measurements, the mean velocity profile still exhibits a log-linear region. Direct measurements of total shear stress (turbulent shear stress and viscous shear stress) confirm the validity of the log-law at high freestream turbulence levels. Velocity defects in the outer region of the boundary layer were significantly decreased resulting in negative wake parameters. Fluctuating rms velocities were only affected when the freestream turbulence levels exceeded the levels of the boundary layer generated rms velocities. Length scales and power spectra measurements showed large scale turbulent eddies penetrate to within y+ = 15 of the wall.