bluuebunny/crossref_metadata_2025 huggingface dataset:
Segmented Domain Decomposition Multigrid Solutions for Two and Three-Dimensional Viscous Flows
Authors: Srinivasan Kumar , Rubin Stanley G.
Published: May 2008

Several viscous incompressible two and three-dimensional flows with strong inviscid interaction and/or axial flow reversal are considered with a segmented domain decomposition multigrid (SDDMG) procedure. Specific examples include the laminar flow recirculation in a trough geometry and in a three-dimensional step channel. For the latter case, there are multiple and three-dimensional recirculation zones. A pressure-based form of flux-vector splitting is applied to the Navier-Stokes equations, which are represented by an implicit, lowest-order reduced Navier-Stokes (RNS) system and a purely diffusive, higher-order, deferred-corrector. A trapezoidal or box-like form of discretization insures that all mass conservation properties are satisfied at interfacial and outflow boundaries, even for this primitive-variable non-staggered grid formulation. The segmented domain strategy is adapted herein for three-dimensional flows and is extended to allow for disjoint subdomains that do not share a common boundary.
An Integral Method for Turbulent Boundary Layers on Rotating Disks
Authors: Abrahamson S. , Lonnes S.
Published: May 2008

An integral method for computing turbulent boundary layers on rotating disks has been developed using a power law profile for the tangential velocity and a new model for the radial profile. A similarity solution results from the formulation. Radial transport, boundary layer growth, and drag on the disk were computed for the case of a forced vortex frees tream flow. The results were compared to previous similarity solutions. The method was extended to a Rankine vortex freestream flow. Differential equations for boundary layer parameters were developed and solved for different Reynolds numbers to look at the net entrainment, boundary layer growth, and drag on the disk.
Noniterative Solution for Pressure in Parabolic Flows
Authors: Truman C. R. , Shirazi S. A. , Blottner F. G.
Published: May 2008

Parabolic flows in which the pressure variation in the streamwise (or marching) direction is unknown a priori include internal thin shear layers, shock-boundary layer interactions, and inverse boundary layers with specified displacement thickness or shear stress. The pressure is typically obtained through an additional iteration beyond that required to determine the velocity components (and other dependent variables). A generalized block-tridiagonal procedure is discussed in which pressure is determined within the iteration for velocity components to substantially reduce computation time. The increase in algebraic complexity in the solution procedure is small; no increase in the size of the block matrices is required. The method applies to any marching solution in which a scalar dependent variable is constant across the flow, but varies in the streamwise or marching direction.
Turbulent Shear Flow and Heat Transfer Over the Repeated Two-Dimensional Square Ribs on Ground Plane
Authors: Okamoto Shiki , Seo Shozo , Nakaso Kouichirou , Kawai Itsurou
Published: May 2008

This paper describes the flow structure over the repeated two-dimensional square ribs of side length D, placed at a pitch S on a ground plane. The value of S/D which most augments the turbulence of the free stream and, hence the heat transfer is calculated. The region of interest in this investigation is far downstream where the velocity and temperature distributions follow similarity rules. The time-mean velocity, static pressure, and the velocity vectors were measured by Pivot-and static pressure tubes and a three hole cylindrical yawmeter. The turbulence intensities and integral scale were obtained using a hot wire anemometer. The mean temperature distribution was measured by thermocouples and the local heat transfer coefficient was then calculated. It is found that at S/D=9 the turbulence intensity is maximized. As a result of this effect and the fact that for S/D=9 the flow reattaches within a groove, the heat transfer is also maximized. The measurements show how the location of reattachment depends on S/D and that high local heat transfer coefficient coincides with the reattachment point. The average heat transfer coefficient and the pressure drop correlation is quantified.
Turbulent Flow in Two-Inlet Channels
Authors: Kao Hsiao C.
Published: May 2008

The problem of turbulent flows in two-inlet channels has been studied numerically by solving the Reynolds-averaged Navier-Stokes equations with the k–ε model in a mapped domain. Both the high Reynolds number and the low Reynolds number form were used for this purpose. In general, the former predicts a weaker and smaller recirculation zone than the latter. Comparisons with experimental data, when applicable, were also made. The bulk of the present computations used, however, the high Reynolds number form to correlate different geometries and inflow conditions with the flow properties after turning.
Prediction of Turbulent Flows in Rotating Rectangular Ducts
Authors: Younis B. A.
Published: May 2008

This paper reports on progress made in the development of a numerical predictive procedure for turbulent flows in rotating rectangular ducts. The unknown turbulent stresses were approximated with the k–ε model of turbulence while Speziale’s (1987) nonlinear stress-strain relationship was utilized to capture the turbulence-driven secondary motions. Data from stationary rectangular ducts of aspect ratios in the range 1:1 to 10:1 were used to check the model. It was found that the occurrence and consequences of these motions were well reproduced. Application of the model to rotating rectangular ducts yielded results which suggest that the interactions between the pressure and turbulence-driven motions are far more important than hitherto suspected.
Large Amplitude Wall Pressure Events Beneath a Turbulent Boundary Layer
Authors: Karangelen C. C. , Wilczynski V. , Casarella M. J.
Published: May 2008

Experimental data on the temporal records of the wall pressure fluctuations beneath a turbulent boundary layer have been acquired in a low-noise flow facility. The pressure data were first analyzed using long-time averaging techniques to determine the statistical properties and the results were compared to the baseline data of Scheme (1983). Next, the pressure records were conditionally sampled at various k threshold levels (p′w ≥ k·Prms) to detect large amplitude, positive and negative events which were then averaged and analyzed to determine their shape, duration, and frequency of occurrence. The intermittent large amplitude events are very short in duration, occur rather infrequently in time, but are a major contributor to the high frequency content of the wall pressure fluctuations. As an example, events where p′w ≥ |3·prms| have an average duration of 14 viscous time units, occur 5 percent of the time and contribute 49 percent to the RMS value. The time between events appears to have a lognormal statistical distribution. The frequency of occurrence of the large amplitude events are consistent with the burst rate for flow structures and thus support the conjecture that the large amplitude events are associated with the near-wall bursting process.
On the Compressible Flow Through an Orifice
Authors: Brower W. B. , Eisler E. , Filkorn E. J. , Gonenc J. , Plati C. , Stagnitti J.
Published: May 2008

A new theory for the compressible flow through an orifice is presented which provides a significant advance over the approach currently employed. As the flow approaches the critical regime (local sonic condition), measurements diverge from the theoretical result due to the non-one-dimensionality of the flow. Nevertheless, a straightforward correlation is available, and the measurements for different pipe/orifice geometries all appear to lie in the vicinity of a single, universal curve. As the flow approaches the incompressible condition the correlation factor (the discharge coefficient) becomes unity.
Acoustic Properties of Solid-Liquid Mixtures and the Limits of Ultrasound Diagnostics—I: Experiments (Data Bank Contribution)
Authors: Atkinson C. M. , Kyto¨maa H. K.
Published: May 2008

Ultrasound as a technique for interrogating two-phase mixtures has the advantages of being nonintrusive, it has a very high frequency response, and is able to penetrate typically opaque highly concentrated mixtures. There exists, however, an inherent compromise in the choice of the frequency of the ultrasound between maximizing spatial resolution and ensuring adequate beam penetration. To this end, the propagation of ultrasound in solid-liquid mixtures has been investigated experimentally for a range of frequencies and concentrations of the dispersed phase. The measured attenuation has been shown to depend roughly linearly on frequency for 0.1<kr<0.75 (where the wavenumber k = 2π/λ, and λ and r are the wavelength and particle radius, respectively), and quadratically for kr > 0.75. As a function of solids concentration, the attenuation displays a maximum at a solids fraction of about 30 percent for the present system of silica beads in water. This robust and reproducible result contradicts models of attenuation that rely on linear superposition of single particle effects. The intensity field produced by a circular disk transducer in a two phase medium at kr~1 shows excellent agreement with the Rayleigh integral with a modified wavenumber and attenuation parameter, and it allows for the prediction of the transducer beam geometry in two phase mixtures for a wide range of frequencies and solids fractions. The limitations of ultrasonic wave propagation as a nonintrusive diagnostic technique, in terms of spatial resolution, have been discussed. Acknowledging these limitations, an ultrasonic instrument for determining the velocity of moving particles at or near maximum packing was built. Preliminary results from this prototypical ultrasonic Doppler velocimeter show good agreement with observations of the settling velocity of silica beads at high concentrations.
Correcting for Response Lag in Unsteady Pressure Measurements in Water
Authors: Conger Rand N. , Ramaprian B. R.
Published: May 2008

There is not much information available on the use of diaphragm-type pressure transducers for the measurement of unsteady pressures in liquids. A procedure for measuring the dynamic response of a pressure transducer in such applications and correcting for its inadequate response is discussed in this report. An example of the successful use of this method to determine unsteady surface pressures on a pitching airfoil in a water channel is presented.
Study of Laminar, Unsteady Piston-Cylinder Flows
Authors: Stro¨ll H. , Durst F. , Peric´ M. , Pereira J. C. F. , Scheuerer G.
Published: May 2008

The present paper concerns numerical investigation of a piston-driven, axisymmetric flow in a pipe assembly containing a sudden expansion. The piston closes the larger of the two pipes. The impulsively starting intake flow is the topic of this investigation. Results of numerical calculations and laser-Doppler measurements are presented to provide an insight into the features of the flow. The calculation procedure employed in this study is based on a finite-volume method with staggered grids and SIMPLE-algorithm for pressure-velocity coupling (Patankar and Spalding, 1972). The convection and diffusion fluxes in the Navier-Stokes equations are discretized with first order upwind and second order central differences, respectively. A fully implicit Euler scheme is used to discretize the temporal derivatives. The Navier-Stokes equations were suitably transformed to allow prediction of the flow within the inlet pipe (fixed grid) and cylinder region (moving grid) simultaneously (once-through procedure). Laser-Doppler measurements of both axial and radial velocity components were performed. Refractive index matching was used to eliminate the wall curvature effects. For each measuring point 20 cycles were measured, showing high repetition rates. Comparison of measured and predicted velocity profiles shows good agreement.
Numerical Simulation of Laminar Flow of Yield-Power-Law Fluids in Conduits of Arbitrary Cross-Section
Authors: Azouz Idir , Shirazi Siamack A. , Pilehvari Ali , Azar J. J.
Published: May 2008

A numerical model has been developed to simulate laminar flow of Power-law and Yield-Power law fluids in conduits of arbitrary cross-section. The model is based on general, nonorthogonal, boundary-fitted, curvilinear coordinates, and represents a new approach to the solution of annular flow problems. The use of an effective viscosity in the governing equation of the flow allows the study of the flow behavior of any fluid for which the shear stress is a function of shear rate only. The model has been developed primarily to simulate annular flow of fluids used in drilling and completion operations of oil or gas wells. Predicted flow rates versus pressure gradient for laminar flow of Newtonian fluids in concentric and eccentric annuli, and Power-law fluids in concentric annuli compare very well with results derived from analytical expressions. Moreover, the predictions for laminar flow of Power-law and Yield-Power-law fluids in eccentric annuli are in excellent agreement with numerical and experimental data published in the literature. The model was also successfully applied to the case of laminar flow of Power-law fluids in an eccentric annulus containing a stationary bed of drilled cuttings and the results are presented herein.
Analysis of an Electromagnetic Boundary Layer Probe for Low Magnetic Reynolds Number Flows
Authors: Langston L. S. , Kasper R. G.
Published: May 2008

Electromagnetic (EM) flow meters are used to measure volume flow rates of electrically conductive fluids (e.g., low magnetic Reynolds number flows of seawater, milk, etc.) in pipe flows. The possibility of using a modified form of EM flow meter to nonobtrusively measure boundary-layer flow characteristics is analytically investigated in this paper. The device, named an electromagnetic boundary layer (EBL) probe, would have a velocity integral-dependent voltage induced between parallel wall-mounted electrodes, as a conductive fluid flows over a dielectric wall and through the probe’s magnetic field. The Shercliff-Bevir integral equation, taken from EM flow meter theory and design, is used as the basis of the analytical model for predicting EBL probe voltage outputs, given a specified probe geometry and boundary layer flow conditions. Predictions are made of the effective range of the nonobtrusive EBL probe in terms of electrode dimensions, the magnetic field size and strength, and boundary layer velocity profile and thickness. The analysis gives expected voltage calibration curves and shows that an array of paired electrodes would be a beneficial feature for probe design. A key result is that the EBL probe becomes a displacement thickness meter, if operated under certain conditions. That is, the output voltage was found to be directly proportional to the boundary layer displacement thickness, δ1, for a given free stream velocity.
Numerical Modeling of Waveguide Heated Microwave Plasmas
Authors: Venkateswaran S. , Schwer D. A. , Merkle C. L.
Published: May 2008

Waveguide-heated microwave plasmas for space propulsion applications are analyzed by a two-dimensional numerical solution of the combined Navier-Stokes and Maxwell equations. Two waveguide configurations—one purely transmitting and the other with a reflecting end wall—are considered. Plasma stability and absorption characteristics are studied and contrasted with the characteristics of resonant cavity heated plasmas. In addition, preliminary estimates of the overall efficiency and the thrust and specific impulse of the propulsion system are also made. The computational results are used to explain experimental trends and to better understand the working of these devices.
Computation of Flow Fields Induced by Water Spraying of an Unconfined Gaseous Plume
Authors: Fthenakis V. M. , Schatz K. W. , Rohatgi U. S. , Zakkay V.
Published: May 2008

Flow fields induced by the interaction of water sprays and a gaseous plume have been studied in the context of absorbing and dispersing an accidental release of toxic gas in the air. The effectiveness of water sprays in absorbing highly water soluble gases was recently demonstrated in extended laboratory and field tests. In this paper, computer simulations are presented of the Hawk, Nevada Test Site, series of water spray/HF mitigation field tests. The model used, HFSPRAY, is a Eulerean/Lagrangian model which simulates the momentum, mass and energy interactions between a water spray and a turbulent plume of HF in air; the model can predict the flow velocities, temperature, water vapor, and HF concentration fields in two-dimensional large-geometries for spraying in any direction, (i.e., down-flow, inclined-down-flow, up-flow, and co-current horizontal flow). The model was validated against recent data on spraying of water on large releases of HF. It can provide a direct input to the design of water spray systems for HF mitigation.
Numerical Versus Experimental Cavitation Tunnel (A Supercavitating Hydrofoil Experiment)
Authors: Kinnas S. A. , Mazel C. H.
Published: May 2008

A supercavitating hydrofoil experiment is performed at the MIT Variable Pressure water tunnel, at several angles of attack and cavitation numbers. The velocity is measured on a rectangular contour surrounding the foil and cavity as well as in the vicinity of the cavity surface. The measurements are compared to the predicted results from a nonlinear cavity analysis method in which the effects of the tunnel walls are included via images. Forces are inferred from momentum integrations of the measured velocities and compared to those predicted from the analysis.
Three-Dimensional Calculation of Air-Water Two-Phase Flow in Centrifugal Pump Impeller Based on a Bubbly Flow Model
Authors: Minemura Kiyoshi , Uchiyama Tomomi
Published: May 2008

To predict the behavior of gas-liquid two-phase flows in a centrifugal pump impeller, a three-dimensional numerical method is proposed on the basis of a bubbly flow model. Under the assumption of homogeneous bubbly flow entraining fine bubbles, the equation of motion of the mixture is represented by that of liquid-phase and the liquid velocity is expressed as a potential for a quasi-harmonic equation. This equation is solved with a finite element method to obtain the velocities, and the equation of motion of an air bubble is integrated numerically in the flow field to obtain the void fraction. These calculations are iterated to obtain a converged solution. The method has been applied to a radial-flow pump, and the results obtained have been confirmed by experiments within the range of bubbly flow regime.
Mach Number Scaling of Single-Component, Two-Phase Flow
Authors: Nikitopoulos D. E.
Published: May 2008

A simple two-fluid formulation is used to investigate compressibility effects and Mach number scaling for equilibrium, evaporating two-phase flow. The definition of the local two-phase Mach number emerges from a critical flow analysis. Comparisons of the theoretical critical mass flux with existing experimental data obtained in steam-water flows show very good agreement for moderate and high qualities over a wide critical pressure range. Within this quality range the predicted critical mass flux is quite insensitive to the velocity ratio. The analysis confirms previous observations, based on homogeneous flow models, indicating that in variable area ducts the critical state does not occur at a geometrical throat. Results of existing critical flow experiments in slowly diverging ducts are discussed in the light of this conclusion. A way from the neighborhood of the flash horizon, pressure-drop and kinetic energy changes are shown to scale with similar local Mach functions as those of single-phase compressible flow. Existing experimental data from vertical-upwards and horizontal two-phase flows in pipes indicate that the Mach number calculated on the basis of the local homogeneous state provides the optimum scaling performance. Scaling of the same experimental data using a Mach number based on the local nonhomogeneous state provides results that are in reasonably good agreement with the theoretical scaling guidelines and predictions, but is handicapped by considerable scatter in the scaled experimental variables.
Prediction of Pump Performance Under Air-Water Two-Phase Flow Based on a Bubbly Flow Model
Authors: Minemura Kiyoshi , Uchiyama Tomomi
Published: May 2008

This paper is concerned with the determination of the performance change in centrifugal pumps operating under two-phase flow conditions using the velocities and void fractions calculated under the assumption of an inviscid bubbly flow with slippage between the two phases. The estimated changes in the theoretical head are confirmed with experiments within the range of bubbly flow regime.
Mass Imbalance Error of Waterhammer Equations and Leak Detection
Authors: Liou Chyr Pyng
Published: May 2008

The waterhammer equations can be used to compute mass inventory changes for pipeline leak detection. In such applications the pipeline is monitored continuously in time and there is no clear demarcation between steady-state and transients. This paper addresses a mass imbalance error of the waterhammer equations. We establish that the net mass influx error for the waterhammer equations at the steady-state with maximum flow-rate bounds the same error during transients. A method to evaluate this bound is presented. Using petroleum products pipelines as an example, we show that the waterhammer equations are not accurate enough for leak detection in some systems.
Performance Prediction by Viscous Flow Analysis for Francis Turbine Runner
Authors: Vu T. C. , Shyy W.
Published: May 2008

Validation of a three-dimensional computational algorithm for viscous flow analysis has been conducted for two types of Francis turbine runner geometry, one low head and one high head, using experimental measurement. Assessment has been made for both qualitative features of flow behavior, as well as quantitative distribution of blade pressure and head loss. The influence of the grid size on the accuracy of the numerical solution is also discussed. Effort has been made to address some of the design issues, and to demonstrate that the present computational algorithm can make useful contributions to help improve the current design practices.
An Investigation of Nucleating Flows of Stream in a Cascade of Turbine Blading—Wake Traverses
Authors: Bakhtar F. , Ebrahimi M. , Webb R. A.
Published: May 2008

During the course of expansion in turbines, steam first supercools and then nucleates to become a two-phase mixture consisting of a very large number of minute droplets suspended in the parent vapor. To reproduce turbine two-phase flow conditions realistically requires a supply of supercooled vapor, which can be achieved under blow-down conditions. This paper is one of a set describing the equipment which has been constructed and the first family of results obtained on a short duration cascade tunnel working on the blow-down principle. The arrangements for traversing downstream of a cascade of nozzle blades and the results obtained are described in the paper.
Development of a k-ε Model for Bubbly Two-Phase Flow
Authors: Lopez de Bertodano M. , Lahey R. T. , Jones O. C.
Published: May 2008

An extension of the k-ε model for bubbly two-phase flow is proposed and tested against experimental data. The basic assumption made is that the shear-induced turbulence and bubble-induced turbulence may be linearly superposed. This assumption results in a model with two time constants that matches both homogeneous two-phase turbulence data (Lance and Bataille, 1991) and pipe data (Serizawa, 1986). The coefficients of the single-phase k-ε model have not been modified and only one additional coefficient is required: the virtual volume coefficient of the bubbles, which may be determined from first principles. This model not only agrees with the data trends, but it also predicts the turbulence suppression which has been measured for high Reynolds number bubbly air/water flows in pipes.
Analysis of Linear Encroachment in Two-Immiscible Fluid Systems in a Porous Medium
Authors: Srinivasan Vijayaraghavan , Vafai Kambiz
Published: May 2008

The flow of two immiscible fluids in a porous medium was analyzed accounting for boundary and inertia effects. This problem was first solved by Muskat using Darcy’s equation for fluid flow in a saturated porous medium. In the present analysis the boundary and inertia effects have been included to predict the movement of the interfacial front that is formed as one fluid displaces the other. In the present work a theoretical study that accounts for the boundary and inertia effects in predicting the movement of the interface for linear encroachment in two immiscible fluid system in a porous material is presented for the first time. The results of the present study when compared with the Muskat’s model show that consideration of the boundary and inertia effects becomes important for low values of mobility ratio (ε<1.0) and higher values of permeability (K>1.0 × 1.0−10 m2).
Determination and Characteristics of the Transition to Two-Phase Slug Flow in Small Horizontal Channels
Authors: Wambsganss M. W. , Jendrzejczyk J. A. , France D. M.
Published: May 2008

Two-phase pressure drop and fluctuating static pressures were measured in a small horizontal rectangular channel (hydraulic diameter = 5.44 mm). The two-phase fluid was an air/water mixture at atmospheric pressure tested over a mass flux range of 50 to 2000 kg/m2˙s. Two-phase flow patterns were identified and an objective method was found for determining the flow pattern transition from bubble or plug flow to slug flow. The method is based on an RMS static pressure measurement. In particular, it is shown that the transition is accompanied by a clear and abrupt increase in the RMS pressure when plotted as a function of mass quality. Use of the RMS pressure as a two-phase flow pattern transition indicator is shown to have advantages over pressure-versus-time trace evaluations reported in the literature. The transition is substantiated by a clear local change in slope in the curve of two-phase pressure drop plotted as a function of either Martinelli parameter or mass quality. For high mass fluxes, the change in slope is distinguished by a local peak. Some degree of substantiation was found in previous work for both of the results (the RMS static pressure change and the local pressure drop change) at the transition to slug flow.
Boundary Layer Theory for a Particulate Suspension
Authors: Chamkha A. J. , Peddieson J.
Published: May 2008

Order of magnitude considerations are employed to develop boundary layer equations for two phase particle/fluid suspension flows. It is demonstrated that a variety of possibilities exist and three of these are examined in detail. Two are applied to the problem of flow past a semi-infinite flat plate.
Numerical Simulation of Heavy Particle Dispersion—Scale Ratio and Flow Decay Considerations
Authors: Wang Lian-Ping , Stock David E.
Published: May 2008

Lagrangian statistical quantities related to the dispersion of heavy particles were studied numerically by following particle trajectories in a random flow generated by Fourier modes. An experimental fluid velocity correlation was incorporated into the flow. Numerical simulation was performed with the use of nonlinear drag. The simulation results for glass beads in a nondecaying turbulent air showed a difference between the horizontal dispersion coefficient and vertical dispersion coefficient. This difference was related to the differences of both the velocity scale and the time scale between the two direction. It was shown that for relatively small particle sizes the particle time scale ratio dominates the value of the diffusivity ratio. For large particles, the velocity scale ratio reaches a value of 1/2 and thus fully determines the diffusivity ratio. Qualitative explanation was provided to support the numerical findings. The dispersion data for heavy particles in grid-generated turbulences were successfully predicted by the simulation when flow decay was considered. As a result of the reduction in effective inertia and the increase in effective drift caused by the flow decay, the particle dispersion coefficient in decaying flow decreases with downstream location. The particle rms fluctuation velocity has a slower decay rate than the fluid rms velocity if the drift parameter is large. It was also found that the drift may substantially reduce the particle rms velocity.
Flow Through Porous Media of Packed Spheres Saturated With Water
Authors: Kececioglu Ifiyenia , Jiang Yuxiang
Published: May 2008

The existing literature on the flow of fluids through porous packed beds gives very limited quantitative information on the criteria employed in marking the applicability of the different flow regimes. It is the objective of this paper to provide experimental evidence for determining the demarcation criteria during the flow of water through a bed of randomly packed spherical beads. Two different sizes of glass beads, 3 mm and 6 mm, were employed as the porous matrix through which water flowed at rates varying from 5.07 × 10−6 m3/s to 4920 × 10−6 m3/s. Our dimensionless pressure drop data showed less variation when the characteristic length of the porous medium was taken to be proportional to the square root of the permeability over the porosity and not the bead diameter. Curves of properly nondimensionalized pressure drop (P’K/μv) plotted against the actual flow Reynolds number based on the porous medium permeability (RˆeK) provided the following information. It was found that Darcy’s law has very limited applicability and is valid for a small range of Reynolds numbers (0.06<RˆeK<0.12). This leads to a pre-Darcy flow that is valid for a much broader range of Reynolds numbers than expected (RˆeK<0.06). Alternatively, the range of validity of the post-Darcy laminar Forchheimer flow is also found to be of much more limited applicability (0.34<RˆeK<2.30) than previous studies (Fand et al., 1987) have indicated (0.57<RˆeK Fand et al. <9.00). Transition to turbulence takes place earlier than expected and turbulent flow prevails from then on (RˆeK>3.40). The dimensionless pressure drop in both the Forchheimer and turbulent flow regimes can be modeled by an appropriately nondimensionalized Ergun’s equation (Carman, 1937), i.e., a first-order inertia term correction is sufficient in both flow regimes. However, the magnitude of the correction coefficients in the Forchheimer regime differs significantly from that in the turbulent flow regime (AˆF=1.00, BˆF=0.70, BˆT=1.90, BˆT=0.22). Again, this differs from previous findings (Fand et al., 1987). The effect of the angle of inclination of the porous medium with respect to the horizontal on the transition mechanisms was also experimentally investigated. No changes other than the correction in the pressure drop due to the static liquid column height were observed.
Asymptotic Wake Behavior of Swept, Blunt Trailing-Edge Airfoils
Authors: Selby G. V. , Miandoab F. H.
Published: May 2008

The effect of base sweep and the addition of passive flow-control devices at constant base sweep angle (30 deg) on the asymptotic behavior of turbulent wakes produced by flatplate airfoils was experimentally examined. It was determined that values of the nondimensional streamwise velocity defect and wake thickness parameters for the grooved model with 30 deg swept base at fourteen base thicknesses downstream of the base at mid-span were closer to asymptotic values from empirical plane wake predictions than values for the 0, 30, and 45 deg swept baseline models and the 30 deg swept model with Wishbone vortex generators. The grooves apparently inhibited the three-dimensionality of the resulting wake flow.
Cavity Flow Predictions Based on the Euler Equations
Authors: Deshpande Manish , Feng Jinzhang , Merkle Charles L.
Published: May 2008

An Euler solver based on artificial-compressibility and pseudo-time stepping is developed for the analysis of partial sheet cavitation in two-dimensional cascades and on isolated airfoils. The computational domain is adapted to the evolution of the cavity surface and the boundary conditions are implemented on the cavity interface. This approach enables the cavitation pressure condition to be incorporated directly without requiring the specification of the cavity length or the location of the inception point. Numerical solutions are presented for a number of two-dimensional cavity flow problems, including both leading edge cavitation and the more difficult mid-chord cavitation condition. Validation is accomplished by comparing with experimental measurements and nonlinear panel solutions from potential flow theory. The demonstrated success of the Euler cavitation procedure implies that it can be incorporated in existing incompressible CFD codes to provide engineering predictions of cavitation. In addition, the flexibility of the Euler formulation may allow extension to more complex problems such as viscous flows, time-dependent flows and three-dimensional flows.
Fluid Flow Behavior in the Curved Annular Sector Duct
Authors: Yang G. , Ebadian M. A.
Published: May 2008

A numerical analysis of the axial and secondary flow behavior in a curved annular sector duct is presented in the paper. The flow is considered to be fully developed laminar flow with constant physical properties. Five parameters have been identified as major variables in controlling the flow behavior. The study indicates that with a moderate Dean number and when the sector angle is smaller than π/2, only two vortices will appear in the cross section of the curved annular sector duct. When the sector angle is larger than π/2, the vortex structure can be very complex, and is often determined by other parameters, especially by the angle between the annular sector duct centerline and the curvature radius direction. The friction coefficient of the curved annular sector duct is affected mainly by the radius ratio, curvature, and axial pressure gradient. The radius ratio of the inner/outer walls can affect the vortex structure only when the radius ratio is very small. When the radius ratio is larger than 0.6, the friction coefficient is only slightly higher than that of a straight annular sector duct. Nevertheless, for the small radius ratio duct, the friction coefficient can be tripled, as compared with a straight annular sector duct. Although the holding pipe curvature and the axial pressure gradient cannot significantly change the vortex structure of the secondary flow, they can however, remarkably increase the friction coefficient by increasing the velocity gradient near the solid boundary.
Perspective: Numerical Simulations of Wakes and Blade-Vortex Interaction
Authors: Mook Dean T. , Dong Bonian
Published: May 2008

A method for simulating incompressible flows past airfoils and their wakes is described. Vorticity panels are used to represent the body, and vortex blobs (vortex points with their singularities removed) are used to represent the wake. The procedure can be applied to the simulation of completely attached flow past an oscillating airfoil. The rate at which vorticity is shed from the trailing edge of the airfoil into the wake is determined by simultaneously requiring the pressure along the upper and lower surface streamlines to approach the same value at the trailing edge and the circulation around both the airfoil and its wake to remain constant. The motion of the airfoil is discretized, and a vortex is shed from the trailing edge at each time step. The vortices are convected at the local velocity of fluid particles, a procedure that renders the pressure continuous in an inviscid fluid. When the vortices in the wake begin to separate they are split into more vortices, and when they begin to collect they are combined. The numerical simulation reveals that the wake, which is originally smooth, eventually coils, or wraps, around itself, primarily under the influence of the velocity it induces on itself, and forms regions of relatively concentrated vorticity. Although discrete vortices are used to represent the wake, the spatial density of the vortices is so high that the computed velocity profiles across a typical region of concentrated vorticity are quite smooth. Although the computed wake evolves in an entirely inviscid model of the flowfield, these profiles appear to have a viscous core. The computed spacing between the regions of concentrated vorticity in the wake and the circulations around them are in good agreement with the experimental results. As an application, a simulation of the interaction between vorticity in the oncoming stream and a stationary airfoil is also discussed.
Steady Laminar Fluid Flow Through Variable Constrictions in Vascular Tube
Authors: Lee T. S.
Published: May 2008

Steady laminar flow fields in the neighborhoods of two consecutive constrictions in a vascular tube were studied for approaching Reynolds number Re in the range of 5 to 200. The upstream stenosis was set at a dimensionless diameter constriction c1 of 0.5 while the downstream stenoses were allowed to vary from c2 = 0.2 to 0.6. The proximity of the constrictions was determined by the spacing ratio of S/D = 1, 2, 3, and ∞. When c2 > c1, a recirculation zone filled the valley between the two constrictions with little changes to the separation and reattachment points as Re was further increased. For c2 < c1 and when Re was increased, the recirculating eddy formed downstream of the first constriction tended to spread beyond the region of the second constriction. This resulted in negative wall vorticity peak occurring in the region of the second constriction for smaller S/D at high Re.
Toward a Pointwise Turbulence Model for Wall-Bounded and Free Shear Flows
Authors: Goldberg Uriel C.
Published: May 2008

A modified version of the Baldwin-Barth k-ℜ two-equation turbulence model is proposed, in which the near-wall function fμ is based on the ratio of the large eddy and the Kolmogorov time scales. This results in a model applicable to both wall-bounded and free shear flows which, nevertheless, does not require explicit knowledge of local distance to walls, rendering it useful within both structured and unstructured computational frameworks for flow predictions involving complex geometries. The new model’s predictive capability is demonstrated through a number of flow cases.
Efficient Simulation of Short and Long-Wave Interactions With Applications to Capillary Waves
Authors: Dommermuth D. G.
Published: May 2008

A perturbation expansion and a multigrid technique are developed for simulating the fully-nonlinear unsteady-interaction of short waves riding on long gravity waves. Both numerical techniques are capable of simulating wave slopes near breaking and wavelength ratios greater than thirty, but the multigrid technique converges more rapidly and it is more efficient. The results of numerical simulations agree qualitatively with experimental measurements of ripple formation on the front face of a gravity-capillary wave.
The Prediction of Velocity and Temperature Profiles in Gravity Currents for Use in Chilled Water Storage Tanks
Authors: Nakos J. T.
Published: May 2008

It has been demonstrated that one way of producing thin thermoclines (temperature gradients) in a chilled water storage tank is by introducing the water horizontally in the form of a gravity current. A gravity current is a fluid intrusion into a body of stagnant fluid at a different density. The incoming fluid is introduced at the bottom of the body of fluid if it is more dense; it is introduced at the top if it is less dense. In the application considered here, chilled water is to be stored in an efficient manner under the original body of warmer water. Vertical profiles of velocity and temperature in transient, two-dimensional, laminar, thermally driven, constant inflow gravity currents are studied. This provides a basis for understanding the initial stages of the formation of a thermocline in a chilled water storage tank. Two laminar flow formulations were developed to predict velocity and temperature profiles in the inertia-buoyancy regime. One formulation uses a strictly numerical approach, while the other uses a singular perturbation method to analyze the flow. Experimental temperature profiles are compared with the results from both formulations, and show good agreement.
Interpreting Vortex Interactions With a Free Surface
Authors: Rood E. P.
Published: May 2008

An understanding of the process by which vorticity interacts with a free surface is sought by analytical examination of the free-surface condition for the vorticity flux. A novel mechanism is suggested that permits closed vortex loops to evolve into open loops terminating at the free surface. It is hypothesized that abrupt vortex “disconnection,” observed in physical experiments, arises from a smooth diffusion of vorticity through the interface, with a necessary coincident tangential acceleration of the interface attributed to viscous forces.
The Initialization of Vortical Free-Surface Flows
Authors: Dommermuth D. G.
Published: May 2008

Numerical simulations of vortical free-surface flows are prone to developing spurious high-frequency dispersive waves unless the flow field is given sufficient time to adjust. At low Froude numbers, the high-frequency waves obscure the true hydrostatic balance of the free-surface elevation with the component of the pressure that is induced by the vortical portion of the flow. The high-frequency waves must be eliminated for remote sensing applications because otherwise the roughening of the free surface and the predicted radar backscatter will be incorrect. A procedure is developed for reducing the initial impulse of the pressure and the subsequent generation of high-frequency waves. Numerical simulations of whirls illustrate the effectiveness of the procedure. The pressure field of the whirls forms dimples on the free surface.
Review: Laminar-to-Turbulent Transition of Three-Dimensional Boundary Layers on Rotating Bodies
Authors: Kobayashi Ryoji
Published: May 2008

The laminar-turbulent transition of three-dimensional boundary layers is critically reviewed for some typical axisymmetric bodies rotating in still fluid or in axial flow. The flow structures of the transition regions are visualized. The transition phenomena are driven by the compound of the Tollmien-Schlichting instability, the crossflow instability, and the centrifugal instability. Experimental evidence is provided relating the critical and transition Reynolds numbers, defined in terms of the local velocity and the boundary layer momentum thickness, to the local rotational speed ratio, defined as the ratio of the circumferential speed to the free-stream velocity at the outer edge of the boundary layer, for the rotating disk, the rotating cone, the rotating sphere and other rotating axisymmetric bodies. It is shown that the cross-sectional structure of spiral vortices appearing in the transition regions and the flow pattern of the following secondary instability in the case of the crossflow instability are clearly different than those in the case of the centrifugal instability.
Estimating Uncertainty in Computations of Two-Dimensional Separated Flows
Authors: Demuren Ayodeji O. , Wilson Robert V.
Published: May 2008

The present paper investigates sources of uncertainties in two-dimensional flow computations and presents methods for estimating them. A sample problem is used for illustration. The following categories are explored in detail: (i) Uncertainty due to truncation error in numerical schemes; (ii) Uncertainty due to discretization error; (iii) Uncertainty due to outflow boundary conditions; (iv) Uncertainty due to incomplete iterative convergence; (v) Uncertainty due to computational grid aspect ratio. The error estimates are based on requirements for internal consistencies in computed results. Therefore, they provide better judgement of the numerical solution integrity than comparisons to experimental data or “benchmark” solutions whose reliability may sometimes be questionable. Ideally, both approaches should be employed. A new method is presented for estimating the optimum grid-cell aspect ratio for computational accuracy and efficiency.
Optimum Suppression of Fluid Forces Acting on a Circular Cylinder
Authors: Sakamoto H. , Haniu H.
Published: May 2008

The objective of this paper is to investigate the suppression of the fluid forces acting on a circular cylinder (hereafter called the main cylinder) by controlling the flow around it. Flow control was established by introducing a fine circular cylinder (hereafter called the control cylinder) near the main cylinder. Measurements were carried out with variation of the position of the control cylinder in the ranges of G/d = 0.004 ~ 0.20 (G is the gap between main cylinder and control cylinder, d is diameter of main cylinder) and α = 0 ~ 180 deg (α is the angle along circumference from the front stagnation point of main cylinder) at a Reynolds number of 6.5 × 104. Subsequently, the steady and unsteady fluid forces, vortex shedding frequency and flow pattern were systematically examined. Furthermore, such matters as the mechanism of the flow control, the nature of the controlled wake, the relationship between the characteristics of the controlled fluid forces, and the behavior of the flow were discussed in detail on the basis of the obtained results regarding fluid forces, vortex shedding frequency and flow pattern.
Three-Dimensional Recirculation Flow in a Backward Facing Step
Authors: Shih Chiang , Ho Chih-Ming
Published: May 2008

The flow field behind a small aspect ratio (channel width/step height = 3) backward-facing step is examined using laser Doppler anemometer. All three velocity components inside the separation region are surveyed in detail. The velocity profile just upstream of the step is laminar and two-dimensional. The velocity field reveals that the reattachment and the flow in the recirculation zone are highly three-dimensional due to the small aspect ratio.
Flow in a Channel With Longitudinal Ribs
Authors: Wang C. Y.
Published: May 2008

The laminar, viscous flow between parallel plates with evenly spaced longitudinal ribs is solved by an eigenfunction expansion and point-match method. The ribs on both plates may be symmetrically placed or staggered. For a given pressure gradient, the mean velocity is plotted as a function of the geometric parameters. We find the wetted perimeter and the friction factor—Reynolds number product are unsuitable parameters for the flow through ducts of complex geometry.
Turbulent Flow Past a Surface-Mounted Two-Dimensional Rib
Authors: Acharya S. , Dutta S. , Myrum T. A. , Baker R. S.
Published: May 2008

The ability of the nonlinear k–ε turbulence model to predict the flow in a separated duct flow past a wall-mounted, two-dimensional rib was assessed through comparisons with the standard k–ε model and experimental results. Improved predictions of the streamwise turbulence intensity and the mean streamwise velocities near the high-speed edge of the separated shear layer and in the flow downstream of reattachment were obtained with the nonlinear model. More realistic predictions of the production and dissipation of the turbulent kinetic energy near reattachment were also obtained. Otherwise, the performance of the two models was comparable, with both models performing quite well in the core flow regions and close to reattachment and both models performing poorly in the separated and shear-layer regions close to the rib.