bluuebunny/crossref_metadata_2025 huggingface dataset:
Development of a Two-Dimensional Turbulent Wake in a Curved Channel With a Positive Streamwise Pressure Gradient
Authors: John J. , Schobeiri M. T.
Published: February 2008

The development of turbomachinery wake flows is greatly influenced by streamline curvature and streamwise pressure gradient. This paper is part of a comprehensive experimental and theoretical study on the development of the steady and periodic unsteady turbulent wakes in curved channels at different streamwise pressure gradients. This paper reports on the experimental investigation of the two-dimensional wake behind a stationary circular cylinder in a curved channel at positive streamwise pressure gradient. Measurements of mean velocity and Reynolds stress components are carried out using a X-hot-film probe. The measured quantities obtained in probe coordinates are transformed to a curvilinear coordinate system along the wake center line and are presented in similarity coordinates. The results show strong asymmetry in velocity and Reynolds stress components. The Reynolds stress components have higher values at the inner half of the wake than at the outer half of the wake. However, the mean velocity defect profiles in similarity coordinates are almost symmetric and follow the same Gaussian function for the straight wake data. A comparison with the wake development in a curved channel at zero streamwise pressure gradient suggests the decay rate of velocity defect is slower and the growth of wake width is faster for a positive streamwise pressure gradient.
Effect of Dispersed Phase on Modification of Turbulent Flow in a Wall Jet
Authors: Sato Yohei , Hishida Koichi , Maeda Masanobu
Published: February 2008

The interaction between dispersed particles and fluid turbulence for a vertical down-flow turbulent wall jet embedded in a uniform stream was investigated experimentally and numerically. Three kinds of size classified spherical particles, which were smaller than the Kolmogorov lengthscale of the flow, were dispersed in the jet upstream of the test section. The particle mass loading ratios were set at up to 0.3. Particle and gas-phase velocities were measured by laser Doppler velocimetry with particle size discrimination, and numerical simulations were carried out considering momentum exchange between both phases. Motion of small particles with Stokes number of around unity was influenced by strong shear in the developing region. Streamwise turbulence intensity was strongly attenuated by the addition of particles in the free shear layer region, while transverse turbulence intensity was suppressed in the fully-developed region of both the free and wall shear regions. Modifications of the mean fluid velocity by the particles induced reduction in the Reynolds stress, which alters the turbulence production. Turbulence modification by particles, with Stokes number of order of unity, is due primarily to the extra dissipation which is a function of particle mean concentration and fluid turbulence in the fully-developed region.
Effect of Nonaxisymmetric Forcing on a Swirling Jet With Vortex Breakdown
Authors: Gursul Ismet
Published: February 2008

Nonaxisymmetric forcing was applied to the vortex breakdown flowfield of a strongly swirling jet and its effects were investigated experimentally. It is shown that the spreading rate of the jet can be increased due to the excitation of the helical mode instability of the breakdown flowfield. This can be achieved for extremely low amplitudes of excitation.
The Experimental Investigation of Jet Fan Aerodynamics Using Wind Tunnel Modeling
Authors: Mutama K. R. , Hall A. E.
Published: February 2008

Jet fan aerodynamic behavior was investigated using wind tunnel modeling. Conditions were created to simulate mine and vehicular tunnel ventilation where these fans are finding increased application. Results showed that the ability of a jet fan to entrain air depends on its proximity to the tunnel wall. Moving the jet fan toward the wall increased the initial pressure drop below ambient in a significant length of the tunnel. This increased the volume of air entrained despite the existence of a large recirculation eddy or back flow whose size diminished as the jet fan was traversed toward the tunnel axis. When the jet fan was located at the tunnel axis the flow was very unstable close to the walls of the tunnel and it had a tendency to reverse itself with periods coinciding with the jet oscillation behavior. The complete set of measurements obtained are suitable for CFD code validation and modeling.
Friction Factor In Sine-Pipe Flow
Authors: Popiel C. O. , van der Merwe D. F.
Published: February 2008

Measurements of pressure losses in a sine-waved hydraulically smooth pipe, having a centerline described by the formula y = h sin (2πx/λ), are presented. The effect of the dimensionless wave-length (λ/d) and amplitude (h/d) on the Darcy friction factor was investigated in the range of the Reynolds number from about 100 to 10 000, and in the ranges of wavelength from λ/d = 17.7 to 150 and amplitude from h/d = 1.5 to 32. The Dean number, based on the minimum radius of the sine-waved centerline curvature, below which the influence of the sine-pipe shape in comparison to the straight tube was not seen is De ≤ ~20. A smooth transition from laminar to turbulent region in the friction factor versus Reynolds number plot was observed. In turbulent region the effect of the sine-pipe shape can be neglected for λ/d ≥ 1.36*h/d + 36.2.
Arbitrary Blade Section Design Based on Viscous Considerations. Background Information
Authors: Bouras B. , Karagiannis F. , Leoutsakos G. , Giannakoglou K. C. , Papailiou K. D.
Published: February 2008

Background information is presented on an arbitrary blade section design method which is outlined in a joint paper. This information concerns the assumptions, the development, and the predictive capabilities of the viscous flow calculation tool used in the design procedure. General properties of laminar and turbulent, unseparated or separated compressible shear layers, necessary for the blade optimization procedure, are discussed.
Arbitrary Blade Section Design Based on Viscous Considerations. Blade Optimization
Authors: Bouras B. , Karagiannis F. , Chaviaropoulos P. , Papailiou K. D.
Published: February 2008

A blade design and optimization procedure is presented in this work, which is based on viscous flow considerations. This procedure concerns the design of optimum rotating arbitrary compressible high subsonic compressor and turbine blade shapes. It takes into account the effects of wall curvature and Coriolis force on turbulence, while it allows the variation of stream surface radius, along which the blade shape is placed, as well as streamtube width, with meridional distance. In order to establish the inverse part of the viscous optimization procedure, aspects such as laminar stability, transition, optimum deceleration and, more generally, the behaviour of compressible attached and separated shear layers are discussed. A plane on which all the general properties of the compressible laminar and turbulent shear layers appear, is constructed and the generation of optimum shear layers for the critical side of the blade shape is established. The complete optimization (design) procedure is then described and discussed, while various designs realized by the present procedure are presented at the end of this paper.
Gas-Liquid Bubbly Flow in Vertical Pipes
Authors: Nakoryakov V. E. , Kashinsky O. N. , Randin V. V. , Timkin L. S.
Published: February 2008

Gas-liquid bubbly flow was investigated in vertical pipes for different flow conditions: fully developed turbulent downward flow in a 42.3 mm diameter pipe and upward flow in a 14.8 mm diameter pipe with liquid of elevated viscosity. Wall shear stress, local void fraction, and liquid velocity profiles, shear stress, and velocity fluctuations were measured using an electrodiffusional method. Results obtained demonstrate the existence of “universal” near-wall velocity distribution in a downward bubbly flow. The reduction of turbulent fluctuations is observed in downward flow as compared to a single-phase turbulent flow. The development of bubble-induced liquid velocity fluctuations in a “laminar” bubbly flow was studied.
Experimental and Computational Assessment of Windage Losses in Rotating Machinery
Authors: Wild P. M. , Djilali N. , Vickers G. W.
Published: February 2008

A combined experimental and computational investigation of the flow between a rotating cylinder and a fixed enclosure is presented. The configuration considered is related to the design of a centrifugal desalinator, and includes the flow in the annular as well as in the axial gap regions. The computed flowfield shows significant variations in the axial distribution of the azimuthal shear stress due to the secondary flow associated with Taylor vortices. The averaged azimuthal shear stress (or torque) is however not very sensitive to the number of vortices. Computational results also show that, where the aspect ratio of the annulus (rotor length to radial gap width) is relatively small, this azimuthal variation results in a higher average azimuthal shear stress than for the case where the aspect ratio of the annulus is relatively large. Various functional relations for windage torque of infinite cylinders, developed by other researchers on the basis of power law relations or assuming a log-law velocity distribution, are evaluated. Three such relations are found to be in good agreement with the experimental results when coupled with a correction for end effects. These relations are also the most useful with respect to the design of rotating equipment.
Unstable Flow in Centrifugal Fans
Authors: Chen P. , Soundra-Nayagam M. , Bolton A. N. , Simpson H. C.
Published: February 2008

Rotating stall and the inlet vortex in centrifugal fans with inlet vane control has been studied. The advances in stall research in aero-engine compressors are discussed. The present study shows that stall in centrifugal fans can be quite different from that in axial compressors, in that stall can occur in a progressive and intermittent fashion. The study also shows that a discontinuity in the fan characteristic is not necessarily accompanied by rotating stall, unlike the axial machines. Experimental results indicate that the positive prewhirl created by inlet vanes tends to delay the occurrence of stall. Also, dorsal fin devices that are used to control the inlet vortex do not seem to affect the stall point unfavorably. The inlet vortex frequency was found to invariably exhibit a linear relation with the flow rate even when dorsal fins were used. This offers a practical method to distinguish between the inlet vortex and rotating stall.
Deformation and Rebounding Processes of a Water Droplet Impinging on a Flat Surface Above Leidenfrost Temperature
Authors: Fujimoto Hitoshi , Hatta Natsuo
Published: February 2008

This paper treats numerical analyses of the deformation and rebounding processes of a water droplet impinging on a flat solid surface above the Leidenfrost temperature with a speed in the order of a few [m/s], as well as the flow field inside the droplet. These calculations were performed using the MAC-type solution method to solve a finite differencing approximation of the axisymmetric Navier-Stokes equations governing incompressible fluid flows. Also, the whole dynamic process of a droplet from the moment of collision with a hot surface including the rebound from it was recorded by using a video camera equipped with a macro lens. First, the water film formed by the droplet impinging on the surface spreads radially in a fairly thin discoid-like shape until it reaches a maximum. Next, the water film begins to recoil backwards towards the center and the recoiling process continues to occur owing to the surface tension effect at the periphery. Subsequently, the center part of the liquid drop begins to elongate upwards and the liquid near the top of the drop pulls up the lower part of the remaining liquid. Finally, a vortical ring structure appearing at the bottom of the elongated droplet induces the rotative motion in such a way as to form the rising flow and the droplet rebounds from the surface as a bowling pin-shaped mass. The numerical model to predict the deformation and rebounding processes was built up by accounting for the presence of viscous and surface tension effects. The numerical results obtained by the model were compared with the experimental data and discussed from a practical point of view.
Buoyant Convection During the Growth of Compound Semiconductors by the Liquid-Encapsulated Czochralski Process With an Axial Magnetic Field and With a Non-Axisymmetric Temperature
Authors: Ma Nancy , Walker J. S.
Published: February 2008

This paper treats the buoyant convection of a molten semiconductor in a cylindrical crucible with a vertical axis, with a uniform vertical magnetic field, and with a non-axisymmetric temperature. Most previous treatments of melt motions with vertical magnetic fields have assumed that the temperature and buoyant convection were axisymmetric. In reality, the temperature and resultant buoyant convection often deviate significantly from axisymmetry. For a given non-axisymmetric temperature, the electromagnetic suppression of the axisymmetric part of the buoyant convection is stronger than that of the non-axisymmetric part, so that the deviation from an axisymmetric melt motion increases as the magnetic field strength is increased. The non-axisymmetric part of the buoyant convection includes relatively strong azimuthal velocities adjacent to the electrically insulating vertical crucible wall, because this wall blocks the radial electric currents needed to suppress azimuthal velocities.
Modeling Hydrodynamic Nonequilibrium in Cavitating Flows
Authors: Chen Yongliang , Heister S. D.
Published: February 2008

A nonlinear numerical model has been developed to assess nonequilibrium effects in cavitating flows. The numerical implementation involves a two-phase treatment with the use of a pseudo-density which varies between the liquid and gas/vapor extremes. A new constitutive equation for the pseudo-density is derived based on the bubble response described by a modified form of the Rayleigh-Plesset equation. Use of this constitutive equation in a numerical procedure permits the assessment of nonequilibrium effects. This scheme provides a quantitative description of scaling effects in cavitated flows. With minimal modifications, the model can also be used for bubbly two-phase flows.
Calculations of Wall Shear Stress in Harmonically Oscillated Turbulent Pipe Flow Using a Low-Reynolds-Number k-ε Model
Authors: Ismael J. O. , Cotton M. A.
Published: February 2008

The low-Reynolds-number k-ε turbulence model of Launder and Sharma is applied to the calculation of wall shear stress in spatially fully-developed turbulent pipe flow oscillated at small amplitudes. It is believed that the present study represents the first systematic evaluation of the turbulence closure under consideration over a wide range of frequency. Model results are well correlated in terms of the parameter ω+ = ωv/Uτ2 at high frequencies, whereas at low frequencies there is an additional Reynolds number dependence. Comparison is made with the experimental data of Finnicum and Hanratty.
Influences of Fluidic Interfaces on the Formation of Fine Scale Structure by Chaotic Mixing
Authors: Zhang D. F. , Zumbrunnen D. A.
Published: February 2008

A numerical model has been developed of two-dimensional chaotic mixing of immiscieble Newtonian fluids. A computationally efficient numerical methodology is employed which is well-suited to complex, evolving interfaces. Mixing was confined to a rectangular cavity with periodically driven upper and lower surfaces. Interfacial forces and the transient flow fields in each phase were considered to assess specifically the influences on interfacial morphology of interfacial tension and phase viscosity ratio under creeping flow conditions. Predicted morphologies are compared to those of solidified specimens synthesized by chaotic mixing in companion studies.
Flows Generated by the Impingement of a Jet on a Rotating Surface: Part I—Basic Flow Patterns
Authors: Brodersen S. , Metzger D. E. , Fernando H. J. S.
Published: February 2008

The results of an experimental study of the flow field resulting from the interaction between an impinging jet and a rotating disk are presented. The resulting flow configuration has applications in turbomachinery, for example, to intensify the local heat transfer at turbine disks. The experiments cover separate measurements of the disk-wall flow, the jet flow and interaction between the two. The flow patterns are investigated over a range of jet Reynolds numbers Rejet = 0.66-104 - 6.80-104 (based on jet diameter) and disk Reynolds numbers Redisk 3.4·105 − 6.2· 105 (based on impingement radius), achieved by varying the jet nozzle diameter, jet flow rate, rotational disk speed and the impingement radius. The measurements included the depth of jet penetration into the wall boundary layer, the travel distance of the jet against the direction of disk rotation, and the turbulent and mean velocity distribution. Another objective of this study concerns the type of flow at the impingement region (jet dominated flow or rotation dominated flow) and the conditions for the transition from one to the other. In the first part of the study, the flow structure of the jet and disk flows as well as the three-dimensional flow fields resulting from the interaction are presented.
Numerical Study of an Unsteady Confined Jet. Application to Crossflow Filtration
Authors: Maranges C. , Arroyo G. , Fonade C. , Estivalezes J. L.
Published: February 2008

This paper deals with the influence of a new flow unsteadiness on the permeate flux in crossflow filtration. A pneumatically controlled valve generates intermittent jets from the main flow, leading to the formation of large vortices moving downstream along the tubular membrane. A numerical calculation of this flow is achieved using a finite difference scheme. Influences of tube diameter and jet velocities are discussed. Application of this technique is carried out by filtering a bentonite suspension through an ultrafiltration membrane.
Exponential-Lagrangian Tracking Schemes Applied to Stokes Law
Authors: Barton I. E.
Published: February 2008

The exponential-Lagrangian tracking scheme applied to Stokes Law is developed by introducing a predictor-corrector formulation. The new predictor-corrector schemes are more accurate than the original scheme and are estimated to give a better performance taking into account the increased computational effort. The schemes are tested on two simple problems and the results are compared with the analytical solutions.
Unsteady Heat Transfer From a Sphere at Small Peclet Numbers
Authors: Feng Zhi-Gang , Michaelides E. E.
Published: February 2008

In a recent paper by Michaelides and Feng (1994) it was discovered that there are history terms in the unsteady heat conduction equation from a small sphere. The history terms are analogous to the terms found in the equation of motion of the sphere, which sometimes are called “Basset terms.” An analysis is presented here for the unsteady heat transfer from a sphere, when its surface temperature undergoes a step change. The singular perturbation technique is used to obtain short and longtime solutions, first in the vicinity of the sphere and then far from the sphere. The solution obtained is for finite but low Peclet numbers. It appears that in the case of the step temperature, change, the history terms are reduced to an analytical expression of the error function.
REVIEW: MEMS and Its Applications for Flow Control
Authors: Ho Chih-Ming , Tai Yu-Chong
Published: February 2008

Emerging micromachining technology enables us to fabricate mechanical parts on the order of micron size. It provides us with micro-sensors and micro-actuators which facilitate the exploration of all areas of science. Furthermore, these miniature transducers can be integrated with microelectronics. With an integrated system, it then becomes possible to complete the loop of sensing, information processing, and actuation. This type of system enables us to perform real-time control of time varying events which are common in fluid dynamics. In this review paper, we will first briefly introduce Micro-Electro-Mechanical-Systems (MEMS) technology. Then, the applications of MEMS to flow control will be discussed.
Small Silicon Pressure Transducers for Space-Time Correlation Measurements in a Flat Plate Boundary Layer
Authors: Lo¨fdahl L. , Ka¨lvesten E. , Stemme G.
Published: February 2008

Silicon based pressure sensors have been used to measure turbulent wall-pressure fluctuations in a two-dimensional flat plate boundary layer, Reθ = 5072. The side lengths of the diaphragms were 100 μm (d+ = 7.2) and 300 μm (d+ = 21.6), giving a ratio of the boundary layer thickness to the diaphragm side length of the order of 240 and a resolution of eddies with wave numbers less than ten viscous units. Power spectra were measured for the frequency range 13 Hz < f < 13 kHz. Scaled in outer and inner variables a clear overlap region between the mid and high frequency parts of the spectrum is shown. In this overlap region the slope was found to be ω−1, while in the high frequency part it was ω−5. Correlation measurements in both the longitudinal and transversal directions were performed and compared to other investigations. Longitudinal space time correlations, including the high frequency range, indicated an advection velocity of the order of half the freestream velocity. A broad band filtering of the longitudinal correlation showed that the high frequency part of the spectrum is associated with the smaller eddies from the inner part of the boundary layer, resulting in a reduction of the correlation.
Numerical Modeling of Micromechanical Devices Using the Direct Simulation Monte Carlo Method
Authors: Piekos E. S. , Breuer K. S.
Published: February 2008

A direct simulation Monte Carlo (DSMC) investigation of flows related to microelectromechanical systems (MEMS) is detailed. This effort is intended to provide tools to facilitate the design and optimization of micro-devices as well as to probe the effects of rarefaction, especially in regimes not amenable to other means of analysis. The code written for this purpose employs an unstructured grid, a trajectory-tracing particle movement scheme, and an “infinite channel” boundary formulation. Its results for slip-flow and transition regime micro-channels and a micro-nozzle are presented to demonstrate its capabilities.
Nonlinear Stability of the Thin Micropolar Liquid Film Flowing Down on a Vertical Plate
Authors: Hung Chen-I , Tsai Jung-Shun , Chen Cha’o-Kuang
Published: February 2008

A perturbation method is used to investigate analytically the nonlinear stability behavior of a thin micropolar liquid film flowing down a vertical plate. In this analysis, the conservation of mass, momentum, and angular momentum are considered and a corresponding nonlinear generalized kinematic equation for the film thickness is thereby derived. Results show that both the supercritical stability and the subcritical instability can be found in the micropolar film flow system. This analysis shows that the effect of the micropolar parameter R(=κ/μ) is to stabilize the film flow, that is, the stability of the flowing film increases with the increasing magnitude of the micropolar parameter R. Also, the present analysis shows that the micropolar coefficients, Δ(=h02/j) and Λ(=γ/μj), have very little effects on the stability of the micro-polar film.
Low Pressure Differential Discharge Characteristics of Saturated Liquids Passing Through Orifices
Authors: Rohloff T. J. , Catton I.
Published: February 2008

An experimental investigation has been performed to determine the effect of variation in the length-to-diameter ratios on the discharge characteristics of saturated liquids passing through square edge orifices subjected to low pressure differentials. Experiments were performed to confirm reported results for sharp edge orifices and for round edge orifices with appreciable ratios of inlet corner radius to orifice diameter.
Free-Surface Wave-Induced Separation
Authors: Zhang Z. J. , Stern F.
Published: February 2008

Free-surface wave-induced separation is studied for a surface-piercing NACA 0024 foil over a range of Froude numbers (0, .2, .37, .55) through computational fluid dynamics of the unsteady Reynolds-averaged Navier-Stokes and the continuity equations with the Baldwin-Lomax turbulence model, exact nonlinear kinematic and approximate dynamic free-surface boundary conditions, and a body/free-surface conforming grid. The flow conditions and uncertainty analysis are discussed. A topological rule for a surface-piercing body is derived and verified. Steady-flow results are presented and analyzed with regard to the wave and viscous flow and the nature of the separation.
Angular Bias Errors in Three-Component Laser Velocimeter Measurements
Authors: Chen Chao-Yi , Kim P. J. , Walker D. T.
Published: February 2008

For three-component laser velocimeter systems, the change in projected area of the coincident measurement volume for different flow directions will introduce an “angular” bias in naturally sampled data. In this study, the effect of turbulence level and orientation of the measurement volumes on angular bias errors was examined. The operation of a typical three-component laser velocimeter was simulated using a Monte Carlo technique. Results for the specific configuration examined show that for turbulence levels less than 10 percent no significant bias errors in the mean velocities will occur and errors in the root-mean-square (r.m.s.) velocities will be less than 3 percent for all orientations. For turbulence levels less than 30 percent, component mean velocity bias errors less than 5 percent of the mean velocity vector magnitude can be attained with proper orientation of the measurement volume; however, the r.m.s. velocities may be in error as much as 10 percent. For turbulence levels above 50 percent, there is no orientation which will yield accurate estimates of all three mean velocities; component mean velocity errors as large as 15 percent of the mean velocity vector magnitude may be encountered.
Particle-Surface Interactions in Heat Exchanger Fouling
Authors: Bouris D. , Bergeles G.
Published: February 2008

The problem of fouling is of vital importance to heat exchanger efficiency and should be considered during the design phase of the heat exchanger. The purpose of the present paper is to introduce a novel method to aid in the evaluation of the various parameters that affect the fouling phenomenon. The method considers the particle-surface interaction from the energy balance at the point of impaction and takes into consideration the material properties of the particle and surface as well as the effect of the surrounding flow field on the particle movement and impaction. The calculated deposition flux is used to form the deposit evolution in time considering the removal mechanisms resulting from fluid shear stress and the eroding impacts of the particles. The model is validated against experimental measurements of particle deposition from a two phase flow of hot gases around a circular cylinder. The experimentally measured flow field around a staggered tube bundle is also predicted and a fouling analysis is performed regarding the validity of fouling studies in scaled down model configurations. It is concluded that the actual dimensions of the heat exchanger configuration are of vital importance to its fouling behavior and scaling laws are difficult to apply.
Possibility of Quantitative Prediction of Cavitation Erosion Without Model Test
Authors: Kato Hiroharu , Konno Akihisa , Maeda Masatsugu , Yamaguchi Hajime
Published: February 2008

A scenario for quantitative prediction of cavitation erosion was proposed. The key value is the impact force/pressure spectrum on a solid surface caused by cavitation bubble collapse. As the first step of prediction, the authors constructed the scenario from an estimation of the cavity generation rate to the prediction of impact force spectrum, including the estimations of collapsing cavity number and impact pressure. The prediction was compared with measurements of impact force spectra on a partially cavitating hydrofoil. A good quantitative agreement was obtained between the prediction and the experiment. However, the present method predicted a larger effect of main flow velocity than that observed. The present scenario is promising as a method of predicting erosion without using a model test.
Measurements of the Velocity Field Downstream of an Impeller
Authors: Petersson Per , Larson Magnus , Jo¨nsson Lennart
Published: February 2008

The velocity field downstream of a model impeller operating in water was measured using a two-component laser doppler velocimeter. The investigation focussed on the spatial development of the mean velocity in the axial, radial, and circumferential direction, although simultaneous measurements were performed of the velocity unsteadiness from which turbulence characteristics were inferred. The measurements extended up to 12 impeller diameters downstream of the blades displaying the properties of the generated swirling jet both in the zone of flow establishment and the zone of established flow. The division between these zones was made based on similarity of the mean axial velocity profile. Integral properties of the flow such as volume and momentum flux were computed from the measured velocity profiles. The transverse spreading of the impeller jet and its development towards self-similarity were examined and compared with non-swirling jets and swirling jets generated by other means.
Stokes Flow Through a Transversely Finned Channel
Authors: Wang C. Y.
Published: February 2008

The efficient method of eigenfunction expansion and point match is used to solve the Stokes flow through a channel with transverse fins. Both in-phase and staggered fins are considered. Streamlines and resistances are found in terms of fin height and fin spacing. Extrapolating to large spacings, the added resistances due to a single pair of aligned fins and that of a single fin in a channel are obtained.
Computation of Unsteady Viscous Marine-Propulsor Blade Flows—Part 1: Validation and Analysis
Authors: Paterson E. G. , Stern F.
Published: February 2008

In this two-part paper, time-accurate solutions of the Reynolds-averaged Navier-Stokes equations are presented, which address through model problems, the response of turbulent propeller-blade boundary layers, and wakes to external-flow traveling waves. In Part 1, the Massachusetts Institute of Technology flapping-foil experiment is simulated and the results validated through comparisons with data. The physics of unsteady blade flows are shown to be complex with analogy to Stokes layers and are explicated through visualization and Fourier analysis. It is shown that convection induced steady/unsteady interaction causes deformation of the external-flow waves and is responsible for the upstream- and downstream-traveling pressure-gradient waves over the foil and in the wake, respectively. The nature of the unsteady displacement thickness suggests viscous-inviscid interaction as the mechanism for the response. In Part 2, a parametric study is undertaken to quantify the effects of frequency, foil geometry, and waveform.
Review: Hot-Wire Anemometry in Transonic and Subsonic Slip Flows
Authors: Stainback P. C. , Nagabushana K. A.
Published: February 2008

In the past, only a relatively small number of investigations were conducted using hot-wire anemometry in transonic and subsonic slip flows. This was due to the complexity of the applicable equations. Now it appears that the developments in electronics and computational methods have made it practical to reconsider this problem. A review of past work performed in these flow regimes using hot-wire anemometry will be made along with a description of recent efforts made to develop methods which might be used to make measurements in these flow regimes. A detailed version of this review article is available through the electronic JFE.
A Capacitance Sensor for Two-Phase Liquid Film Thickness Measurements in a Square Duct
Authors: Thorncroft G. E. , Klausner J. F.
Published: February 2008

The use of capacitance sensors for measuring liquid film thickness or phase concentration in two-phase flow has gained popularity in recent years. In designing such sensors, there are many issues which must be considered in order to optimize performance: desired temporal and spatial resolution, two-phase flow regime, permittivity of the phases, duct geometry, electrical shielding, and temperature variation in the flow field. These issues are discussed, and the design of a 12.7 mm square cross section capacitance sensor which measures liquid film thickness in either stratified are annular two-phase flow is presented. Using a composite material analysis and an effective permittivity ratio, predictive relations for capacitance as a function of liquid film thickness have been derived for stratified and annular film patterns. The analysis eliminates the need for calibrating the sensor for stratified and annular flow regimes. Optical measurements of liquid film thickness using a high resolution CCD camera are compared against those using the capacitance sensor in conjunction with the predictive relations. The sensor was tested on a bench top for a stratified film pattern with no flow and two different electrode configurations (upwards and side configurations) using FC-87, a low-permittivity (εr = 1.72) dielectric fluid. The standard deviations between the film thicknesses measured optically and those predicted using the capacitance sensor and analysis are 0.014 and 0.019 mm for the respective upward and side electrode configurations. The sensor was also implemented in a vertical flow boiling facility, which uses FC-72 (εr = 1.75) as the working fluid. Time-averaged film thicknesses measured using the capacitance sensor are compared against ensemble-averaged measurements using the CCD camera for annular vertical upflow and downflow. The upflow and downflow standard deviations are 0.17 and 0.093 mm, respectively. As expected, the agreement for vertical flow is not as good as that for the horizontal no-flow case, because large fluctuations in film thickness are characteristic of annular two-phase flow, and the uncertainty of the photographic measurement is increased.
Turbulent Dispersion of Heavy Particles With Nonlinear Drag
Authors: Mei Renwei , Adrian R. J. , Hanratty T. J.
Published: February 2008

The analysis of Reeks (1977) for particle dispersion in isotropic turbulence is extended so as to include a nonlinear drag law. The principal issue is the evaluation of the inertial time constants, βα−1, and the mean slip. Unlike what is found for the Stokesian drag, the time constants are functions of the slip velocity and are anisotropic. For settling velocity, VT, much larger than root-mean-square of the fluid velocity fluctuations, u0, the mean slip is given by VT. For VT→0, the mean slip is related to turbulent velocity fluctuation by assuming that fluctuations in βα are small compared to the mean value. An interpolation formula is used to evaluate βα and VT in regions intermediate between conditions of VT→0 and VT≫ u0. The limitations of the analysis are explored by carrying out a Monte-Carlo simulation for particle motion in a pseudo turbulence described by a Gaussian distribution and Kraichnan’s (1970) energy spectrum.
On End-Wall Corner Vortices in a Lid-Driven Cavity
Authors: Chiang T. P. , Hwang Robert R. , Sheu W. H.
Published: February 2008

We conducted a flow simulation to study the laminar flow in a three-dimensional rectangular cavity. The ratio of cavity depth to width is 1:1, and the span to width aspect ratio (SAR) is 3:1. The governing equations defined on staggered grids were solved in a transient context by using a finite volume method, in conjunction with a segregated solution algorithm. Of the most apparent manifestation of three-dimensional characteristics, we addressed in this study the formation of corner vortices and its role in aiding the transport of fluid flows in the primary eddy and the secondary eddies.
Oscillatory Momentum Transport Mechanisms in Transitional Complex Geometry Flows
Authors: Majumdar D. , Amon C. H.
Published: February 2008

This work reports direct numerical simulations of transitional flows in communicating channels. Above a critical Reynolds number, the flow becomes fluctuating and self-sustained with vortical motions temporally synchronized with channel traveling waves. The energy transfer mechanism between the mean and the fluctuating flow is investigated along with the distributions of oscillatory shear stress and transitional viscosity. The kinetic energy equation for the fluctuating velocity is solved from DNS data to evaluate the contributions of the production term, viscous dissipation, work of dynamic pressure and work of viscous shear stresses.
Boundary Layer Flow About and Inside a Liquid Sphere
Authors: El-Shaarawi Maged A. I. , Al-Farayedhi Abdulghani , Antar Mohamed A.
Published: February 2008

A finite-difference scheme has been developed to solve the boundary-layer equations governing laminar flows around and inside a spherical fluid droplet moving steadily in another immiscible fluid. Using this scheme, results not available in the literature have been obtained for circulating droplets at intermediate and high interior-to-exterior viscosity ratios (μ*) and large values of the external flow Reynolds number (Re). Detailed results over the range 1.01 ≤ μ* ≤ ∞ (solid sphere) and 100 ≤ Re ≤ 10000 are presented for the velocity profiles outside and inside the droplet, the interface shear stress, the external flow separation angle, the droplet surface velocity and the drag coefficient.
Effects of Bulk Flow Pulsations on Film-Cooled Boundary Layer Structure
Authors: Ligrani P. M. , Gong R. , Cuthrell J. M. , Lee J. S.
Published: February 2008

Experimental results are presented which describe the effects of bulk flow pulsations on film cooled boundary layer structure. The film is produced by a single row of simple angle film cooling holes and the pulsations are in the form of sinusoidal variations of static pressure and streamwise velocity. Such pulsations are important in turbine studies because: (i) static pressure pulsations result in significant periodic variations of film cooling flow rates, coverage, and trajectories, and (ii) static pressure pulsations occur near blade surfaces in operating engines from passing shock waves and potential flow interactions between moving blade rows. Distributions of ensemble-averaged and time-averaged Reynolds stress tensor components are presented for x/d of 4.5, 9.8, 16.4, and 24.1 along with distributions of streamwise mean velocity and streamwise mean vorticity, where x is streamwise distance from the downstream edge of the holes and d is hole diameter. Important changes from the imposed bulk flow pulsations are evident in all measured quantities, especially just downstream of the holes at x/d = 4.5. Here, Maximum Reynolds shear stresses −2u′v′/u∞2 are reduced by the pulsations in regions containing the largest film concentrations. This is because the shear layer produced by the injectant oscillates its position as each pulsations is imposed. This causes the shear layer to become more diffused as it is spread over a larger spatial volume.
On the Slightly Reduced Navier-Stokes Equations
Authors: Xu Jian-Zhong , Yu Wen-Sheng
Published: February 2008

In this paper the so-called slightly reduced Navier-Stokes (SRNS) equations with most streamwise viscous diffusion and heat conduction terms are investigated in detail. It is proved that the SRNS equations are hyperbolic-parabolic in mathematics, which is the same as the current RNS or PNS equations. The numerical methods for solving the RNS equations are, therefore, applicable to the present SRNS equations. It is further proved that the SRNS equations have a uniformly convergent solution with accuracy of 0 (ε2) or 0 (Re−1) which is higher than that of the RNS equations, and for a laminar flow past a flat plate the SRNS solution is regular at the point of separation and is a precise approximation to that of the complete Navier-Stokes equations. The numerical results demonstrate that the SRNS equations may give accurate picture of the flow and are an effective tool in analyzing complex flows.
Numerical Analysis of Unsteady Flow in the Weis-Fogh Mechanism by the 3D Discrete Vortex Method With GRAPE3A
Authors: Ro Kideok , Tsutahara Michihisa
Published: February 2008

The three-dimensional flows in the Weis-Fogh mechanism are studied by flow visualization and numerical simulation by a discrete vortex method. In this mechanism, two wings open, touching their trailing edges (fling), and rotate in opposite directions in the horizontal plane. At the “fling” stage, the flow separates at the leading edge and the tip of each wing. Then they rotate, and the flow separates also at the trailing edges. The structure of the vortex systems shed from the wings is very complicated and their effect on the forces on the wings have not yet been clarified. Discrete vortex method, especially the vortex stick method, is employed to investigate the vortex structure in the wake of the two wings. The wings are represented by lattice vortices, and the shed vortices are expressed by discrete three-dimensional vortex sticks. In this calculation, the GRAPE3A hardware is used to calculate at high speed the induced velocity of the vortex sticks and the viscous diffusion of fluid is represented by the random walk method. The vortex distributions and the velocity field are calculated. The pressure is estimated by the Bernoulli equation, and the lift and moment on the wing are also obtained. However, the simulations, especially those for various Reynolds numbers, should be treated with caution, because there is no measurement to compare them with and the discrete vortex method is approximate due to rudimentary modeling of viscosity.
Review—The Transient Equation of Motion for Particles, Bubbles, and Droplets
Authors: Michaelides E. E.
Published: February 2008

The development, form, and engineering applications of the transient equation of motion of rigid particles, bubbles, and droplets are presented. Some of the early work on the equation of motion, as well as recent advances, are exposed. Particular emphasis is placed on the semiempirical forms of the equation, which are widely used in engineering practice. The creeping flow assumption, on which most of the known applications are based, is critically examined and its limitations are pointed out. Recent results on particle flow, which include the effect of the advection of a downstream wake and are applicable to finite (but small) Reynolds numbers are also presented. The form of the history (Basset) term is discussed, in the light of recent work and its effect on the integrated results of the equation of motion is examined. Recommendations are given on the appearance, importance, and significance of the history and added mass terms for those who may use the semiempirical form of the transient equation of spheres in a differential or integrated form.
Experimental Study of Turbulent Concentration Flow Field in the Wake of a Bluff Body
Authors: Balachandar R. , Chu Vincent H. , Zhang Jianbo
Published: February 2008

Experiments were conducted to determine the turbulent concentration field in the wake of a normal flat plate. Dye was introduced behind the plate as the tracer. The measurement of the dye concentration was carried out using an optical colorimeter in the near wake, covering a distance of five to thirty plate widths downstream of the body. The mean, r.m.s., and intermittency profiles of the concentration field were obtained at a number of cross sections. Phase averaging was also carried out to determine the structure of the large scale eddies. Typical maximum concentration levels in the core of the eddies were found to be two to four times larger than the corresponding mean values. The widths of the concentration profiles in the near wake are 1.5 to 2.5 times wider than the corresponding widths of the velocity profiles. The present dye concentration data obtained for the normal flat plate are unique and complement a small number of previous experimental investigations of the scalar field based on temperature measurements in the wake of circular cylinders.
The Use of Pattern Recognition and Proper Orthogonal Decomposition in Identifying the Structure of Fully-Developed Free Turbulence
Authors: Kopp G. A. , Ferre´ J. A. , Giralt Francesc
Published: February 2008

The eigenvectors obtained from proper orthogonal decomposition (POD) are used as the selection criteria to classify the individual events contained in date files of two-component velocity signals recorded in the non-homogeneous (vertical) and homogeneous (horizontal) planes of a fully-developed turbulent cylinder wake. This procedure uses the dominant eigenvectors from POD as initial templates to perform a pattern recognition (PR) analysis of the signals so that the individual coherent events appearing randomly in the signals can be educed more objectively. It is found that the prototype or ensemble average of the group of events classified as the large-scale structure in the vertical plane has a circulatory motion with strong negative streamwise and outward lateral velocity fluctuations. In the horizontal plane, the average structure is a double roller with negative streamwise velocity fluctuations in its centerplane. The class of instantaneous events selected contribute significantly to the variance in the outer intermittent region, but much less in the fully turbulent core. While the first eigenvector from POD captures some of the features shown by this prototype, it is not enough to depict by itself all the characteristics shown by the ensemble average of the repetitive three-dimensional instantaneous events that occur in the fully-developed turbulent wake.
Aero-Thermodynamic Loss Analysis in Cases of Normal Shock Wave-Turbulent Shear Layer Interaction
Authors: Kaldellis J. K.
Published: February 2008

Transonic-supersonic decelerating flow cases appearing in modern turbomachines are some of the most complex flow cases in fluid mechanics which also present practical interest. In the present work, a closed and coherent shock loss model is proposed based on the complete viscous flow simulation using some fast and reliable computational tools. The resulting model describes accurately the entropy rise and the total pressure loss in cases of strong shock-shear layer interaction and cancels the need to use low speed correlations modified for compressibility effects and extrapolated to transonic-supersonic flow cases. The accuracy and the reliability of the proposed shock-loss model are verified using detailed experimental data concerning various flow cases which present either flow separation or industrial interest.
Transient Fluid-Structure Interaction in a Control Valve
Authors: Kerh Tienfuan , Lee J. J. , Wellford L. C.
Published: February 2008

Numerical investigation on the interaction of a viscous incompressible fluid with a control valve is conducted by using the finite element method and Newmark approach. Variables including displacement, velocity, and acceleration are presented as a function of time to describe movements of the solid system. The velocity vectors, pressure variations, pressure loss coefficients, and energy losses are displayed to reveal the resulting flow fields. It is observed that the motion of the structural element which becomes a moving boundary had a significant influence on the flow fields. In a periodic inlet flow, the motion of the structural system and the resulting flow fields were almost identical for each cycle after the third cycle. The simulated results presented in this paper provide a good description of the induced flow field in a piping system under the effect of a pulsating piston or a wave.