bluuebunny/crossref_metadata_2025 huggingface dataset:
Turbulence Model for Steady and Unsteady Boundary Layers in Strong Pressure Gradients
Authors: Hytopoulos E. , Schetz J. A. , Simpson R. L.
Published: February 2008

A new turbulence model for two-dimensional, steady and unsteady boundary layers in strong adverse pressure gradients is described. The model is developed in a rational way based on understanding of the flow physics obtained from experiments. The turbulent shear stress is given by a mixing length model, but the mixing length in the outer region is not a constant times the boundary layer thickness; it varies according to an integral form of the turbulence kinetic energy equation. This approach accounts for the history effects of the turbulence. The form of the near-wall mixing length model is derived based on the distribution of the shear stress near the wall, and it takes into account the pressure and convection terms which become important in strong adverse pressure gradients. Since the significance of the normal stresses in turbulent kinetic energy production increases as separation is approached, a model accounting for this contribution is incorporated. Experimental data indicate a change in turbulence structure near and through separation. Such a change can be significant and is accounted for here using an empirical function. The complete model was tested against steady and unsteady, two-dimensional experimental cases with adverse pressure gradients up to separation. Improved predictions compared to those obtained with other turbulence models were demonstrated.
Calculation of Fully-Developed Turbulent Flow in Rectangular Ducts With Nonuniform Wall Roughness
Authors: Naimi M. , Gessner F. B.
Published: February 2008

The predictive capabilities of four transport-type turbulence models are analyzed by comparing predictions with experimental data for fully-developed flow in (1) a rectangular duct with a step change in roughness on one wall (Case 1), and (2) a square duct with one rib-roughened wall (Case 2). The models include the Demuren-Rodi (DR) k-ε model, the Sugiyama et al. (S) k-ε model, the Launder-Li (LL) Reynolds stress transport equation model, and the differential stress (DS) model proposed recently by the authors. For the first flow situation (Case 1), the results show that the DS model yields improved agreement between predicted and measured primary and secondary mean velocity distributions in comparison to the DR and LL models. For the second flow situation (Case 2), the DS model is superior to the DR and S models for predicting experimentally observed mean velocity, turbulence kinetic energy, and Reynolds stress anisotropy behavior, especially in the vicinity of a corner formed by the juncture of adjacent smooth and rough walls. The results are analyzed in order to explain why the DR model leads to the formation of a spurious secondary flow cell near this corner that is not present in the experimental flow.
Measurements in a Transitional Boundary Layer With Go¨rtler Vortices
Authors: Volino R. J. , Simon T. W.
Published: February 2008

The laminar-turbulent transition process has been documented in a concave-wall boundary layer subject to low (0.6 percent) free-stream turbulence intensity. Transition began at a Reynolds number, Rex (based on distance from the leading edge of the test wall), of 3.5 × 105 and was completed by 4.7 × 105. The transition was strongly influenced by the presence of stationary, streamwise, Go¨rtler vortices. Transition under similar conditions has been documented in previous studies, but because concave-wall transition tends to be rapid, measurements within the transition zone were sparse. In this study, emphasis is on measurements within the zone of intermittent flow. Twenty-five profiles of mean streamwise velocity, fluctuating streamwise velocity, and intermittency have been acquired at five values of Rex, and five spanwise locations relative to a Go¨rtler vortex. The mean velocity profiles acquired near the vortex downwash sites exhibit inflection points and local minima. These minima, located in the outer part of the boundary layer, provide evidence of a “tilting” of the vortices in the spanwise direction. Profiles of fluctuating velocity and intermittency exhibit peaks near the locations of the minima in the mean velocity profiles. These peaks indicate that turbulence is generated in regions of high shear, which are relatively far from the wall. The transition mechanism in this flow is different from that on flat walls, where turbulence is produced in the near-wall region. The peak intermittency values in the profiles increase with Rex, but do not follow the “universal” distribution observed in most flat-wall, transitional boundary layers. The results have applications whenever strong concave curvature may result in the formation of Go¨rtler vortices in otherwise 2-D flows.
Air Entrainment in the Developing Flow Region of Plunging Jets—Part 2: Experimental
Authors: Cummings P. D. , Chanson H.
Published: February 2008

When a water jet impinges a pool of water at rest, air bubbles may be entrained and carried away below the pool free surface: this process is called plunging jet entrainment. The study presents new experimental data obtained with a vertical supported jet. Distributions of air concentration and mean air-water velocity, and bubble chord length distributions measured in the developing shear layer are presented. The results indicate that the distributions of void fraction follow closely analytical solution of the diffusion equation. Further, the momentum shear layer and the air bubble diffusion layer do not coincide. Chord length data show a wide range of air bubble sizes and overall the experimental results suggest strong interactions between the entrained air bubbles and the momentum transfer mechanisms.
The Influence of the Regulator Diameter and Injection Nozzle Geometry on the Flow Structure in Pneumatic Dimensional Control Systems
Authors: Crnojevic C. , Roy G. , Bettahar A. , Florent P.
Published: February 2008

The present paper describes an experimental investigation of the various parameters affecting the operation of industrial pneumatic controllers based on the jet nozzle principle. A test rig was built to monitor supply pressure, air temperature, airflow characteristics, and the static pressure distribution over the flat plate on which the jet impinges. The results demonstrate the existence of a low pressure, separated flow zone, subject to fouling, which subsequently was eliminated by appropriate changes of the injection nozzle geometry. The previous experimental findings were also confirmed by numerical simulation of the flow. Experimental results also show that the internal diameter of the regulator, situated inside the measuring branch, has an important influence on the sensitivity of the apparatus, as well as influencing its range.
A Digital Particle Image Velocimetry Investigation of Flow Field Instabilities of Axial-Flow Impellers
Authors: Myers K. J. , Ward R. W. , Bakker Andre´
Published: February 2008

Digital particle image velocimetry (DPIV) has been used to examine the flow field in a vessel agitated by an axial-flow impeller in turbulent operation. Both a pitched-blade turbine and a high-efficiency impeller were studied. Time series analysis indicates that the flow field is not steady; rather, it is subject to transients with frequencies well below the blade passage frequency (periods ranging from 40 to over 300 impeller revolutions have been observed). This result has important implications for computational modeling because current descriptions of agitated vessels are based upon time-averaged flow fields with superimposed turbulence. This modeling approach may not accurately capture the mixing associated with the low-frequency phenomena observed in this study.
Visualization Measurement and Numerical Analysis of Internal Flow in Cross-Flow Fan
Authors: Tsurusaki Hiromu , Tsujimoto Yoshinobu , Yoshida Yoshiki , Kitagawa Koichi
Published: February 2008

Measurements of the internal flow velocity distributions in a cross-flow fan were made by particle-tracking velocimetry (PTV). The flow paths (pathlines) were photographed using a digital camera and analyzed using a computer program developed by Tsurusaki and Urata (1989) in order to obtain the velocity distributions. From the velocity distributions, the vorticity distributions were calculated and the vorticity supply and diffusion in the eccentric vortex region was discussed based on an axisymmetric vortex model. A numerical simulation was carried out using a random walk model. The work done by the impeller was also discussed based on the measured velocity distributions.
Prediction of Flow Behavior and Performance of Squirrel-Cage Centrifugal Fans Operating at Medium and High Flow Rates
Authors: Kind R. J.
Published: February 2008

This paper describes a method for predicting flow behavior and performance for centrifugal fans of the squirrel-cage type. The work is directed at improving under standing of the factors affecting performance of these fans. A simulation approach has been adopted. That is, the fan is subdivided into a number of zones (inlet zone, blading zone, volute zone) and the zones are divided into elements. Flow behavior in the zones and elements and interactions between them are modeled using appropriate equations and correlations. The blading correlations make use of new experimental data for high-solidity cascades of bent sheet metal blades, typical of squirrel-cage fans. Predicted fan performance characteristics are in reasonable agreement with experimental results for flow rates at and above the best-efficiency operating point. Although relatively simple, the method recognizes the main flow phenomena and interactions that occur in squirrel-cage fans and it thus represents a substantial advance over what is currently available in the literature. Together with earlier experimental work, development of the method has provided considerable insight into the relative importance of various aspects of flow behavior. The ability to deal with extensive reverse flow through the rotor blading has not yet been incorporated and it is evident that this ability is essential for realistic prediction of flow behavior and performance at below-design flow rates.
Development and Optimization of Screw Machines With a Simulation Model—Part I: Profile Generation
Authors: Stosˇic´ N. , Hanjalic´ K.
Published: February 2008

This paper presents a method for the design of twin screw compressors and expanders, which is based on a differential algorithm for defining the rotor profiles and an analytical model of the fluid flow and thermodynamic processes within the machine. Part I of the paper describes the algorithm for screw rotor profile generation. It demonstrates the conjugacy condition which, when solved explicitly, enables a variety of primary arcs to be defined either analytically or by discrete point curves. Its use greatly simplifies the design since only primary arcs need to be specified and these can be located on either the main or gate rotor or even on any other rotor including a rack, which is a rotor of infinite radius. Secondary arcs are then generated automatically from this. By such means any profile combination may be considered. The most efficient were obtained from a combined rotor-rack generation procedure. An example of this combination is given which produces a rotor profile with stiff lobes and a higher throughput than any other known type. Part II describes a mathematical model of the compression and expansion processes within positive displacement machines which has been well proven in its use for the design of reciprocating and screw compressors and screw expanders.
Development and Optimization of Screw Machines With a Simulation Model—Part II: Thermodynamic Performance Simulation and Design Optimization
Authors: Hanjalic´ K. , Stosˇic´ N.
Published: February 2008

This paper presents a method for the design of twin screw compressors and expanders, which is based on a differential algorithm for defining the rotor profile and an analytical model of the fluid flow and thermodynamic processes within the machine. Part I of the paper presents a method for screw rotor profile generation which simplifies and improves design procedures. An example is given of its use in the development of a new “N” rotor profile, which is shown to be superior to other well-known types. Part II describes a numerical model of the thermodynamic and fluid flow processes within screw machines, which is valid for both the compressor and expander modes of operation. It includes the use of the equations of conservation of mass and energy applied to an instantaneous control volume of trapped fluid within the machine with allowance for fluid leakage, oil or other fluid injection, heat transfer, and the assumption of real fluid properties. By simultaneous solution of these equations, pressure-volume diagrams may be derived of the entire compression or expansions, process within the machine. The procedure has been developed over a period of fifteen years and validated with experimental results obtained from both reciprocating and screw compressors and screw expanders, some of which are included. The rotor profile generation processor, thermofluid solver and optimizer, together with preprocessing facilities for the input data and graphical post-processing and CAD interface, have been incorporated into a design package which provided a suitable tool for analysis and optimization of twin screw machine design. An example of its use is given in the optimization of the gate tip radius of a selected compressor design.
Riser-Relief Valve Dynamic Interactions
Authors: Botros K. K. , Dunn G. H. , Hrycyk J. A.
Published: February 2008

The dynamic stability behavior of a typical pilot-operated relief valve has been investigated both numerically and experimentally. The valve piston equation of motion has been solved in connection with the conservation equations of the oscillating flow in the attached riser. The boundary condition at the riser tip is of a choked flow type and had to be given special attention in oscillatory flows. Pertinent quasi-steady flow equations have been formulated to describe the different phases of relief along the piston lift. A model describing the process of pressure relief is described briefly, followed by a parametric study of the effects of various factors on the oscillation levels and characters. Field measurements were taken on an NFS 8 pilot-operated dual-outlet relief valve mounted on a riser assembly of L/D = 15 in a natural gas compressor station.
Two Types of Nonlinear Pressure-Drop Versus Flow-Rate Relation Observed for Saturated Porous Media
Authors: Lage J. L. , Antohe B. V. , Nield D. A.
Published: February 2008

Previous reports of experiments performed with water (Fund et at., 1987 and Kececioglu and Jiang, 1994) indicated that beyond the Forchheimer regime the rate of change of the hydrostatic pressure gradient along a porous medium suddenly decreases. This abnormal behavior has been termed “transition to turbulence in a porous medium.” We investigate the relationship between the hydrostatic pressure gradient of a fluid (air) through a porous medium and the average seepage fluid velocity. Our experimental results, reported here, indicate an increase in the hydrostatic pressure rate beyond a certain transition speed, not a decrease. Physical arguments based on a consideration of internal versus external incompressible viscous flow are used to justify this distinct behavior, a consequence of the competition between a form dominated transition and a viscous dominated transition. We establish a criterion for the viscous dominated transition from consideration of the results of three porous media with distinct hydraulic characteristics. A theoretical analysis based on the semivariance model validation principle indicates that the pressure gradient versus fluid speed relation indeed departs from the quadratic Forchheimer-extended Darcy flow model, and can be correlated by a cubic function of fluid speed for the velocity range of our experiments.
A Two-Phase Cinematic PIV Method for Bubbly Flows
Authors: Oakley T. R. , Loth E. , Adrian R. J.
Published: February 2008

A cinematic particle image velocimeter (PIV) for measurement of time varying fields in two-phase flow is described. This system has the capability to track detailed characteristics of individual bubbles moving through a turbulent flow field, e.g., size, shape, velocity, and acceleration, and simultaneously to measure the instantaneous fluid velocity field on a two-dimensional plane. The system employs an argon-ion laser, a rotating polygonal mirror and a single 35 mm movie camera. Two features of this technique are the ability to capture bubble trajectories for long periods of time and the use of bubble images which appear as two fine point images for each instant, from which centroid and diameter can be deduced. The instrument is evaluated in an experiment on the dispersion of nominally 3.5 mm diameter bubbles from a point source in a two stream, turbulent, planar free-shear layer. Characteristics of the fluid field and the bubble motion are described.
The Effect of Negative Spanwise Rotation on Dean Vortices
Authors: Wang Liqiu
Published: February 2008

A finite-volume numerical analysis is performed to examine effect of spanwise rotation on Dean vortices. The specific problem considered is fully developed flow in a square channel with streamwise curvature and spanwise rotation in a negative sense, i.e., the Coriolis force counteracts the centrifugal force. The rotation of the channel is found to have a significant effect on the flow structure in general, and the Dean vortices in particular. The nonlinear interaction of the Coriolis instability and the centrifugal instability results in several new flow structures, including one with coexistence of Ekman, Dean, and corner vortices, one with coexistence of Ekman and corner vortices, one with one-pair vortices and an agostrophic, virtually inviscid core, and one with Coriolis vortices on the inner convex wall. These are in agreement with the experimental findings.
Perturbation Solution for Flow in Two-Cavity Dies
Authors: Ruschak K. J. , Weinstein S. J.
Published: February 2008

Dies having two distribution cavities and slots are sometimes used in film-forming applications demanding high uniformity. The flow distribution in the primary slot is evened by the secondary cavity and slot. The flow equations for such a die are solved in closed form for coat-hanger and T-shaped dies by a perturbation method. Limiting cases of the solutions reveal the operating mechanism, and the use of the solutions in a design problem is demonstrated.
Dynamical Nuclei Measurement: On the Development and the Performance Evaluation of an Optimized Center-Body Meter
Authors: Pham T. M. , Michel J. M. , Lecoffre Y.
Published: February 2008

This work is concerned with the development of a center-body venturi for nuclei measurements of novel design, the Venturix. Our project aims to: 1. Define a specially tailored geometry for cavitation nuclei measurement. This design study takes into consideration the following main aspects: the venturi mean flow in subcavitating regime, the viscous effects, the bubble dynamics. 2. Evaluate the performance of the meter: After testing the proposed design concepts, the venturi operating characteristics, in particular its operational limits, are assessed. Finally, the performance of the acoustic method used for detecting and counting the active nuclei in the venturi is discussed.
Tip Vortex Cavitation on an Oscillating Hydrofoil
Authors: Boulon O. , Franc J. P. , Michel J. M.
Published: February 2008

This paper discusses tests conducted in the hydrodynamic tunnel of the University of Grenoble on a 3D oscillating hydrofoil. Visualization of unsteady tip vortex cavitation indicates a strong influence of the water nuclei content. The investigation was focused on the influence of the oscillation frequency on tip vortex cavitation inception. For very low nuclei content, cavitation inception is strongly delayed as compared to the steady-state results at very small oscillation frequencies. This delay is significantly reduced by nuclei seeding. The results can be explained by assuming that the time required for the inception of cavitation in the tip vortex corresponds to the time necessary for a cavitation nucleus to be captured by the vortex core.
Rotordynamic Forces in Cavitating Inducers
Authors: Bhattacharyya A. , Acosta A. J. , Brennen C. E. , Caughey T. K.
Published: February 2008

This paper reports an experimental investigation of rotordynamic forces in a whirling axial flow inducer under the influence of cavitation at various flow coefficients. The results show the occurrence of large destabilizing peaks in the force tangential to the whirl orbit for positive whirl frequency ratios. The magnitude of the destabilizing forces increased with a decrease in cavitation number and flow coefficient. The rotordynamic data obtained do not exhibit quadratic functional behavior normally assumed in many rotordynamic models. Consequently, conventional generalized stiffness, damping, and inertia matrices cannot be determined for the inducer. The results demonstrate the complexity of rotordynamic forces and their consequences on stability of axial flow inducers.
Observations of Oscillating Cavitation of an Inducer
Authors: Tsujimoto Yoshinobu , Yoshida Yoshiki , Maekawa Yasukazu , Watanabe Satoshi , Hashimoto Tomoyuki
Published: February 2008

Oscillating cavitation of an inducer was observed through unsteady inlet pressure measurements and by use of high speed video picture, covering a wide range of flow coefficient and cavitation number. One of the purposes of the study is to identify a mode of rotating cavitation predicted by a linear analysis, and the other is to obtain a general view of oscillating cavitation. The number of rotating cavitation cells and their propagation velocity were carefully determined from the phase difference of pressure fluctuations at various circumferential locations. Various kinds of oscillating cavitation were observed: rotating cavitation rotating faster/slower than impeller rotation, cavitation in backflow vortices, and surge mode oscillations. Effects of inlet and outlet (effective) pipelength were also studied.
Diffusion Induced Bubble Populations Downstream of a Partial Cavity
Authors: Yu Po-Wen , Ceccio Steven L.
Published: February 2008

Bubble populations downstream of stable partial cavities were examined using in-line holography, and the bubble number distribution and void fraction of the bubbly clouds were determined. The measured flux of bubbles downstream of the cavity was used to determine the diffusion rate of noncondensable gas into the cavity. Four models of gas diffusion into partial cavities from Parkin and Kermeen (1963), Brennen (1969), and Parkin and Ravindra (1993) are reviewed, and their predictions are compared with the experimentally determined mass diffusion rates. The models, which recognize a turbulent boundary layer over the cavity interface, over-predict the diffusion rate by one order of magnitude for cavities characterized by L/δ > 25.
A Perspective on the Construction of Nonsymmetric Uncertainty Intervals
Authors: Maciejewski Paul K.
Published: February 2008

This paper presents a new method for constructing nonsymmetric uncertainty intervals, one that is based on estimates of “expected values” and “variances” associated with deterministic errors that one constructs from estimates of “upper bias limits” and “lower bias limits” for measured variables. On the assumption that upper bias limits and lower bias limits specified by the user correspond to 95 percent confidence intervals for normally distributed deterministic errors, the uncertainty intervals determined by the new method reduce to approximate 95 percent confidence intervals for the true value of the measured variables.
Parametric Studies on a Numerical, Nonlinear Pulse Tube Flow
Authors: Kirkconnell C. S. , Colwell G. T.
Published: February 2008

The need for high reliability, low cost, low vibration cryocoolers, for both military and commercial applications, has spawned and continues to drive the development of pulse tube cryogenic refrigerators. The expander contains no moving parts, yielding the potential for marked improvements in these areas. Though pulse tube refrigeration has been thoroughly studied, more accurate analytic and numerical modeling tools are needed to facilitate the development of thermodynamically efficient pulse tube cryocoolers to meet the needs of the user community. At present, one of the primary areas of modeling uncertainty is in the calculation of the dissipative losses occurring within the pulse tube itself. Toward this end, a numerical model was developed to solve the one-dimensional, nonlinear governing equations for heat and mass flow in a pulse tube. The governing equations are scaled for high-frequency (>60 Hz) pulse lube operation. The resulting system of nonlinear, time-dependent equations was solved directly using the method of lines. The numerical model was verified analytically using a representative set of equations with a known solution. A sensitivity analysis was performed to investigate the influence of different parameters on the solution.
Friction Factor Determination for Flow Through Finite Wire-Mesh Woven-Screen Matrices
Authors: Sodre´ J. R. , Parise J. A. R.
Published: February 2008

Experiments were carried out to determine the pressure drop through an annular conduit filled with a plain square wire-mesh woven-screen matrix. The tests involved turbulent fully developed flow of air at steady-state conditions, with the modified Reynolds number (M(1−ε)/Re), based on the hydraulic radius of the packed bed, ranging from 5 × 10−4 to 5 × 10−3. The test section was built according to the geometry of a Stirling engine, simulating an annular regenerator with a radius ratio of 1.369 and a screen of mesh size 10. A corrected Ergun equation was used to correlate the experimental data, considering the wall effects. Comparisons with results obtained by other authors extended the validation of the correlation obtained to a wider range of modified Reynolds numbers (1 × 10−4 ≤ M(1 − ε)/Re ≤ 1) and to different screen mesh sizes. The correlation has been found to work for annular and circular cross-section beds.
Application of Turbulence Models to Bypass Transition
Authors: Westin K. J. A. , Henkes R. A. W. M.
Published: February 2008

The present study considers the use of low-Reynolds number, single-point closures for transition prediction at high levels of free-stream turbulence. The work is focused on two differential Reynolds Stress Transport models, which are compared with the k – ε model of Launder and Sharma. Calculations are carried out for attached boundary layer flow on a flat plate equipped with a sharp leading edge at zero-pressure gradient and for various free-stream turbulence levels. Comparisons with results from a large eddy simulation reveal significant shortcomings in the modeling of the dissipation, with a large overprediction in the pretransitional boundary layer. The present results are in some cases in conflict with other results reported in the literature, and it is shown that the discrepancies can be ascribed to the implementation of the free-stream boundary conditions.
A Modified Low-Reynolds-Number k-ω Model for Recirculating Flows
Authors: Peng Shia-Hui , Davidson Lars , Holmberg Sture
Published: February 2008

A modified form of Wilcox’s low-Reynolds-number k-ω model (Wilcox, 1994) is proposed for predicting recirculating flows. The turbulent diffusion for the specific dissipation rate, ω, is modeled with two parts: a second-order diffusion term and a first-order cross-diffusion term. The model constants are re-established. The damping functions are redevised, which reproduce correct near-wall asymptotic behaviors, and retain the mechanism describing transition as in the original model. The new model is applied to channel flow, backward-facing step flow with a large expansion ratio (H/h = 6), and recirculating flow in a ventilation enclosure. The predictions are considerably improved.
Open Channel Flow Over Submerged Obstructions: An Experimental and Numerical Study
Authors: Fadda Dani , Raad Peter E.
Published: February 2008

This paper reports on experimental and computational investigations of water flow over two-dimensional obstacles in an open channel. Both triangular and semi-circular obstacles are considered in order to study the effects of obstacle type and size on the flow. The upstream flow is subcritical while the downstream flow is supercritical in all the cases discussed. The results of the experiments indicate that the downstream Reynolds number increases and appears to approach an asymptotic value as the obstacle height is increased. The upstream Reynolds number, on the other hand, decreases linearly as the obstacle height is increased. For the cases involving triangular obstacles, comparisons are presented between the results of the experimental measurements and the computational simulations as well as with available analytical solutions for inviscid flow. The comparisons point to the conclusion that the fluid rotational activity has a negligible effect on the overall flow in the open channel.
Nearfield Evolution of a Longitudinal Vortex Generated by an Inclined Jet in a Turbulent Boundary Layer
Authors: Zhang X. , Collins M. W.
Published: February 2008

The nearfield evolution of a longitudinal vortex embedded in a turbulent boundary layer was examined in a wind-tunnel test. The Vortex was produced by an inclined round jet, the diameter of the jet being 0.4δ0.99. Flow properties were measured at a freestream velocity of 20 m/s using a laser Doppler anemometer. Control parameters are jet skew and pitch angles, and speed ratio. It was observed that a single vortex is generated at x = 10D. The effects of the control parameters on the strength and location of the vortex were examined. For an embedded vortex and favorable velocity distribution, the test results suggested a skew angle of 60 deg, a pitch angle of α = 30 deg, and speed rations of γ = 1.0. The study provided insight into the flow physics and a database.
Wall Pressure and Effective Wall Shear Stresses in Heat Exchanger Tube Inlets With Application to Erosion-Corrosion
Authors: Elvery D. G. , Bremhorst K.
Published: February 2008

Inclined flow into tube inlets is studied in order to identify flow characteristics associated with enhanced erosion-corrosion rates at tube inlets. Measured wall pressures and shear stresses are presented for inlet flow with inclination angles up to 60 deg for a tube Reynolds number of 71,000. These show that the areas with most potential for wear are located near the reattachment point of the recirculation bubble as well as in regions at the downstream side of the tube inlet. The latter are located opposite the recirculation region but away from the symmetry plane due to strong swirl of the flow in that region. The results are related to erosion-corrosion patterns observed in practice.
Numerical Study of Sail Aerodynamics
Authors: Guilbaud Michel , Rajaona D. R.
Published: February 2008

A boundary-element method to compute steady flows around a set of sails at close hauled conditions. (i.e., the wind is coming from upstream of the boat at a moderate angle of attack for the sails) is presented. It is based on a mixed potential-velocity formulation in order to solve the problem of zero thickness sail conditions. An iterative procedure is proposed to account for the interaction between sails; the shape of the wake of the leading sail is computed as and equilibrium stream surface in such a way that the plane wake assumption is not used. Numerical results are compared with tests conducted on isolated jibs and sets of jib and main sails in wind tunnels.
Flow Analysis in a Pump Diffuser—Part 2: Validation and Limitations of CFD for Diffuser Flows
Authors: Muggli F. A. , Eisele K. , Casey M. V. , Gu¨lich J. , Schachenmann A.
Published: February 2008

This paper describes an investigation into the use of CFD for highly loaded pump diffuser flows. A reliable commercial Navier-Stokes code with the standard k-ε turbulence model was used for this work. Calculations of a simple planar two-dimensional diffuser demonstrate the ability of the k-ε model to predict the measured effects of blockage and area ratio on the diffuser static pressure recovery at low loading levels. At high loading levels with flow separation the k-ε model underestimates the blockage caused by the recirculation in the flow separation region and overestimates the pressure recovery in the diffuser. Three steady-state calculations of a highly loaded vaned diffuser of a medium specific speed pump have been carried out using different inlet boundary conditions to represent the pump outlet flow. These are compared to LDA measurement data of the flow field and demonstrate that although the Navier-Stokes code with the standard k-ε turbulence model is able to predict the presence of separation in the flow, it is not yet able to accurately predict the static pressure rise of this highly loaded pump diffuser beyond the flow separation point.
Unsteady Total Pressure Field Downstream of an Embedded Stator in a Multistage Axial Flow Compressor
Authors: Suryavamshi N. , Lakshminarayana B. , Prato J. , Fagan J. R.
Published: February 2008

The results from measurements of the unsteady total pressure field downstream of an embedded stage of a three stage axial flow compressor are presented in this paper. The measurements include area traverses of a high response kulite total pressure probe and a pneumatic five hole probe downstream of stator 2 at the peak efficiency operating point for the compressor. These data indicate that both the shaft-resolved and unresolved fluctuations contribute to the unsteadiness of the total pressure field in multistage compressors. Specifically, regions associated with high levels of unsteadiness and, consequently, high levels of mixing including both the hub and casing end walls and the airfoil wakes have significant levels of shaft resolved and unresolved unsteadiness. Temporal variations of stator exit flow are influenced by both shaft resolved and unresolved unsteadiness distributions. The limitations of state-of-the-art instrumentation for making measurements in moderate and high speed turbomachinery and the decomposition used to analyze these data are also discussed.
Wing Section Optimization for Supersonic Viscous Flow
Authors: Item Cem C. , Baysal Oktay
Published: February 2008

To improve the performance of a highly swept supersonic wing, it is desirable to have an automated design method that also includes a higher fidelity to the flow physics. With this impetus, an aerodynamic optimization methodology incorporating the thin-layer Navier-Stokes equations and sensitivity analysis had previously been developed. Prior to embarking upon the full wing design task, the present investigation concentrated on the identification of effective optimization problem formulations and testing the feasibility of the employed methodology, by defining two-dimensional test cases. Starting with two distinctly different initial airfoils, two independent optimizations resulted in shapes with similar features: cambered, parabolic profiles with sharp leading- and trailing-edges. Secondly, an outboard wing section normal to the subsonic portion of the leading edge, which had a high normal angle-of attack, was considered. The optimization resulted in a shape with twist and camber that eliminated the adverse pressure gradient, hence, exploiting the leading-edge thrust. The wing section shapes obtained in all the test cases included the features predicted by previous studies. This was considered as a strong indication that the flow field analyses and sensitivity coefficients were computed and provided to the present gradient-based optimizer correctly. Also, from the results of the present study, effective optimization problem formulations could be deduced to start a full wing shape optimization.
Combustors for Micro-Gas Turbine Engines
Authors: Waitz Ian A. , Gauba Gautam , Tzeng Yang-Sheng
Published: February 2008

The development of a hydrogen-air microcombustor is described. The combustor is intended for use in a 1 mm2 inlet area, micro-gas turbine engine. While the size of the device poses several difficulties, it also provides new and unique opportunities. The combustion concept investigated is based upon introducing hydrogen and premixing it with air upstream of the combustor. The wide flammability limits of hydrogen-air mixtures and the use of refractory ceramics enable combustion at lean conditions, obviating the need for both a combustor dilution zone and combustor wall cooling. The entire combustion process is carried out at temperatures below the limitations set by material properties, resulting in a significant reduction of complexity when compared to larger-scale gas turbine combustors. A feasibility study with initial design analyses is presented, followed by experimental results from 0.13 cm3 silicon carbide and steel microcombustors. The combustors were operated for tens of hours, and produced the requisite heat release for a microengine application over a range of fuel-air ratios, inlet temperatures, and pressures up to four atmospheres. Issues of flame stability, heat transfer, ignition and mixing are addressed. A discussion of requirements for catalytic processes for hydrocarbon fuels is also presented.
Numerical Modeling of Non-Newtonian Fluid Flow in a Porous Medium Using a Three-Dimensional Periodic Array
Authors: Inoue Masahiko , Nakayama Akira
Published: February 2008

Three-dimensional numerical experiments have been conducted to investigate the viscous and porous inertia effects on the pressure drop in a non-Newtonian fluid flow through a porous medium. A collection of cubes placed in a region of infinite extent has been proposed as a three-dimensional model of microscopic porous structure. A full set of three-dimensional momentum equations is treated along with the continuity equation at a pore scale, so as to simulate a flow through an infinite number of obstacles arranged in a regular pattern. The microscopic numerical results, thus obtained, are processed to extract the macroscopic relationship between the pressure gradient-mass flow rate. The modified permeability determined by reading the intercept value in the plot showing the dimensionless pressure gradient versus Reynolds number closely follows Christopher and Middleman’s formula based on a hydraulic radius concept. Upon comparing the results based on the two- and three-dimensional models, it has been found that only the three-dimensional model can capture the porous inertia effects on the pressure drop, correctly. The resulting expression for the porous inertia possesses the same functional form as Ergun’s, but its level is found to be only one third of Ergun’s.
Turbulent Friction Factor for Two-Phase: Air-Powerlaw Fluid Flows Through Horizontal Tubes
Authors: Rao B. K.
Published: February 2008

Turbulent experimental friction factors for air and power-law fluid flows through a horizontal tube are reported. The power-law fluids studied were aqueous solutions of Carbopol® (at concentrations 1000 and 2000 wppm). The two-phase friction factors were correlated in terms of the generalized Reynolds number (Re*). Over a range of the Re* from 6000 to 80,000, the simpler homogeneous model is accurate enough for engineering prediction of turbulent friction factor for air and power-law fluid flows through straight tubes.
Flow Visualization in Bubbly Two-Phase Hydraulic Jump
Authors: Mossa Michele , Tolve Umberto
Published: February 2008

The present study investigates bubbly two-phase flow in a hydraulic jump using a flow visualization technique. Bubbly two-phase flow is encountered in many engineering problems; however, mainly because of experimental difficulties, little is known on the internal structure of these flows, although such knowledge is clearly essential to a thorough understanding of the mass transfer between the two component phases. In the past, some authors measured the distribution of void ratio in a hydraulic jump using hot-film anemometry. Nowadays this interesting technique may be improved using a flow visualization technique, which enables one to obtain the percentage of air across each vertical section of the jump. This is possible by evaluating the gray levels of the first principal axes of transformed images starting from RGB images. The experiments considered the phenomenon of air concentration in a hydraulic jump, which was studied and analyzed using image processing techniques, aimed at obtaining reliable quantitative measurements. To achieve this, the processing system was planned and tested at the hardware level and a procedure for managing the processing was set up. The calibration curve was obtained using the McCorquodale and Khalifa law (1983). The results permit the visualization of flow structures and the estimation of air concentration of the flow along all the jump and to show the position in which the air concentration reaches the maximum value versus time.
One-Dimensional Bubbly Cavitating Flows Through a Converging-Diverging Nozzle
Authors: Wang Yi-Chun , Brennen C. E.
Published: February 2008

A nonbarotropic continuum bubbly mixture model is used to study the one-dimensional cavitating flow through a converging-diverging nozzle. The nonlinear dynamics of the cavitation bubbles are modeled by the Rayleigh-Plesset equation. Analytical results show that the bubble/bubble interaction through the hydrodynamics of the surrounding liquid has important effects on this confined flow field. One clear interaction effect is the Bernoulli effect caused by the growing and collapsing bubbles in the nozzle. It is found that the characteristics of the flow change dramatically even when the upstream void fraction is very small. Two different flow regimes are found from the steady state solutions and are termed: quasi-steady and quasi-unsteady. The former is characterized by large spatial fluctuations downstream of the throat which are induced by the pulsations of the cavitation bubbles. The quasi-unsteady solutions correspond to flashing flow. Bifurcation occurs as the flow transitions from one regime to the other. An analytical expression for the critical bubble size at the bifurcation is obtained. Physical reasons for this quasi-static instability are also discussed.
Quantitative Evaluation of Cavitation Erosion
Authors: Hattori Shuji , Mori Hiroyuki , Okada Tsunenori
Published: February 2008

In order to evaluate the quantitative cavitation-erosion resistance of materials, a pressure-detector-installed specimen was developed, which can measure both the impact load produced by cavitation bubble collapse and the volume loss simultaneously. Test specimens (pressure-detection rod) used were nine kinds of metals and were exposed to vibratory cavitation. A linear relation was obtained for all materials between the accumulated impact energy ∑Fi2 calculated from the distribution of impact loads and the volume loss, independent of test conditions. Impact energy accumulated during the incubation period and the energy for a unit material removal in steady-state period were obtained from the relation. These values are very Important concerning quantitative erosion resistance evaluation. That is, when the distribution of impact loads is acquired for different cavitation conditions, the volume loss can be estimated. This idea was applied to the venturi cavitation erosion. The experimental results for venturi test corresponds well with the prediction using these impact energy values. It was concluded that the quantitative impact energy values of materials can be determined independent of the apparatus and the test condition by using the newly developed pressure-detector-installed specimen.
Simple Stochastic Model for Particle Dispersion Including Inertia, Trajectory-Crossing, and Continuity Effects
Authors: Etasse Emmanuel , Meneveau Charles , Poinsot Thierry
Published: February 2008

An eddy-lifetime, stochastic Lagrangian model for particle dispersion in weakly laden turbulent flows is proposed, in which the interaction time-scale between particles and turbulent eddies is parametrized so as to include several physical effects. It takes into account particle inertia, crossing-trajectory effect, the possible difference in lateral and longitudinal dispersion, and some Reynolds number effects. The parametrization is based on previous results, from a theoretical dispersion model in isotropic turbulence using the trajectory-velocity independence and Gaussian approximations, as well as from Large-Eddy-Simulation. Simple fits are introduced to efficiently capture the main results from these prior studies, allowing practical implementation within the context of k – ε engineering codes. Results from simulations using the proposed approach are compared with experimental data of dispersion in decaying isotropic turbulence.
Alternate Eddy Shedding Set Up by the Nonaxisymmetric Recirculation Zone at the Exhaust of a Cyclone Dust Separator
Authors: Griffiths A. J. , Yazdabadi P. A. , Syred N.
Published: February 2008

Two cyclone dust separators with geometric swirl numbers of 3.324 and 3.043 were used to analyze the motion of the complex three-dimensional time dependent motion set up in the free exhaust. A quantitative analysis of the flow was carried out, obtaining time dependent velocity measurements with the use of laser Doppler anemometry (L.D.A.) techniques. The investigations highlighted a eddy or vortex shedding mechanism in two distinct areas of the flow. This was in part caused by a reverse flow zone and a precessing vortex core within the exhaust region of the separator. Changes in the Reynolds number by a factor of 2 were observed to have no effect on the main characteristics of the flow. Some changes were seen in the flow structure with change in swirl number, particularly the size of the reverse flow zone and the position of the large engulfment vortices.
The Systematic Uncertainty of Laboratory Flow Calibrations
Authors: Keyser D. R.
Published: February 2008

There are two axioms applicable to this paper: the first is that no flowmeter can be more accurate than the laboratory which calibrated it; the second that no uncalibrated flowmeter can be more accurate than a like one that is calibrated in a recognized standards laboratory. The fundamental systematic uncertainty in flow measurement is that inherited from the flow calibration laboratory. This paper focuses on ascertaining this fundamental systematic uncertainty by deriving from the published instances of interlaboratory comparisons a value in which one may be 95 percent confident. For water flow measurements, this value turns out to be ±0.28 percent, and for liquid hydrocarbon fuels it is ±0.77 percent. Cold water calibrations are the most accurate.
An Experimental Study of Swirling Flow Pneumatic Conveying System in a Vertical Pipeline
Authors: Li Hui , Tomita Yuji
Published: February 2008

A swirling flow is adopted for a vertical pneumatic conveying system to reduce conveying velocity, pipe wear, and particle degradation. An experimental study has addressed the characteristics of swirling flow pneumatic conveying (SFPC) for the total pressure drop, solid flow patterns, power consumption, and additional pressure drop. Polystyrene, polyethylene, and polyvinyl particles with mean diameters of 1.7, 3.1, and 4.3 mm, respectively, were transported as test particles in a vertical pipeline 12.2 m in height with an inside diameter of 80 mm. The initial swirl number was varied from 0.38 to 0.94, the mean air velocity was varied from 9 to 23 m/s, and the mass flow rate of the solids was varied from 0.3 to 1.25 kg/s. The minimum and critical air velocities decreased as much as 20 and 13 percent, respectively, when using SFPC. The total pressure drop and power consumption of SFPC are close to those of axial flow pneumatic conveying in the low air velocity range.