bluuebunny/crossref_metadata_2025 huggingface dataset:
Laminar Fluid Flow in a Planar 90 Degree Bifurcation With and Without a Protruding Branching Duct
Authors: Travers T. G. , Worek W. M.
Published: February 2008

The laminar flow field in a planar, ninety degree bifurcation is examined. This numerical study uses the computational-fluid-dynamics software Fluent Version 4.11. First, the velocity field in a bifurcation without a protruding branching duct is modeled, and the results are successfully compared to experimental data. Next, the flow field is studied in bifurcations that have branching ducts that protrude into the main duct. The velocity field and pressure drop are documented, and are found to be strongly influenced by the extent of the branching duct protrusion.
Rarefaction and Compressibility Effects in Gas Microflows
Authors: Beskok Ali , Karniadakis George Em , Trimmer William
Published: February 2008

Gas microflows are encountered in many applications of Micro-Electro-Mechanical Systems (MEMS). Computational modeling and simulation can provide an effective predictive capability for heat and momentum transfer in microscales as well as means of evaluating the performance of a new microdevice before hardware fabrication. In this article, we present models and a computational methodology for simulating gas microflows in the slip-flow regime for which the Knudsen number is less than 0.3. The formulation is based on the classical Maxwell/Smoluchowski boundary conditions that allow partial slip at the wall. We first modify a high-order slip boundary condition we developed in previous work so that it can be easily implemented to provide enhanced numerical stability. We also extend a previous formulation for incompressible flows to include compressibility effects which are primarily responsible for the nonlinear pressure distribution in micro-channel flows. The focus of the paper is on the competing effects of compressibility and rarefaction in internal flows in long channels. Several simulation results are presented and comparisons are provided with available experimental data. A specific set of benchmark experiments is proposed to systematically study compressibility, rarefaction and viscous heating in microscales in order to provide validation to the numerical models and the slip-flow theory in general as well as to establish absolute standards in this relatively young field of fluid mechanics.
Transonic and Boundary Layer Similarity Laws in Dense Gases
Authors: Cramer M. S. , Whitlock S. T. , Tarkenton G. M.
Published: February 2008

We discuss the validity of similarity and scaling laws for transonic flow and compressible boundary layers when dense gas effects are important. The physical mechanisms for the failure of each class of scaling law are delineated. In the case of transonic flow, a new similitude based on a modified small disturbance equation is presented.
Flow Visualization by Velocity-Pressure Cross-Correlation
Authors: Hijikata Kunio , Suzuki Yuji , Iwana Kenji
Published: February 2008

For a better understanding of turbulent structure, a new flow visualization method of the time averaged structure of the turbulence has been developed by using wall pressure fluctuations to detect the velocity fluctuations of the flow. Velocity fluctuations were visualized by the measured velocity-wall pressure cross-correlation with the whole flow field for various time-delays between two measured values. Turbulence structures of a backward-facing step flow were investigated for three different flow regions—the recirculating flow just behind the step, the reattachment flow, and the boundary layer flow far down stream from the step. Sequential contour maps of cross-correlation values were calculated to display movement and deformation of turbulent structures both qualitatively and quantitatively.
Active Control of Instabilities in Laminar Boundary Layers—Overview and Concept Validation
Authors: Joslin R. D. , Erlebacher G. , Hussaini M. Y.
Published: February 2008

This paper (the first in a series) focuses on using active-control methods to maintain laminar flow in a region of the flow in which the natural instabilities, if left unattended, lead to turbulent flow. The authors review previous studies that examine wave cancellation (currently the most prominent method) and solve the unsteady, nonlinear Navier-Stokes equations to evaluate this method of controlling instabilities. It is definitively shown that instabilities are controlled by the linear summation of waves (i.e., wave cancellation). Although a mathematically complete method for controlling arbitrary instabilities has been developed, the review, duplication, and physical explanation of previous studies are important steps for providing an independent verification of those studies, for establishing a framework for the work which will involve automated transition control, and for detailing the phenomena by-which the automated studies can be used to expand knowledge of flow control.
Turbulent Flow in Longitudinally Finned Tubes
Authors: Edwards D. P. , Hirsa A. , Jensen M. K.
Published: February 2008

An experimental investigation of fully developed, steady, turbulent flow in longitudinally finned tubes has been performed. A two-channel, four-beam, laser-Doppler velocimeter was used to measure velocity profiles and turbulent statistics of air flow seeded with titanium dioxide particles. Mean velocities in axial, radial, and circumferential directions were measured over the tube cross sections and pressure drop in the tubes was measured at six stations along the test section length in order to calculate the fully developed friction factor. Four experimental tube geometries were studied: one smooth tube; two 8-finned tubes (fin height-to-radius ratios of 0.333 and 0.167), and one 16-finned tube (fin height-to-radius ratio of 0.167); Detailed measurements were taken at air flow rates corresponding to Reynolds numbers of approximately 5000, 25,000, and 50,000. Friction factor data were compared to literature results and showed good agreement for both smooth and finned tubes. The wall shear stress distribution varied significantly with Reynolds number, particularly for Reynolds numbers of 25,000 and below. Maximum wall shear stress was found at the fin tip and minimum at the fin root. Four secondary flow cells were detected per fin (one in each interfin spacing and one in each core region for each fin); secondary flows were found to be small in comparison to the mean axial flow and relative magnitudes were unaffected by axial flow rate at Reynolds numbers above 25,000. The fluctuating velocities had a structure similar to that of the smooth tube in the core region while the turbulence in the interfin region was greatly reduced. The principal, primary shear stress distribution differed considerably from that of the smooth tube, particularly in the interfin region, and the orientation was found to be approximately in the same direction as the gradient of the mean axial velocity, supporting the use of an eddy viscosity formulation in turbulence modeling.
A Comparison of Some Recent Eddy-Viscosity Turbulence Models
Authors: Menter F. R.
Published: February 2008

The performance of recently developed eddy-viscosity turbulence models, including the author’s SST model, is evaluated against a number of attached and separated adverse pressure gradient flows. The results are compared in detail against experimental data, as well as against the standard k-ε model. Grid convergence was established for all computations. The study involves four different, state-of-the-art finite difference (finite volume) codes.
An Experimental Study of Swirling Flow Pneumatic Conveying System in a Horizontal Pipeline
Authors: Li Hui , Tomita Yuji
Published: February 2008

In order to reduce power consumption, pipe wear and particle degradation in pneumatic conveying system, a swirling flow pneumatic conveying (SFPC) system is proposed in this paper, and an experimental study focuses on the SFPC system in a horizontal pipeline in terms of the overall pressure drop, solid flow patterns, power consumption and the additional pressure drop. Polystyrene, polyethylene, and polyvinyl pellets with mean diameters of 1.7, 3.1, and 4.3 mm, respectively, were transported as test particles in a horizontal pipeline of 13 m length and 80 mm inside diameter. The initial swirl number was varied from 0.58 to 1.12, the mean air velocity from 9 m/s to 24 m/s, and the solid mass flow rate from 0.43 kg/s to 1.17 kg/s. It is found that in the lower gas velocity range, the pressure drop, the power consumption and additional pressure drop for SFPC were lower than those for axial flow pneumatic conveying. The critical and minimum air velocities were decreased by SFPC, the maximum reduction rates being 13 and 17 percent, respectively. The fluctuation of wall static pressure for SFPC was also decreased.
Stokes Layers in Horizontal-Wave Outer Flows
Authors: Choi J. E. , Sreedhar M. K. , Stern F.
Published: February 2008

Results are reported of a computational study investigating the responses of flat plate boundary layers and wakes to horizontal wave outer flows. Solutions are obtained for temporal, spatial, and traveling waves using Navier Stokes, boundary layer, and perturbation expansion equations. A wide range of parameters are considered for all the three waves. The results are presented in terms of Stokes-layer overshoots, phase leads (lags), and streaming. The response to the temporal wave showed all the previously reported features. The magnitude and nature of the response are small and simple such that it is essentially a small disturbance on the steady solution. Results are explainable in terms of one parameter ξ (the frequency of oscillation). For the spatial wave, the magnitude and the nature of the response are significantly increased and complex such that it cannot be considered simply a small disturbance on the without-wave solution. The results are explainable in terms of the two parameters λ−1 and x/λ (where λ is the wavelength). A clear asymmetry is observed in the wake response for the spatial wave. An examination of components of the perturbation expansion equations indicates that the asymmetry is a first-order effect due to nonlinear interaction between the steady and first-harmonic velocity components. For the traveling wave, the responses are more complex and an additional parameter, c (the wave speed), is required to explain the results. In general, for small wave speeds the results are similar to a spatial wave, whereas for higher wave speeds the response approaches the temporal wave response. The boundary layer and perturbation expansion solutions compares well with the Navier Stokes solution in their range of validity.
Three-Dimensional Analysis of Partially Open Butterfly Valve Flows
Authors: Huang Chendong , Kim Rhyn H.
Published: February 2008

A numerical simulation of butterfly valve flows is a useful technique to investigate the physical phenomena of the flow field. A three-dimensional numerical analysis was carried out on incompressible fluid flows in a butterfly valve by using FLUENT, which solves difference equations. Characteristics of the butterfly valve flows at different valve disk angles with a uniform incoming velocity were investigated. Comparisons of FLUENT results with other results, i.e., experimental results, were made to determine the accuracy of the employed method. Results of the three-dimensional analysis may be useful in the valve design.
Viscosity Correction Factor for Rotameter
Authors: Wojtkowiak J. , Popiel Cz. O.
Published: February 2008

This paper describes how the developed formula for rotameter flow coefficient using the rotameter scale provided by a manufacturer and valid for a specific fluid and for design (or calibration) flow conditions allow us to determine the actual flow rate at measurement conditions and for different fluid having different viscosity or allow us to determine the viscosity correction factor. The developed theory has been verified experimentally using typical rotameter equipped with the plumb type float, for water of 15°C and having a flow rate range from 10 to 100 l/hr. A very good agreement between theoretical calculation and calibration results was obtain for water flow at 88°C. In this case the kinematic viscosity ratio was v15/v88 = 3.46 and the density correction factor was Fdens = 1.019 and the viscosity correction factor varied from Fvisc = 1.095 at 100 l/hr to Fvisc = 1.93 at 10 l/hr.
Sequential Production of mm-Sized Spherical Shells in Liquid-Liquid Gas Systems
Authors: Kawano Satoyuki , Hashimoto Hiroyuki , Ihara Akio , Shin Keiji
Published: February 2008

A new device has been developed for sequential production of mm-sized solid spherical shells using liquid-liquid gas systems. This device comprises a cylindrical vessel, for containing two kinds of immiscible liquids, and a gas injection orifice, set at the center of the vessel’s bottom. Solid spherical shells are successfully and sequentially produced by solidifying rising liquid spherical shells, formed sequentially at the horizontal interface between two immiscible liquids.
Comparisons of Magnetic Resonance Imaging Velocimetry With Computational Fluid Dynamics
Authors: Newling B. , Gibbs S. J. , Derbyshire J. A. , Xing D. , Hall L. D. , Haycock D. E. , Frith W. J. , Ablett S.
Published: February 2008

The flow of Newtonian liquids through a pipe system comprising of a series of abrupt expansions and contractions has been studied using several magnetic resonance imaging (MRI) techniques, and also by computational fluid dynamics. Agreement between those results validates the assumptions inherent to the computational calculation and gives confidence to extend the work to more complex geometries and more complex fluids, wherein the advantages of MRI (utility in opaque fluids and noninvasiveness) are unique. The fluid in the expansion-contraction system exhibits a broad distribution of velocities and, therefore, presents peculiar challenges to the measurement technique. The MRI protocols employed were a two-dimensional tagging technique, for rapid flow field visualisation, and three-dimensional echo-planar and gradient-echo techniques, for flow field quantification (velocimetry). The Computational work was performed using the FIDAP package to solve the Navier-Stokes equations. The particular choice of parameters for both MRI and computational fluid dynamics, which affect the results and their agreement, have been addressed.
Flow Driven by a Torsionally-Oscillating Shrouded Endwall Disk
Authors: Lim Tae Gyu , Hyun Jae Min
Published: February 2008

A study is made of time-dependent flow of a viscous fluid driven by an oscillating shrouded disk in finite geometry. Numerical solutions to the Navier-Stokes equations are obtained for the flow in a cylindrical cavity with its upper endwall disk executing torsional oscillation at a velocity Ω cos λt. Details of the three-component velocity field are examined at high Reynolds number. The value of the nondimensional amplitude of disk oscillation, ε = Ω/λ, encompasses a range up to ε ≳ O(1). The numerical results for the azimuthal flow for ε ≪ 1 are consistent with the predictions of the earlier analytical model. The azimuthal flow is largely confined to the Stokes layer thickness. The analytical predictions of the meridional flow, based on a straightforward expansion technique, display discrepancies from the numerical results. The steady meridional streaming at finite values of ε is exhibited. The qualitative patterns of meridional steady streaming are verified by laboratory flow visualizations. The explicit effect of Re on the overall flow character is scrutinized. The numerical data are processed to describe the behavior of the torque coefficient at the oscillating disk.
Flow Field Unsteadiness in the Tip Region of a Transonic Compressor Rotor
Authors: Puterbaugh S. L. , Copenhaver W. W.
Published: February 2008

An experimental investigation concerning tip flow field unsteadiness was performed for a high-performance, state-of-the-art transonic compressor rotor. Casing-mounted high frequency response pressure transducers were used to indicate both the ensemble averaged and time varying flow structure present in the tip region of the rotor at four different operating points at design speed. The ensemble averaged information revealed the shock structure as it evolved from a dual shock system at open throttle to an attached shock at peak efficiency to a detached orientation at near stall. Steady three-dimensional Navier Stokes analysis reveals the dominant flow structures in the tip region in support of the ensemble averaged measurements. A tip leakage vortex is evident at all operating points as regions of low static pressure and appears in the same location as the vortex found in the numerical solution. An unsteadiness parameter was calculated to quantify the unsteadiness in the tip cascade plane. In general, regions of peak unsteadiness appear near shocks and in the area interpreted as the shock-tip leakage vortex interaction. Local peaks of unsteadiness appear in mid-passage downstream of the shock-vortex interaction. Flow field features not evident in the ensemble averaged data are examined via a Navier-Stokes solution obtained at the near stall operating point.
Airfoil Section Characteristics at a Low Reynolds Number
Authors: Sunada Shigeru , Sakaguchi Akitoshi , Kawachi Keiji
Published: February 2008

The aerodynamic characteristics of airfoils operating at Re = 4 × 103 were examined, varying the parameters related to the airfoil shape such as thickness, camber, and roughness. Airfoils with good aerodynamic performance at this Re have the following shape characteristics: (1) they are thinner than airfoils for higher Re numbers, (2) they have a sharp leading edge, and (3) they have a camber of about five percent with its maximum camber at about mid-chord. The characteristics of airfoils are strongly affected by leading edge vortices. The measured two-dimensional airfoil characteristics indicate that the planform, which greatly affects the flight performance of the three-dimensional wing at high Reynolds numbers, has little effect on the flight performance at this Reynolds number.
Low-Speed Maneuvering Hydrodynamics of Fish and Small Underwater Vehicles
Authors: Bandyopadhyay P. R. , Castano J. M. , Rice J. Q. , Philips R. B. , Nedderman W. H. , Macy W. K.
Published: February 2008

The low-speed maneuvering by fish and small underwater vehicles is considered. The focus is on fluid engineering rather than on biology. An attempt is made to learn from aquatic animals and apply the distilled knowledge to build maneuvering devices. The work is described in three parts. In the first, the morphology of twenty eight species of fish is considered. They are classified into three categories: low speed highly maneuverable, high speed poorly maneuverable, and an overlapping category, viz., high speed highly maneuverable. The qualitative relationship between the length scales of their fins and maneuvering ability is examined. Next, an obstacle-filled aquarium is built and the maneuvering trajectories of two species of fish that are fast yet maneuverable, are video-taped and digitized. Their performance are compared with those of small underwater vehicles. In this manner, the maneuvering “gap” between nature and engineering which appears to be large, is quantified. Finally, based on their length scales in species offish that are deft in maneuvering, a dorsal-fin based maneuvering device is built and its behavior is studied.
The Effects of Salt Water on Bubble Cavitation
Authors: Ceccio S. , Gowing S. , Shen Y. T.
Published: February 2008

Experiments were performed to examine differences in bubble cavitation inception, form, and acoustic emission in fresh and salt water. The tests were conducted on an axisymmetric headform known as the ITTC body, and acoustic emission, cavitation bubble size, and cavitation event rates were measured for a variety of cavitation numbers and dissolved air contents at a fixed dynamic head for both fresh and salt water. Differences were detected in the cavitation inception index, cavitation bubble size distribution, and the resulting overall acoustic emission. These differences may be attributed to variations in the freestream nuclei population, and it is hypothesized that the solution of salt reduced the number and size of freestream nuclei. Differences in the overall acoustic emission were attributed to variation in the bubble event rate and average maximum bubble size between the fresh and salt water cavitation.
The Axisymmetric Sudden Expansion Flow of a Non-Newtonian Viscoplastic Fluid
Authors: Vradis G. C. , O¨tu¨gen M. V.
Published: February 2008

The flow of a non-Newtonian viscoplastic Bingham fluid over an axisymmetric sudden expansion is studied by numerically solving the governing fully-elliptic continuity and momentum equations. Solutions are obtained for a wide range of Reynolds and yield numbers in the laminar flow regime with constant fluid properties. The present work demonstrates that the finite-difference technique can successfully be employed to obtain solutions to separating/reattaching internal flows of Bingham plastics. The results demonstrate the strong effects of the yield and Reynolds numbers on both the integral and the local structure of the separating and reattaching flow. Higher yield numbers result in larger overall effective viscosities and thus faster flow recovery downstream of the sudden expansion. The reattachment length decreases with increasing yield numbers, eventually reaching an asymptotic nonzero value which, in turn, is dependent on the Reynolds number. The strength of the recirculating flow also decreases with increasing yield numbers.
Turbulent Near Wake Behind a Partially Grooved Circular Cylinder
Authors: Lo K. W. , Ko N. W. M.
Published: February 2008

The near wake of a partially V-grooved circular cylinder is investigated. The flow over the smooth half is within the high subcritical regime, while that over the grooved half is within the transcritical regime. Secondary and Strouhal vortices are found in the near wakes of both halves. The formation of the secondary vortices, due to the separated shear layers, is coupled to the formation of the Strouhal vortices. These secondary vortices of the two halves are of the horse shoe type. On the grooved half, another type of secondary vortices is found to be caused by the flow over the triangular blunt end of the V grooves.
Effects of N+ Ion Implantation on Surface Modification of Cavitation Damage for 0Cr13Ni9Ti SS of Turbine Materials
Authors: Wu Jian Hua , Chai Gong Chun
Published: February 2008

In this paper, six technologies have been used to implant N+ into 0Cr13Ni9Ti SS, which is a kind of general turbine material in China, and their comparative experiments have been done to get a much better technology of modification of cavitation damage. The results show that the ability of cavitation resistance for 0Cr13Ni9Ti SS by N+ ion implantation can reach more than 2 times of that before treatment. Meanwhile, for various technologies of N+ ion implantation the changes of the surface compounds and their organizations are analysed by X-ray diffraction (XRD).
The Effect of Pressure Gradients on Transition Zone Length in Hypersonic Boundary Layers
Authors: Kimmel R. L.
Published: February 2008

Boundary layer transition was measured in zero, favorable, and adverse pressure gradients at Mach 8 using heat transfer. Models consisted of 7° half angle forecones 0.4826 m long, followed by flared or ogive aft bodies 0.5334 m long. The flares and ogives produced constant pressure gradients. For the cases examined, favorable pressure gradients delay transition and adverse pressure gradients promote transition, but transition zone lengths are shorter in favorable pressure gradient. Results of the effect of adverse pressure gradient on transition zone lengths were inconclusive.
Perspective: Measurements and Analyses of Nonlinear Wave Interactions With Higher-Order Spectral Moments
Authors: Hajj M. R. , Miksad R. W. , Powers E. J.
Published: February 2008

In fluid flows, fluid-structure interactions and other fluids-related problem, nonlinear dynamics play an important role in determining the development, response or output. Understanding these dynamics is essential for development of analytical models and prediction and control purposes. Higher-order statistical analysis has been shown to be an effective tool that can be applied to identify nonlinear couplings and measure energy transfer rates. These techniques have been applied by our group and others to investigate transition of shear flows, energy cascading in turbulence, oceanographic and geophysical flows, and fluid-structure interactions. The results of these investigations revealed important nonlinear characteristics regarding these problems. In this paper, we review these techniques and explain their usefulness in identification and quantification of nonlinear dynamics.
Effects of Convection and Decay of Turbulence on the Wall Pressure Wavenumber-Frequency Spectrum
Authors: Keith W. L. , Abraham B. M.
Published: February 2008

Cross-spectral and cross-correlation data from experiments and numerical simulations have shown the turbulent wall pressure convection velocity to vary with the streamwise sensor spatial separation. This variation is due to the spatial decay rates of turbulent structures in the inner and outer regions of the boundary layer. Its effect is shown to have a significant impact on the distribution of energy in the wavenumber-frequency spectrum Φ (k1, ω). The standard Corcos model is known to over predict the wavenumber-frequency spectrum at low wavenumbers. This is shown to result from its constant convection velocity assumption. The spectral levels at sub convective and lower wavenumbers are shown to be directly influenced by the spatial variation in convection velocity. Convection velocity measurements from past investigations that cover the range 285 ≤ Rθ ≤ 29,000 are compared, and an outer variable scaling is shown to effectively collapse the data.
Numerical Verification of Scaling Laws for Shock-Boundary Layer Interactions in Arbitrary Gases
Authors: Cramer M. S. , Park S. H. , Watson L. T.
Published: February 2008

The steady, two-dimensional interaction of an oblique shock with a laminar flat-plate boundary layer has been examined through use of the Beam-Warming implicit scheme. A wide range of fluids is considered as are freestream pressures corresponding to dense gases, i.e., gases at pressures which are so large that the ideal gas law is no longer accurate. The results, when combined with the triple-deck theory of Kluwick (1994), provides strong support for the idea that the classical scaling laws can be extended to dense gases.
Energy Separation in a Jet Flow
Authors: Seol W. S. , Goldstein R. J.
Published: February 2008

Fluids in motion can separate into regions of higher and lower energy (temperature); this is called “energy separation.” The present study concerns the mechanism of energy separation in a free, circular, air jet, including the effects of acoustic excitation. Starting with the initial energy separation occurring in the boundary layer inside the nozzle, the energy separation in a jet begins to be affected by the action of vortices from an axial location, measured from the jet exit, of about 0.3D (D is the diameter of nozzle exit), becomes intensified at about 0.5D, begins to be diffused from about 1D, and there is no discernible energy separation at about 14D. The entrainment of the ambient fluid considerably affects the energy separation, and its effects appear at axial locations between about 6D and 8D. The present definition of the energy separation factor renders its distribution independent of the jet Reynolds number; except for axial locations between about 0.3D and 4D. The development of energy separation in the region close to the nozzle exit is faster when the jet Reynolds number is higher. Acoustic excitation not only enhances the energy separation, but also accelerates its diffusion. This effect is greatest for axial locations between about 1D and 4D. The fact that the acoustic excitation has a strong effect on the vortex structure and the energy separation provides good evidence that the convective that the convective that the convective movement of vortices is the cause of energy separation in jets.
Predictions of Impacting Sonic and Supersonic Jets
Authors: Cumber P. S. , Fairweather M. , Falle S. A. E. G. , Giddings J. R.
Published: February 2008

A mathematical model of sonic and supersonic jets, validated previously by the present authors for the prediction of moderately and highly underexpanded free jets, is used to simulate the near field structure of jets which impact a flat surface orthogonally, and its accuracy assessed by comparing model predictions with experimental data available in the literature. For impacting, moderately underexpanded jets, results derived from the model are found to be in close agreement with data on the location of both free jet shocks, and the stand-off shock formed adjacent to the impacted surface. In addition, the model provides reasonable estimates of density within the free jet and stagnation regions of such flows, with the existence, or otherwise, of bubbles being successfully predicted. Measurements of pressure occurring on the surface of the impacted plate, produced by the impingement of both sonic and supersonic jets, are also predicted with reasonable accuracy, although the decaying amplitude of spatially periodic pressure oscillations within the wall jet region of these flows is slightly over predicted in some cases.
The Case of the Singing Vortex
Authors: Maines B. , Arndt R. E. A.
Published: February 2008

A relatively high amplitude, discrete tone is radiated from fully developed tip vortex cavitation under certain conditions. The phenomenon of the “singing vortex” was first reported by Higuchi et al. (1989). This study more closely examines the singing phenomenon by varying the hydrofoil cross-section, scale, angle of attack, water quality, and cavitation number in two different facilities. Noise data were collected for each condition with visual documentation using both still photography and high speed video in an effort to explain the mechanism of vortex singing. The theory of Kelvin (1880) provides a framework for correlating all the data obtained.
Turbulent Wall-Pressure Fluctuations: A New Model for Off-Axis Cross-Spectral Density
Authors: Singer B. A.
Published: February 2008

Models for the distribution of the wall-pressure under a turbulent boundary layer often estimate the coherence of the cross-spectral density in terms of a product of two coherence functions. One such function describes the coherence as a function of separation distance in the mean-flow direction, the other function describes the coherence in the cross-stream direction. Analysis of data from a large-eddy simulation of a turbulent boundary layer reveals that this approximation dramatically underpredicts the coherence for separation directions that are neither aligned with nor perpendicular to the mean-flow direction. These models fail even when the coherence functions in the directions parallel and perpendicular to the mean flow are known exactly. A new approach for combining the parallel and perpendicular coherence functions is presented. The new approach results in vastly improved approximations for the coherence.
Wavenumber Spectra of High Magnitude Wall Pressure Events in a Numerically Simulated Turbulent Boundary Layer
Authors: Abraham B. M. , Keith W. L.
Published: February 2008

A method for conditionally sampling the spatial field of the wall pressure beneath a turbulent boundary layer in order to search for high magnitude events and calculate the corresponding wavenumber spectrum is presented. The high magnitude events are found using a simple peak detection algorithm at a fixed instant in time and the wavenumber spectra are calculated using discrete Fourier transforms. The frequency of occurrence for high magnitude positive events is found to be approximately the same as for high magnitude negative events. The contribution of the high magnitude events to the rms wall pressure for various trigger levels is calculated and compared with results from similar experimental studies performed in the time domain. The high magnitude events are shown to occur infrequently and to contribute significantly to the rms wall pressure. Wavenumber spectra from the high magnitude positive and negative events are calculated and compared with the unconditionally sampled spectra. The high magnitude events contain energy focused around a particular stream-wise wavenumber and have high broadband spectral levels.
Near-Wall Modeling of Plane Turbulent Wall Jets
Authors: Gerodimos G. , So R. M. C.
Published: February 2008

In most two-dimensional simple turbulent flows, the location of zero shear usually coincides with that of vanishing mean velocity gradient. However, such is not the case for plane turbulent wall jets. This could be due to the fact that the driving potential is the jet exit momentum, which gives rise to an outer region that resembles a free jet and an inner layer that is similar to a boundary layer. The interaction of a free-jet like flow with a boundary-layer type flow distinguishes the plane wall jet from other simple flows. Consequently, in the past, two-equation turbulence models are seldom able to predict the jet spread correctly. The present study investigates the appropriateness of two-equation modeling; particularly the importance of near-wall modeling and the validity of the equilibrium turbulence assumption. An improved near-wall model and three others are analyzed and their predictions are compared with recent measurements of plane wall jets. The jet spread is calculated correctly by the improved model, which is able to replicate the mixing behavior between the outer jet-like and inner wall layer and is asymptotically consistent. Good agreement with other measured quantities is also obtained. However, other near-wall models tested are also capable of reproducing the Reynolds-number effects of plane wall jets, but their predictions of the jet spread are incorrect.
A Reynolds Stress Function for Wall Layers
Authors: Panton R. L.
Published: February 2008

Data for the Reynolds stress near a wall are correlated by employing the theory of perturbation methods to form a composite expansion. This absorbs, to first order, the effects of Reynolds number. Published data from three experimental and three direct numerical simulations of channel flow and one experimental and two direct numerical simulations of pipe flow were examined. A single curve fits the results to within the scatter of the data. If Reynolds number effects exist, they are hidden in the scatter. A closed-form equation, with one arbitrary constant, represents the data and has the proper limiting behavior both very near and very far from the wall.
Assessment of Artificial Dissipation Models for Three-Dimensional Incompressible Flow Solutions
Authors: Lin F. B. , Sotiropoulos F.
Published: February 2008

Various approaches for constructing artificial dissipation terms for three-dimensional artificial compressibility algorithms are presented and evaluated. Two, second-order accurate, central-differencing schemes, with explicitly added scalar and matrix-valued fourth-difference artificial dissipation, respectively, and a third-order accurate flux-difference splitting upwind scheme are implemented in a multigrid time-stepping procedure and applied to calculate laminar flow through a strongly curved duct. Extensive grid-refinement studies are carried out to investigate the grid sensitivity of each discretization approach. The calculations indicate that even the finest mesh employed, consisting of over 700,000 grid nodes, is not sufficient to establish grid independent solutions. However, all three schemes appear to converge toward the same solution as the grid spacing approaches zero. The matrix-valued dissipation scheme introduces the least amount of artificial dissipation and should be expected to yield the most accurate solutions on a given mesh. The flux-difference splitting upwind scheme, on the other hand, is more dissipative and, thus, particularly sensitive to grid resolution, but exhibits the best overall convergence characteristics on grids with large aspect ratios.
A Finite-Element Study of Newtonian and Power-Law Fluids in Conical Channel Flow
Authors: Garrioch S. H. , James D. F.
Published: February 2008

A numerical study of Newtonian and shear-thinning fluids in high-speed, laminar flow through a conical channel is presented. Using a variety of cone-angles and Reynolds numbers on the order of 100, converging flow is mapped according to several characteristics: the angle at which separation occurs at the cone outlet, the extent to which sink-flow is approximated, and the pressure drop through the cone. While the data provides a fundamental description of conical flow, it may be of particular usefulness to rheologists in establishing an inelastic baseline for a converging-flow rheometer.
An Experimental Investigation Into the Breakdown of Low Reynolds Number Pulsed Flows at a Pipe Orifice
Authors: Addison P. S. , Ervine D. A. , Chan A. H. C. , Williams K. J.
Published: February 2008

An experimental investigation into the behavior of pulsed vortex flows in water at a pipe orifice is outlined. An experimental apparatus is described whereby flow pulsations are generated using an electronically controlled motor-piston arrangement. Preliminary investigations are made into the unpulsed system to determine the natural vortex shedding frequency for various Reynolds numbers and orifice diameters. The downstream development of initially axisymmetric, periodically pulsed vortices shed from the orifice plate are then examined. The growth, evolution, and eventual breakdown of the initially axisymmetric vortices are investigated for a range of flow control parameters. Various flow regimes are discerned and an attempt is made to categorize them.
Design of Homologous Ram Pumps
Authors: Young Brian
Published: February 2008

The hydraulic ram pump was invented over 200 years ago but became obsolete with the availability of mains water. Ram pumps make use of a renewable energy resource and are environmentally friendly. They are particularly appropriate for use in the developing world as their construction is basic and robust and they are inexpensive and easy to install and maintain. Unfortunately, simple rational design procedures applicable to any size of pump are not available. The paper analyses the behavior of ram pumps under zero recoil conditions from which three nondimensional design relationships, applicable at any delivery head, are derived for beat frequency, quantity delivered, and source capacity in terms of the independent variables. These relationships incorporate constants consistent with existing experimental results. Recommendations are given for the maximum length of drive pipe which supersede traditional rules-of-thumb for the ratios L/D and L/H. The procedure allows selection of all system parameters, including the drive pipe length, for a set of homologous ram pumps.
Fluid Force Moment on a Centrifugal Impeller Shroud in Precessing Motion
Authors: Tsujimoto Yoshinobu , Yoshida Yoshiki , Ohashi Hideo , Teramoto Norihiro , Ishizaki Shin
Published: February 2008

Experimental results of fluid moment on a centrifugal impeller shroud in precessing motion are discussed based on the bulk flow model to elucidate the fundamental flow mechanism. It is shown that the backshroud/casing clearance flow and the destabilizing fluid force moment can be simulated by the bulk flow model fairly well if the measured behavior of the resistance is correctly incorporated in the model. From the calculations with and without steady and unsteady wall shear stresses, the unsteady component of the clearance flow is shown to be basically a two-dimensional inviscid flow induced by the change in the flow thickness. The effects of the leakage flow rate and the resistance at the leakage flow entry are discussed, paying attention to the steady tangential velocity of the leakage flow.
Axisymmetric Inertial Oscillations in Transient Rotating Flows in a Cylinder
Authors: Kim Jae Won , Hyun Jae Min
Published: February 2008

A numerical study is made of axisymmetric inertial oscillations in a fluid-filled cylinder. The entire cylinder undergoes a spin-up process from rest with an impulsively started rotation rate Ω(t) = Ω0 + εω cos(ωt). Numerical solutions are obtained to the axisymmetric, time-dependent Navier-Stokes equations. Identification of the inertial oscillations is made by inspecting the evolution of the pressure difference between two pre-set points on the central axis, Cp. In the limit of large time, the inertial frequency thus determined is in close agreement with the results of the classical inviscid theory for solid-body rotation. As in previous experimental studies, the t* − (Ω0/ω) plots are constructed for inertial oscillations, where t* indicates the time duration until the maximum Cp is detected. These detailed numerical results are in broad agreement with the prior experimental data. Flow intensifications under the resonance conditions are illustrated based on the numerical results. Depictions are made of the increase in the amplitude of oscillating part of the total angular momentum under the resonance conditions. Also, the patterns of t* − (Ω0/ω) curves are displayed for different inertial frequency modes.
A Numerical Study on Motion of a Sphere Coated With a Thin Liquid Film at Intermediate Reynolds Numbers
Authors: Kawano S. , Hashimoto H.
Published: February 2008

The steady viscous flow past a sphere coated with a thin liquid film at low and intermediate Reynolds numbers (Re ≤ 200) was investigated numerically. The influences of fluid physical properties, film thickness, and Reynolds number on the flow pattern were clarified. Temperature field around the compound drop was also analyzed. The strong dependence of flow pattern on the characteristics of heat transfer was recognized. The empirical equation of the drag coefficient for the compound drop was proposed. Furthermore, the explicit adaptability of the drag coefficient equation for a gas bubble, a liquid drop, and a rigid, sphere in the range of Reynolds number Re ≤ 1000 was confirmed.
The Rise of Bubbles in a Vertical Shear Flow
Authors: Ervin E. A. , Tryggvason G.
Published: February 2008

Full numerical simulations of two- and three-dimensional bubbles in a shear flow, by a finite difference front tracking method, are presented. The effects of inertial, viscous, gravitational, and surface forces on the lift of a deformable bubble rising due to buoyancy in a vertical shear flow, are examined. Bubbles with a large surface tension coefficient migrate toward the downward moving fluid, as predicted analytically for a cylinder or a sphere in a shear flow. Bubbles with smaller surface tension deform, and generally migrate in the opposite direction. The combined effects of the shear flow and the buoyancy deform the bubble in such a way that the circulation around the deformed bubbles is opposite to that of undeformed bubbles.
Flow Characteristics of a Bluff Body Cut From a Circular Cylinder
Authors: Aiba S. , Watanabe H.
Published: February 2008

This is a report on an investigation of the flow characteristics of a bluff body cut from a circular cylinder. The volume removed from the cylinder is equal to d/2(1 − cos θs), where d and θs are the diameter and the angular position (in the case of a circular cylinder, θs, = 0 deg), respectively. θs, ranged from 0 deg to 72.5 deg and Re (based on d and the upstream uniform flow velocity U∞) from 2.0 × 104 to 3.5 × 104. It is found that a singular flow around the cylinder occurs at around θs = 53 deg when Re > 2.5 × 104, and the base pressure coefficient (−Cpb,) and the drag coefficient CD take small values compared with those for otherθs.
Flow Past Large Obstructions Between Corotating Disks in Fixed Cylindrical Enclosures
Authors: Suzuki Hiroshi , Humphrey Joseph A. C.
Published: February 2008

Numerical calculations have been performed for isothermal, laminar, three-dimensional flow past one or two fixed obstructions radially aligned and symmetrically located between a pair of disks corotating in a fixed cylindrical enclosure. The single-obstruction cases respectively model the influence on the flow of (a) a magnetic head arm support and (b) an air lock. The dual-obstruction cases model the simultaneous presence of these two objects. The air lock produces an interdisk cross-stream plane blockage of 62 percent while the two head arm supports produce blockages of 31 percent and 62 percent, respectively. For the cases with the air lock and arm support simultaneously present, the circumferential angle between them is fixed to 40 or 80 deg. Velocity, pressure, shear stress and the disk torque coefficient are predicted mostly for a Reynolds number (Re=ΩR22/v) corresponding to 10,000, approximately, where R2, Ω, and v are the disk radius, the disk angular velocity in rad/s, and the kinematic viscosity of air at 300 K, respectively. The calculations show that a large blockage significantly alters the interdisk flow characteristics by markedly raising the pressure ahead of an obstruction and accelerating the flow through the empty space around it. This induces a detached region of reversed flow ahead of the obstruction, quite distinct from that in its wake. The disk surface pressure distributions point to a potential source of dynamical instability in rotating disk flows with obstructions. By redefining the torque coefficient and Reynolds number to account for dual blockage effects the relationship between these two quantities generally follows the theoretical expression of Humphrey et al. (1992). It is shown that the bulk of the drag on an obstruction is form drag as opposed to friction drag.
Suppression of Fluid Forces Acting on a Square Prism by Passive Control
Authors: Sakamoto H. , Tan K. , Takeuchi N. , Haniu H.
Published: February 2008

Suppression of fluid forces acting on a square prism by passive control of the approaching flow was investigated in the present study. Flow was controlled using a small flat plate upstream of the prism. The position of the flat plate was varied within the range of S/W = 0 ~ 3.0 (S: distance between the flat plate and square prism, W: width of square prism) and the width h of the flat plate ranged from 2 mm to 8 mm (h/W = 0.05 ~ 0.19). Steady and unsteady fluid forces, vortex shedding frequency, and flow pattern were systematically investigated. The maximum reduction of time-averaged drag was 75 percent, and the maximum reduction in fluctuating lift and drag was 95 and 80 percent, respectively, using a flat plate 1/10 of the size of the square prism.
Prediction of Transpired Turbulent Boundary Layers With Arbitrary Pressure Gradients
Authors: Oljaca Miodrag , Sucec James
Published: February 2008

An integral method, using Coles combined inner and outer law as the velocity profile, is developed for calculation of turbulent boundary layers with blowing or suction and pressure gradients. The resulting ordinary differential equations are solved numerically for the distribution of skin friction coefficient and integral thickness along the surface. Comparisons of predicted skin friction coefficients with experimental data are made for a wide range of blowing and suction rates and for various pressure gradients, including adverse, zero and a strong favorable gradient. In addition to good agreement with experimental data for constant blowing fractions F, the method is also successfully tested on cases where the blowing fraction is variable with position. Predictions, in general, exhibit satisfactory agreement with the data. The integral method predictions are comparable to, or better than, a number of finite difference procedures in a limited number of cases where comparisons were made.