bluuebunny/crossref_metadata_2025 huggingface dataset:
Flow Distribution and Pressure Drop in Diffuser-Monolith Flows
Authors: Kim J. Y. , Lai M.-C. , Li P. , Chui G. K.
Published: February 2008

Most current automotive catalytic converters use diffusers to distribute the flow field inside the monolithic bricks where catalysis takes place. While the characteristics and performance of a simple diffuser flow are well documented, the influence of downstream brick resistance is not clear. In this paper, the trade-off between flow-uniformity and pressure drop of an axisymmetric automotive catalytic converters is studied numerically and experimentally for selected cases. The monolithic brick resistance is formulated from the pressure gradient of fully developed laminar duct-flow and corrected for the entrance effect. The monolithic brick downstream stabilizes the diffuser flows both physically and computationally. A distribution index was formulated to quantify the degree of nonuniformity in selected test cases. The test matrix covers a range of different diffuser angles and flow resistance (brick types). For simplicity, an axisymmetric geometry is chosen. Flow distribution within the monolith was found to depend strongly on diffuser performance, which is modified strongly by brick resistance. Pressure drop due to the headers and brick resistance and their relative roles is also identified. The implications of these data for converter design are discussed in terms of the trade-off between flow-uniformity and pressure drop.
Deformation Process of a Water Droplet Impinging on a Solid Surface
Authors: Hatta Natsuo , Fujimoto Hitoshi , Takuda Hirohiko
Published: February 2008

This paper is concerned with numerical simulations of the deformation behavior of a liquid droplet impinging on a flat solid surface, as well as the flow field inside the droplet. In the present situation, the case where a droplet impinges on the surface at room temperature with a speed in the order of a few [m/s], is treated. These simulations were performed using the MAC-type solution method to solve a finite-differencing approximation of the Navier-Stokes equations governing an axisymmetric and incompressible fluid flow. For the first case where the liquid is water, the liquid film formed by the droplet impinging on the solid surface flows radially along it and expands in a fairly thin discoid-like shape. Thereafter, the liquid flow shows a tendency to stagnate at the periphery of the circular film, with the result that water is concentrated there is a doughnut-like shape. Subsequently, the water begins to flow backwards toward the center where it accumulates in the central region. For the second case where a n-heptane droplet impinges the surface, the film continues to spread monotonically up to a maximum diameter and there is no recoiling process to cause a backwards flow towards the central region. In this study the whole deformation process was investigated from numerical as well as experimental points of view. We find that the results obtained by the present mathematical model give fairly good agreement with the experimental observations. The effects of the viscous stresses and the surface tension on the deformation process of the droplets are estimated and discussed from a practical standpoint.
Effect of Reynolds Number on Isotropic Turbulent Dispersion
Authors: Mei Renwei , Adrian Ronald J.
Published: February 2008

The influence of the spatio-temporal structure of isotropic turbulence on the dispersion of fluid and particles with inertia is investigated. The spatial structure is represented by an extended von Ka´rma´n energy spectrum model which includes an inertial sub-range and allows evaluation of the effect of the turbulence Reynolds number, Reλ. Dispersion of fluid is analyzed using four different models for the Eulerian temporal auto-correlation function D(τ). The fluid diffusivity, normalized by the integral length scale L11 and the root-mean-square turbulent velocity u0, depends on Reλ. The parameter cE = T0u0/L11, in which T0 is the Eulerian integral time scale, has commonly been assumed to be constant. It is shown that cE strongly affects the value of the fluid diffusivity. The dispersion of a particle with finite inertia and finite settling velocity is analyzed for a large range of a particle inertia and settling velocity. Particle turbulence intensity and diffusivity are influenced strongly by turbulence structure.
Vorticity Transport Analysis of Turbulent Flows
Authors: Gorski J. J. , Bernard P. S.
Published: February 2008

Turbulence closure for the Reynolds averaged Navier-Stokes equations based on vorticity transport theory is investigated. General expressions for the vorticity transport correlation terms in arbitrary two-dimensional mean flows are derived. Direct numerical simulation data for flow in a channel is used to evaluate the modeled terms and set unknown scales. Results are presented for a channel, flat plate boundary layer, and flow over a hill. The computed mean flow and kinetic energy compares well with numerical and physical experiments. The vorticity transport model appears to perform better than conventional Boussinesq eddy viscosity Reynolds stress models near the separated flow region on the leeward side of the hill.
Structure of Turbulence in an Incipient-Separating Axisymmetric Flow
Authors: Singh Rakesh K. , Azad Ram S.
Published: February 2008

The relative intensity, skewness, and flatness of fluctuating streamwise velocity along the centerline of an 8 deg included angle conical diffuser show dramatic rapid growth in the final stages of the flow under the increasing influence of growing instantaneous reversals in the wall-layer. Pulsed-wire anemometry was effectively used for the measurement of quantitative instantaneous reversals and the turbulent flow field. In the severe adverse pressure gradient of the diffuser flow, the maxima of the streamwise and transverse fluctuating velocities, Reynolds shear stress, and turbulent energy production coincide and move away from their near-wall position in the pipe, also the velocity triple products show completely opposite nature as compared to the pipe flow. These measurements reveal the strong influence of instantaneous backflow on the structure of turbulence. The present results further corroborate the ability of the “structural” turbulence model of Nagano and Tagawa (1990) to predict velocity triple products in an axisymmetric diffuser flow.
Oscillatory Electrohydrodynamic Gas Flows
Authors: Lai F. C. , McKinney P. J. , Davidson J. H.
Published: February 2008

Prior numerical solutions of electrohydrodynamic flows in a positive-corona, wire-plate electrostatic precipitator are extended to reveal steady-periodic electrohydrodynamic flows. Previously, only steady solutions were reported. The present study includes results for flows with Reynolds numbers from 0 to 4800 and with dimensionless electric number ranging from 0.06 to ∞. Results indicate that two regimes of low frequency oscillatory flow occur. The first regime is characterized by a single recirculating vortex that oscillates in strength between one and five Hertz. The second regime is characterized by two counter-rotating vortices that oscillate in strength at a frequency near one Hertz.
Testing Three-Dimensional Bluff-Body Models in a Low-Speed Two-Dimensional Adaptive Wall Test Section
Authors: Sumner D. , Brundrett E.
Published: February 2008

Thin, sharp-edged disk models were evaluated in a low-speed two-dimensional adaptive flexible wall test section to determine the optimum adaptive wall testing environment for three-dimensional bluff-body models, by providing model testing recommendations for nominal solid blockage ratio and model span ratio. Drag coefficient measurements obtained under straight wall and adapted wall conditions showed that for a two-dimensional adaptive wall test section, the model span ratio imposes a more severe restriction upon model size than does the nominal solid blockage ratio. Minimum wall interference conditions were achieved with adapted walls for nominal solid blockage ratios less than 3 percent and model span ratios less than 21 percent, independent of the nominal test section aspect ratio, based on favorable comparison with previously-published experimental data. Data obtained under straight wall conditions confirmed that wall interference effects can only be neglected in conventional, straight-walled test sections for solid blockage ratios less than 0.5 percent and model span ratios less than 10 percent. The post-test boundary correction method of Maskell was successfully used to adjust the straight wall test section drag coefficient measurements of the larger models for wall interference effects, but no direct measurements of wall interference are used with this method. The results support the careful use of a two-dimensional wall adjustment strategy for three-dimensional nonlifting flows.
Implications of Correlated Bias Uncertainties in Single and Comparative Tests
Authors: Coleman Hugh W. , Steele W. Glenn , Taylor Robert P.
Published: February 2008

The effects of correlated bias uncertainties are considered for both single and comparative tests, and the implications of these effects on experimental results and their uncertainties are investigated. In single tests, correlated bias uncertainties can cause a significant increase or a significant decrease in uncertainty in the result as compared to the uncertainty when there are no correlation effects. In comparative tests, the experimental result is the difference of two test results or the ratio of two test results. If the same test apparatus and instrumentation are used to obtain the two test results, the systematic uncertainty in the difference or the ratio can be significantly less than the systematic uncertainly in the individual test results. However, the effects of systematic uncertainties do not cancel out in comparative tests in which the result is the difference in the results of two tests, contrary to assertions made in some engineering standards.
The Interaction of Rolling Vortices With a Turbulent Boundary Layer
Authors: Donnelly M. J. , Rediniotis O. K. , Ragab S. A. , Telionis D. P.
Published: February 2008

Laser-Doppler velocimetry is employed to measure the periodic field created by releasing spanwise vortices in a turbulent boundary layer. Phase-averaged vorticity and turbulence level contours are estimated and presented. It is found that vortices with diameter of the order of the boundary layer quickly diffuse and disappear while their turbulent kinetic energy spreads uniformly across the entire boundary layer. Larger vortices have a considerably longer life span and in turn feed more vorticity into the boundary layer.
Prediction of Compressible Flow Pressure Losses in 30–150 Deg Sharp-Cornered Bends
Authors: Haidar Nia
Published: February 2008

This paper considers the measurement and prediction of the additional total pressure losses of subsonic steady air flow in sharp-cornered bends, similar to those present in the secondary air cooling systems of gas turbine engines. The bends examined ranged between 30 to 150 in 30 deg increments and were circular in cross section. Experimental results covering a wide speed range up to choking are presented for five different bend geometries. An analytical flow model provided results in fairly good agreement with the measurements obtained and equally compared favourably with the experimental findings of other researchers at low Mach numbers. The highest attainable upstream Mach number (MU) of the average upstream flow was 0.57 for the 30 deg bend. The maximum possible values of MU represent a limiting condition dictated by downstream choking of the flow. The compressible flow coefficients, caused by the presence of the bends, can be expected to be between 10 to 20 percent higher than those for incompressible flow.
Predictions of the Structure of Turbulent, Highly Underexpanded Jets
Authors: Cumber P. S. , Fairweather M. , Falle S. A. E. G. , Giddings J. R.
Published: February 2008

A mathematical model capable of predicting the shock and flow structure of turbulent, underexpanded jets is described. The model is based on solutions of the fluid flow equations obtained using a second-order accurate, finite-volume integration scheme together with an adaptive grid algorithm. Closure of these equations is achieved using a k-ε turbulence model coupled to the compressible dissipation rate correction proposed by Sarkar et al. (1991a). Extending earlier work which demonstrated the ability of this model to predict the structure of moderately underexpanded jets, the present paper compares model predictions and experimental data, reported in the literature, on a number of highly underexpanded releases. The results obtained demonstrate that the model yields reliable predictions of shock structure in the near field, inviscid region of such jets, while in the far field results derived using the compressibility corrected turbulence model are adequate for predicting mean flow properties, and are superior to those obtained using a standard k-ε approach.
Three-Dimensional Boundary Layers Over an Infinite Swept Bump and Free Wing
Authors: Wu Xiaohua , Squires Kyle D.
Published: February 2008

Three-dimensional laminar boundary layers past an infinite swept bump and free wing were investigated numerically using the fractional step method. The objective of the work was to study the effect of surface curvature induced changes in pressure gradient and changes in the freestream flow on boundary layer skewness and growth. Simulation results demonstrate that for flows over the bump the first transition from adverse to favorable pressure gradient occurs at the front concave/convex inflexion and the second transition from favorable to adverse pressure gradient occurs at the summit. For flows past a free wing, the only transition from favorable to adverse pressure gradient occurs in front of the summit and the subsequent adverse pressure gradient is larger than the corresponding value for the bump. For both the bump and wing, the increase of initial skewing angle from 0 to 30 deg causes a 10 percent reduction in the length of the wake; the wake behind the wing is about 12 percent longer in streamwise extent than the corresponding wake behind the bump. Integral parameters in the flows over the bump display a wavy trend due to the two transitions of the pressure gradient. On the other hand, the single transition from favorable to adverse pressure gradient brings about a monotonic increase of the integral parameters for flows past the wing. Near separation and reattachment, surface-streamlines are skewed strongly in the spanwise direction. Conditions of flow detachment for the bump and wing are in good agreement with correlations for laminar separating flows with power-law velocity profiles as well as correlations for wall-curvature-induced turbulent separating flows.
Mass Flow and Thrust Performance of Nozzles With Mixed and Unmixed Nonuniform Flow
Authors: Decher Reiner
Published: February 2008

The calculated thrust and mass flow rate of a nozzle depend on the uniformity of the entering flow. The one-dimensional flow equations are extended to arrive at analytic expressions for the predicted performance of a nozzle processing two streams whose properties are determined ahead of the throat. The analysis approach forms the basis for the understanding of flows which have more complex distributions of total pressure and temperature. The uncertainty associated with mixing is examined by the consideration of the two limiting cases: compound flow with no mixing and completely mixed flow. Nozzle discharge and velocity coefficients accounting for non-uniformity are derived. The methodology can be extended to experimentally measured variations of flow properties so that proper geometric design variables may be obtained.
Tumbling Flow in Loop-Scavenged Two-Stroke Engines
Authors: Tsui Yeng-Yung , Cheng Hong-Ping
Published: February 2008

A multidimensional calculation procedure is used to investigate the flow in loop-scavenged two-stroke engine with curved cylinder heads. Five different cylinder heads are considered. The curvature of cylinder head increases from case I to case IV. In case V the head curvature is further increased, but it is shaped in the radially outer region. Calculations reveal that a tumbling vortex forms after the exhaust port is closed and the vortex constantly dominates the flow structure in the cylinder throughout the compression period. With high head curvatures the vortex is well organized and occupies the entire cylinder volume in the late compression stage. Due to compression of the better organized tumbling vortex by the moving piston more energy cascades from mean flow to turbulence in the high curvature cases III and IV. As for case V, the larger clearance in the bowl center region leads to lower shear stresses and, thus, the turbulence augmentation phenomenon is less prominent than that for cases III and IV.
The Ducted Tip—A Hydrofoil Tip Geometry With Superior Cavitation Performance
Authors: Green S. I. , Duan S. Z.
Published: February 2008

A novel hydrofoil design, consisting of a small diameter flow-through duct affixed to the tip, has been studied. The tip vortex cavitation inception index, σi, of this hydrofoil geometry is about a factor of 2 lower than that of a conventional rounded hydrofoil tip. This inception improvement comes with little associated performance penalty. For angles of attack greater than 8 deg the noncavitating lift-drag ratio is actually superior to that of an unducted hydrofoil of equal span, although with lower wing loadings the hydrofoil performance is diminished by application of the ducted tip. The ducted tip is effective at reducing the tip vortex inception index because, in contrast with the rounded tip, for which vorticity in the Trefftz plane is confined to a line, the ducted tip shed vorticity at the trailing edge is distributed over a line and circle. Distributing the vorticity in this fashion causes the trailing vortex to roll up less tightly, and hence have a higher core pressure and lower σi, than a conventional hydrofoil tip. It is also suspected that the interaction at the microscale level between the flow through the duct, and the flow around it, makes the vortex core size larger, and therefore σi smaller. The ducted tip design has many potential marine applications, including to ship and submarine propellers, submarine control fins, and ship rudders.
Capillary Instability of a Liquid Film on a Wire
Authors: Teng H. , Kinoshita C. M. , Masutani S. M.
Published: February 2008

Capillary instability of a liquid film on a wire in another fluid (gas or liquid) is investigated. The liquid film is found to be unstable whenever the wavelength of the surface disturbance exceeds the circumference of the film surface—this criterion for instability of the liquid film is the same as that established by Rayleigh for a liquid thread. Analysis shows that instability of the liquid film is strongly influenced by film thickness and the properties of the film and the ambient fluid, and that thin, highly viscous liquid films are more stable than thick, low viscosity liquid films.
Review—Advances in Three-Dimensional Turbulent Boundary Layers With Emphasis on the Wall-Layer Regions
Authors: Johnston J. P. , Flack K. A.
Published: February 2008

Current information concerning three-dimensional turbulent boundary layers is discussed. Several topics are presented including (i) a detailed description of eleven experiments published since 1990. In nine cases cross flows are controlled by pressure gradients imposed from the freestream, but in two cases the cross flows are wall-shear-driven. The other topics include (ii) an examination of state of the art in measurement techniques; (iii) a look at some issues and ideas in turbulence modeling; and (iv) an introduction to new work on the visualization and description of quasicoherent structures (high/low-speed streaks and turbulent vortices) in three-dimensional turbulent boundary layers.
Perspective: The Experimentalist and the Problem of Turbulence in the Age of Supercomputers
Authors: Gharib Morteza
Published: February 2008

Due to the rising capabilities of computational fluid mechanics (CFD), the role of the experimentalist in solving the problem of turbulence has come under serious question. However, after much initial excitement by the prospect of CFD, the basic understanding of non-linear fluid phenomena such as turbulence still remains a grand challenge and will remain so into the unforeseeable future. It appears that in order to accelerate the development of a comprehensive and practical understanding and modeling of turbulence, it is required that a constructive synergism between experiments and simulations be created. Moreover, the digital revolution has helped experimental fluid mechanics to acquire new capabilities in the whole-field flow mapping technique which enables it to efficiently interface with CFD. This new horizon is promising in its capabilities to guide, validate and actively interact in conducting reliable simulations of turbulent flows.
A New Low-Reynolds-Number k-ε Model for Turbulent Flow Over Smooth and Rough Surfaces
Authors: Zhang Hanzhong , Faghri Mohammad , White Frank M.
Published: February 2008

A new low-Reynolds-number k-ε model is proposed to simulate turbulent flow over smooth and rough surfaces by including the equivalent sand-grain roughness height into the model functions. The simulation of various flow experiments shows that the model can predict the log-law velocity profile and other properties such as friction factors, turbulent kinetic energy and dissipation rate for both smooth and rough surfaces.
Measurements of the Turbulence Structure in the Vicinity of a 3-D Separation
Authors: Chesnakas C. J. , Simpson R. L.
Published: February 2008

The flow in the cross-flow separation region of a 6:1 prolate spheroid at 10 deg angle of attack, ReL = 4.20 × 106, was investigated using a novel, miniature, 3-D, fiber-optic Laser Doppler Velocimeter (LDV). The probe was used to measure three simultaneous, orthogonal velocity components from within the model, from approximately y+ = 7 out to the boundary layer edge. Velocity, Reynolds stress, and velocity triple product measurements are presented. These measurements are used to calculate the skin friction and to examine the convection, production, and diffusion of turbulent kinetic energy (TKE) about the three-dimensional separation. Comparisons of the measured production and diffusion of TKE in the cross-flow separation region—as well as in nonseparated regions of the flow—to the production and diffusion predicted by several models for these terms are shown.
Turbulent Flow Simulation of a Runner for Francis Hydraulic Turbines Using Pseudo-Compressibility
Authors: Arakawa Chuichi , Qian Yi , Kubota Takashi
Published: February 2008

A three-dimensional Navier-Stokes code with pseudo-compressibility, an implicit formulation of finite difference, and a k – ε two-equation turbulence model has been developed for the Francis hydraulic runner. The viscous flow in the rotating field can be simulated well in the design flow operating condition as well as in the off-design conditions in which a strong vortex occurs due to the separation near the leading edge. Because the code employs an implicit algorithm and a wall function near the wall, it does not require a large CPU time. It can therefore be used on a small computer such as the desk-top workstation, and is available for use as a design tool. The same kind of algorithm that is used for compressible flows has been found to be appropriate for the simulation of complex incompressible flows in the field of turbomachinery.
Jet Interaction in Liquid-Liquid Coaxial Injectors
Authors: Sivakumar D. , Raghunandan B. N.
Published: February 2008

Interaction between two conical sheets of liquid formed by a coaxial swirl injector has been studied using water in the annular orifice and potassium permanganate solution in the inner orifice. Experiments using photographic techniques have been conducted to study the influence of the inner jet on outer conical sheet spray characteristics such as spray cone angle and break-up length. The core spray has a strong influence on the outer sheet when the pressure drop in the latter is low. This is attributed to the pressure variations caused by ejector effects. This paper also discusses the merging and separation behavior of the liquid sheets which exhibits hysteresis effect while injector pressure drop is varied.
Numerical Study of Viscous Flows Inside Partially Filled Spinning and Coning Cylinders
Authors: Selmi Mohamed
Published: February 2008

This paper is concerned with the solution of the 3-D-Navier-Stokes equations describing the steady motion of a viscous fluid inside a partially filled spinning and coning cylinder. The cylinder contains either a single fluid of volume less than that of the cylinder or a central rod and a single fluid of combined volume (volume of the rod plus volume of the fluid) equal to that of the cylinder. The cylinder rotates about its axis at the spin rate ω and rotates about an axis that passes through its center of mass at the coning rate Ω. In practical applications, as in the analysis and design of liquid-filled projectiles, the parameter ε = τ sin θ, where τ = Ω/ω and θ is the angle between spin axis and coning axis, is small. As a result, linearization of the Navier-Stokes equations with this parameter is possible. Here, the full and linearized Navier-Stokes equations are solved by a spectral collocation method to investigate the nonlinear effects on the moments caused by the motion of the fluid inside the cylinder. In this regard, it has been found that nonlinear effects are negligible for τ ≈ 0.1, which is of practical interest to the design of liquid-filled projectiles, and the solution of the linearized Navier-Stokes equations is adequate for such a case. However, as τ increases, nonlinear effects increase, and become significant as ε surpasses about 0.1. In such a case, the nonlinear problem must be solved. Complete details on how to solve such a problem is presented.
Lift and Drag Distributions of Yacht Sails Using Wake Surveys
Authors: Locke N. J. , Jackson P. S. , Flay R. G. J.
Published: February 2008

The theory and numerical methods used to determine the lift and drag of a yacht model from wind-tunnel wake measurements are discussed. The results from wake surveys taken downstream of a l/40th scale IACC model yacht are presented. The lift and drag distributions are given as well as plots of the total pressure loss, vorticity, and stream-function, which are helpful in understanding the flow structure.
Three-Dimensional Particle Tracking Velocimetry With Laser-Light Sheet Scannings
Authors: Ushijima Satoru , Tanaka Nobukazu
Published: February 2008

This paper describes three-dimensional particle tracking velocimetry (3D PTV), which enables us to obtain remarkably larger number of velocity vectors than previous techniques. Instead of the usual stereoscopic image recordings, the present 3D PTV visualizes an entire three-dimensional flow with the scanning laser-light sheets generated from a pair of optical scanners and the images are taken by a high-speed video system synchronized with the scannings. The digital image analyses to derive velocity components are based on the numerical procedure (Ushijima and Tanaka, 1994), in which several improvements have been made on the extraction of particle images, the determination of their positions, the derivation of velocity components and others. The present 3D PTV was applied to the rotating fluids, accompanied by Ekman boundary layers, and their complicated secondary flow patterns, as well as the primary circulations, are quantitatively captured.
The Simulation of Mixing Layers Driven by Compound Buoyancy and Shear
Authors: Snider D. M. , Andrews M. J.
Published: February 2008

Fully developed compound shear and buoyancy driven mixing layers are predicted using a k-ε turbulence model. Such mixing layers present an exchange of equilibrium in mixing flows. The k-ε buoyancy constant Cε3 = 0.91, defined in this study for buoyancy unstable mixing layers, is based on an approximate self-similar analysis and an accurate numerical solution. One-dimensional transient and two-dimensional steady calculations are presented for buoyancy driven mixing in a uniform flow field. Two-dimensional steady calculations are presented for compound shear and buoyancy driven mixing. The computed results for buoyancy alone and compound shear and buoyancy mixing compare well with measured data. Adding shear to an unstable buoyancy mixing layer does not increase the mixing growth rate compared with that from buoyancy alone. The nonmechanistic k-ε model which balances energy generation and dissipation using constants from canonical shear and buoyancy studies predicts the suppression of the compound mixing width. Experimental observations suggest that a reduction in growth rate results from unequal stream velocities that skew and stretch the normally vertical buoyancy plumes producing a reduced mixing envelope width.
Application of Chaos Theory in Identification of Two-Phase Flow Patterns and Transitions in a Small, Horizontal, Rectangular Channel
Authors: Cai Y. , Wambsganss M. W. , Jendrzejczyk J. A.
Published: February 2008

Various measurement tools that are used in chaos theory were applied to analyze two-phase pressure signals with the objective of identifying and interpreting flow pattern transitions for two-phase flows in a small, horizontal rectangular channel. These measurement tools included power spectral density function, autocorrelation junction, pseudo-phase-plane trajectory, Lyapunov exponents, and fractal dimensions. It was demonstrated that the randomlike pressure fluctuations characteristic of two-phase flow in small rectangular channels are chaotic, and governed by a high-order deterministic system. The correlation dimension is potentially a new approach for identifying certain two-phase flow patterns and transitions.
A Review of Electrical Impedance Techniques for the Measurement of Multiphase Flows
Authors: Ceccio S. L. , George D. L.
Published: February 2008

Various developments in the use of electrical impedance methods in multiphase flow are reviewed. Because the components of a multiphase flow often exhibit different electrical properties, a variety of probes have been developed to study such flows by measuring impedance in the region of interest. Nonintrusive devices are used to measure spatially averaged flow properties, such as void fraction. Local probes have been developed to measure a variety of pointwise flow quantities, including film thickness in annular flow, local void fraction in dispersed flows, bubble and particle sizes, and flow velocities. Such impedance probes are usually easy to build and use, and can have a high frequency response. However, the spatial resolution of the probes may be limited, calibration may be difficult, and the accuracy of some probes may be limited to specific phase distributions. Researchers are now using electric fields to reconstruct the impedance distribution within a measurement volume via Electrical Impedance Tomography (EIT). EIT systems employ voltage and current measurements on the boundary of a domain to create a representation of the impedance distribution within the domain. EIT inversion algorithms are discussed, and the application of EIT to multiphase flows is reviewed. The benefits and limitations of EIT systems are also discussed.
Frequency Dependence of Mass Flow Gain Factor and Cavitation Compliance of Cavitating Inducers
Authors: Otsuka S. , Tsujimoto Y. , Kamijo K. , Furuya O.
Published: February 2008

Unsteady cavitation characteristics are analyzed based on a closed cavity model in which the length of the cavity is allowed to oscillate. It is shown that the present model blends smoothly into quasi-steady calculations at the low frequency limit, unlike fixed cavity length models. Effects of incidence angle and cavitation number on cavitation compliance and mass flow gain factor are shown as functions of reduced frequency. The cavity volume is evaluated by three methods and the results are used to confirm the accuracy and adequacy of the numerical calculations. By comparison with experimental data on inducers, it is shown that the present model can simulate the characteristics of unsteady-cavitation qualitatively.
On the Virtual Nonexistence of Multiple Instability Regions for Some Heat-Exchanger Arrays in Crossflow
Authors: Pai¨doussis M. P. , Price S. J. , Mureithi N. W.
Published: February 2008

In fluidelastic analyses involving a time delay (or phase lag) between the motions of cylinders in an array and the resultant unsteady fluid forces on the cylinders, a succession of instability-stability regions is predicted theoretically at low values of the mass-damping parameter, mδ/ρD2, below the “ultimate” fluidelastic instability, beyond which the system is not restabilized. However, as experimenters have had difficulty in verifying the existence of these regions of instability, it is legitimate to ask (i) do these regions really exist, and (ii) why are they so rarely observed? In this paper, with the aid of the quasi-steady model of Price and Pai¨doussis and with expanded measurements of lift and drag coefficients for a parallel triangular array with P/D = 1.375, it is shown that (a) the stability of the array strongly depends on geometric asymmetries; (b) whereas for a perfectly symmetric geometry the system may have several sub-ultimate instability regions, an asymmetry of as little as 0.02D may quench them and leave only the ultimate instability region intact. This suggests a possible explanation as to why the instability regions in question are so difficult to “find” experimentally. It also suggests that they may be of rather less practical importance for operating engineering systems than had heretofore been assumed, at least for some array geometries.
Study of Vaneless Diffuser Rotating Stall Based on Two-Dimensional Inviscid Flow Analysis
Authors: Tsujimoto Yoshinobu , Yoshida Yoshiki , Mori Yasumasa
Published: February 2008

Rotating stalls in vaneless diffusers are studied from the viewpoint that they are basically two-dimensional inviscid flow instability under the boundary conditions of vanishing velocity disturbance at the diffuser inlet and of vanishing pressure disturbance at the diffuser outlet. The linear analysis in the present report shows that the critical flow angle and the propagation velocity are functions of only the diffuser radius ratio. It is shown that the present analysis can reproduce most of the general characteristics observed in experiments: critical flow angle, propagation velocity, velocity, and pressure disturbance fields. It is shown that the vanishing velocity disturbance at the diffuser inlet is caused by the nature of impellers as a “resistance” and an “inertial resistance,” which is generally strong enough to suppress the velocity disturbance at the diffuser inlet. This explains the general experimental observations that vaneless diffuser rotating stalls are not largely affected by the impeller.
Numerical and Experimental Study of the Propulsive Speed of the Three Joint Bending Propulsion Mechanism
Authors: Nakashima Motomu , Ono Kyosuke
Published: February 2008

The three joint bending propulsion mechanism as a simplified model offish propulsion is investigated. A numerical method for a coupled system of the bending propulsion mechanism and the fluid is developed, based on the two-dimensional discrete vortex method. The characteristics of the propulsive speed of the three joint bending propulsion mechanism are numerically and experimentally investigated. It is found that the propulsive speed has a maximum numerical and experimental value when all the phase differences of the joints are about zero for four amplitude patterns. The experimental propulsive speeds are found to be 65 ∼ 80 percent of the numerical ones.
General Equation for the Design of Capillary Tubes
Authors: Yilmaz Tuncay , U¨nal Saban
Published: February 2008

Capillary tubes are used widely in small refrigeration systems. It is necessary to design the capillary tube, but there does not exist any analytical equation which allows the determination of capillary tube length or mass flow rate for all refrigerants. In this work, an analytical equation is derived which allows to design the capillary tubes. The comparison with existing methods and experimentally obtained values using the refrigerants R12, R22, R113, R114, R134a, and R600a has turned out to be satisfactory.
Compressible Dusty-Gas Boundary-Layer Flow Over a Flat Surface
Authors: Chamkha Ali J.
Published: February 2008

Equations governing compressible boundary-layer laminar flow of a two-phase particulate suspension are developed based on a continuum representation of both phases. These equations include such effects as particle-phase viscous stresses, variable position-dependent particle slip coefficient, and general power-law viscosity-temperature and thermal conductivity-temperature relations. The dimensionless form of the equations are applied to the problem of flow over a semi-infinite flat surface. An appropriate transformation is employed to allow proper comparison with previously published results for special cases of this problem. The full coupled system of equations is solved numerically via an implicit finite-difference method. Graphical results for the density, and temperature profiles as well as the displacement thicknesses, skin-friction coefficients, and the wall heat transfer coefficient for both the fluid and particle phases are presented and discussed in detail. In addition, a parametric study is performed to illustrate the influence of the particle to fluid viscosity ratio and the viscosity-temperature power exponent on the flow properties.
On the Application of the Method of Landau and Lifshitz to Sonic Velocities in Homogeneous Two-Phase Mixtures
Authors: Leung Joseph C.
Published: February 2008

A theoretical sonic velocity correlation for homogeneous two-phase mixtures under thermal equilibrium is proposed. By application of the method of Landau and Lifshitz, a dimensionless correlating parameter ω is found whereby different fluid properties and vapor mass fraction (quality) can be adequately accounted for by the simple correlation.
The Effects of Turbulence and Unsteadiness on Vortex Shedding From Sharp-Edged Bluff Bodies
Authors: Wolochuk M. C. , Plesniak M. W. , Braun J. E.
Published: February 2008

Motivated by a desire to evaluate vortex shedding flow meters for measuring velocity in unsteady turbulent flow applications, the objective of our work was to study the effects of flow disturbances on vortex shedding from sharp-edged bluff bodies. In particular, the combined effects of turbulence and unsteadiness were examined, as well as their separate effects using controlled wind tunnel tests. After causing an initial and sudden decrease in the Strouhal number, increasing turbulence intensity from 2.5 to 10 percent resulted in only a 2.4 percent increase in the Strouhal number, for turbulence with a length scale of 0.5 bluff body diameters. Turbulence integral length scale had a significant influence on the Strouhal number, with the greatest effect exhibited for length scales near 3 bluff body diameters. Turbulence of this length scale caused a 26 percent decrease in the Strouhal number, as compared to a low-turbulence base case. Fluctuating pressure amplitude and signal-to-noise ratio were also affected by turbulence, and decreased significantly when the integral length Kali was increased from 0.25 to 0.75 bluff body diameters for a turbulence intensity of 10 percent. Unsteadiness caused lock-on for forcing at the Strouhal frequency, twice and four times the Strouhal frequency, while no lock-on was observed for forcing at half the Strouhal frequency. The range of lock-on increased with increasing perturbation amplitude and was asymmetric about the resonant frequency. For the cases investigated, the effects of combined turbulence and unsteadiness were additive, with the turbulence shifting the Strouhal frequency, and the unsteadiness causing lock-on about the shifted Strouhal frequency. The results of this study suggest that vortex shedding flow meters should be calibrated in turbulent flows and turbulence length scale must be controlled at the bluff body. Lock-on can be avoided by sizing the bluff body so that the shedding frequency is always much greater than any disturbance frequency in the flow.
An Experimental Investigation of Buoyant Transient Cavity Collapse Near Rigid Cylindrical Boundaries
Authors: Best J. P. , Soh W. K. , Yu C. F.
Published: February 2008

An experimental investigation was carried out upon spark discharge generated transient cavities undergoing motion in the neighborhood of a rigid cylinder, in order to investigate the interaction between the Bjerknes attraction of the rigid cylinder and the buoyancy force. It was found that upon collapse, the direction of migration and liquid jet formation occurs at a characteristic angle. The experimental observations support the postulate that the direction of the Kelvin impulse at collapse bears close correlation with the direction of migration and jet formation.
Effects of Flow Pulsation on a Single-Rotor Turbine Meter
Authors: Jungowski Wiktor M. , Weiss Marvin H.
Published: February 2008

The effects of pulsating flow on a single-rotor turbine meter with a rotor response parameter greater than unity were investigated experimentally. Oscillating velocities and pressures at various pipe cross-sections were measured which enabled characterization of flow properties at the turbine meter and their effect on meter error. Measurements confirmed that meter error depends primarily on the velocity amplitude of the flow pulsation at the rotor. Moreover, it was shown that the velocity amplitude at the rotor results from the system response, which is determined by the system geometry, pulsation frequency, flow velocity, and sound speed. Lastly, a technique was developed to determine the meter error from measurements of dynamic pressure in the pipe upstream or downstream of the turbine meter.
Experiments on Turbulence Management Using Screens and Honeycombs
Authors: Farell Cesar , Youssef Sadek
Published: February 2008

The results of some experiments on turbulence management using combinations of honeycombs of a different lengths and coarse and fine screens, carried out in highly nonuniform and turbulent flows generated in a 127-mm plexiglas pipe by an upstream fan, are presented. The performance of the devices as single manipulators and in combination is evaluated through hot-wire measurements of the mean and rms values of the longitudinal velocities over the pipe cross section. The results show that relatively short honeycombs, preceded by a coarse screen and followed by one or more fine screens, can be used for effective management of such flows.
Wall Suction Effects on the Structure of Fully Developed Turbulent Pipe Flow
Authors: Sofialidis D. , Prinos P.
Published: February 2008

The effects of wall suction on the structure of fully developed pipe flow are studied numerically by solving the Reynolds averaged Navier-Stokes equations. Linear and nonlinear k-ε or k-ω low-Re models of turbulence are used for “closing” the system of the governing equations. Computed results are compared satisfactorily against experimental measurements. Analytical results, based on boundary layer assumptions and the mixing length concept, provide a law of the wall for pipe flow under the influence of low suction rates. The analytical solution is found in satisfactory agreement with computed and experimental data for a suction rate of A = 0.46 percent. For the much higher rate of A = 2.53 percent the above assumptions are not valid and analytical velocities do not follow the computed and experimental profiles, especially in the near-wall region. Near-wall velocities, as well as the boundary shear stress, are increased with increasing suction rates. The excess wall shear stress, resulting from suction, is found to be 1.5 to 5.5 times the respective one with no suction. The turbulence levels are reduced with the presence of the wall suction. Computed results of the turbulent shear stress uv are in close agreement with experimental measurements. The distribution of the turbulent kinetic energy k is predicted better by the k-ω model of Wilcox (1993). Nonlinear models of the k-ε and k-ω type predict the reduction of the turbulence intensities u’, v’, w’, and the correct levels of v’ and w’ but they underpredict the level of u’.
Evaluation of Algebraic Stress Modeling in Free-Surface Jet Flows
Authors: Walker David T. , Chen Chao-Yi
Published: February 2008

This study evaluated three algebraic stress models for predicting turbulent stresses near the free surface in a free-surface jet at nonzero Froude number, by comparing to experiments. The models examined included one with no explicit near-surface modeling, one which specified model coefficients in terms of invariants of the anisotropy tensor, and a third model which employed a surface correction with an ad-hoc damping function. Experiment showed that at low Froude number, the anisotropy near the free surface did not attain the limiting behavior characteristic of two-dimensional turbulence and the anisotropy increased with streamwise distance. At high Froude number the surface can have little effect on the anisotropy. Far from the free surface, all the models performed well. For the model with no explicit free-surface modeling, the turbulence near the free surface was predicted to be isotropic. For the “anisotropy-invariant” model, the predicted anisotropy was too small and confined to locations too near the free surface. The model with the ad-hoc damping function captured the anisotropy near the free surface best, but specification of the decay constant in the damping function is an open question.
The Structure of Shear Driven Mixing With an Unstable Thermal Stratification
Authors: Snider D. M. , Andrews M. J.
Published: February 2008

A water channel experiment was used to study the turbulent mixing structure of shear and buoyancy. Density differences between fluid layers were produced by thermal stratification. The experiment was statistically steady and provided a long time period to study the mixing behavior and record data. The mixing process was studied using dye and digitized photographs. Individual images gave a two-dimensional view of the mixing structure. An ensemble average of images gave averaged mixing layer growth rate and the distribution of light and heavy fluid in the mixing layer. The experiment gave a space-time transformation in the streamwise direction which permitted a continuous study of the mixing evolution. The structure of the early growth of a compound shear and buoyancy mixing layer is presented. For the range of flows (δU; = 0 to 1 cm/s) and density differences (0.38 to 2.4 kg/m3), buoyancy was the dominant mixing mechanism.
Flows Generated by the Impingement of a Jet on a Rotating Surface: Part II—Detailed Flow Structure and Analysis
Authors: Brodersen S. , Metzger D. E. , Fernando H. J. S.
Published: February 2008

In Part I of this investigation, the basic flow patterns resulting from the interaction of a turbulent jet and a rotating disk flow were presented paying particular attention to mean and turbulent velocity fields around the jet impingement point on the disk and the flow patterns above the disk in the absence of the jet. The latter was used as a base flow pattern, on which the influence of the jet impingement could be assessed. In Part II, further details of the jet/disk flow interaction problem are discussed in the context of potential turbomachinery cooling applications and previous experimental results.