bluuebunny/crossref_metadata_2025 huggingface dataset:
A Multizone Time-Marching Technique for Unsteady Separating Three-Dimensional Boundary Layers and Its Application to the Symmetry-Plane Solution of an Impulsively Started Prolate Spheroid
Authors: Wu Tzuyin , Shen Shan-Fu
Published: May 2008

Recent interest in unsteady separation and separated flows brings up the need of an accurate and efficient computational scheme for general unsteady three-dimensional boundary-layer flows. Resolution of the singular behavior at separation is a delicate problem. The task is further complicated by the geometrical singularity and the nonstationary stagnation point. The present paper proposes a numerical scheme to sidestep these difficulties. At the first stage of development, the simpler problem of the symmetry-plane solution of the laminar boundary-layer over an impulsively-started prolate spheroid is calculated. Results show that the present Eulerian calculation satisfactorily captures the singular behavior of the boundary layer when separation is approached. Comparison with Xu and Wang’s recent results and those for the two-dimensional elliptic cylinder calculated by the Lagrangian method are also made. Discussions of the results for unsteady separation at zero, small and large incidences are presented.
Experimental Investigation of the Flow With a Free Surface in an Impulsively Rotating Cylinder
Authors: Choi Sangmin , Kim Jae Won , Hyun Jae Min
Published: May 2008

An experimental investigation was made of the impulsive spin-up from rest of a liquid in a partially filled cylindrical container. The main impetus was placed on delineating the effects of the presence of a free surface on the transient development of flow. Of particular interest were the situations in which the free surface intersected either or both of the endwall disks during the course of spin-up. Extensive flow visualizations were carried out by using suspended metal particles. An image processing technique was utilized to determine accurately the propagating velocity shear front as well as the time-dependent free surface contour. Precise measurements of the fluids velocities were obtained by using a laser Doppler velocimeter. The transient velocity profiles were mapped out, and they were found to be in satisfactory agreement with the predictions based on the simplified analysis. The radial location, Rs(t), of the propagating shear front was measured by applying the image processing technique to the visualized azimuthal flow field. The experimental data were found to be consistent with the numerical predictions.
Velocity Bias Associated With Laser Doppler Anemometer Controlled Processors
Authors: Winter A. R. , Graham L. J. W. , Bremhorst K.
Published: May 2008

The controlled processor has been proposed as a means of avoiding velocity bias in laser Doppler anemometry. A theoretical model is presented to show that results free of bias can be obtained if both the ratio of integral time scale to measurement time scale (integral scale data density) and the ratio of sampling time to the measurement time scale (normalized sample interval) are greater than five. Further, by separation of the integral scale data density and normalized sample interval parameters, it is shown that at any integral scale data density the controlled processor will not produce any less bias than a sample and hold processor and no more bias than the free running (unweighted) processor. In some situations it may be considered superior to the sample and hold processor as the data processing and hardware requirements are reduced without any loss of measurement accuracy. Experimental data confirming the theoretical results are also shown. Some of these data are contrary to at least one model available in the literature.
A Method for Correcting Wall Pressure Measurements in Subsonic Compressible Flow
Authors: Ducruet C.
Published: May 2008

A theoretical and experimental investigation has been made of the static pressure hole problem in subsonic flow. Thanks to a linearization, the effects of the boundary layer, of the velocity gradient and of the wall curvature could be separated so that a formula of correction containing three influence functions has been obtained. These functions were determined in the case of practical requirements by means of experiments made on appropriate models for two values of the depth-to-diameter ratio and for at least three values of the Mach number. Then, the method of correction has been applied to the flow around an airfoil at zero angle of attack.
A Cavitation Susceptibility Meter With Optical Cavitation Monitoring—Part One: Design Concepts
Authors: d’Agostino L. , Acosta A. J.
Published: May 2008

This work is concerned with the design of a Cavitation Susceptibility Meter based on the use of a venturi tube for the measurement of the active cavitation nuclei concentration in water samples as a function of the applied tension. The operation of the Cavitation Susceptibility Meter is analyzed and the main considerations leading to the proposed design are illustrated and critically discussed. The results of this analysis indicate that the operational range is mainly limited by nuclei interference, flow separation and saturation (choking), and suggest to develop a Cavitation Susceptibility Meter where: (a) the flow possesses a laminar potential core throughout the venturi throat section in all operational conditions; (b) the pressure at the venturi throat is determined from the upstream pressure and the local flow velocity; (c) the detection of cavitation and the measurement of the flow velocity are carried out optically by means of a Laser Doppler Velocimeter; (d) a custom-made electronic Signal Processor incorporating a frequency counter is used for real time data generation and temporary storage; (e) a computerized system performs the final acquisition and reduction of the data.
A Cavitation Susceptibility Meter With Optical Cavitation Monitoring—Part Two: Experimental Apparatus and Results
Authors: d’Agostino L. , Acosta A. J.
Published: May 2008

This work is concerned with the development and operation of a Cavitation Susceptibility Meter based on the use of a venturi tube for the measurement of the active cavitation nuclei concentration in water samples as a function of the applied tension. The pressure at the venturi throat is determined from the upstream pressure and the local flow velocity without corrections for viscous effects because the flow possesses a laminar potential core in all operational conditions. The detection of cavitation and the measurement of the flow velocity are carried out optically by means of a Laser Doppler Velocimeter. A custom-made electronic Signal Processor is used for real time data generation and temporary storage and a computerized system for final data acquisition and reduction. The implementation of the whole system is described and the results of the application of the Cavitation Susceptibility Meter to the measurement of the water quality of tap water samples are presented and critically discussed with reference to the current state of knowledge on cavitation inception.
Some Characteristics of Flow Past Backward Facing Steps Including Cavitation Effects
Authors: Ramamurthy A. S. , Balachandar R. , Govinda Ram H. S.
Published: May 2008

Backward facing steps were used to study experimentally the effects of expansion ratio and Reynolds numbers on the incipient cavitation indices. For small changes in the expansion ratio, significant changes in incipient cavitation index were observed. The effect of cavitation and expansion ratio on the frequency of vortex shedding and pressure distribution downstream of flow separation were also obtained. The dependence of the flow characteristics like the reattachment length, the frequency of vortex shedding and the pressure distribution downstream of the step were obtained under noncavitating conditions. In the present study, the range of Reynolds numbers has been extended to values which are normally encountered in prototype installations.
Unsteady Gaseous Diffusion Associated With a Fully Cavitating Oscillating Flat-Plate Hydrofoil
Authors: Ravindra K. , Parkin B. R.
Published: May 2008

This paper gives an analysis of convective gaseous diffusion into a full cavity behind an oscillating flat-plate hydrofoil in a turbulent flow. The unsteady diffusion theory accounts for fluctuations of cavity gas pressure and length which are assumed to be harmonic oscillations but are not necessarily in phase with the hydrofoil motion. A diffusive lag function is found which, for a given reduced frequency, determines the instantaneous diffusion rate as a product of the lag function and the quasisteady mass diffusion. The present results can be used to study the rate of gas entrainment from the cavity into the wake behind the oscillating cavity.
Homogeneous Turbulence in Bubbly Flows
Authors: Lance M. , Marie´ J. L. , Bataille J.
Published: May 2008

The present study is devoted to the interaction between a swarm of bubbles and a turbulent field in a linear shear flow. The transversal and longitudinal evolutions of the void fraction and of the Reynolds stress tensor have been measured. When the air bubbles are blown uniformly into the shear, the void fraction profiles exhibit a strong gradient which can be explained by kinematical effects. No void migration has been observed. The behavior of the Reynolds tensor indicates that the nonisotropy induced by the mean velocity gradient decreases when the void fraction increases. A simple mechanism is proposed to interpret this fact, and a turbulence model based on one-point closures is compared to the experimental data.
Nonideal Gas Effects for the Venturi Meter
Authors: Bober W. , Chow W. L.
Published: May 2008

A method for treating nonideal gas flows through a venturi meter is described. The method is an extension of a previous study reported in an earlier paper. The method involves the determination of the expansion factor which may then be used to determine the mass flow rate through the venturi meter. The method also provides the means for determining the critical pressure ratio as well as the maximum flow rate per unit throat area. The Redlich-Kwong equation of state is used, which allows for closed form expressions for the specific heat at constant volume and the change in entropy. The Newton-Raphson method is used to determine the temperature and specific volume at the throat. It is assumed that the following items are known: the upstream temperature and pressure and the ratio of the throat pressure to the upstream pressure. Results were obtained for methane gas. These results indicate that for the cases considered, the use of the ideal gas expression for the expansion factor would lead to an error in the determination of the mass flow rate; the error increases as the throat to inlet pressure ratio decreases. For the example reported in this study, the maximum percent difference in the critical pressure ratio between the ideal and nonideal gases was 5.81 percent, while the maximum percent difference in the maximum flow rate per unit throat area was 7.62 percent.
Modulation of Turbulence by a Dispersed Phase
Authors: Gore R. A. , Crowe C. T.
Published: May 2008

The results of an earlier investigation that demonstrated the significance of the particle diameter/fluid length scale ratio in determining whether or not the addition of a dispersed phase would cause an increase or decrease in the carrier phase turbulent intensity are extended to radial locations other than the centerline. Presently, it is shown that similar trends are valid except the results become less correlated approacthing the wall for the case of pipe flows. The results for jets are independent of radial location. It is also shown that other possible non-dimensional parameters do not correlate the data as well.
An Assessment of Computational Fluid Dynamic Techniques in the Analysis and Design of Turbomachinery—The 1990 Freeman Scholar Lecture
Authors: Lakshminarayana B.
Published: May 2008

The objective of this paper is to review and assess various computational fluid dynamic techniques used for the analysis and design of turbomachinery. Assessments of accuracy, efficiency, range of applicability, effect of physical approximations, and turbulence models are carried out. Suggestions are made as to the most appropriate technique to be used in a given situation. The emphasis of the paper is on the Euler and Navier-Stokes solvers with a brief assessment of boundary layer solutions, quasi three-dimensional and quasi-viscous techniques. A brief review of the techniques and assessment of the following methods are carried out: pressure-based method, explicit and implicit time marching techniques, pseudo-compressibility technique for incompressible flow, and zonal techniques. Recommendations are made with regard to the most appropriate technique for various flow regimes and types of turbomachinery, incompressible and compressible flows, cascades, rotors, stators, liquid-handling and gas-handling turbomachinery. Computational fluid dynamics has reached a high level of maturity; Euler codes are routinely used in design and analysis, and the Navier-Stokes codes will also be commonplace before the end of this decade. But to capture the realism in turbomachinery rotors and multi-stage turbomachinery, it is necessary to integrate the physical models along with the computational techniques. Turbulence and transition modeling, grid generation, and numerical techniques play a key role. Finally, recommendations are made for future research, including the need for validation data, improved acceleration schemes, techniques for two-phase flow, improved turbulence and transition models, development of zonal techniques, and grid generation techniques to handle complex geometries.
A Finite-Element Perturbation Approach to Fluid/Rotor Interaction in Turbomachinery Elements. Part 1: Theory
Authors: Baskharone E. A. , Hensel S. J.
Published: May 2008

The vibrational characteristics of a rotor that is in contact with a fluid in an annular clearance gap, as dictated by the fluid forces in the gap, are investigated. The “rotor” here is a general term that may refer to the shaft segment within the housing of an annular seal, on the simple end of the application spectrum, or the shroud-seal assembly in a shrouded-impeller stage of a turbomachine, on the complex end. The disturbance under consideration involves the axis of rotation, and includes a virtual lateral eccentricity, together with a whirling motion around the housing centerline. Uniqueness of the computational model stems from the manner in which the rotor eccentricity is physically perceived and subsequently incorporated. It is first established that the fluid reaction components arise from infinitesimally small deformations with varied magnitudes which are experienced by an assembly of finite elements in the rotor-to-housing gap as the gap becomes distorted due to the rotor virtual eccentricity. The idea is then cast into a perturbation model in which the perturbation equations emerge from the flow-governing equations in their discrete finite-element form as opposed to the differential form, which is traditionally the case. As a result, restrictions on the rotor-to-housing gap geometry, or the manner in which the rotor virtual eccentricity occurs are practically nonexisting. While the emphasis in this paper is on the theoretical model, a representative application of the model and assessment of the numerical results are the focus of a companion paper that is being published concurrently.
A Finite-Element Perturbation Approach to Fluid/Rotor Interaction in Turbomachinery Elements. Part 2: Application
Authors: Baskharone E. A. , Hensel S. J.
Published: May 2008

A newly devised perturbation model for the fluid-induced vibration of turbomachinery rotating elements is used to compute the rotordynamic coefficients of an annular seal. First, the finite element-based solution of the flow field in the centered-rotor operation mode is verified and its grid dependency tested for different seal configurations. The rotordynamic behavior of a hydraulic seal with a clearance gap depth/length ratio of 0.01, as a representative case, is then analyzed under a cylindrical type of rotor whirl and several running speeds. The direct and cross-coupled rotordynamic coefficients dictating the rotor instability mechanism in this case are compared to experimental and analytical data, and the outcome is favorable. The numerical results are also used to discuss the validity of a common assumption in existing computational models in regard to the circumferential distribution of the perturbed flow variables in the eccentric rotor operation mode.
Navier-Stokes Calculations of Transonic Flows Past Cavities
Authors: Srinivasan S. , Baysal O.
Published: May 2008

Presented in this paper is a computational investigation of subsonic and transonic flows past three-dimensional deep and transitional cavities. Simulations of these self-induced oscillatory flows have been generated through time-accurate solutions of the Reynolds averaged, full Navier-Stokes equations, using the explicit MacCormack scheme. The Reynolds stresses have been included through the Baldwin-Lomax algebraic turbulence model with certain modifications. The computational results include instantaneous and time averaged flow properties. The results of an experimental investigation have been used not only to validate the time-averaged results, but also to investigate the effects of varying the Mach number and the incoming boundary-layer thickness. Time series analyses have been performed for the instantaneous pressure values on the cavity floor and compared with the results obtained by a predictive formula. While most of the comparisons have been favorable, some discrepancies have been observed, particularly on the rear face. The present results help understanding the three-dimensional and unsteady features of the separations, vortices, the shear layer, as well as some of the aeroacoustic phenomena of compressible cavity flows.
Oscillating Flow Past a Rigid Circular Cylinder: A Finite-Difference Calculation
Authors: Wang Xuegeng , Dalton Charles
Published: May 2008

A finite-difference study of the sinusoidally oscillating flow past a fixed circular cylinder is made using vorticity and stream function as the dependent variables. Calculations are performed for conditions which lead to both a symmetric wake and an unsymmetric wake. The Reynolds number ranges from 100 to 3000 and the Keulegan-Carpenter number ranges from 1 to 12. A hybrid differencing scheme is introduced to provide a stable for large values of the parameters. Good comparison to flow visualization results and calculated force coefficients is found. The results are given a physical interpretation for the various vortex patterns observed.
Velocity Profiles for Turbulent Boundary Layers Under Freestream Turbulence
Authors: Hoffmann J. A. , Mohammadi K.
Published: May 2008

Correlations for changes of skin friction coefficients (Δcf) and wake parameters (ΔΠ), relative to the low freestream turbulence condition, are presented for the case of turbulent boundary layers under freestream turbulence with zero and adverse pressure gradients. The turbulent boundary layers were evaluated on a plate in a wind tunnel using a monoplane rod set turbulence generator; comparisons were also made using the data of several other investigators. The results, which define the velocity profiles within the boundary layers, were found to collapse for a large range of the pressure gradient parameter.
Three-Dimensional Vortex Structure in a Leading-Edge Separation Bubble at Moderate Reynolds Numbers
Authors: Sasaki Kyuro , Kiya Masaru
Published: May 2008

This paper describes the results of a flow visualization study which concerns three-dimensional vortex structures in a leading-edge separation bubble formed along the sides of a blunt flat plate. Dye and hydrogen bubbles were used as tracers. Reynolds number (Re), based on the plate thickness, was varied from 80 to 800. For 80 < Re < 320, the separated shear layer remains laminar up to the reattachment line without significant spanwise distortion of vortex filaments. For 320 < Re < 380, a Λ-shaped deformation of vortex filaments appears shortly downstream of the reattachment and is arranged in-phase in the downstream direction. For Re > 380, hairpin-like structures are formed and arranged in a staggered manner. The longitudinal and spanwise distances of the vortex arrangement are presented as functions of the Reynolds number.
Computation of Turbulent Flow in a Thin Liquid Layer of Fluid Involving a Hydraulic Jump
Authors: Rahman M. M. , Faghri A. , Hankey W. L.
Published: May 2008

Numerically computed flow fields and free surface height distributions are presented for the flow of a thin layer of liquid adjacent to a solid horizontal surface that encounters a hydraulic jump. Two kinds of flow configurations are considered: two-dimensional plane flow and axisymmetric radial flow. The computations used a boundary-fitted moving grid method with a k-ε model for the closure of turbulence. The free surface height was determined by an optimization procedure which minimized the error in the pressure distribution on the free surface. It was also checked against an approximate procedure involving integration of the governing equations and use of the MacCormack predictor-corrector method. The computed film height also compared reasonably well with previous experiments. A region of recirculating flow as found to be present adjacent to the solid boundary near the location of the jump, which was caused by a rapid deceleration of the flow.
The Performance of Two Butterfly Valves Mounted in Series
Authors: Morris M. J. , Dutton J. C.
Published: May 2008

The results of an experimental investigation concerning the operating characteristics of two similar butterfly valves mounted in series are reported. Emphasis is given to the influence of the upstream valve disk angle, the downstream valve disk angle, the relative valve orientation, and the spacing between the valves. The dimensionless pressure drop, the mass flowrate coefficient, and the aerodynamic torque coefficient of each valve are used to characterize the system performance. The results show that the operating characteristics are strongly tied to the combined effect of the two valve disk angles. With noted exceptions, the valve disk orientation and spacing are secondary influences.
The Influence of Structural Damping on Internal Pressure During a Transient Pipe Flow
Authors: Budny Dan D. , Wiggert D. C. , Hatfield F. J.
Published: May 2008

A four equation model of axial wave propagation with Poisson coupling which includes viscous damping to account for structural energy dissipation is evaluated. Comparison of the predictions with experimental data indicates that the model can satisfactorily predict fluid pressure and structural velocity. The results show that structural damping reduces the pressure peaks during a transient event by eliminating the high frequency components. For the conditions studied, this reduction was 20 to 25 percent for a piping system with no axial constraints. A saddle-type support increases the equivalent viscous damping, and this increased damping can be modeled as either distributed damping or as an external damper.
Anisotropy in a Turbulent Jet Near a Free Surface
Authors: Swean T. F. , Ramberg S. E. , Miner E. W.
Published: May 2008

Laboratory measurements of the turbulent flow in a plane jet issuing at the free surface are reported and analyzed. Single point hot-film data were obtained for mean velocities and turbulent fluctuations of velocity and shear stress in the developing jet to X1/b0 = 240 with an emphasis on the near surface turbulent behavior. Anisotropy distributions indicate two layers within the jet. In a surface-influenced layer of thickness comparable to 1.5 times the local jet half-width, the vertical anisotropy is constant and the primary net energy transfer is from the streamwise component of turbulent kinetic energy to the spanwise component. In a near-surface layer of about 1/4 the local half-width, the vertical fluctuations decrease rapidly with a concomitant redistribution of energy to the horizontal components. Analysis of one-dimensional spectra reveal integral scales that are consistent with the anisotropy data and in qualitative agreement with analyses and experiments for isotropic turbulence near moving walls and free surfaces but differs by exhibiting a wider range of surface-influenced turbulent length scales.
A New Way to Represent Jet Pump Performance
Authors: Elger D. F. , McLam E. T. , Taylor S. J.
Published: May 2008

Researchers commonly represent jet pump performance with a Head-Ratio, Flow-Ratio curve (HR/FR curve). Our goals are to show that the HR/FR curve has several problems and to propose a new way to represent jet pump performance. Analysis shows that a HR/FR curve does not give enough information to predict the operating point of a system which includes a jet pump. Moreover, the way a HR/FR curve specifies data can force a designer to estimate head losses inside a jet pump and can make data acquisition very difficult. We propose using two curves to represent jet pump performance. Use of the two curves eliminates the problems of the HR/FR curve.
Mean Flow Field and Reynolds Stress Behavior in Coannular Jet Flow With Swirl Along a Centerbody
Authors: Frey M. O. , Gessner F. B.
Published: May 2008

An experimental study was conducted of an incompressible turbulent flow which exits from two concentric annular nozzles and develops along an unconfined centerbody. The operating Reynolds number based on centerbody diameter and the axial bulk velocity of the inner stream at the nozzle exit was 8 × 104. Swirl was imparted only to the inner stream, and the outer-to-inner stream mass flow rate ratio was fixed at unity. The results show that streamwise oscillations exist in the mean flow which apparently arise when vortices shed at the nozzle lip separating the two streams interact with the centerbody boundary layer. A comparison of Reynolds shear stress profiles with mean strain rates in the flow indicates that departures from local equilibrium exist in the mixing layer downstream of the nozzle exit. Local law-of-the-wall behavior is observed, however, near the centerbody surface. Analysis of the results shows that the use of conventional wall functions for the turbulence kinetic energy may not be appropriate for this flow situation, and that closure at the full Reynolds stress transport equation level is required for prediction purposes.
Initial Drop Size and Velocity Distributions for Airblast Coaxial Atomizers
Authors: Eroglu H. , Chigier N.
Published: May 2008

Initial drop size and velocity distributions, after complete disintegration of coaxial liquid jets, were determined by phase Doppler measurements. The measured radial distributions of Sauter mean diameter (SMD) were compared with the photographs of the disintegrating liquid jet. The SMD distribution was found to be strongly affected by the structure and behavior of the preceding liquid intact jet. The results showed that SMD increases with increasing liquid supply pressure as well as with decreasing air supply pressure. The axial measurement stations were determined from the photographs of the coaxial liquid jet at very short distances (1–2 mm) downstream of the observed break-up locations. The droplets accelerated at these regions under the influence of the air velocity. Smaller droplets were found to reach higher velocities because of their larger drag-to-momentum ratio. In general, minimum droplet mean velocities were found at the center, and the maximum velocities were near the spray boundary. Size velocity correlations show that the velocity of larger drops did not change with drop size. Drop rms velocity distributions have double peaks whose radial positions coincide with the maximum mean velocity gradients.
Characterization of Injection Nozzles for Gas-Solid Flow Applications
Authors: Dodge F. T. , Green S. T. , Johnson J. E.
Published: May 2008

Laser phase-doppler velocimetry measurements have been used to characterize the particle-gas sprays produced by straight-tube nozzles that simulate idealized fuel injectors for solid fuel combustion systems. Tests were conducted on two nozzle sizes, for two particle sizes, two loading ratios, and two gas velocities. The Reynolds numbers was varied from 9500 to 19000, and the Stokes number from 1.9 to 61.4. It was found that the velocities of the particles in the spray decelerate more slowly, and the velocity profiles are generally more narrow, than for a single-phase free-jet. The turbulence level of the particles in the sprays was found to be less than half the turbulence level of a single-phase free-jet, and the turbulent velocity profiles were not yet fully developed at X = 40D. The hydrodynamic characteristics of the nozzles that are the most important for combustion systems were found to be: (a) the particle spray expands radially at a cone angle of 2° (measured at the radius corresponding to the peak of the particle mass flux distribution); and (b) the nozzle pressure drop and particle mass flow can be related by a correlation that depends on loading ratio, Reynolds number, Stokes number, and the pressure drop coefficient of the nozzle for a single phase flow.
Computational Analysis of the Transonic Flow Field of Two-Dimensional Minimum Length Nozzles
Authors: Argrow B. M. , Emanuel G.
Published: May 2008

The method of characteristics is used to generate supersonic wall contours for two-dimensional, straight sonic line (SSL) and curved sonic line (CSL) minimum length nozzles for exit Mach numbers of two, four and six. These contours are combined with subsonic inlets to determine the influence of the inlet geometry on the sonic-line shape, its location, and on the supersonic flow field. A modified version of the VNAP2 code is used to compute the inviscid and laminar flow fields for Reynolds numbers of 1,170, 11,700, and 23,400. Supersonic flow field phenomena, including boundary-layer separation and oblique shock waves, are observed to be a result of the inlet geometry. The sonic-line assumptions made for the SSL prove to be superior to those of the CSL.
The Oscillation of Horseshoe Vortex Systems
Authors: Baker C. J.
Published: May 2008

The oscillation of horseshoe vortex systems around the base of wall mounted obstacles in the transition range is considered. Existing experimental data from a variety of sources is reanalyzed in the light of two hypotheses—that vortex oscillations are caused by (a) oscillations of the entire vortex system, and (b) by vortex core instabilities. It is shown that both types of oscillation exist, and the regions in parameter space in which each exist are determined. The interaction between the different types of oscillation is also described.
The Structure of a Three-Dimensional Tip Vortex at High Reynolds Numbers
Authors: Stinebring D. R. , Farrell K. J. , Billet M. L.
Published: May 2008

The tip vortex structure of a three-dimensional hydrofoil at high Reynolds number was measured experimentally in both the 48 in. (1.22 m) diameter water tunnel and the 48 in. (1.22 m) wind tunnel at the Applied Research Laboratory, Penn State. The flow on and near the hydrofoil was measured in both facilities using a number of flow visualization techniques and laser velocimetry. The downstream tip vortex was measured with a three-component laser velocimeter at a number of streamwise positions. A detailed planar mapping of the flow in the water tunnel was completed near the region of cavitation inception. The effect of roughness on the downstream tip vortex was investigated. In this paper, these measurements are presented with discussion relative to the occurrence of cavitation.
Studies of Scaling of Tip Vortex Cavitation Inception on Marine Lifting Surfaces
Authors: Hsu C. C.
Published: May 2008

The roll up of vortex sheet on a lifting surface in early stages is studied. The structures of tip vortex flow, both in the outer inviscid and inner viscous regions, are examined. The velocity in the viscous core is determined and used as basis for the prediction of tip vortex cavitation. Some comparisons between the calculated and measured tip vortex cavitation inception numbers are made, and the results are generally in good agreement.
Oscillatory Fluid Flow Through a Porous Medium Channel Bounded by Two Impermeable Parallel Plates
Authors: Khodadadi J. M.
Published: May 2008

In the absence of the inertia effects, the analytic solution to the fully developed oscillatory fluid flow through a porous medium channel bounded by two impermeable parallel plates is presented. For the limiting case when a highly viscous fluid undergoes slow pulsation in a high porosity medium, the phase lag vanishes and similar velocity profiles are observed. At the other extreme limiting situation, fluid flow near the symmetry plane has a phase lag of 90 deg from the pressure gradient wave. Moreover, the velocity profiles exhibit maxima next to the wall which is similar to the “channeling” phenomenon observed in variable-porosity studies. It is shown that the temporal average of the frictional drag over a period vanishes, indicating no net energy losses due to oscillations.
Visualization of the Wing-Tip Vortex in Temporal and Spatial Pressure Gradients
Authors: Liang X. , Ramaprian B. R.
Published: May 2008

The evolution of the longitudinal vortex in the near field of a rectangular wing has been visualized using smoke and laser light sheet. The smoke photographs have been used to obtain qualitative and some quantitative information on the effect of small temporal and spatial pressure gradients on the evolution of the vortex in this region. The experiments indicate that even small temporal retardation of the flow can produce a significant increase in the vortex size. This effect becomes larger with increase in retardation, angle of incidence and distance downstream from the trailing edge. Also, vortex “breakdown” was found to occur in many temporal-deceleration experiments. The vortex evolution was, however, found to be relatively insensitive to the presence of small spatial adverse pressure gradients along the flow direction. The study is preliminary in nature and needs to be supported by more extensive quantitative measurements.
Numerical Simulation of Heavy Particle Dispersion Time Step and Nonlinear Drag Considerations
Authors: Wang Lian-Ping , Stock D. E.
Published: May 2008

Numerical experiments can be used to study heavy particle dispersion by tracking particles through a numerically generated instantaneous turbulent flow field. In this manner, data can be generated to supplement physical experiments. To perform the numerical experiments efficiently and accurately, the time step used when tracking the particles through the fluid must be chosen correctly. After finding a suitable time step for one particular simulation, the time step must be reduced as the total integration time increases and as the free-fall velocity of the particle increases. Based on the numerical calculations, we suggest that the nonlinear drag be included in a numerical simulation if the ratio of the particle’s Stokes free-fall velocity to the fluid rms velocity is greater than two.
Thin Flowing Liquid Film Thickness Measurement by Laser Induced Fluorescence
Authors: Driscoll D. I. , Schmitt R. L. , Stevenson W. H.
Published: May 2008

An optical technique is described for determining, in a physically nondisturbing manner, the thickness of a thin flowing liquid film. The technique is based on measurement of the fluorescent emission induced in the liquid film by absorption of a focused laser beam. Although the technique was developed for measuring liquid film thicknesses on the prefilming surface of an airblast fuel atomizer, it is not limited to this application.
Airblast Atomization of Viscous Newtonian Liquids Using Twin-Fluid Jet Atomizers of Various Designs
Authors: Tsai S. C. , Viers B.
Published: May 2008

Airblast atomization of viscous Newtonian liquids is carried out using coaxial twin-fluid jet atomizers of different nozzle sizes, slit angles, and slit cross sections for air flow. As the atomizing air swirls downstream along the liquid jet, waves form on the surface of the liquid jet. As a result, the liquid jet sheds ligaments which rapidly collapse into small drops. The atomized drop sizes can be described in terms of three dimensionless groups, namely, liquid-to-air mass ratio (M˙L/M˙A), Weber number (We), and Ohnesorge number (Z) in simple forms whose exponents and coefficients are determined by the best least square fit to the experimental results using the generalized inverse method. In addition, we found that the atomized drop sizes substantially decrease as the atomizing air pressure exceeds a threshold value which varies from less than 170 to 220 kPa depending on the nozzle size and the slit cross section.
The Effect of Friction on Flow Distribution in Dividing and Combining Flow Manifolds
Authors: Shen P. I.
Published: May 2008

An analytical solution was obtained to evaluate the effect of friction on flow distribution in both dividing and combining flow manifolds. The governing equation follows the derivation given by Bajura (1971). A constant cross-sectional area of the manifold header and a constant friction factor along the manifold were assumed in order to obtain the analytical solution, expressed in terms of the lateral flow distribution as a function of two performance parameters, α (the friction parameter) and β (momentum loss parameter). Numerical results show that friction always increases flow imbalance in the combining flow configuration, but it may either increase or decrease flow imbalance in the dividing flow configuration, depending on the ratio of the lateral area to the cross-sectional area of the manifold.
On Stability Analysis of a Flexible Cylinder in an Array of Rigid Cylinders
Authors: Marn J. , Catton I.
Published: May 2008

The concept of an unsteady control volume is used to predict the onset of instability for a simple array of cylinders. The array consists of a flexible cylinder placed amidst rigid cylinders. The fluid is assumed to be incompressible with a “slip” boundary condition used on the solid/liquid interface. The equations derived for the model from first principles are solved in the complex plane. The results are compared to experimental data. The paper is concluded with a discussion of the advantages and disadvantages of the model and an assessment of the accuracy of the predictions.
Bounded Energy States in Homogeneous Turbulent Shear Flow—An Alternative View
Authors: Bernard P. S. , Speziale C. G.
Published: May 2008

The equilibrium structure of homogeneous turbulent shear flow is investigated from a theoretical standpoint. Existing turbulence models, in apparent agreement with physical and numerical experiments, predict an unbounded exponential time growth of the turbulent kinetic energy and dissipation rate; only the anisotropy tensor and turbulent time scale reach a structural equilibrium. It is shown that if a residual vortex stretching term is maintained in the dissipation rate transport equation, then there can exist equilibrium solutions, with bounded energy states, where the turbulence production is balanced by its dissipation. Illustrative calculations are presented for a k–ε model modified to account for net vortex stretching. The calculations indicate an initial exponential time growth of the turbulent kinetic energy and dissipation rate for elapsed times that are as large as those considered in any of the previously conducted physical or numerical experiments on homogeneous shear flow. However, vortex stretching eventually takes over and forces a production-equals-dissipation equilibrium with bounded energy states. The plausibility of this result is further supported by independent calculations of isotropic turbulence which show that when this vortex stretching effect is accounted for, a much more complete physical description of isotropic decay is obtained. It is thus argued that the inclusion of vortex stretching as an identifiable process may have greater significance in turbulence modeling than has previously been thought and that the generally accepted structural equilibrium for homogeneous shear flow, with unbounded energy growth, could be in need of re-examination.
On the Motion of Solid Spheres Falling Through Viscous Fluids in Vertical and Inclined Tubes
Authors: Humphrey Joseph A. C. , Murata Hiroyuki
Published: May 2008

Little is known about the rotational motion of spheres falling through viscous fluids in inclined tubes. Most studies have investigated translational and rotational motions in vertical tubes. These works show that in creeping flow a sphere’s translational and rotational velocities are independent. Rotation is predicted and observed for eccentric spheres while concentric spheres fall without rotation. Experiments were performed by us with steel spheres of radius r falling through glycerine in a tube of variable inclination angle and of radius R such that r/R = 0.882, 0.757, 0.442. For the cases involving two or three spheres falling together various modes of motion were observed. Especially interesting was the finding that the rotation direction of a sphere gradually changes from positive (opposite to downhill rolling) to negative (in the sense of downhill rolling) as the tube inclination angle is increased. This is allowed by the inertia-induced lift force which maintains a sphere at a very small but finite distance from the inclined tube wall. However, by further increasing the inclination angle the lift force eventually becomes smaller than the apparent weight of the sphere which, upon finally contacting the tube wall, descends by rolling along it. Examination of our findings in the light of earlier results for vertical and inclined tubes suggests that, through its effect on sphere eccentricity, inertia indirectly affects the rotational motion of a falling sphere when Rep10−3 but it does not significantly affect the translational motion when Rep<1. None of the inclined tube studies performed to date has been completely devoid of inertia-induced lift effects.
Turbulence Modeling and Simulation of Atmospheric Boundary Layers
Authors: Abou-Arab T. W. , Serag-Eldin M. A.
Published: May 2008

The essence and shortcoming of turbulence modeling and simulation of atmospheric boundary/surface layers are discussed. The present approach rests on the extensively tested and widely used two-equation k-ε model to predict such flows. All features and constants of the standard version of the k-ε model as it is used for shear flows are retained here. This eliminates the requirement of rigorous experimental validation. However, the model with its set of boundary conditions features compatibility and realizability with the commonly reported stable, unstable and neutral atmospheric boundary/surface layer data. The paper presents also a comparison with experimental data and other models and the need for future research in this direction.
A Frequency-Domain Filtering Technique for Triple Decomposition of Unsteady Turbulent Flow
Authors: Brereton G. J. , Kodal A.
Published: May 2008

A new technique is presented for decomposing unsteady turbulent flow variables into their organized unsteady and turbulent components, which appears to offer some significant advantages over existing ones. The technique uses power-spectral estimates of data to deduce the optimal frequency-domain filter for determining the organized and turbulent components of a time series of data. When contrasted with the phase-averaging technique, this method can be thought of as replacing the assumption that the organized motion is identically reproduced in successive cycles of known periodicity by a more general condition: the cross-correlation of the organized and turbulent components is minimized for a time series of measurement data, given the expected shape of the turbulence power spectrum. The method is significantly more general than the phase average in its applicability and makes more efficient use of available data. Performance evaluations for time series of unsteady turbulent velocity measurements attest to the accuracy of the technique and illustrate the improved performance of this method over the phase-averaging technique when cycle-to-cycle variations in organized motion are present.
On Turbulent Flows Dominated by Curvature Effects
Authors: Cheng G. C. , Farokhi S.
Published: May 2008

A technique for improving the numerical predictions of turbulent flows with the effect of streamline curvature is developed. Separated flows and the flow in a curved duct are examples of flow fields where streamline curvature plays a dominant role. New algebraic formulations for the eddy viscosity μt incorporating the k–ε turbulence model are proposed to account for various effects of streamline curvature. The loci of flow reversal (where axial velocities change signs) of the separated flows over various backward-facing steps are employed to test the capability of the proposed turbulence model in capturing the effect of local curvature. The inclusion of the effect of longitudinal curvature in the proposed turbulence model is validated by predicting the distributions of the longitudinal velocity and the static pressure in an S-bend duct and in 180 deg turn-around ducts. The numerical predictions of different curvature effects by the proposed turbulence models are also reported.
Pressure Oscillation in the Leakage Annulus Between a Shrouded Impeller and Its Housing Due to Impeller-Discharge-Pressure Disturbances
Authors: Childs D. W.
Published: May 2008

An analysis is presented for the perturbed flow in the leakage path between a shrouded-pump impeller and its housing caused by oscillations in the impeller-discharge pressure. A bulk-flow model is used for the analysis consisting of the path-momentum, circumferential-momentum, and continuing equations. Shear stress at the impeller and housing surfaces are modeled according to Hirs’ turbulent lubrication model. In the present analysis, perturbations of the impeller discharge pressure are used to excite the fluid annulus. The circumferential variation of the discharge pressure is expanded in a Fourier series up to order n1, where n1 is the number of impeller blades. A precession of the impeller wave pattern in the same direction or opposite to pump rotation is then assumed to completely define the disturbance excitation. Predictions show that the first (lowest-frequency) “centrifugal-acceleration” mode of the fluid within the annulus has its peak pressure amplitude near the wearing-ring seal. Pressure oscillations from the impeller can either be attenuated or (sharply) magnified depending on: (a) the tangential velocity ratio of the fluid entering the seal, (b) the order of the fourier coefficient, and (c) the closeness of the precessional frequency of the rotating pressure field to the first natural frequency of the fluid annulus, and (d) the clearance in the wearing-ring seal.
Flow Development and Analysis of MHD Generators and Seawater Thrusters
Authors: Doss E. D. , Roy G. D.
Published: May 2008

The flow characteristics inside magnetohydrodynamic (MHD) plasma generators and seawater thrusters are analyzed and are compared using a three-dimensional computer model that solves the governing partial differential equations for fluid flow and electrical fields. Calculations have been performed for a Faraday plasma generator and for a continuous electrode seawater thruster. The results of the calculations show that the effects caused by the interaction of the MHD forces with the fluid flow are strongly manifested in the case of the MHD generator as compared to the flow development in the MHD thruster. The existence of velocity overshoots over the sidewalls confirm previously published results for MHD generators with strong MHD interaction. For MHD thrusters, the velocity profile is found to be slightly flatter over the sidewall as compared to that over the electrode wall. As a result, distinct enhancement of the skin friction exists over the sidewalls of MHD generators in comparison to that of MHD thrusters. Plots of velocity profiles and skin friction distributions are presented to illustrate and compare the flow development in MHD generators and thrusters.