bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct Measurements of Turbulent Boundary Layer Wall Pressure Wavenumber-Frequency Spectra
Authors: Abraham B. M. , Keith W. L.
Published: February 2008

Direct measurements of streamwise wavenumber-frequency spectra of turbulent wall pressure fluctuations were made in an acoustically quiet water tunnel. A linear array of evenly spaced flush mounted pressure sensors was used to measure the wall pressure field at 48 streamwise locations. This array provided over 24 dB of resolution (sidelobe rejection) in the wavenumber domain, leading to an accurate estimate of the “convective ridge” and part of the subconvective and low wavenumber portions of the spectrum at discrete frequencies. Boundary layer parameters, including the mean wall shear stress, boundary layer thickness, displacement thickness, and momentum thickness, were derived from mean streamwise velocity measurements for 8100 < Rθ < 16,700. Time and length scales derived from these parameters were used to nondimensionalize the measured spectra. The effectiveness of different scalings for nondimensionalizing the low and convective wavenumber regions at discrete frequencies was evaluated.
Turbulence in Compressible Mixing Layers
Authors: Ladeinde Foluso , Liu Wei , O’Brien Edward E.
Published: February 2008

The direct numerical simulation (DNS) of two-dimensional compressible turbulent mixing layers is reported in this paper for convective Mach numbers Mc = 0.5, 0.8 and 1.0. All scales of flow are resolved with a 2562 grid, although results are also obtained for 642, 962 and 1282 grids for the purpose of determining the effective accuracy and grid-independence of our calculations. The effect of Mach number is also reported for all the Reynolds stress tensor components and for the “shear” components of the anisotropy tensor, the dissipation tensor, pressure-strain, and the triple correlation tensor. The short-time behaviors of some of these quantities are similar to those reported by Sarkar (1995) for homogeneous shear flow, in spite of the differences in the problem type and initial and boundary conditions. The relative magnitudes and signs of the unclosed terms in the Reynolds stress equations provide information on those that have to be retained for turbulence modeling as well as the sense of their contribution.
Analytical Model of a Side-Heated Free Convection Loop Placed in a Transverse Magnetic Field
Authors: Ghaddar Nesreen
Published: February 2008

The hydrodynamic characteristics of a buoyancy-driven convection loop containing an electrically-conducting fluid in a transverse magnetic field are investigated analytically using a one-dimensional model. One side of the loop is isothermally heated and the other side isothermally cooled, and the upper and lower sections are insulated. The model which is based on the use of the Hartmann Plane-Poiseuille flow solution for estimating loop shear stress, predicts the flow velocity and the induced current of the magnetohydrodynamic generator in terms of the flow and geometric parameters. The study covers ranges of Grashof number, Gr, from 102 to 106, the Hartmann number, Ha, from 0 to 20, the Prandtl number, Pr, from .003 to 7, and loop height to thickness ratio, L/d, from 10 to 50. It is shown that at low Prandtl numbers, Pr ≪ 1, there exists an optimal Hartmann number, Haopt, that maximizes the induced electric current. This Haopt depends weakly on the Grashof number. The side-heated loop performance is also compared with the bottom heated loop model of Ghaddar, (1997a). It is found that at a low Prandtl number, side heated loop induces the higher velocity whereas at high Prandtl numbers the bottom heated loop induces higher velocity.
Application of Neural Networks and Fuzzy Logic to the Calibration of the Seven-Hole Probe
Authors: Rediniotis O. K. , Chrysanthakopoulos G.
Published: February 2008

The theory and techniques of Artificial Neural Networks (ANN) and Fuzzy Logic Systems (FLS) are applied toward the formulation of accurate and wide-range calibration methods for such flow-diagnostics instruments as multi-hole probes. Besides introducing new calibration techniques, part of the work’s objective is to: (a) apply fuzzy-logic methods to identify systems whose behavior is described in a “crisp” rather than a “linguistic” framework and (b) compare the two approaches, i.e., neural network versus fuzzy logic approach, and their potential as universal approximators. For the ANN approach, several network configurations were tried. A Multi-Layer Perceptron with a 2-node input layer, a 4-node output layer and a 7-node hidden/middle layer, performed the best. For the FLS approach, a system with center average defuzzifier, product-inference rule, singleton fuzzifier, and Gaussian membership functions was employed. The Fuzzy Logic System seemed to outperform the Neural Network/Multi-Layer Perceptron.
A Large-Eddy Simulation of the Near Wake of a Circular Cylinder
Authors: Jordan S. A. , Ragab S. A.
Published: February 2008

The formation and the downstream transport of the Strouhal vortices in the near wake of a circular cylinder are investigated using the large-eddy simulation (LES) method. The governing equations are formulated in curvilinear coordinates to accommodate a nonorthogonal grid with formal development of a dynamic model to account for the subgrid turbulent scales. Results were produced with and without use of the model. The focus of the investigation is at a subcritical Reynolds number of 5600. Using the dynamic model, the LES results compared best to the published experimental data in terms of both the global and local wake characteristics such as the drag and base pressure coefficients, shedding and detection frequencies, peak vorticity, and the downstream mean velocity-defect and Reynolds stresses. The results further showed streamwise filaments that connect subsequent Strouhal vortices. Qualitatively, the time-averaged Reynolds stresses of the formation region revealed similar symmetric characteristics over the range 525 ≤ Re ≤ 140,000.
Enhanced Power Spectrum Estimates Using Kalman Reconstruction
Authors: Benedict Lance H. , Gould Richard D.
Published: February 2008

A number of power spectral density (PSD) estimators were assessed using real laser Doppler anemometer (LDA) data from grid generated turbulence. PSD estimates from the raw data via the slotting technique and direct transform method were compared to those estimated from sample and hold, linear, and Kalman reconstructed velocity time histories. Of the reconstruction schemes, only Kalman reconstruction was shown to significantly reduce the effects of noise on the measurements leading to an additional 2 decades in power of high frequency information.
An Implicit Multigrid Scheme for the Compressible Navier-Stokes Equations With Low-Reynolds-Number Turbulence Closure
Authors: Gerlinger Peter , Bru¨ggemann Dieter
Published: February 2008

A multigrid method for convergence acceleration is used for solving coupled fluid and turbulence transport equations. For turbulence closure a low-Reynolds-number q-ω turbulence model is employed, which requires very fine grids in the near wall regions. Due to the use of fine grids, convergence of most iterative solvers slows down, making the use of multigrid techniques especially attractive. However, special care has to be taken on the strong nonlinear turbulent source terms during restriction from fine to coarse grids. Due to the hyperbolic character of the governing equations in supersonic flows and the occurrence of shock waves, modifications to standard multigrid techniques are necessary. A simple and effective method is presented that enables the multigrid scheme to converge. A strong reduction in the required number of multigrid cycles and work units is achieved for different test cases, including a Mack 2 flow over a backward facing step.
Numerical Calculation of the Fully Developed Turbulent Flow in an Axially Rotating Pipe With a Second-Moment Closure
Authors: Rinck K.-J. , Beer H.
Published: February 2008

The effect of axial tube rotation on the fully developed pipe flow is analyzed by a low-Reynolds-number turbulence closure and compared with experimental results. A flow which is initially turbulent is stabilized by the rotation leading to a laminarized mean axial velocity distribution. The applied second-moment closure reveals an encouraging ability to capture this phenomenon as well as other features of the mentioned flow configuration. The “rapid” part of the pressure-strain correlation model is found to have a significant influence on the numerical results and seems to be the key for further improvements concerning highly swirling flows.
Numerical Simulation of Turbulent Jets With Rectangular Cross-Section
Authors: Wilson R. V. , Demuren A. O.
Published: February 2008

Three-dimensional turbulent jets with rectangular cross-section are simulated with a finite-difference numerical method. The full Navier-Stokes equations are solved at a low Reynolds number, whereas at a higher Reynolds number filtered forms of the equations are solved along with a sub-grid scale model to approximate effects of the unresolved scales. A 2-N storage, third-order Runge-Kutta scheme is used for temporal discretization and a fourth-order compact scheme is used for spatial discretization. Divergence-free velocity field is obtained by solving a Poisson equation for pressure with the same spatial discretization scheme for consistent accuracy. Computations are performed for different inlet conditions which represent different types of jet forcing within the shear layer. The phenomenon of axis-switching is observed in some cases. At low Reynolds numbers, it is based on self-induction of the vorticity field, whereas at higher Reynolds numbers, the turbulent structure becomes the dominant mechanism in natural jets. Budgets of the mean streamwise velocity show that convection is balanced by gradients of the Reynolds stresses and the pressure.
Study on the Deposition Profile Characteristics in the Micron-Scale Trench Using Direct Simulation Monte Carlo Method
Authors: Ikegawa Masato , Kobayashi Jun’ichi , Maruko Morihisa
Published: February 2008

As integrated circuits are advancing toward smaller device features, step-coverage in submicron trenches and holes in thin film deposition are becoming of concern. Deposition consists of gas flow in the vapor phase and film growth in the solid phase. A deposition profile simulator using the direct simulation Monte Carlo method has been developed to investigate deposition profile characteristics on small trenches which have nearly the same dimension as the mean free path of molecules. This simulator can be applied to several deposition processes such as sputter deposition, and atmospheric- or low-pressure chemical vapor deposition. In the case of low-pressure processes such as sputter deposition, upstream boundary conditions of the trenches can be calculated by means of rarefied gas flow analysis in the reactor. The effects of upstream boundary conditions, molecular collisions, sticking coefficients, and surface migration on deposition profiles in the trenches were clarified.
CFD Simulation and Experimental Validation of a New Closed Circuit Wind/Water Tunnel Design
Authors: Gordon Robert , Imbabi Mohammed S.
Published: February 2008

A new closed-circuit wind/water tunnel to support flow visualization research was designed and constructed at The University of Aberdeen. Review of existing closed-circuit tunnel designs revealed that they are large, expensive, difficult to set up and maintain, and typically employ a single working fluid. Key objectives of the work reported here were to reduce the overall cost and size of the tunnel, facilitate the use of alternative working fluids (air or water), and provide high quality flow within the test section. Conventional design methods were used initially, and computational fluid dynamics (CFD) was then employed to simulate the flow within critical sections of the tunnel. The results from CFD played a decisive role in identifying the modifications needed to achieve the compact, cost-effective tunnel design eventually built and tested. Flow quality within the test section was established using measured velocity profiles, and these are also presented.
Prediction of Solid/Free-Surface Juncture Boundary Layer and Wake of a Surface-Piercing Flat Plate at Low Froude Number
Authors: Sreedhar Madhu , Stern Fred
Published: February 2008

Results are reported of a RANS simulation investigation on the prediction of turbulence-driven secondary flows at the free-surface juncture of a surface-piercing flat plate at low Froude numbers. The turbulence model combines a nonlinear eddy viscosity model and a modified version of a free-surface correction formula. The different elements of the model are combined and the model constants calibrated based on the premises that the anisotropy of the normal stresses is mainly responsible for the dynamics of the flow in the juncture region, and an accurate modeling of the normal-stress anisotropy as obtained from the data is a primary requirement for the successful prediction of the overall flow field. The predicted mean velocity, streamwise vorticity, turbulent kinetic energy, and other quantities at the juncture are then compared with data and analyzed with regard to findings of related studies. In agreement with the experimental observations, the simulated flow at large depths was essentially two-dimensional and displayed all the major features of zero pressure gradient boundary layer and wake, including the anisotropy of normal stresses in the near-wall region. In the boundary-layer free-surface juncture region, the major features of interest that were predicted include the generation of secondary flows and the thickening of the boundary layer near the free surface. In the wake free-surface juncture region, even though secondary flows and a thickening of the wake width near the free surface were predicted in accordance with the experimental observations, the overall comparison with the experiment was not as satisfactory as the boundary-layer juncture. This is partly due to the lack of a strong coherent flow structure in the wake juncture and the presence of possible wave effects in the wake in the experiments. An examination of the terms in the Reynolds-averaged streamwise vorticity equation reconfirmed the importance of the anisotropy of the normal Reynolds stresses in the production of streamwise vorticity. The free-surface wave elevations were negligible for the present model problem for the nonzero Froude number studied. Finally, concluding remarks are presented with regards to extensions for practical geometries such as surface ship flows.
First and Second-Order Accurate Schemes for Two-Fluid Models
Authors: Tiselj Iztok , Petelin Stojan
Published: February 2008

The six-equation two-fluid model of two-phase flow taken from the RELAP5/MOD3 computer code has been used to simulate three simple transients: a two-phase shock tube problem, the Edwards Pipe experiment, and water hammer due to rapid valve closure. These transients can be characterized as fast transients, since their characteristic time-scales are determined by the sonic velocity. First and second-order accurate numerical methods have been applied both based on the well-known, Godunov-type numerical schemes. Regarding the uncertainty of the two-fluid models in today’s large computer codes for the nuclear thermal-hydraulics, use of second-order schemes is not always justified. While this paper shows the obvious advantage of second-order schemes in the area of fast transients, first-order accurate schemes may still be sufficient for a wide range of two-phase flow transients where the convection terms play a minor role compared to the source terms.
A Methodology for Determining Experimental Uncertainties in Regressions
Authors: Brown K. K. , Coleman H. W. , Steele W. Glenn
Published: February 2008

A methodology to determine the experimental uncertainties associated with regressions is presented. When a regression model is used to represent experimental information, the uncertainty associated with the model is affected by random, systematic, and correlated systematic uncertainties associated with the experimental data. The key to the proper estimation of the uncertainty associated with a regression is a careful, comprehensive accounting of systematic and correlated systematic uncertainties. The methodology presented in this article is developed by applying uncertainty propagation techniques to the linear regression analysis equations. The effectiveness of this approach was investigated and proven using Monte Carlo simulations. The application of that methodology to the calibration of a venturi flowmeter and its subsequent use to determine flowrate in a test is demonstrated. It is shown that the previously accepted way of accounting for the contribution of discharge coefficient uncertainty to the overall flowrate uncertainty does not correctly account for all uncertainty sources, and the appropriate approach is developed, discussed, and demonstrated.
A Wall-Distance-Free k-ε Model With Enhanced Near-Wall Treatment
Authors: Goldberg U. , Peroomian O. , Chakravarthy S.
Published: February 2008

This paper evaluates a wall-distance-free low-Re k-ε turbulence closure model which incorporates an extra source term in the ε transport equation designed to increase the level of ε in nonequilibrium flow regions, thus reducing the kinetic energy and length scale magnitudes to improve prediction of adverse pressure gradient flows, including those involving separated flows regions. Two such cases are used here to test the model: one in the low speed flow regime, the other a supersonic one. Comparisons with experimental data and with an earlier version of the k-ε model, as well as with a variant of the k-ω model (both also wall-distance-free) reveal that the proposed model enables improved prediction of adverse pressure gradient flows relative to more traditional k-ε models.
Supersonic Jet Noise Reductions Predicted With Increased Jet Spreading Rate
Authors: Dahl Milo D. , Morris Philip J.
Published: February 2008

In this paper, predictions are made of noise radiation from single, supersonic, axisymmetric jets. We examine the effects of changes in operating conditions and the effects of simulated enhanced mixing that would increase the spreading rate of the jet shear layer on radiated noise levels. The radiated noise in the downstream direction is dominated by mixing noise and, at higher speeds, it is well described by the instability wave noise radiation model. Further analysis with the model shows a relationship between changes in spreading rate due to enhanced mixing and changes in the far field radiated peak noise levels. The calculations predict that enhanced jet spreading results in a reduction of the radiated peak noise level.
Three-Dimensional Laminar Flow in a Rotating Multiple-Pass Square Channel With Sharp 180-Deg Turns
Authors: Hwang Jenn-Jiang , Lai Dong-Yuo
Published: February 2008

This paper presents a study of three-dimensional laminar flow in a rotating multiplepass channel connected with 180-deg sharp bends. Fluid-flow fields are calculated for the entire domain via the Navier-Stokes equations through a finite-difference scheme. For closure of this elliptic-type problem, periodical fully developed conditions are employed between the entrance and exit of the two-pass module. Experiments for the stationary two-pass channel are conducted to validate the numerical procedure and data. The emphasis of the present prediction is on the rotating and through-flow rate effects on the fluid-flow and friction characteristics in the straight channel as well as in the turn region. It is found that the rotation-induced Coriolis force significantly raises the wall-friction losses in the straight channel. However, the head loss of the sharp turn is decreased with increasing rotation speed, because the flow discrepancy between the inlet and outlet of the sharp turn is less significant for the higher rotation speed. Moreover, overall pressure-drop penalty across the two-pass channel is found to be enhanced by the rotation speed as well as the duct through-flow rate.
Methods to Set Up and Investigate Low Reynolds Number, Fully Developed Turbulent Plane Channel Flows
Authors: Durst F. , Fischer M. , Jovanovic´ J. , Kikura H.
Published: February 2008

The tripping of fully developed turbulent plane channel flow was studied at low Reynolds number, yielding unique flow properties independent of the initial conditions. The LDA measuring technique was used to obtain reliable mean velocities, rms values of turbulent velocity fluctuations and skewness and flatness factors over the entire cross-section with emphasis on the near-wall region. The experimental results were compared with the data obtained from direct numerical simulations available in the literature. The analysis of the data indicates the important role of the upstream conditions on the flow development. It is shown that the fully developed turbulent state at low Reynolds number can be reached only by significant tripping of the flow at the inlet of the channel. Effects related to the finite size of the LDA measuring control volume and an inaccuracy in the estimation of the wall shear stress from near-wall velocity measurements are discussed in detail since these can yield systematic discrepancies between the measured and simulated results.
Turbulent Flow Past an Array of Bluff Bodies Aligned Along the Channel Axis
Authors: Liou Tong-Miin , Chen Shih-Hui
Published: February 2008

Computations and measurements of time mean velocities, total fluctuation intensities, and Reynolds stresses are presented for spatially periodic flows past an array of bluff bodies aligned along the channel axis. The Reynolds number based on the channel hydraulic diameter and cross-sectional bulk mean velocity, the pitch to rib-height ratio, and the rib-height to channel-height ratio were 2 × 104, 10, and 0.13, respectively. The unsteady phase-averaged Navier-Stokes equations were solved using a Reynolds stress model with wall function and wall-related pressure strain treatment to reveal the feature of examined unsteady vortex shedding flow. Laser Doppler velocimetry measurements were performed to measure the velocity field. Code verifications were performed through comparisons with others’ measured developing single-rib flow and our measured fully developed rib-array flow. The possible causes for the differences between the experiments and computations are discussed. The calculated phase-averaged flow field clearly displays the vortex shedding behind the rib and is characterized in terms of shedding Strouhal number, vortex trajectory, vortex celerity, and vortex travelling distance in a phase cycle. Furthermore, the difference between the computed developing single-rib flow and fully developed rib-array flow is addressed.
Three-Dimensional Blade Boundary Layer and Endwall Flow Development in the Nozzle Passage of a Single Stage Turbine
Authors: Ristic D. , Lakshminarayana B.
Published: February 2008

The three-dimensional viscous flow field development in the nozzle passage of an axial flow turbine stage was measured using a “x” hot-wire probe. The measurements were carried out at two axial stations on the endwall and vane surfaces and at several spanwise and pitchwise locations. Static pressure measurements and flow visualization, using a fluorescent oil technique, were also performed to obtain the location of transition and the endwall limiting streamlines. The boundary layers on the vane surface were found to be very thin and mostly laminar, except on the suction surface downstream of 70 percent axial chord. Strong radial pressure gradient, especially close to the suction surface, induces strong radial flow velocities in the trailing edge regions of the blade. On the endwalls, the boundary layers were much thicker, especially near the suction corner of the casing surface, caused by the secondary flow. The secondary flow region near the suction surface-casing corner indicated the presence of the passage vortex detached from the vane surface. The boundary layer code accurately predicts the three-dimensional boundary layers on both vane surfaces and endwall in the regions where the influence of the secondary flow is small.
Drop Size and Velocity Measurements in an Electrostatically Produced Hydrocarbon Spray
Authors: Shrimpton J. S. , Yule A. J.
Published: February 2008

Liquid hydrocarbons are difficult to atomize electrostatically at practical flow rates due to their high resistivities and low concentration of charge carriers. However special “charge injection” techniques have been used in this investigation to produce combustible sprays of oils. An experimental investigation of the drop size and velocity distributions within a charged kerosine spray is presented, using a PDA system and photographic methods. Bimodal size distributions are found with a central core of larger drops or ligament formations near the nozzle surrounded by a sheath of smaller drops. Because of the bimodal character the concept of average diameter for the spray is difficult to apply so that there is little practical use defining a relationship between mean drop diameter and mean specific charge without a knowledge of a relationship between charge and size of individual drops. Examination of the velocity component distributions showed the processes which control the two-zone characteristics of the spray. The larger drops have a high inertia and were less deflected by the space charge force within the spray and it is argued that the larger drops possess a smaller specific charge compared with the smaller drops which reinforces the tendency for the large drops to remain along the spray centreline. For the smaller drops the converse is the case, to the extent that at low flow rates their trajectories have a negligible axial velocity component and recirculation toward the earthed injector body is observed.
Studies on the Behavior of Droplets and the Air Flow in a Hollow-Cone Spray
Authors: Okamoto Tatsuyuki , Takagi Toshimi , Kaji Toshikazu , Shimazaki Katsunori , Nakanishi Kenji
Published: February 2008

Experimental and numerical investigations are made on the behavior of droplets in a hollow-cone spray paying attention to the liquid sheet formed at the orifice of pressure-swirl atomizer. Simultaneous measurements of droplet sizes and velocities are made by phase-Doppler technique and numerical simulations are carried out based on the transient Eulerian equations for the gas and the Lagrangian equation for the droplets, taking account of the liquid sheet formed at the atomizer orifice. It is shown that the simulation gives good predictions by incorporating the existence of the liquid sheet. The predicted results indicate that the movement of the liquid sheet induces a strong air stream which acts as a strong side wind against the droplets immediately after breakup. This air stream selectively transports small droplets toward the central region and plays an essential role in the classification of droplets by size. Accordingly, the existence of the liquid sheet is significant for the characteristics of droplet dispersion and it should not be neglected in the prediction of hollow-cone spray flows. In addition, the shape of the liquid sheet is theoretically computed based on the simplified equations of motion. The comparison between the theoretical computation and the experimental result suggests that the surface tension of liquid is predominant in determining the shape of the liquid sheet.
On the Choking of the Flow of Piezoviscous Liquids
Authors: Bair Scott , Khonsari M. M.
Published: February 2008

Experimental evidence for choking of a piezoviscous fluid in a capillary tube is presented. It is shown that under a nearly isothermal condition, it is possible for the flow to experience choking whereby increasing the inlet pressure ceases to affect the flow rate. The occurrence of this phenomenon can be predicted by noting a singularity in the flowrate equation in a capillary tube under typical conditions encountered in injection molding (cf. Denn, 1981). Further examination of the general equations governing the shear flow of laminar, incompressible, linearly viscous (Newtonian) liquids under high pressures reveals a singularity in the solution for the pressure gradient. This apparently unreported phenomenon is shown to occur in liquids whose viscosity, μ, increases exponentially with pressure, p, as μ = μ0eαp, where α denotes the pressure-viscosity coefficient. For a two-dimensional configuration with the shearing action taking place in the x direction and with p = p(x, y), the singularity is shown to take place when: τyx=±(1/α)1+α2τxxτyy, at which case both δp/δx and δp/δy →∞. The occurrence of such a singularity in the pressure gradient may have an important physical implication in lubrication flows particularly in concentrated contacts such as elastohydrodynamic lubrication of rolling element bearings and gears.
Coupling Between a Viscoelastic Gas/Liquid Interface and a Swirling Vortex Flow
Authors: Lopez J. M. , Chen J.
Published: February 2008

While the structure and dynamics of boundary layers on rigid no-slip walls in rotation dominated enclosed flows are still an area of active research, the interactions between rotating or swirling flows with a free surface have received comparatively less attention. For the most part, investigations in this area have been focused on clean free surfaces, which may be treated as stress-free. However, in most practical situations the surface is rarely clean, and even under laboratory conditions, it is quite difficult to achieve a clean free surface. Most impurities in liquids are surface active, and hence the name surface active agent or surfactant. These surfactants tend to establish an equilibrium surface concentration which alters the interfacial tension and interfacial viscoelastic properties of the gas/liquid interface. The coupling between the bulk swirling flow and the interface is provided via the tangential stress balances, and these stresses on the interface are dependent upon the surface concentration of surfactant, which in turn is altered by the interfacial flow. Forces acting on the interface include surface tension gradients (elastic) and the viscous resistance to shear and dilatation. These viscoelastic properties vary with the surfactant concentration on the surface. Here, we present numerical studies of flow in a cylinder driven by the constant rotation of the bottom endwall with the top free surface being contaminated by a Newtonian surfactant. Comparisons with a clean free surface and a no-slip stationary top endwall provide added insight into the altered dynamics that result from the presence of a small amount of surfactant.
Nonlinear Spin-Up of a Rotating Stratified Fluid: Experimental Method and Preliminary Results
Authors: Hewitt Richard E. , Davies Peter A. , Duck Peter W. , Foster Michael R. , Smith Fraser W.
Published: February 2008

We consider the nonlinear spin-up of a rotating stratified fluid in a conical container. An analysis of similarity-type solutions to the relevant boundary-layer problem (Duck et al, 1997) has revealed three types of behavior for this geometry. In general, the boundary-layer evolves to either a steady state, a growing boundary-layer, or a finite-time singularity depending on the initial to final rotation rate ratio, and a “modified Burger number.” We emphasize the experimental aspects of our continuing spin-up investigations and make some preliminary comparisons with the boundary-layer theory, showing good agreement. The experimental data presented is obtained through particle tracking velocimetry. We briefly discuss the qualitative features of the spin-down experiments which, in general, are dominated by nonaxisymmetric effects. The experiments are performed using a conical container filled with a linearly stratified fluid, the generation of which is nontrivial. We present a general method for creating a linear density profile in containers with sloping boundaries.
Flow Structure in a Rayleigh-Be´nard Cell Upon Impulsive Spin-Up
Authors: Vorobieff Peter , Ecke Robert E.
Published: February 2008

We investigate convection in a cylindrical Rayleigh-Be´nard cell with radius-to-height ratio Γ = 1/2. The cell is subjected to impulsive spin-up about its vertical axis. We use TLC (thermochromic liquid crystal) imaging for temperature field measurements and PIV (particle image velocimetry) for velocity reconstruction of the transition in the range of Rayleigh numbers R from 5 × 107 to 5 × 108 and dimensionless rotation rates Ω from 0 up to 8 × 104. The initial (at rest) and the final (in steady rotation) states of the system are those of turbulent convection. The most persistent transient feature is a well-defined ring pattern characterized by a decrease in temperature, axial velocity directed downward and high azimuthal shear. The latter leads to formation of an azimuthally regular structure of Kelvin-Helmholz vortices. During the final stage of the transition, this vortical structure loses azimuthal regularity and an irregular pattern of vortices characteristic of turbulent rotating convection forms.
Recirculation Zones in a Cylindrical Container
Authors: Jahnke Craig C. , Valentine Daniel T.
Published: February 2008

The flow field induced inside a cylindrical container by the rotation of the two end walls is described. It is shown that stagnation points leading to separation bubbles occur on the axis of rotation and/or the bottom end wall for certain ranges of the characteristic parameters; the Reynolds number, the aspect ratio of the container, and the ratio of the rotation rates of the end walls. Flow fields in a container of aspect ratio 2.0 are examined for Reynolds numbers from 100 to 3000 and ratios of the rotation rates of the top and bottom end walls from −0.10 to 1.0. For a range of ratios of the rotation rates of the top and bottom end walls and Reynolds numbers it is shown that ring vortices surrounding a columnar vortex core exist.
On the Creation of Stagnation Points in a Rotating Flow
Authors: Mullin T. , Tavener S. J. , Cliffe K. A.
Published: February 2008

We report the results of a numerical study of the creation of stagnation points in a rotating cylinder of fluid where both endwalls are rotated. Good agreement is found with previous results where the stagnation points are formed on the core of the primary columnar vortex. Novel phenomena have been uncovered at small aspect ratios where stagnation occurs off-axis directly and the secondary vortex which is created forms a toroid. The case is then considered of a small cylinder placed along the center of the flow and despite the qualitatively different boundary condition, the phenomena are found to be robust.
Centrifugal Pump Performance Drop Due to Leading Edge Cavitation: Numerical Predictions Compared With Model Tests
Authors: Hirschi R. , Dupont Ph. , Avellan F. , Favre J.-N. , Guelich J.-F. , Parkinson E.
Published: February 2008

The aim of this paper is to present the results obtained with a 3-D numerical method allowing the prediction of the cavitation behavior of a centrifugal pump and to compare this prediction to model tests. The influence of the diffuser on the pump performances, for a cavitating flow, is taken into account by performing coupled computations. The proposed method, which allows the performance drop prediction, consists of assuming the cavity interface as a free surface boundary of the computation domain and in computing the single phase flow. The unknown shape of the interface is determined using an iterative procedure matching the cavity surface to a constant pressure boundary (pv). The originality of the method is that the adaptation process is done apart from the flow calculation, allowing to use any available code.
Prediction of Cavitation Erosion: An Energy Approach
Authors: Pereira F. , Avellan F. , Dupont Ph.
Published: February 2008

The objective is to define a prediction and transposition model for cavitation erosion. Experiments were conducted to determine the energy spectrum associated with a leading edge cavitation. Two fundamental parameters have been measured on a symmetrical hydrofoil for a wide range of flow conditions: the volume of every transient vapor cavity and its respective rate of production. The generation process of transient vapor cavities is ruled by a Strouhal-like law related to the cavity size. The analysis of the vapor volume data demonstrated that vapor vortices can be assimilated to spherical cavities. Results are valid for both the steady and unsteady cavitation behaviors, this latter being peculiar besides due to the existence of distinct volumes produced at specific shedding rates. The fluid energy spectrum is formulated and related to the flow parameters. Comparison with the material deformation energy spectrum shows a remarkable proportionality relationship defined upon the collapse efficiency coefficient. The erosive power term, formerly suggested as the ground component of the prediction model, is derived taking into account the damaging threshold energy of the material. An erosive efficiency coefficient is introduced on this basis that allows to quantify the erosive potential of a cavitation situation for a given material. A formula for localization of erosion is proposed that completes the prediction model. Finally, a procedure is described for geometrical scale and flow velocity transpositions.
Instability of Taylor Vortex and Nonaxisymmetric Modes in Flow Between Rotating Porous Cylinders
Authors: Kong Chin-Hwa , Lee Chi-Kuo
Published: February 2008

A numerical solution of linear differential equations governing the instability of Taylor vortex and nonaxisymmetric modes inflow between rotating porous cylinders is present. Solutions take into account the presence of a radial flow between the two rotating cylinders. The critical Reynolds number and corresponding critical axial and azimuthal wavenumber are shown for different values of radius ratio, ratio of angular velocities of the inner and outer cylinders. The results show that not only the critical Reynolds number but the oscillatory onset mode of nonaxisymmetric disturbances can be altered when a radial flow is superimposed on the circular Couette flow. The weak inward flow has a destablizing effect for wide-gap, corotating system of positive and large μ (ratio of angular velocities ω1 and ω2) and the weak outward flow has a destablizing effect for small gap, co-rotating system and all counter-rotating system. The most unstable mode of instability depends not only on the angular speed ratio of both cylinders but also the strength of radial flow.
The Influence of Atomizing Gas Molecular Weight on Low Mass Flowrate Effervescent Atomizer Performance
Authors: Lund M. T. , Jian C. Q. , Sojka P. E. , Gore J. P. , Panchagnula M. V.
Published: February 2008

The relationship between atomizing gas molecular weight and spray mean drop size, Rosin-Rammler distribution parameter, and number averaged drop velocity is reported for a low mass flowrate effervescent atomizer-produced spray. Experimental data at lower gas-liquid ratios (GLR’s) demonstrate that an increase in the molecular weight of the atomizing gas increases mean drop size and decreases number averaged drop velocity. The increase in mean drop size is attributed to an increase in the thickness of the liquid annulus at the nozzle exit and a subsequent increase in the diameter of ligaments formed there. The decrease in number averaged velocity results from a decrease in jet momentum rate. A model developed to explain the atomization process indicates that the gas flow is choked at higher GLR’s.
Spatial Structure of Negative ∂u˜/∂y in a Low Rθ Turbulent Boundary Layer
Authors: Klewicki Joseph C. , Hill Rodney B.
Published: February 2008

Multiple line Molecular Tagging Velocimetry (MTV) measurements are used to explore the flow fields associated with wall region negative ∂u˜/∂y in a low Rθ turbulent boundary layer. Velocity and velocity gradient statistical profiles establish the present flow as a canonical flat plate boundary layer. Spatial correlations of ∂u/∂y in the (x, y) plane, and the probability of observing negative ∂u˜+/∂y+ exceeding a negative threshold of −0.25 reveal the highly significant presence of negative ∂u˜/∂y in the region 25 ≤ y+ ≤ 35. Examination of the instantaneous flow fields underlying the conditional pdf indicates that there are two primary contributing motions: (i) streamwise shear layers and (ii) compact regions containing ±∂u˜/∂y. The importance of these motions to wall region turbulence production is discussed.
Turbulent Flow Through Spacer Grids in Rod Bundles
Authors: Yang Sun Kyu , Chung Moon Ki
Published: February 2008

The effects of the spacer grids with mixing vanes in rod bundles on the turbulent structure were investigated experimentally. The detailed hydraulic characteristics in subchannels of a 5 × 5 rod bundle with mixing spacer grids were measured upstream and downstream of the spacer grid by using a one component LDV (Laser Doppler Velocimetry). Axial velocity and turbulent intensity, skewness factor, and flatness factor were measured. The turbulence decay behind spacer grids was obtained from measured data. The trend of turbulence decay behaves in a similar way as turbulent flow through mesh grids or screens. Pressure drop measurements were also performed to evaluate the loss coefficient for the spacer grid and the friction factor for a rod bundle.
A Time-Marching Method for the Calculation of Nonsimilar 3D Boundary Layers on Turbomachinery Blades
Authors: Asvapoositkul W. , Zangeneh M.
Published: February 2008

A method is presented for the computation of three-dimensional boundary layers on turbomachinery blades. The method is based on a finite difference approach on a body-fitted curvilinear coordinate system, in which the time-dependent 3D boundary layer equations are marched in time until the steady-state solution is found. The method does not employ a similarity law and can therefore be applied to nonsimilar boundary layers. The method not only enables one to compute the skin friction and displacement of the boundary, but also provides information on the sources of entropy generation on the blades. The entropy generation is in fact split into three main components, which correspond to heat conduction, streamwise shear stress, and cross-flow shear stress. By considering each of the components of shear stress, at the design stage considerable insight can be found on the best way of modifying the blade geometry in order to reduce blade losses. The method is validated by comparison with analytical data for a laminar flat plate, experimental results for a helical blade in turbulent flow, and an axial compressor blade. Finally, the method is applied to the prediction of boundary layers on a subsonic centrifugal compressor impeller blade.
Pipeline Leak Detection by Impulse Response Extraction
Authors: Liou Chyr Pyng
Published: February 2008

A system’s response to an impulse can be used to detect and diagnose abnormalities. The impulse response can be extracted by using cross-correlations between a low amplitude pseudo random binary disturbance input and the system’s output. This fact is applied to pipeline hydraulics as a means of real-time non-interruptive integrity monitoring. A method of generating the pseudo random binary disturbance is proposed. The extraction of a pipeline’s impulse response with the presence of noise is investigated. The features of the response of an intact pipeline and characteristic changes in the impulse response as a result of a leak are established. The feasibility of using impulse response to detect and to locate a leak in real-time is demonstrated.
Forced Convection During Liquid Encapsulated Crystal Growth With an Axial Magnetic Field
Authors: Ma Nancy , Walker John , Bliss David , Bryant George
Published: February 2008

This paper treats the forced convection, which is produced by the rotation of the crystal about its vertical centerline during the liquid-encapsulated Czochralski or Kyropoulos growth of compound semiconductor crystals, with a uniform vertical magnetic field. The model assumes that the magnetic field strength is sufficiently large that convective heat transfer and all inertial effects except the centripetal acceleration are negligible. With the liquid encapsulant in the radial gap between the outside surface of the crystal and the vertical wall of the crucible, the forced convection is fundamentally different from that with a free surface between the crystal and crucible for the Czochralski growth of silicon crystals. Again unlike the case for silicon growth, the forced convection for the actual nonzero electrical conductivity of an indium-phosphide crystal is virtually identical to that for an electrically insulating crystal. The electromagnetic damping of the forced convection is stronger than that of the buoyant convection. In order to maintain a given balance between the forced and buoyant convections, the angular velocity of the crystal must be increased as the magnetic field strength is increased.
On the Suitability of the Low Mach Number Assumption in the Modeling of the Damage Induced by Pressure Transients in Piping Systems
Authors: Freitas Rachid Felipe B. , Costa Mattos Heraldo S.
Published: February 2008

One-dimensional models for predicting the damage induced by pressure transients in piping systems conveying liquids have been proposed and analysed recently. However, such works have been concerned mainly with the adequacy of the constitutive equations adopted for different pipe materials and with the numerical techniques used for approximating the solution of the resulting mathematical problems. In the present paper the suitability of the simplifying low Mach number assumption adopted in the modeling is investigated. The analysis is carried out based on the eigenvalue problem associated to the governing equations, without appealing to any specific mechanical behavior of the pipe material. Numerical results obtained for the most used pipe materials show that this simplifying assumption is adequate for metallic tubes, but may fail when plastic tubes are considered.