bluuebunny/crossref_metadata_2025 huggingface dataset:
Testing of Transition-Region Models
Authors: Singer B. A. , Dinavahi P. G.
Published: May 2008

An approach for the comprehensive testing of transition-region models is described. This approach emphasizes the need to evaluate the models in all of the flow scenarios for which they are likely to be used. As an example of the procedure, seven different types of transitioning flows are described and experiments illustrating the different scenarios are discussed. Two transition-region models are used to predict mean-flow quantities for all of the cases and selected results are reported.
Pressure Drop in Single-Phase and Two-Phase Couette-Poiseuille Flow
Authors: Salhi A. , Rey C. , Rosant J. M.
Published: May 2008

This paper is concerned with axial pressure gradient in single-phase and two-phase flow at low void fraction in a narrow annular space between two concentric cylinders, the inner one rotating. From experimental results, the coupling function (inertial forces/centrifugal forces) is parameterized by Taylor or Rossby numbers for two values of the intercylindrical width (clearance). The results are discussed with regard to different flow regimes and it is shown in particular that transition from the turbulent vorticed regime to the turbulent regime occurs at Ro ≃ 1. The proposed correlation agrees in a satisfactory manner to all the regimes studied in our experiments and in those given in the bibliography. In addition, original tests with a two-phase liquid/gas flow at 5 percent G.O.R. (gas oil ratio), for a finely dispersed gas phase are also reported. These results indicate a similar behavior to single-phase flows, justifying the transposition of the same correlation in the framework of the homogeneous model.
Studies on Cavitation Inception Process in Separated Flows
Authors: Milton S. G. , Arakeri V. H.
Published: May 2008

Studies related to cavitation inception process in separated flows are reported. Experimental observations of bubble appearance in grooves with laminar or turbulent boundary layer over them have clearly shown that gaseous diffusion process is significantly enhanced in turbulent flow. This process can lead to local nuclei size modification in environment similar to that of flow over a groove, like laminar separation “bubbles.” Cavitation inception modeling including this aspect is carried out for predicting inception conditions associated with “bubble-ring” cavitation commonly observed on hemispherically nosed axisymmetric body. Qualitative dependence of predicted inception numbers with velocity is found to agree very well with experimental observations of Carroll (1981).
Dynamics of Attached Cavities on Bodies of Revolution
Authors: Ceccio S. L. , Brennen C. E.
Published: May 2008

Attached cavitation was generated on two axisymmetric bodies, a Schiebe body and a modified ellipsoidal body (the I. T. T. C. body), both with a 50.8 mm diameter. Tests were conducted for a range of cavitation numbers and for Reynolds numbers in the range of Re = 4.4 × 105 to 4.8 × 105. Partially stable cavities were observed. The steady and dynamic volume fluctuations of the cavities were recorded through measurements of the local fluid impedance near the cavitating surface suing a series of flush mounted electrodes. These data were combined with photographic observations. On the Schiebe body, the cavitation was observed to form a series of incipient spot cavities which developed into a single cavity as the cavitation number was lowered. The incipient cavities were observed to fluctuate at distinct frequencies. Cavities on the I. T. T. C. started as a single patch on the upper surface of the body which grew to envelope the entire circumference of the body as the cavitation number was lowered. These cavities also fluctuated at distinct frequencies associated with oscillations of the cavity closure region. The cavities fluctuated with Strouhal numbers (based on the mean cavity thickness) in the range of St = 0.002 to 0.02, which are approximately one tenth the value of Strouhal numbers associated with Ka´rma´n vortex shedding. The fluctuation of these stabilized partial cavities may be related to periodic break off and filling in the cavity closure region and to periodic entrainment of the cavity vapor. Cavities on both headforms exhibited surface striations in the streamwise direction near the point of cavity formation, and a frothy mixture of vapor and liquid was detected under the turbulent cavity surface. As the cavities became fully developed, the signal generated by the frothy mixture increased in magnitude with frequencies in the range of 0 to 50 Hz.
Numerical Simulation of Flows Driven by a Torsionally Oscillating Lid in a Square Cavity
Authors: Iwatsu Reima , Hyun Jae Min , Kuwahara Kunio
Published: May 2008

Numerical studies were made of the flow of a viscous fluid in a two-dimensional square container. The flows are driven by the top sliding wall, which executes sinusoidal oscillations. Numerical solutions were acquired by solving the time-dependent, two-dimensional incompressible Navier-Stokes equations. Results are presented for wide ranges of two principal physical parameters, i.e., Re, the Reynolds number and ω′, the nondimensional frequency of the lid oscillation. Comprehensive details of the flow-structure are presented. When ω′ is small, the flow bears qualitative similarity to the well-documented steady driven-cavity flow. The flow in the bulk of cavity region is affected by the motion of the sliding upper lid. On the contrary, when ω′ is large, the fluid motion tends to be confined within a thin layer near the oscillating lid. In this case, the flow displays the characteristic features of a thin-layer flow. When ω′ is intermediate, ω′ ~ O(1), the effect of the side walls is pronounced; the flow pattern reveals significant changes between the low-Re and high-Re limits. Streamline plots are constructed for different parameter spaces. Physically informative interpretations are proposed which help gain physical insight into the dynamics. The behavior of the force coefficient Cf has been examined. The magnitude and phase lag of Cf are determined by elaborate post-processings of the numerical data. By utilizing the wealth of the computational results, characterizations of Cf as functions of Re and ω′ are attempted. These are in qualitative consistency with the theoretical predictions for the limiting parameter values.
Decay of Temperature Variance in the Presence of Nonhomogeneous Strain
Authors: Rahai H. R. , LaRue J. C.
Published: May 2008

The effect of nonhomogeneous strain caused by two-dimensional objects on single point statistical properties of the turbulent velocity and turbulent scalar (temperature) and their cross moments are investigated experimentally. The heated turbulent flow is produced by means of a biplane array of round rods and a biplane array of heated, thin wires placed downstream of the turbulence producing grid. The experiments are performed for three different ratios of the integral scale, L0, to the object dimension, D, of 0.38, 1.90 and 3.1. Measurements of time resolved temperature and longitudinal and transverse velocity components are obtained using a triple wire probe consisting of a cold wire and two hot wires. Results indicate that, for L0 =1.9 and 3.1, the decay rate of temperature variance is nearly the same in the region of maximum shear stress (mss) and near the centerline. However, the decay rate in these regions is less than the decay rate of the temperature variance outside the cylinder wake. For L0/D = 0.38, the decay rates of temperature variance are the same in the regions of mss, near the centerline and outside the wake. Transverse growth of the wake is found to be either increased or nearly unchanged depending on, respectively, whether the free stream integral scale is larger or less than that of the wake.
Viscous Simulation Method for Unsteady Flows Past Multicomponent Configurations
Authors: Fouladi Kamran , Baysal Oktay
Published: May 2008

An algorithm is developed to obtain numerical simulations of flows about complex configurations composed of multiple and nonsimilar components with arbitrary geometries. The algorithm uses a hybridization of the domain decomposition techniques for grid generation and to reduce the computer memory requirement. Three-dimensional Reynolds-averaged, unsteady, compressible, and full Navier-Stokes equations are solved on each of the subdomains by a fully vectorized, finite-volume, upwind-biased, approximately factored, and multigrid method. The effect of Reynolds stresses is incorporated through an algebraic turbulence model with several modifications for interference flows. The algorithm is applied to simulate supersonic flows past an ogive-nose-cylinder near or inside a cavity. The cylinder is attached to an offset L-shaped sting when placed above the cavity opening. The unsteady nature of these flowfields and the interaction of the cavity shear layer with the cylinder are simulated. These cases illustrate two significantly different and important interference characteristics for an internally carried store separating from its parent body. Unsteadiness of the cavity flow has a more pronounced effect on the normal forces acting on the cylinder when the cylinder is placed inside the cavity. The time averaged surface pressures compare favorably with the wind tunnel data, despite the averaging time period for the computations being three orders of magnitude smaller than that of the experimental measurements.
Simulation of Three-Dimensional Shear Flow Around a Nozzle-Afterbody at High Speeds
Authors: Baysal Oktay , Hoffman Wendy B.
Published: May 2008

Turbulent shear flows at supersonic and hypersonic speeds around a nozzle-afterbody are simulated. The three-dimensional, Reynolds-averaged Navier-Stokes equations are solved by a finite-volume and implicit method. The convective and the pressure terms are differenced by an upwind-biased algorithm. The effect of turbulence is incorporated by a modified Baldwin-Lomax eddy viscosity model. The success of the standard Baldwin-Lomax model for this flow type is shown by comparing it to a laminar case. These modifications made to the model are also shown to improve flow prediction when compared to the standard Baldwin-Lomax model. These modifications to the model reflect the effects of high compressibility, multiple walls, vortices near walls, and turbulent memory effects in the shear layer. This numerically simulated complex flowfield includes a supersonic duct flow, a hypersonic flow over an external double corner, a flow through a non-axisymmetric, internal-external nozzle, and a three-dimensional shear layer. The specific application is for the flow around the nozzle-afterbody of a generic hypersonic vehicle powered by a scramjet engine. The computed pressure distributions compared favorably with the experimentally obtained surface and off-surface flow surveys.
Effect of Inlet Flow Distortion on Performance of Vortex Controlled Diffusers
Authors: Sullerey R. K. , Ashok V. , Shantharam K. V.
Published: May 2008

The present experimental investigations are concerned with diffusers employing the concept of vortex control to achieve high pressure recovery in a short length. Two types of two-dimensional diffusers have been studied, namely, vortex controlled and hybrid diffusers. Investigations have been carried out on such short diffusers with symmetrically and asymmetrically distorted inlet velocity profiles for area ratios 2.0 and 2.5 and divergence angle of 30 and 45 deg at a Reynolds number of 105. For each of the above configurations, experiments have been carried out for a range of fence subtended angles and bleed rates. The results indicate improvement in diffuser effectiveness up to a particular bleed off for both types of diffusers. It was observed that the nature of exit velocity profiles could be controlled by differential bleed.
A Flux-Split Solution Procedure for Unsteady Inlet Flow Calculations
Authors: Pordal H. S. , Khosla P. K. , Rubin S. G.
Published: May 2008

The solution of the reduced Navier Stokes (RNS) equations is considered using a flux-split procedure. Unsteady flow in a two dimensional engine inlet is computed. The problems of unstart and restart are investigated. A sparse matrix direct solver combined with a domain decomposition strategy is used to compute the unsteady flow field. Strong shock-boundary layer interaction, time varying shocks, and time varying recirculation regions are efficiently captured.
Blocked Flow Subchannel Simulation Comparison With Single-Phase Flow Data
Authors: Teyssedou A. , Tapucu A. , Camarero R.
Published: May 2008

The time integral of the local instantaneous conservation laws has been used as the starting point to write a subchannel model. The assumption of an ergodic stationary process has made possible further spatial integration of the resulting equations. The SIMPLER algorithm is adapted for the numerical solution. An adequate blockage model assures an overall momentum balance over the discretized cell elements. Comparison of the predictions of the proposed model with the experimental data confirms that it is capable of handling blockages of up to 90 percent of the flow area. Presently, this model only accounts for the hydrodynamic aspect of vertical adiabatic flows.
Experimental Validation of a Ducted Propeller Analysis Method
Authors: Hughes M. J. , Kinnas S. A. , Kerwin J. E.
Published: May 2008

A ducted propeller model was tested in the MIT water tunnel. A hub apparatus was designed which allowed for the duct and propeller forces to be measured separately. The forces on the duct and propeller were measured over a range of advance coefficients. Velocities were measured upstream and downstream from the duct using a Laser Doppler Velocimetry system. Using these velocities the experimental values for the spanwise distribution of circulation on the propeller blades were then calculated. The experimental results were compared to the results from a propeller lifting surface/duct and hub surface panel analysis code over the same range of advance coefficients showing very good agreement for the duct and propeller forces and the circulation in the region of attached flow.
Velocity Characteristics of Multiple Impinging Jets Through a Crossflow
Authors: Barata J. M. M. , Dura˜o D. F. G. , Heitor M. V.
Published: May 2008

The mean and turbulent velocity characteristics of the flowfield resulting from the impingement of two and three jets against a wall through a low-velocity crossflow are quantified in detail making use of a laser-Doppler velocimeter and are discussed together with the visualization of the flow. The experiments have been carried out for a velocity ratio between the jets and the crossflow of 30, for a Reynolds number based on the jet exit of 105,000, and for the jet exit five jet-diameters above the ground plate, and provided a basis to improve knowledge of several related complex flow fields in engineering applications. The results characterize the turbulent transport typical of multiple impinging flows associated with a large penetration of the impinging jets through a crossflow, and quantify the formation of fountain flows due to collision of consecutive wall jets. The turbulence measurements include those of Reynolds shear stress and identify large effects of flow distortion on the turbulence structure parameters that determine the empirical constants in engineering models of turbulence.
Acoustics of Transonic Flow Around an Oscillating Flap
Authors: Lyrintzis A. S. , Xue Y.
Published: May 2008

Investigation of noise mechanisms due to unsteady transonic flow is important for aircraft noise reduction. In this work, the near-field impulsive noise due to an oscillating flap is simulated numerically. The problem is modeled by the two-dimensional high frequency transonic small disturbance equation (VTRAN2 code). The three types of unsteady shock wave motion have been identified. Two different important disturbances exist in the pressure signal. The disturbances are related to the unsteady motion of the supersonic pocket and fluctuating lift, and drag forces. Pressure wave signatures, noise frequency spectra, and noise directivity are shown.
Application of Digital Moire Interferometry for Mapping Conical Flows
Authors: Srikanth R. , Rathakrishnan E.
Published: May 2008

Digital Moire Interferometry (DMI)—a low cost and sufficiently sensitive geometric interference technique with very low stability requirement analyzed through digital image processing, has been applied to compressible conical flows. An inverse Abel transform algorithm for image reconstruction from moire deflectograms has been developed.
Dynamic Criterion for Cavitation of Bubbles
Authors: Dugue´ C. , Fruman D. H. , Billard J.-Y. , Cerrutti P.
Published: May 2008

The classical criterion for cavitation of bubbles is based on the assumption that the bubbles are subjected to a quasi-steady pressure change and that the non condensible gas within the cavity behaves isothermally. No simple criterion accounting for the dynamic terms in the Rayleigh-Plesset equation is presently available. In this paper, a criterion for cavitation of bubbles, which takes into account the contribution of the dynamic terms in the Rayleigh-Plesset equation, is obtained and presented in a new useful form which allows rapid computation of the critical pressure whatever the fluid and the nuclei initial conditions are. The impact of these results on different flow situations are analyzed and discussed.
Occurrence of Bubbles in a Thin Wire at Low Reynolds Number
Authors: Sato K.
Published: May 2008

Thin wires of various diameters from 0.07 to 0.7 mm are examined about appearances and characteristics of bubble occurrence behind them in the range of low Reynolds numbers. The appearance of bubbles is very dependent on diameters of wires. Two different types of bubbles can be observed in the present experiment. One is a streamer-type bubble for smaller wires and the other is a small unspherical bubble for larger wires. The incipient and the desinent values of cavitation number also change greatly with the bubble types. The streamer-type bubble is related to the presence of laminar separation zone and the growth due to air diffusion. The small unspherical bubble can be mainly attributed to the motion of rolled-up vortices and the growth due to vaporization.
Simultaneous Cavitation Susceptibility Meter and Holographic Measurements of Nuclei in Liquids
Authors: d’Agostino Luca , Green S. I.
Published: May 2008

Cavitation Susceptibility Meter (CSM) and holographic measurements of cavitation nuclei distributions are compared in this paper. The CSM optically detects cavitation in water samples flowing through a venturi and relates the unstable nuclei concentration to the applied tension in the fluid. A ruby laser holographic system measures the nuclei size distribution directly. Microbubbles have been used as the dominant nuclei source. The data from the two detection schemes are correlated by accounting for the dynamic response of the cavities in the venturi throat. The active nuclei distributions predicted by the holographic data compare favorably with those measured by the CSM. Both detection methods show that the nuclei concentration rises approximately exponentially as the applied tension is increased and then, with further reduction in the pressure, tends to a nearly constant maximum due to the shortage of remaining cavitatable nuclei. The CSM consistently underestimates the concentration of active cavitation nuclei, due to limited electro-optical resolution and mutual interference effects between cavities in the venturi. The good qualitative agreement of the two techniques supports the validity of the data correlation model and clearly indicates that any practical interpretation of measured nuclei size distributions for cavitations predictions is highly dependent of the specific flow conditions. Attempts to cavitate saturated water of the California Institute of Technology Low Turbulence Water Tunnel in the CSM were unsuccessful even at the lowest attainable CSM throat pressures (about −40kPa). This is thought to be due to insufficient throat tension and, at least partially, to the short time available for cavity growth in the CSM.
Fluid-Structure Interaction With Cavitation in Transient Pipe Flows
Authors: Fan D. , Tijsseling A.
Published: May 2008

The interactions between axial wave propagation and transient cavitation in a closed pipe are studied. Definitive experimental results of the phenomenon are produced in a novel apparatus. The apparatus is characterized by its simplicity and its capability of studying transient phenomena in a predictable sequence. The influence due to friction is small and the representations of the boundary conditions are straightforward. Measurements with different severity of cavitation are provided to enable other researchers in the area to compare with their theoretical models. A new cavitating fluid/structure interaction cavitation model is proposed. The measurements are compared with the column separation model of Tijsseling and Lavooij (1989) and the new model to validate the experimental results.
Brief Reviews of Some Time-Dependent Flows
Authors: Sarpkaya T.
Published: May 2008

Separated flows in general and time-dependent flows in particular provide fertile territory for fundamental research in fluid dynamics and account for much of the subject matter. Thus, it is thought appropriate to review some of these unsteady flows with special emphasis on hydrodynamic applications which are, admittedly, of special interest to this writer: unsteady separation; characteristics of impulsively and nonimpulsively-started flow about cylinders; excursion of separation points on circular cylinders in oscillating flow; separation and other flow phenomena governing the unsteady maneuvers of large submerged bodies, and, finally, the three-dimensional footprints of subsurface vortical structures rising toward the free surface. It is hoped that these concise reviews will enhance communication between various groups of researchers, draw attention to many exciting phenomena in naval hydrodynamics, and inspire new research topics.
Simultaneous Variable Solutions of the Incompressible Steady Navier-Stokes Equations in General Curvilinear Coordinate Systems
Authors: Vradis G. , Zalak V. , Bentson J.
Published: May 2008

A simultaneous variable solution technique for the incompressible, steady, two-dimensional Navier-Stokes equations in primitive formulation and general curvilinear orthogonal and nonorthogonal coordinate systems has been developed. The governing equations are discretized using finite difference approximations. The formulation is fully second order accurate and the well-known staggered grid of Welch and Harlow is used. The solution algorithm is based on an iterative marching technique in which the algebraic equations are linearized by evaluating the coefficients at the previous iteration level. The resulting system of linear equations is solved in a marching fashion by employing a block tridiagonal solution algorithm to obtain the solution along lines transverse to the main flow direction. The strong pressure-velocity coupling inherent in the present formulation results in high convergence rates. Flows in channels of different geometries have been computed and the results have been compared to available data in the literature. In all cases the method has demonstrated to be accurate, robust and computationally efficient.
The Linear Stability of a Flat Plate Boundary-Layer Approaching a Cylindrical Obstacle
Authors: Spall R. E. , Malik M. R.
Published: May 2008

The linear stability of the low-speed three-dimensional flow over a flat plate with an attached cylinder is studied. The region of interest is upstream of the initial separation point and includes the effects of both adverse and favorable pressure gradients, as well as crossflow. The resulting boundary-layer is subject to both the Tollmien-Schlichting (TS) and crossflow instabilities. Linear stability calculations, using N-factor correlations, indicate that the transition process would be dominated by TS instabilities, although for low frequencies crossflow-type disturbances are important.
Boundary-Layer Transition in Accelerating Flows With Intense Freestream Turbulence: Part 2—The Zone of Intermittent Turbulence
Authors: Blair M. F.
Published: May 2008

Hot-wire anemometry was employed to examine the laminar-to-turbulent transition of low-speed, two-dimensional boundary layers for two (moderate) levels of flow acceleration and various levels of grid-generated freestream turbulence. Flows with an adiabatic wall and with uniform-flux heat transfer were explored. Conditional discrimination techniques were employed to examine the zones of flow within the transitional region. This analysis demonstrated that as much as one-half of the streamwise-component unsteadiness, and much of the apparent anisotropy, observed near the wall was produced, not by turbulence, but by the steps in velocity between the turbulent and inter-turbulent zones of flow. Within the turbulent zones u′/v′ ratios were about equal to those expected for equilibrium boundary-layer turbulence. Near transition onset, however, the turbulence kinetic energy within the turbulent zones exceeded fully turbulent boundary-layer levels. Turbulent-zone power-spectral-density measurements indicate that the ratio of dissipation to production increased through transition. This suggests that the generation of the full equilibrium turbulent boundary-layer energy cascade required some time (distance) and may explain the very high TKE levels near onset.
A Three-Layer Model for Separated Turbulent Flows
Authors: Goldberg U. C.
Published: May 2008

A model for turbulent backflows is presented and evaluated. The model is based on experimental observations, in particular, the wake-like behavior of the turbulence and the diffusion/dissipation energy balance observed in detached flow regions. These lead to an ODE for the eddy viscosity, based on a reduced form of the turbulence kinetic energy equation. This ODE is solvable analytically. The model is applied as a module within a “host” eddy viscosity model (such as a k–ε model) using a three-layer framework wherein the host model predicts the eddy viscosity distribution in the inner and outer layers, while the backflow model provides the eddy viscosity field in the middle layer. Predictions of several flow cases, using this approach, are shown.
A Comparison of Turbulent Boundary Layer Wall-Pressure Spectra
Authors: Keith W. L. , Hurdis D. A. , Abraham B. M.
Published: May 2008

Turbulent boundary layer wall-pressure spectra from various experimental investigations and a recent numerical simulation are presented. The spectra are compared in nondimensional form with three commonly used scaling laws. Attenuations resulting from inadequate sensor spatial resolution are shown to be of primary importance at the higher frequencies. The dependence of the scaling laws on momentum thickness Reynolds number is discussed. The ratio of the outer to the inner boundary layer length scale is shown to provide insight into the observed trends in the spectra.
On the Organization of Flow and Heat Transfer in the Near Wake of a Circular Cylinder in Steady and Pulsed Flow
Authors: Telionis D. P. , Gundappa M. , Diller T. E.
Published: May 2008

Skin friction, pressure, and heat transfer gages are employed to monitor the flow and heat transfer field along the periphery of a circular cylinder in steady and pulsed flow at Reynolds numbers, Re = 23,000 to 50,000. Averaged distributions, RMS, and power spectra of all measurements are displayed. Special attention is directed at the organization of the near wake, as detected by the three types of surface gages. The response of the wake to pulsing of the oncoming stream is also examined. It is found that when the wake is locked on the driving frequency, the basic character of the flow is not changed, but the organized motion stands out more clearly. Moreover, the signals become cleaner and background noise in the spectra is reduced. Skin friction and heat transfer gages are shown to respond to local variations of the corresponding quantities, whereas pressure gages respond to global characteristics of the flow.
Three-Dimensional Flow Patterns in Two-Dimensional Wakes
Authors: Triantafyllou G. S.
Published: May 2008

The development of three-dimensional patterns in the wake of two-dimensional objects is examined from the point of view of hydrodynamic stability. It is first shown that for parallel shear flows, which are homogeneous along their span, the time-asymptotic state of the instability is always two-dimensional. Subsequently, the effect of flow inhomogeneities in the spanwise direction is examined. Slow modulations of the time-average flow in the span wise direction, and localized regions of strongly inhomogeneous flow are separately considered. It is shown that the instability modes of an average flow with a slow modulation along the span have a spanwise wavelength equal to twice that of the average flow. Moreover, for the same average flow two instability modes are possible, identical in every respect except from their spanwise structure. Localized inhomogeneities on the other hand can generate through linear resonances inclined vortex filaments in the homogeneous part of the fluid. The theory provides an explanation for the vortex patterns observed in recent flow visualization experiments, and a theoretical justification of the cosine law for the frequency of inclined vortex shedding (Williamson, 1988).
Gas-Solid Flow in a Fluidically Oscillating Jet
Authors: Morris G. J. , Jurewicz J. T. , Palmer G. M.
Published: May 2008

The motion of air and solid particles is examined in a fluidically oscillating slot jet using time-averaged and cycle-resolved laser Doppler anemometry measurements. These measurements reveal the time dependent relative velocity magnitudes between the phases as well as the detailed nature of the flulidcally oscillating slot jet. Temporal phase differences between the gas and solid phases ranged up to 40 degrees for jet oscillation frequencies up to 50 Hz. The results indicate that the fluidic nozzle is an effective particle spreading device and the fuel injector attributes are inherently present such as enhanced mixing and low velocity regions for flame anchoring.
Simulation of the Strongly Swirling Aerodynamic Field in a Vortex Combustor
Authors: Nieh Sen , Zhang Jian
Published: May 2008

This paper presents the simulation of the strongly swirling aerodynamic field in a coal-fired vortex combustor (VC) recently developed for commercial heating applications. A new version of algebraic Reynolds stress model was employed for the closure of non-isotropic turbulence. The calculated results of the 25 cm I.D. bench-scale and the 61 cm I.D. full-scale VC cold test models showed the pertinent aerodynamic features of the VC in terms of strongly swirling, developing, recirculating, and non-isotropic turbulent flow. The dynamic similarity was generally maintained for VCs of different scales.
Novel Optical Fiber Installation by Use of Spiral Airflow
Authors: Horii K. , Matsumae Y. , Ohsumi K. , Cheng X. M. , Kage S. , Hashimoto B.
Published: May 2008

A high performance apparatus for installing optical fiber has been developed. With this apparatus, a 2 mm-diameter optical fiber was blown through a 6 mm-diameter rolled tube over a distance of 1200 meters, which is longer than the 700 meters achieved by the best conventional techniques. The heart of the apparatus is a novel nozzle with an annular slit and a conical cylinder to create a spiral airflow. In the spiral flow, the back flow of air is prevented due to the Coanda effect and the fiber scraping along the tube wall is minimized, owing to the ordered flow structure, leading to effective fiber installation. To obtain a spiral flow, pressurized air is forced into the buffer area of a nozzle and then through the angled annular slit into the tube entrance. Due to the Coanda effect of the annular jet from the slit attaching to the nozzle walls and the conical cylinder, the downstream flow in the tube develops a highly stable spiral structure with a steep axial velocity distribution that is more ordered than typical turbulent tube flows at the same mass-flow rate.
Study of Branched Turboprop Inlet Ducts Using a Multiple Block Grid Calculation Procedure
Authors: Tolpadi A. K. , Braaten M. E.
Published: May 2008

An important requirement in the design of an inlet duct of a turboprop engine is the ability to provide foreign object damage protection. A possible method for providing this protection is to include a bypass branch duct as an integral part of the main inlet duct. This arrangement would divert ingested debris away from the engine through the bypass. However, such an arrangement could raise the possibility of separated flow in the inlet, which in turn can increase pressure losses if not properly accounted for during the design. A fully elliptic three-dimensional body-fitted computational fluid dynamics (CFD) code based on pressure correction techniques has been developed that has the capability of performing multiple block grid calculations compatible with present day turboshaft and turboprop branched inlet ducts. Calculations are iteratively performed between sets of overlapping grids with one grid representing the main duct and a second grid representing the branch duct. Both the grid generator and the flow solver have been suitably developed to achieve this capability. The code can handle multiple branches in the flow. Using the converged flow field from this code, another program was written to perform a particle trajectory analysis. Numerical solutions were obtained on a supercomputer for a typical branched duct for which experimental flow and pressure measurements were also made. The flow separation zones predicted by the calculations were found to be in good agreement with those observed in the experimental tests. The total pressure recovery factors measured in the experiments were also compared with those obtained numerically. Within the limits of the grid resolution and the turbulence model, the agreement was found to be fairly good. In order to simulate the path of debris entering the duct, the trajectories of spherical particles of different sizes introduced at the inlet were determined.
Effect of Density, Size Distribution, and Concentration of Solid on the Characteristics of Centrifugal Pumps
Authors: Gahlot V. K. , Seshadri V. , Malhotra R. C.
Published: May 2008

Experimental data on the performance of the centrifugal pumps pumping mixtures of solids and water have been presented. The solids used were coal of density 1480 kg/m3 and zinc tailings of density 2850 kg/m3. Maximum size of particles was approximately 3 mm. Tests have been conducted with a rubber lined impeller pump and a metal impeller pump. Effects of solid properties (viz: density, size, and size distribution as well as concentration of solids) on the performance of the pumps have been studied. The measured performance of pumps is compared with the predictions based on the correlations available in literature and a modified empirical relationship has been proposed for the prediction of the pump performance with slurries.
Quantitative Visualization of the Flow Within the Volute of a Centrifugal Pump. Part A: Technique
Authors: Dong R. , Chu S. , Katz J.
Published: May 2008

This paper describes a series of quantitative flow visualization experiments within the volute of a centrifugal pump by implementing the “Particle Displacement Velocimetry” method (PDV or PIV). Part A focuses on the measurement procedures and includes an uncertainty analysis. This technique involves illuminating sections of a flow field with pulsed laser sheets while seeding the water with microscopic, neutrally buoyant particles containing imbedded fluorescent dye. By pulsing the laser more than once while recording a single photograph each particle leaves multiple traces on the same film. The analysis procedures consist of dividing the image to a large number of small sections (windows) and computing of the mean shift of all particles within each window. This shift is determined by computing the autocorrelation function of the image within the window. Digital image processing and specially written software are being utilized while analyzing the data. It is demonstrated that by controlling the magnification and particle concentration, the relative error can be maintained at about 1 percent. A sample analyzed image is presented. The rest of the data are included in Part B.
Quantitative Visualization of the Flow Within the Volute of a Centrifugal Pump. Part B: Results and Analysis
Authors: Dong R. , Chu S. , Katz J.
Published: May 2008

PDV is used for measuring the velocity within the volute of a centrifugal pump at different impeller blade orientations, on and off design conditions. It is demonstrated that the flow is “pulsating” and depends on the location of the blade relative to the tongue. The leakage also depends on blade orientation and increases with decreasing flow rate. The velocity near the impeller is dominated by the jet/wake phenomenon. Differences in the outflux from the impeller, resulting from changes inflow rate, occur primarily near the exit. Away from the tongue the distributions of vθ mostly agrees with the assumption that vθ ∝ 1/r. Sites prone to high velocity fluctuations include the blade wake, interface between the jet and the wake and near the tongue. Angular momentum and kinetic energy fluxes, turbulent stresses and tubulence production are also computed. It is shown that at the same θ the momentum flux can increase near the impeller and decrease at the perimeter. Consequently, the mean flux cannot be used for estimating conditions near the impeller. Torques caused by τrθ and τθθ can be as high as 2 and 5 percent of the change in angular momentum flux, respectively.
Numerical Study of the Effects of Longitudinal Acceleration on Solid Rocket Motor Internal Ballistics
Authors: Gottlieb J. J. , Greatrix D. R.
Published: May 2008

The internal ballistics of a solid-propellant rocket motor subjected to both constant and oscillatory longitudinal accelerations are studied. The one-dimensional time-dependent equations of motion governing the unsteady two-phase core flow in the accelerating motor chamber and nozzle are solved numerically by using the random-choice method, along with pressure-dependent and crossflow-dependent burning-rate equations for propellant combustion. A constant forward acceleration produces negligible effects, whereas longitudinal motor vibrations near the natural frequency of waves criss-crossing the length of the motor chamber can produce large but bounded oscillatory motor-chamber pressures.
Motion of a Particle Floating on a Liquid Meniscus Surface
Authors: Katoh Kenji , Fujita Hideomi , Imazu Eiichiro
Published: May 2008

A small particle floating on a liquid meniscus surface near a vessel wall tends to approach the wall spontaneously. This particle behavior was theoretically investigated. Because of the solid-liquid wettability, an unbalanced force is caused in the tangential direction to the liquid surface and the particle moves toward the wall. Since the particle is excessively drawn into the liquid due to the wettability, an excessive buoyant force acts on the particle in the vertical direction. The force caused by the wettability points in the normal downward direction to the liquid surface and is balanced with that component of the buoyant force. Then the tangential component of the buoyant force is not balanced and the particle moves upward along the meniscus surface. The particle trajectory calculation based on the above theoretical consideration agrees with the experimental one observed by a high-speed video camera system.
The Magnitude of Basset Forces in Unsteady Multiphase Flow Computations
Authors: Liang Li , Michaelides Efstathios E.
Published: May 2008

The equation of motion of a small spherical particle moving in a fluid is solved numerically with the radius of the sphere and the ratio of fluid to particle densities being parameters. The Basset force term is computed and compared to the total force on the particle for the case of turbulent flow in a duct. It is found that the Basset force may be neglected in the equation of motion of the particle only when the fluid to particle density ratio is very high and the particle diameter is greater than 1 μm. A dimensional analysis is also performed for the case when the particle size and the characteristic flow dimension are of the same order of magnitude. In the latter case, it is deduced that the Basset force is significant whenever the flow Reynolds number is greater than one.
Numerical Analysis of Gas-Particle Two-Phase Subsonic Freejets
Authors: Hatta N. , Ishii R. , Fujimoto H.
Published: May 2008

This paper describes a numerical analysis of gas-droplet two-phase subsonic free jets in the axisymmetric system. Thermal coupling through heat transfer to droplets, as well as momentum coupling through aerodynamic drag responsible for droplet motion, is taken into account in the present numerical model. The Navier-Stokes equations for a gas-phase interacting with particle phase are solved by a time-dependent difference technique and the particle-phase is solved by a discrete particle cloud model. The jet flow structures of mixture composed of air and water-droplets with 1 μm, 5 μm, and 30 μm, respectively, in diameter are calculated with a single particle size. Some of significant characteristics for the two-phase subsonic free jets are pointed out, in particular, focusing upon the effect of particle size on the flow structure.
Water Quality Effects on Cavitation Inception in a Trailing Vortex
Authors: Arndt Roger E. A. , Keller Andreas P.
Published: May 2008

Tip vortex cavitation studies were made with a hydrofoil that was elliptical in planform, with an aspect ratio of 3, and having a modified NACA 662-415 profile. LDV measurements of the tangential velocity component in the vortex were used to determine that the minimum pressure in the vortex varies with lift coefficient squared, i.e., that the incipient cavitation number σi should follow a Cl2 relation (σi ≈ Cl2). This is in contradiction to previous observations (Arndt et al. 1991) that the tip vortex cavitation index varied approximately with lift coefficient to the power 1.4. By carefully monitoring the tensile strength of the water, i.e., its susceptibility to cavitation, the discrepancy was traced to the capability of the test water to sustain a tensile stress. Cavitation in “weak” water (no tensile strength) does follow the Cl2 relationship, whereas observations in “strong” water (rupture considerably below vapor pressure) more closely followed the previously observed variation, i.e., σi ≈ Cl1.4. Since the structure of the vortex cannot be affected by changes in the water quality, the discrepancy can be explained only by the amount of tension that can be sustained by the test water before inception occurs. Apparently a relatively larger value of tension can be sustained in the vortex is the strength of the vortex is increased (i.e., increasing Cl). This would explain the observed deviation from the expected Cl2 law for water with measurable tensile strength.
A Note on Choking Cavitation Flow Past Bluff Bodies
Authors: Ramamurthy A. S. , Balachandar R.
Published: May 2008

A model is developed to predict the choking cavitation number for sharp edged bluff bodies subject to wall interference effects. The fact that the forebody pressure distribution under cavitating conditions essentially resembles the values obtained in noncavitating flows is made use of in the development of the model. The model is verified using experimental results from present and previous studies for a specific case of choking flow past a two-dimensional prismatic body.
Applicability Tests for the Phase Doppler Anemometer for Cavitation Nuclei Measurements
Authors: Tanger H. , Weitendorf E.-A.
Published: May 2008

A compromise between a reliable phase-diameter relationship and a good imaging of the control volume is necessary for the application of the PDA-instrument when measuring cavitation nuclei. The phase-diameter relationship for bubbles and particles were investigated theoretically, the imaging of the control volume experimentally. Further PDA-measurements were conducted for Latex spheres of different diameters as scattering objects in water and single air bubbles in transparent plastics. The concentrations and diameters measured by the PDA agreed very well with the results obtained by other methods. An investigation of the scattering from background particles in water was conducted with the PDA in the small HSVA cavitation tunnel. Also, nuclei measurements were performed with a gassing system in the tunnel. In 1972 the ITTC-Cavitation Committee defined specifications for the control of the free air content in routine and research cavitation tests. The tests completed so far show that the PDA-instrument fulfills these specifications for the major part.
Hydrodynamics of Two-Phase Flow Across Horizontal In-line and Staggered Rod Bundles
Authors: Dowlati R. , Chan A. M. C. , Kawaji M.
Published: May 2008

The void fraction and friction pressure drop measurements have been made for vertical two-phase flow of air-water across staggered and in-line rod bundles with different pitch-to-diameter ratios. All void fraction data showed a strong mass velocity effect and were significantly less than the values predicted by a homogeneous flow model, but were well correlated using the dimensionless gas velocity, jg*. The two-phase friction multiplier data could be well correlated with the Martinelli parameter for G > 200 kg/m2s. The correlations developed for void fraction and two-phase friction multiplier were successfully tested in predicting the total pressure drop in boiling R-113 experiments.
Experimental Investigation of Uniform-Shear Flow Past a Circular Cylinder
Authors: Kwon Tae Soon , Sung Hyung Jin , Hyun Jae Min
Published: May 2008

Extensive laboratory experiments were carried out to investigate the uniform-shear flow approaching a circular cylinder. The aim was to present the Strouhal number (St)- Reynolds number (Re) diagrams over a broad range of the shear parameter K (0 ≤ K ≤ 0.25) and at higher values of Re (600 ≤ Re ≤ 1600). An image processing technique, in conjunction with flow visualization studies, was used to secure more quantitative depictions of vortex shedding from the cylinder. The Strouhal number increases with increasing shear parameter. The drag coefficient decreases with increasing Re; also, Cd decreases as the shear parameter K increases.
The Measurement of Solid-Liquid Contact Angles
Authors: Katoh Kenji , Fujita Hideomi , Sasaki Hideharu , Miyashita Koichi
Published: May 2008

A new method is proposed for measuring solid-liquid contact angles. The well-known phenomenon where the liquid meniscus formed under a downward facing solid surface spontaneously breaks at a certain height is utilized in the contact angle measurements. The relation between the contact angle and the critical height of the solid surface where the instability occurs was derived theoretically from the solid-liquid wetting behavior using a thermodynamic approach. From the theoretical model the contact angles can be obtained by measuring the critical height of the solid. The validity of the analysis and the usefulness of the method were experimentally confirmed for various solid surfaces and test liquids.
A Note on a Generalized Eddy-Viscosity Hypothesis
Authors: Andreasson P. , Svensson U.
Published: May 2008

The standard eddy-viscosity concept postulates that zero velocity gradient is accompanied by zero shear stress. This is not true for many boundary layer flows: wall jets, asymmetric channel flows, countercurrent flows, for example. The generalized eddy-viscosity hypothesis presented in this paper, relaxes this limitation by recognizing the influence of gradients in the turbulent length scale and the shear. With this new eddy-viscosity concept, implemented into the standard k–ε model, predictions of some boundary layer flows are made. The modelling results agree well with measurements, where predictions with the standard eddy-viscosity concept are known to fail.