bluuebunny/crossref_metadata_2025 huggingface dataset:
Navier-Stokes Simulations of a Novel Viscous Pump
Authors: Sharatchandra M. C. , Sen Mihir , Gad-el-Hak Mohamed
Published: February 2008

A numerical study of flow in a novel viscous-based pumping device appropriate for microscale applications is described. The device, essentially consisting of a rotating cylinder eccentrically placed in a channel, is shown to be capable of generating a net flow against an externally imposed pressure gradient. Navier-Stokes Simulations at low Reynolds numbers are carried out using a finite-volume approach to study the influence of various geometric parameters. Slip effects for gas flows are also briefly investigated. The numerical results indicate that the generated flow rate is a maximum when the cylinder is in contact with a channel wall and that an optimum plate spacing exists. These observations are in excellent agreement, both qualitatively and quantitatively, with a previous experimental study. Furthermore, it is shown that effective pumping is obtained even for considerably higher Reynolds numbers, thereby extending the performance envelope of the proposed device to non-microscale applications as well. Finally, slip-flow effects appear to be significant only for Knudsen numbers greater than 0.1, which is important from the point of view of microscale applications.
LDV Measurements of Spatially Periodic Flows Over a Detached Solid-Rib Array
Authors: Liou Tong-Miin , Yang Chih-Ping , Lee Hsin-Li
Published: February 2008

Measurements of mean velocities, turbulence intensities, and Reynolds stresses are presented for spatially periodic flows in a duct of width-to-height ratio 2 with a detached solid-rib array. The Reynolds number based on the duct hydraulic diameter and cross-sectional bulk mean velocity (Ub), the pitch to rib-height ratio, and the rib-height to duct-height ratio were 2 × 104, 10, and 0.133, respectively. The rib-detached-distance to rib-height ratio was varied from 0 to 3.25 (duct axis) to study its effect on wake length and asymmetry, convective velocity and turbulent kinetic energy immediately behind the rib, maximum turbulent shear stress, and turbulence anisotropy. The results showed that the dominant fluid dynamic factors responsible for the reported peak values of local Nusselt number around the detached rib could be identified. Moreover, the turbulence structure parameter distribution and anisotropy were analyzed to examine the basic assumptions embedded in the turbulence models. Furthermore, the secondary-flow mean velocities were found to be one to two order of magnitude smaller than Ub.
Experimental Determination of Permeability and Inertia Coefficients of Mechanically Compressed Aluminum Porous Matrices
Authors: Antohe B. V. , Lage J. L. , Price D. C. , Weber R. M.
Published: February 2008

A heat exchanger, using mechanically compressed microporous matrices, is being developed for cooling high power electronics. The thermal efficiency of this new device depends on the hydraulic characteristics (porosity φ, permeability K, and Forchheimer coefficient cF) of the matrix inserted in it. These quantities have to be obtained experimentally as predictive models do not exist. Twenty-eight compressed matrices are initially chosen for experimental testing. Based on structural requirements, nine matrices are selected for full hydraulic characterization. The determination of permeability and inertia coefficient of each matrix is performed following a proposed direct methodology based on the curve fitting of the experimental results. This methodology is found to yield more consistent and accurate results than existing methods. The uncertainty of the experimental results is evaluated with a new and general procedure that can be applied to any curve fitting technique. Results indicate that the tested matrices have a unique characteristic, that of a relatively wide porosity range, from 0.3 to 0.7, within a relatively narrow permeability range, from 1.0 × 10−10 m2 to 12 × 10−10 m2. The inertia coefficient varies from 0.3 to 0.9. These hydraulic characteristics lead to a microporous heat exchanger performing within requirements.
Tip Vortex Formation and Cavitation
Authors: Maines B. H. , Arndt R. E. A.
Published: February 2008

This paper summarizes recent research on the relation between boundary layer flow, tip vortex structure for a finite span wing, and cavitation. Three hydrofoils of elliptic planform of aspect ratio 3 were constructed with different NACA cross sections. Using a sprayed oil droplet technique to visualize the boundary layer flow, each foil was found to have dramatically different flow separation characteristics on both the suction and pressure sides. Careful examination of the tip region suggests that while the initial stages of vortex roll-up from the pressure side are similar for each hydrofoil section, the vortex boundary layer interaction on the suction side differs for each section. The degree of interaction was observed to increase as the lifting efficiency decreased. Over the Reynolds number range tested, tip vortex cavitation inception has been observed to follow an almost universal scaling. Differences in this scaling law are correlated with the degree of vortex/boundary layer interaction.
Numerical Modeling of the Thermodynamic Effects of Cavitation
Authors: Deshpande Manish , Feng Jinzhang , Merkle Charles L.
Published: February 2008

A Navier-Stokes solver based on artificial compressibility and pseudo-time stepping, coupled with the energy equation, is used to model the thermodynamic effects of cavitation in cryogenic fluids. The analysis is restricted to partial sheet cavitation in two-dimensional cascades. Thermodynamic effects of cavitation assume significance in cryogenic fluids because these fluids are generally operated close to the critical point and also because of the strong dependence of the vapor pressure on the temperature. The numerical approach used is direct and fully nonlinear, that is, the cavity profile evolves as part of the solution for a specified cavitation pressure. This precludes the necessity of specifying the cavity length or the location of the inception point. Numerical solutions are presented for two-dimensional flow problems and validated with experimental measurements. Predicted temperature depressions are also compared with measurements for liquid hydrogen and nitrogen. The cavitation procedure presented is easy to implement in engineering codes to provide satisfactory predictions of cavitation. The flexibility of the formulation also allows extension to more complex flows and/or geometries.
Ensemble-Average Equations of a Particulate Mixture
Authors: Liljegren L. M.
Published: February 2008

A method of evaluating the transfer terms appearing in the ensemble-average fluid transport equation is developed and applied to obtain the transport equations describing flow of a dilute particulate mixture containing solid spherical particles. The equations apply in the limit where interactions between phases are both quasi-steady and viscous, which is defined as flows that meet the three criterion Ref(a/ℒ)2 ≪ 1, vfτ/a2 ≫ 1, and Rep ≪ 1. In this limit, the terms describing transfer of momentum between the two phases of the mixture are evaluated to O(1) in the particle radius and O(γp) in the particle phase density. The continuity equations and consistency principle are exact. When the first two conditions are not met, the transport equations require the terms that describe virtual mass forces; when the third is not met, the transport equations require terms that describe Oseen corrections to the drag term.
A Numerical Study of Vortex Shedding From a Square Cylinder With Ground Effect
Authors: Hwang Robert R. , Yao Chia-Chi
Published: February 2008

A numerical study has been conducted to investigate the behavior of the vortical wake created by a square cylinder placed in a laminar boundary-layer flow. The calculations are performed by solving the unsteady 2D Navier-Stokes equations with a finite-volume method. The Reynolds-number regime investigated is from 500 to 1500. Another parameter that is varied is the distance of the cylinder from the wall. The initial and subsequent development of the vortex shedding phenomenon are investigated. The presence of the wall is found to have strong effects on the properties of these vortices, as well as lift, drag, and Strouhal number.
Effect of a Crossflow at the Entrance to a Film-Cooling Hole
Authors: Thole K. A. , Gritsch M. , Schulz A. , Wittig S.
Published: February 2008

Understanding the complex flow of jets issuing into a crossflow from an inclined hole that has a short length-to-diameter ration is relevant for film-cooling applications on gas turbine blades. In particular, this experimental study focused on the effect of different velocities in a coflowing channel at the cooling hole entrance. Flows on both sides of the cooling hole (entrance and exit) were parallel and in the same direction. With the blowing ratio and the mainstream velocity at the hole exit remaining fixed, only the flow velocity in the channel at the hole entrance was varied. The Mach number at the hole entrance was varied between 0 < Mac < 0.5, while the Mach number at the hole exit remained constant at Ma∞ = 0.25. The velocity ratio and density ratio of the jet were unity giving a blowing ratio and momentum flux ratio also of unity. The single, scaled-up film-cooling hole was inclined at 30 deg with respect to the mainstream and had a hole length-to-diameter ratio of L/D = 6. Flowfield measurements were made inside the hole, at the hole inlet and exit, and in the near-hole region where the jet interacted with the crossflow at the hole exit. The results show that for entrance crossflow Mach numbers of Mac = 0 and 0.5, a separation region occurs on the leeward and windward side of the cooling hole entrances, respectively. As a result of this separation region, the cooling jet exits in a skewed manner with very high turbulence levels.
Numerical Analysis of Turbulent Flow in Fluid Couplings
Authors: Bai L. , Fiebig M. , Mitra N. K.
Published: February 2008

Numerical simulation of three-dimensional unsteady turbulent flows in fluid couplings was carried out by numerically solving Navier-Stokes equations in a rotating coordinate system. The standard k-ε model was used to take turbulence into account. A finite volume scheme with colocated body-fitted grids was used to solve the basic equations. Computed flow structures show the vortex generation and its effect on the torque transmission. Computed local velocity and torque flow compare well with measurements.
Effects of Efficiency Techniques on Accuracy of Dynamic-Overlapped Grids for Unsteady Flows
Authors: Yen Guan-Wei , Baysal Oktay
Published: February 2008

There is a strong need for advancements to compute unsteady flows involving moving boundaries and bodies. Currently, there appear to be two dynamic approaches and their variants proposed for this problem: unstructured and domain decomposition methods. The present objective was to further explore the overlapped grids when it was enhanced with several enabling and efficiency techniques. Their effect on the accuracy was studied primarily from a qualitative point of view. Unsteady Euler equations were solved by a characteristic-based alternating-direction-implicit scheme on overlapped subdomains capable of moving with respect to each other. Initially, the errors due to discretization, Courant number, and interpolations, were studied by solving the Riemann problem and comparing with its exact solution. It was concluded, that a second- or higher-order method, temporally and spatially, was needed even in the dynamic interpolation regions. Secondly, the effects of intermediate grids and diagonal inversions were demonstrated by simulating the 2-D flowfield history of a store separating from a wing section along a prescribed trajectory. For computational efficiency, using approximate diagonal inversions for the discrete equations produced less accurate but acceptable results. However, to ease the grid generation, overlaying an intermediate subdomain in a region of high flow gradients had significant impact on the accuracy.
Numerical Experiments on Application of Richardson Extrapolation With Nonuniform Grids
Authors: Celik Ismail , Karatekin Ozgur
Published: February 2008

Some unresolved problems related to Richardson extrapolation (RE) are elucidated via examples, and possible remedies are suggested. The method is applied to the case of turbulent flow past a backward facing step using nonuniform grid distributions. It is demonstrated that RE can be used to obtain grid independent solutions using the same grid refinement factors in both coordinate directions. The use of generalized wall functions together with the standard k-ε model seems to work well even if the grid refinement extends into the viscous sublayer. In addition, the grid convergence index and other standard uncertainty measures are compared, and a new uncertainty measure is suggested which seems to be a better indicator for the grid convergence error.
Shape of an Annular Liquid Jet
Authors: Hasan M. Z. , Mitsutake Y. , Monde M.
Published: February 2008

Analytical and experimental studies have been done to determine the shape of a vertical, axisymmetric, annular liquid jet. From a balance of the surface, pressure, gravity, and inertia forces, a nonlinear, second-order, ordinary differential equation is obtained for the shape of the annular jet. This equation is solved numerically by the Runge-Kutta-Nystro¨m method. An annular jet either converges (closes), diverges, or maintains (theoretically) its original radius depending upon the magnitude of the difference between the inside and outside pressure. This corresponds to, in terms of a dimensionless pressure p, whether p is less than, greater than, or equal to 2. An experiment has been performed to verify the analytical solution. The jet velocity, inside pressure and other parameters have been varied to obtain different shapes of the jet, both closing and diverging. Good agreement with the analytical prediction is found.
On the Influence of Liquid Elasticity on Mixing in a Vessel Agitated by a Two-Bladed Impeller
Authors: Youcefi A. , Anne-Archard D. , Boisson H. C. , Sengelin M.
Published: February 2008

The experimental study of the velocity field and power consumption has been conducted in a mixing vessel equipped with a two-bladed impeller for a viscoelastic fluid and Newtonian and non-Newtonian inelastic fluids for comparison. The results are interpreted in the light of a numerical simulation using a finite element method for an “Oldroyd B” fluid. Both numerically and experimentally a significant influence of elasticity is found for an effective Reynolds number greater than a threshold value Re* = 5 to 7. Above this value, the power number is increased and the velocity field deeply modified. This behavior is mainly related to elasticity of the fluid.
Theoretical Study of Pressure Fluctuations Downstream of a Diffuser Pump Impeller—Part 2: Effects of Volute, Flow Rate and Radial Gap
Authors: Qin W. , Tsukamoto H.
Published: February 2008

The fundamental analysis in the first report was extended to calculate the unsteady flow induced by the interaction between impeller blades and diffuser vanes/volute casing in a diffuser pump. The unsteady flow in the diffuser vane passage, as well as the volute casing, is assumed to be induced by the five kinds of singularities—the bound vortices distributed on the impeller blades, diffuser vanes and volute casing wall, the sources at volute outlet, and the free vortices shed from the trailing edge of diffuser vanes. Calculated unsteady pressures agree with the corresponding experimental data. And the calculated results showed the effects of the flow rate, volute casing and the radial gaps between impeller blade trailing edge and diffuser vane leading edge on the magnitude of unsteady pressure downstream of impeller.
Tip Clearance and Tip Vortex Cavitation in an Axial Flow Pump
Authors: Laborde R. , Chantrel P. , Mory M.
Published: February 2008

A combined study of tip clearance and tip vortex cavitations in a pump-type rotating machine is presented. Cavitation patterns are observed and cavitation inception is determined for various gap heights, clearance and blade geometries, and rotor operating conditions. An optimum clearance geometry is seen to eliminate clearance cavitation when the clearance edge is rounded on the blade pressure side. The gap height has a strong effect on clearance cavitation inception, but the trends vary considerably when other parameters are also modified. The gap height and clearance geometry have less influence on tip vortex cavitation but forward and backward blade skew is observed to reduce and increase tip vortex cavitation, respectively, as compared to a blade with no skew.
The Erosive Axial Collapse of a Cavitating Vortex: An Experimental Study
Authors: Dominguez-Cortazar M. A. , Franc J. P. , Michel J. M.
Published: February 2008

The erosive efficiency of cavitating vortices is well known, although its exact mechanism has not been clarified. In order to bring fundamental information to the subject, a new device called “Cavermod” was designed in which axial collapse of a cavitating vortex is produced. We consider in this paper the design principles of the apparatus, the measurement equipment necessary for observing the phenomenon (collapse time of the order of a few milliseconds, axial collapse velocity varying between 70 and 700 m/s), and the main features of the flow. We also study the damage produced on erosion targets in order to correlate the vortex collapse with the erosion indentation features.
Experimental Study of Deformation Mechanism of a Water Droplet Impinging on Hot Metallic Surfaces Above the Leidenfrost Temperature
Authors: Hatta Natsuo , Fujimoto Hitoshi , Kinoshita Kenji , Takuda Hirohiko
Published: February 2008

This paper is concerned with the collision dynamics of a water droplet impinging on three kinds of smooth surfaces (Inconel alloy 625, stainless-steel, and silicon) heated to above the Leidenfrost temperature (500°C). It has been found that the time histories of the droplet diameter, the height and the distance between the bottom of droplet and the hot surface after rebounding are almost unchangeable regardless of the kind of surface material, when the Weber number is kept so low that the droplet does not break up into some parts. However, the critical Weber number, whether or not the droplet is disintegrated into some pieces during deformation, has been confirmed to be changeable depending upon the kind of surface material. For relatively low Weber number cases, but above the critical one, the droplet breaks up into some parts after the droplet reaches a maximum diameter on the surface. As the Weber number is increased further, the droplet disintegration occurs during the spreading process. Also, the droplet disintegration mechanism has been discussed from an experimental point of view.
Equilibrium of Ventilated Cavities in Tip Vortices
Authors: Castro F. , Crespo A. , Manuel F. , Fruman D. H.
Published: February 2008

The behavior of air cavities formed in the tip vortex trailing from rectangular and elliptical planform hydrofoils has been examined both theoretically and experimentally. These cavities have some peculiarities, in particular their ability to move upstream and reach a stable position near the hydrofoil. Detailed measurements of the velocity components and the cavity characteristics have been made. The results are compared with the predictions of a theoretical model based on the equilibrium balance of a cavity developing in a Lamb vortex.
Mechanism and Control of Cloud Cavitation
Authors: Kawanami Y. , Kato H. , Yamaguchi H. , Tanimura M. , Tagaya Y.
Published: February 2008

Generation mechanism of cloud cavitation on a hydrofoil section was investigated in a sequence of experiments through observation of cloud cavitation by high-speed video and high-speed photo as well as pressure measurements by pressure pick-ups and a hydrophone. The mechanism was also investigated by controlling cloud cavitation with an obstacle fitted on the foil surface. From the results of these experiments, it was found that the collapse of a sheet cavity is triggered by a re-entrant jet rushing from the trailing edge to the leading edge of the sheet cavity, and consequently, the sheet cavity is shed in the vicinity of its leading edge and thrown downstream as a cluster of bubbles called cloud cavity. In other words, the re-entrant jet gives rise to cloud cavitation. Moreover, cloud cavitation could be controlled effectively by a small obstacle placed on the foil. It resulted in reduction of foil drag and cavitation noise.
Uncertainties and CFD Code Validation
Authors: Coleman H. W. , Stern F.
Published: February 2008

A new approach to computational fluid dynamics code validation is developed that gives proper consideration to experimental and simulation uncertainties. The comparison error is defined as the difference between the data and simulation values and represents the combination of all errors. The validation uncertainly is defined as the combination of the uncertainties in the experimental data and the portion of the uncertainties in the CFD prediction that can be estimated. This validation uncertainty sets the level at which validation can be achieved. The criterion for validation is that the magnitude of the comparison error must be less than the validation uncertainty. If validation is not accomplished, the magnitude and sign of the comparison error can be used to improve the mathematical modeling. Consideration is given to validation procedures for a single code, multiple codes and/or models, and predictions of trends. Example results of verification/validation are presented for a single computational fluid dynamics code and for a comparison of multiple turbulence models. The results demonstrate the usefulness of the proposed validation strategy. This new approach for validation should be useful in guiding future developments in computational fluid dynamics through validation studies and in the transition of computational fluid dynamics codes to design.
Interaction of a Steady Approach Flow and a Circular Cylinder Undergoing Forced Oscillation
Authors: Zhang Jianfeng , Dalton Charles
Published: February 2008

This paper presents a numerical study on the interaction of a steady approach flow and the forced transverse oscillation of a circular cylinder. The two-dimensional stream-function/vorticity formulation of the Navier-Stokes equations is solved by a semi-implicit finite-difference scheme. Calculations for flows with different amplitude (a) and frequency (fc) of the oscillation of the cylinder show a strong effect of the oscillation when fc is close to fso, the vortex shedding frequency, of the stationary cylinder. Lock-on of vortex shedding, distinct flow patterns, and increase in both drag and lift coefficients from those of a stationary cylinder are observed for Reynolds number Re = 200, a/R (R is the radius of the cylinder) from 1.0 to 2.0, fc/fso from 0.85 to 1.7. For Re = 855, a/R = 0.26, a large eddy simulation model for turbulent flow is used. The results at Re = 855 and a/R = 0.26 show that lock-on has occurred for fc/fso ≥ 0.85. The behavior of the drag and lift coefficients is seen to be influenced by the lock-on phenomenon.
State Estimator of Flow as an Integrated Computational Method With the Feedback of Online Experimental Measurement
Authors: Hayase Toshiyuki , Hayashi Satoru
Published: February 2008

This paper deals with a state estimator or simply an observer of flow field. The observer, being a fundamental concept in the control system theory, also has a potential in the analysis of flow related problems as an integrated computational method with the aid of experiment. In the framework of the observer, the state of physical flow is estimated from the mathematical model with the feedback of on-line experimental measurement. A SIMPLER based flow simulation algorithm is used as the mathematical model of the real flow and partial experimental measurement of flow is fed back to the boundary condition through the feedback controller. The existence of the feedback-loop essentially distinguishes the observer from ordinary flow simulations. Time variation of the computational result of the observer is expected to converge exactly to that of the physical flow in the whole flow domain even for unstable turbulent flows. A numerical experiment has been performed to confirm the validity of the proposed observer for a turbulent flow through a duct of square cross section. The physical flow to be estimated is modeled by a numerical solution. Appropriate choice for the proportional feedback gain of the observer results in accelerated convergence of the simulation by a factor of 0.012 and reduced error in estimation of the perturbation velocity by a factor of 0.6 in the whole domain or a factor of 0.3 behind the output measurement plane in comparison with the ordinary flow simulation without feedback.
Numerical Simulation of Drag Reducing Turbulent Flow in Annular Conduits
Authors: Azouz Idir , Shirazi Siamack A.
Published: February 2008

Inadequate transport of rock cuttings during drilling of oil and gas wells can cause major problems such as excessive torque, difficulty to maintain the desired orientation of the drill string, and stuck or broken pipe. The problem of cuttings transport is aggravated in highly inclined wellbores due to the eccentricity of the annulus which results in nonuniformity of the flowfield within the annulus. While optimum cleaning of the borehole can be achieved when the flow is turbulent, the added cost due to the increased frictional losses in the flow passages may be prohibitive. A way around this problem is to add drag-reducing agents to the drilling fluid. In this way, frictional losses can be reduced to an acceptable level. Unfortunately, no model is available which can be used to predict the flow dynamics of drag-reducing fluids in annular passages. In this paper, a numerical model is presented which can be used to predict the details of the flowfield for turbulent annular flow of Newtonian and non-Newtonian, drag-reducing fluids. A one-layer turbulent eddy-viscosity model is proposed for annular flow. The model is based on the mixing-length approach wherein a damping function is used to account for near wall effects. Drag reduction effects are simulated with a variable damping parameter in the eddy-viscosity expression. A procedure for determining the value of this parameter from pipe flow data is discussed. Numerical results including velocity profiles, turbulent stresses, and friction factors are compared to experimental data for several cases of concentric and eccentric annuli.
Linear Feedback Control of Boundary Layer Using Electromagnetic Microtiles
Authors: Singh S. N. , Bandyopadhyay P. R.
Published: February 2008

This paper presents a system-theory approach to control of a two-dimensional turbulent flow of saltwater on a flat plate using Lorentz forces produced by microtiles of small magnets and electrodes. Beginning with the two-dimensional Navier-Stokes equations of motion, a finite, dimensional, linear state variable, approximate model is obtained using Galerkin’s procedure. Based on this model, linear feedback control laws are obtained to achieve stabilization of the perturbed flow to the base flow. It is shown that spatially distributed longitudinal or surface-normal forces stabilize the flow perturbations. However, for lower wave numbers, longitudinal forces are more effective because surface-normal forces require larger electrode voltages for the same response characteristics. Simulation results are presented to show how stabilization is accomplished in the closed-loop system.
Eddy Viscosity Transport Equations and Their Relation to the k-ε Model
Authors: Menter F. R.
Published: February 2008

A formalism will be presented which allows transforming two-equation eddy viscosity turbulence models into one-equation models. The transformation is based on Bradshaw’s assumption that the turbulent shear stress is proportional to the turbulent kinetic energy. This assumption is supported by experimental evidence for a large number of boundary layer flows and has led to improved predictions when incorporated into two-equation models of turbulence. Based on it, a new one-equation turbulence model will be derived from the k-ε model. The model will be tested against the one-equation model of Baldwin and Barth, which is also derived from the k-ε model (plus additional assumptions) and against its parent two-equation model. It will be shown that the assumptions involved in the derivation of the Baldwin-Barth model cause significant problems at the edge of a turbulent layer.
Correlated Compressible and Incompressible Channel Flows
Authors: Crnojevic´ C. , Djordjevic´ V. D.
Published: February 2008

Compressible flow in channels of slowly varying cross section at moderately high Reynolds numbers is treated in the paper by employing some Stewartson-type transformations that convert the problem into an incompressible one. Both adiabatic flow and isothermal flow are considered, and a Poiseuille-type incompressible solution is mapped onto compressible plane in order to generate some exact solutions of the compressible governing equations. The results show striking effects that viscosity may have upon the flow characteristics in this case, in comparison with more conventional high Reynolds number flows.
Interfacial Area Concentration and Void Fraction of Two-Phase Flow in Narrow Rectangular Vertical Channels
Authors: Wilmarth T. , Ishii M.
Published: February 2008

Adiabatic concurrent vertical two-phase flow of air and water through narrow rectangular channels, gap widths 1 mm and 2 mm, was investigated. This study involved the observation of flow using a charge coupled device (CCD) camera. These images were then digitized and examined by applying an image processing technique to determine local average void fraction and local average interfacial area concentration. The void fraction data were then plotted using a drift flux plot to determine the distribution parameter and vapor drift velocity for each separate flow regime.
A Study on Converging Thin Annular Jets
Authors: Sivakumar D. , Raghunandan B. N.
Published: February 2008

An interesting feature of jets from liquid-liquid coaxial swirl atomizers used in bipropellant rockets or elsewhere is that the outer jet results in a tulip shaped liquid bulb even at operating pressure levels. In this context, experiments have been performed to study the discharge and tulip characteristics of annular jets through qualitative simulation of outer jet flow conditions at which tulip bulb prevails. It is shown that the discharge coefficient increases steeply with Reynolds number, a trend which is distinct from that of circular orifices. The range of flow conditions at which tulip bulb prevails decreases with the annular gap. If a swirl component is introduced into the annular jet, it alters the discharge characteristics and the tulip range with a tendency to form multiple tulips. Variation of tulip length for different annular gaps shows a common trend when plotted against liquid flow rate. The experimental data of tulip length agree reasonably with the theoretical model reported in literature.
Jet-Plate Interaction for Wedge-Shaped Plates of Arbitrary Angles
Authors: Chu S. S. , Chow W. L.
Published: February 2008

An investigation has been undertaken to study the problems of jet-plate interaction through the method of hodograph transformation. The physical flow field is first transformed to a hodograph domain. By using properly selected flow parameters, the solution is established through numerical computations with rectangular grid in the hodograph plane. The resulting plate configuration, the free streamlines, and the flow properties in the physical plane are subsequently obtained through direct numerical integration. Jet flows toward wedge-shaped plates of arbitrary angles are solved to demonstrate the ability of the method. To verify the solutions, momentum principle has been employed in the physical plane for all test cases. It is found that the results obtained through this method are satisfactory.
Chebyshev Collocation Analysis of Axisymmetric Flow and Heat Transfer Between Counter-Rotating Disks
Authors: Hill R. W. , Ball K. S.
Published: February 2008

Unsteady constant property flow between two counter-rotating finite disks is considered for a range of Reynolds numbers. Both disks are taken to be isothermal with an imposed temperature difference between them. The flow is assumed to be axisymmetric, and buoyancy forces are neglected. The conservation equations for momentum and energy are solved using a special Chebyshev collocation technique utilizing a pressure Poisson influence matrix approach to maintain a solenoidal velocity field. Three values of the disk angular velocity ratio, Γ = ω2/ω1, are considered: Γ = −1.0, −0.4, and 0.0. The flow is observed to become more complex, transitioning from steady to periodic to chaotic flow regimes as the Reynolds number is increased. The simulations are found to agree reasonably well with experimental data from the literature for Γ = −1.0 and 0.0, whereas discrepancies exist for Γ = −0.4 that are similar to those observed by others in simulations using turbulence models. The heat transfer rates between the disks are shown to increase with Reynolds number due to increasing velocities and to a lesser extent chaotic mixing over the parameter range considered.
Experimental Comparison of Flow Fields at the Inlet and the Outlet of an Inducer With Shrouded and Unshrouded Configurations
Authors: Offtinger C. , Henry C. , Morel R. , Spettel F.
Published: February 2008

The flow field was measured near the inlet and the outlet of an axial flow inducer with shrouded and unshrouded configurations combined with a centrifugal pump. Investigations were conducted using a five-hole probe for several flow rates. The shroud induces a counterrotating flow at the inlet and a blockage effect at the outlet in the region of the tip and for all flow rates. The radial distribution of losses keeps the same shape in both configurations, however, the loss level is higher with the shroud, giving a lower efficiency for the combined inducer-impeller configuration. Effects of shrouded inducer on part load instability are not clear and deserve more research, which is in progress.
Flow Analysis in a Pump Diffuser—Part 1: LDA and PTV Measurements of the Unsteady Flow
Authors: Eisele K. , Zhang Z. , Casey M. V. , Gu¨lich J. , Schachenmann A.
Published: February 2008

This paper describes experimental research aimed at improving our understanding of the complex unsteady three-dimensional flow field associated with the interaction between a pump impeller and its vaned diffuser. The paper provides the results of experiments carried out using Laser Particle Tracking Velocimetry (LPTV) and Laser Doppler Anemometry (LDA), in which time-resolved details of the unsteady flow field in a vaned diffuser of a medium specific speed pump have been obtained as a function of the local position of the pump impeller blades. Detailed flow field measurements have been carried out at several measurement positions in the diffuser and at a number of operating points along the pump characteristic. The measurement results have been analyzed to elucidate some interesting flow features observed in this typical pump diffuser. These include three-dimensional flow at the impeller outlet, flow separation in the diffuser channel, unsteady recirculation of the flow from the diffuser into the impeller, the passage of vorticity in the impeller blade wakes through the diffuser, and periodic unsteadiness and turbulence in the diffuser flow channel. The relevance of these flow features to the stability of the pump characteristic is discussed.
Two-Phase Void Fraction and Pressure Drop in Horizontal Crossflow Across a Tube Bundle
Authors: Xu G. P. , Tou K. W. , Tso C. P.
Published: February 2008

Void fraction and friction pressure drop measurements were made for an adiabatic, horizontal two-phase flow of air-water, air-oil across a horizontal in-line, 5 × 20 tube bundle with pitch-to-diameter ratio, P/D, of 1.28. For both air-water and air-oil flow, the experimental results showed that the average void fraction were less than the values predicted by a homogenous flow model, but were well correlated with the Martinelli parameter Xtt and liquid-only Froude number FrLO. The two-phase friction multiplier data exhibited an effect of flow pattern and mass velocity, and they could be well-correlated with the Martinelli parameter.
Pressure Drop and Heat Transfer of Magnetohydrodynamic Annular Two-Phase Flow in Rectangular Channel
Authors: Kumamaru H. , Fujiwara Y.
Published: February 2008

An annular two-phase flow model has been proposed to predict the pressure drop and heat transfer of magnetohydrodynamic (MHD) gas-liquid metal two-phase flow in a rectangular channel for the case of high void fraction. The model for a rectangular channel, in which the applied magnetic field is perpendicular to the short side of the channel cross-section, nearly predicts Inoue et al.’s experimental data on the MHD pressure drop. For fusion reactor conditions, the model shows calculated results that the MHD pressure drop for two-phase flow can be lowered to 10 percent of that of the single-phase liquid flow and the heat transfer coefficient can be increased by a factor of two or more over that of the single-phase liquid flow.
Influence of the Nuclei on the Cavitation Inception for Different Types of Cavitation on Ship Propellers
Authors: Gindroz B. , Billet M. L.
Published: February 2008

In order to relate nuclei size distributions with inception cavitation in cavitation facilities, a test program was conducted at the Grand Tunnel Hydrodynamique (GTH) of the Bassin d’Essais des Care`nes. The GTH, which has a complete air control system including dissolved gas and nuclei (microbubbles) control, offers the opportunity to answer this question. The tests were conducted on the three 34 mm diameter propellers used by Kuiper (1981), each of these propellers being characterized by a different cavitation type: bubble, sheet and vortex cavitation. The water nuclei content correspond to strong degassed water (maximum tension), low injection of medium size nuclei (medium tension-low content), large injection of medium size nuclei (medium tension-high content) and large injection of large nuclei (minimum tension). By injecting medium size nuclei for a low content and a high content, we can examine the influence of the number of nuclei on the cavitation inception characteristic. During all the tests, the dissolved air content was kept constant. The GTH online Cavitation Nuclei Counter (Centerbody Venturi) was used to measure both the water nuclei distribution and the liquid tension. Comparisons are made with the calibrated Centerbody Venturi, a Phases Doppler Particles Analyzer (PDPA) and Holographic measurements.
Refractive-Index-Matching Laser Velocimetry for Complex, Isothermal Flows
Authors: Thompson B. E. , Bouchery O. , Lowney K. D.
Published: February 2008

Refractive-Index-Matching Laser Velocimetry (RIMLV) obtains velocity distributions inflow through complex geometries. Laser velocimetry is used to measure the flow of a constant-temperature mixture that has the refractive index of acrylic models. This mixture can be used to obtain duct Reynolds numbers in the turbulent regime at moderate expense. Measurements can be obtained at desired locations, specifically inside complex models even if laser beams pass through multiple curved surfaces, which is advantageous when comparing measured and calculated results.
An Experimental and Numerical Study of Turbulent Swirling Pipe Flows
Authors: Parchen R. R. , Steenbergen W.
Published: February 2008

Both experimental and numerical studies have been performed aimed at the description of the decay of swirl in turbulent pipe flows. Emphasis is put on the effect of the initial velocity distribution on the rate of decay. The experiments show that, even far downstream of the swirl generator, the decay of the integral amount of angular momentum depends on the initial velocity distribution. This suggests that the description of the decay in terms of the widely suggested single exponential, function, is not sufficient. The calculations are based on (i) a standard k – ε model and (ii) models based on an algebraic transport model for the turbulent stresses. It appears that in a weakly swirling pipe flow, second-order models reduce to simple modifications of the standard k – ε model. While the standard k – ε model predicts a decay largely insensitive to the initial velocity distribution, the modified versions of the k – ε model, the ASM and the RSM, predict a strong sensitivity to the initial velocity distribution. Nevertheless, the standard k – ε model seems to predict the rate of decay of the swirl better than the second-order models. It is concluded that the corrections for the streamline curvature introduced by the second-order closures, largely overestimate the effect of rotation on the radial exchange of angular momentum.
Review: Diffuser Design and Performance Analysis by a Unified Integral Method
Authors: Johnston J. P.
Published: February 2008

A computational tool, called a Unified Integral Method (UIM) is reviewed. The method is used for preliminary design and performance analysis of simple diffusers with thin inlet boundary layers and subsonic flow in their inviscid core regions. The assumptions needed for application of a UIM are not very restrictive in many practical cases: straight diffusers with thin, turbulent inlet boundary layers and subsonic, irrotational core flows. The method provides designers with useful results including pressure recovery, location of separation and stalled regions, and exit plane profiles which may be used to evaluate total pressure loss and various flow distortion indices. Besides reviewing some basic concepts concerning stall and separation, describing the basis of the method and some details for making the UIM work, actual cases where it was tested versus data are discussed. In addition, UIM results are compared to results obtained by a RANS method run in a well known duct flow solver for a subsonic diffuser where data are also available. In another case, its output and data were compared to results from a CFD code typical of the many design codes in use in industry today. In both cases, the UIM results were as good, or better than those from the higher level methods, and the UIM is much simpler and easier to use as a design tool.
Turbulence Structure of Plane Surface-Jets in a Weak Coflowing Stream for Different Initial Wake Conditions
Authors: Martinuzzi R. , Zaghloul A. M. , Al-Qaraguli W. , Baddour R. E.
Published: February 2008

Plane turbulent surface-jets, in the presence of a weak coflowing ambient current, were experimentally investigated using a single-component Laser Doppler Velocimeter. This study is concerned with the influence on the turbulent mixing characteristics of the relative speed ratio, U′a, and the wake generated behind the plate separating the jet from the ambient stream at the exit. Data were analyzed for similarity characteristics, surface speed decay, jet growth rate, jet momentum flux, jet volume flux, and turbulence decay. An integral analysis of the governing equations was also conducted to examine the turbulent entrainment properties of the surface jets.
Experimental Study of Flow in a High Aspect Ratio 90 Deg Curved Diffuser
Authors: Majumdar B. , Mohan Ratan , Singh S. N. , Agrawal D. P.
Published: February 2008

Measurements for developing turbulent flow in a high aspect ratio (AS = 6), small area ratio (AR = 2), 90 deg curved diffuser have been made. Mean velocities, static and total pressures, and turbulence intensities were measured using a three-hole pressure probe and a hot-wire probe. Flow visualization studies were also made using a tuft probe. Experiments show that, except for a very small zone near the exit, there is no flow reversal in any part of the diffuser. Streamwise bulk flow is seen to shift toward the concave wall side in the downstream half of the diffuser, under the influence of centrifugal force. One pair of counter-rotating vortical motion was identified at 30 deg turn, which break into more pairs (at least two) of counter-rotating vortices in the downstream. The z-plane velocity profiles show a nearly 2-D flow in the initial part of the diffuser but subsequently the flow becomes increasingly 3-D. A static pressure recovery coefficient of 51 percent was achieved.
A Survey of Time-Averaged Characteristics of Laminar and Turbulent Horseshoe Vortices
Authors: Ballio F. , Bettoni C. , Franzetti S.
Published: February 2008

The time-averaged kinematical and dynamical characteristics of the junction vortex system in front of a symmetrical obstacle are systematically analyzed for both laminar and turbulent flows. A wide set of experimental and numerical results from the literature is coordinated in nondimensional form together with some new computational data. In turbulent flows the dimensions of the vortex system in the symmetry plane depend only on the obstacle geometry; in laminar systems they are also correlated with the Reynolds number and the thickness of the incoming boundary layer. The horseshoe vortices induce shear stresses on the bottom several times higher than those of the undisturbed boundary layer.
Statistical Model of a Self-Similar Turbulent Plane Shear Layer
Authors: Hong Zuu-Chang , Chen Ming-Hua
Published: February 2008

A turbulence probability density function (pdf) equation model is employed to solve a self-similar turbulent plane shear layer. The proper similarity variable was introduced into the problem of interest to reduce the pdf equation into a spatially one-dimensional equation, which is still three dimensional in velocity space. Then the approximate moment method is employed to solve this simplified pdf equation. By the solutions of this equation, the various one-point mean quantities are immediatelly available. Agreement of the calculated mean velocity, turbulent energy and Reynolds stress with the available experimental data is generally satisfactory indicating that the pdf equation model and the moment method can quantitatively describe the statistics of free turbulence. Additionally, the balance of turbulence energy was calculated and discussed subsequently. It shows that the pdf methods are of more potential in revealing turbulence structure than conventional turbulence models.
Experimental and Numerical Investigation Into the Flow Characteristics of Channels Etched in 〈100〉 Silicon
Authors: Flockhart S. M. , Dhariwal R. S.
Published: February 2008

This paper describes the experimental and numerical techniques utilized to determine the flow characteristics of a series of trapezoidal channels. The channels, with hydraulic diameters ranging from 50 to 120 μm, are fabricated in silicon using micro fabrication technology. During this investigation the flow results, using distilled water as the test fluid, were kept well within the laminar flow regime, with the experimental results not exceeding a Reynolds number of 600. In order to compare the experimental results to the theoretical response, a numerical technique is employed to determine the predicted flow. This essentially involves solving the unknown coefficients of the Dirichlet equation using a numerical method called the Householder Transformation. The numerical results, provide a comparison to the experimental results and allows us to determine whether flow in channels of these dimensions, fabricated in silicon, produce results comparable to the predicted theoretical response.