bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical Simulations of Shock-Bubble Interactions Using an Improved Ghost Fluid Method
Authors: Takahira Hiroyuki , Yuasa Shinya
Published: October 2008

The present work is concerned with numerical simulations for the shock-bubble interactions using the Ghost Fluid Method (GFM) in which the interface is captured with level set methods. The GFM is applied to the interactions between an air shock wave and a cylindrical or spherical helium bubble to investigate the numerical diffusion in the reinitialization procedure of the level set function. It is shown that the interface is not captured accurately using the GFM without the reinitialization of the level set function. The numerical diffusion in the reinitialization procedure affects the formation of a re-entrant jet and vortex structures after a shock wave impacts the bubble. It is also shown that the results with the hybrid particle level set method agree with the experiments by Haas and Sturtevant. The hybrid particle level set method is superior in the mass conservation. Also, we have improved the GFM by correcting velocities, pressure and density at boundary nodes using the Riemann solutions to avoid numerical oscillations near the gas-liquid interface. We have succeeded in capturing the sharp interface for the shock-air bubble interaction in water by using the improved GFM coupling with the hybrid particle level set method. Mass conservation in the hybrid particle level set method is better than that in the standard level set method with high order discretization scheme.
Energy Transfer in Nearly Isotropic Perforated-Plate Turbulence
Authors: Liu Rui , Ting David S.-K. , Carriveau Rupp
Published: October 2008

In this empirical study, the turbulence energy transfer is quantified downstream of a novel orificed perforated plate in a wind tunnel. The measurements were taken within the nearly homogeneous and isotropic region in which the turbulence statistics up to the 4th order were measured to be fairly uniform. The three-dimensional turbulence spectra were estimated from their one-dimensional counterparts and the evolution of the turbulence energy transfer was characterized.
Experimental Investigations on Fluid Flow and Heat Transfer Through Rectangular Mini Channels
Authors: Solomon John T. , Sobhan Choondal B.
Published: October 2008

Experimental investigation aimed at characterizing fluid flow and heat transfer in mini channels is presented in this paper. Forced convection flow of water through rectangular mini channels with hydraulic diameters in the range 0.28–3.67 mm and aspect ratio (H/W) 0.33–2.5 was investigated, for different flow regimes, and correlations were proposed for the flow friction and heat transfer characteristics. Fully turbulent convective heat transfer was achieved in mini channels at lower Reynolds numbers compared to conventional sized channels. Critical Reynolds number for flow transition was found to decrease with the hydraulic diameter. A comparison of the results in mini channels and microchannels indicate common trends, especially in the dependence of flow transition on the hydraulic diameter. Experimental results were compared with predictions for flow and convective heat transfer in conventional sized channels. Based on the finding that the frictional and heat transfer characteristics of the mini channels deviated from the predictions, a few new correlations are proposed for the laminar and turbulent flow regimes.
Development of a Trailing Vortex Formed With Spanwise Tip Blowing
Authors: Richardson S. , Bettle J. , Holloway A. G. L.
Published: October 2008

Trailing vortices were formed with strong core jets and high turbulence levels in a wind tunnel using a NACA 0015 rectangular semi-span wing having a full span aspect ratio of 5 and a chord Reynolds number of 105. The jet on the vortex core was generated by air blown from the wing tip through small holes arranged along the periphery of the wing cross-section. Experiments covered a wide range of tip jet blowing rates for a wing incidence of 5°. Measurements of mean velocity, mean vorticity and turbulence kinetic energy were made up to 32 chords downwind using a hot wire probe. At zero wing incidence the flow was observed to have the form of a jet in cross-flow with counter-rotating streamwise vortices of zero net circulation and a very high turbulence level. With the wing at incidence, the tip vortex envelopes the jet and as it develops its diameter grows and the jet velocity and circumferential velocity decline sharply. Further downwind the vortex formed with tip jets evolves into an axisymmetric form similar to a natural vortex but with a diameter up to 5 times larger. The turbulence intensities measured on the core without blowing were ∼2.5% of the free stream velocity while with blowing they were ∼15% in the near wake and ∼9% in the far wake.
RANS Modeling in Gas-Solid Particle Vertical Flow (Keynote)
Authors: Kartushinsky Alexander I. , Michaelides Efstathios E.
Published: October 2008

Results from the mathematical modeling of gas-solid particle flow in an upward pipe flow performed in the framework of RANS are presented. The aim of the study is to implement an original model of closure for the transport equations of dispersed phase along with particles involvement into the turbulence motion into further developed the modeling description using the RANS approach. It allows the retention of the diffusive terms in both streamwise and radial directions. The numerical simulations show that the RANS method captures the main characteristics and overall behavior of the particle and gas-phase motion in the pipe.
Dynamic Simulation of Gas-Liquid Homogeneous Flow in Natural Gas Pipeline Using Two-Fluid Conservation Equations
Authors: Abbaspour Mohammad , Chapman Kirby S. , Glasgow Larry A. , Zheng Zhongquan C.
Published: October 2008

Homogeneous two-phase flows are frequently encountered in a variety processes in the petroleum and gas industries. In natural gas pipelines, liquid condensation occurs due to the thermodynamic and hydrodynamic imperatives. During horizontal, concurrent gas-liquid flow in pipes, a variety of flow patterns can exist. Each pattern results from the particular manner by which the liquid and gas distribute in the pipe. The objective of this study is to simulate the non-isothermal, one-dimensional, transient homogenous two-phase flow gas pipeline system using two-fluid conservation equations. The modified Peng-Robinson equation of state is used to calculate the vapor-liquid equilibrium in multi-component natural gas to find the vapor and liquid compressibility factors. Mass transfer between the gas and the liquid phases is treated rigorously through flash calculation, making the algorithm capable of handling retrograde condensation. The liquid droplets are assumed to be spheres of uniform size, evenly dispersed throughout the gas phase. The method of solution is the fully implicit finite difference method. This method is stable for gas pipeline simulations when using a large time step and therefore minimizes the computation time. The algorithm used to solve the nonlinear finite-difference thermo-fluid equations for two phase flow through a pipe is based on the Newton-Raphson method. The results show that the liquid condensate holdup is a strong function of temperature, pressure, mass flow rate, and mixture composition. Also, the fully implicit method has advantages, such as the guaranteed stability for large time step, which is very useful for simulating long-term transients in natural gas pipeline systems.
An Improved Multiscale Model for Dilute Turbulent Gas-Particle Flows Based on the Equilibration of Energy Concept
Authors: Xu Ying , Subramaniam Shankar
Published: October 2008

The objective of this study is to assess, and possibly improve, models for turbulent particle-laden flows. We begin by understanding the behavior of two existing models—one proposed by Simonin [Von Karman Institute of Fluid Dynamics Lecture Series, 1996], and the other by Ahmadi [Int. J. Multiphase Flow, 1990]—in the limiting case of statistically homogeneous particle-laden turbulent flow. The decay of particle and fluid phase turbulent kinetic energy (TKE) are compared with direct numerical simulation results. Even this simple flow poses a significant challenge to current models which have difficulties in reproducing important physical phenomena, such as the variation of TKE decay with particle Stokes number. Some of these problems can be traced to the model for the interphase TKE transfer timescale. A new model for the interphase transfer timescale is proposed that accounts for the interaction of particles with a range of fluid turbulence scales. A new multiphase turbulence model—the Equilibration of Energy Model (EEM)—is proposed, that incorporates this multiscale interphase transfer concept. The particle and fluid TKE evolution predicted by this new EEM model correctly reproduce the trends with particle Stokes number.
Active Tonal Noise Control for a Computer Cooling Fan
Authors: Wang J. , Huang L.
Published: October 2008

The noise sources on a blade can be divided into a thrust force component along the rotating axis, and a drag component on the fan rotational plane. The radiated noise is a result of acoustic interference among all blades, and the two source components. For a sample fan with B = 7 blades, and S = 7 struts, the drag noise is expected to cancel themselves out. The result is a simple thrust dipole along the fan axis. When B = 7 and S = 4, however, the dominant noise is a rotating drag dipole. This paper demonstrates, experimentally, the global elimination of the blade passing frequency tones from the thrust dipole and the rotating drag dipole for the two sample fans, respectively, by using a simple open-loop, feedforward control scheme. The rig consists of a miniature electret microphone used as a rotation sensor, ordinary loudspeakers, and a bandpass filter with adjustable amplitude and phase delay. The electret microphone is installed flush with the inlet bellmouth of the fan. It picks up the fluctuating aerodynamic pressure caused by the passing rotor blades, and is shown to perform much better than a normal optical tachometer.
Comparison of Different Physical Models for the Simulation of Cavitating Flows Around a Hydrofoil
Authors: De Giorgi Maria Grazia , Ficarella Antonio , Laforgia Domenico
Published: October 2008

The simulation of cavitating flow is a difficult task, due to the large density ratio between liquid and vapor phases. Investigation of currently known cavitation and turbulence models and their mutual interaction can be a significant asset to improving their performance. Several previous works present results on cavitating flows obtained by various physical models, that mainly differ by the treatment of the mass transfer calculation between vapor and liquid phases. In this study different cavitation models have been implemented in a commercial, general-purpose CFD code (Fluent), and numerical results were compared to experimental ones. Different turbulence models have been also compared, in particularly, to improve numerical simulations by taking into account the influence of the compressibility of two-phase medium on turbulence, a modified k-ε RNG model and compared with the standard model. The simulations were carried out on 2D hydrofoil with an Eppler E817 cross section. The inception cavitation number, the cavity length and the pressure coefficients have been compared with the experimental data.
Heat Transfer and Fluid Flow in Porous Media With Two Equations Non-Darcian Model
Authors: Nouri-Borujerdi A. , Nazari M.
Published: October 2008

In the present study criterion for local thermal equilibrium assumption is studied. It concerns with the fluid flow and heat transfer between two parallel plates filled with a saturated porous medium under non-equilibrium condition. A two-equation model is utilized to represent the fluid and solid energy transport. Numerical Finite Volume Method has been developed for solving coupled energy equations and the Non-Darcian effects are considered for description of momentum equation. The effects of suitable non dimensional parameters as Peclet number and conductivity ratio has been studied thoroughly. A suitable non dimensional equation proposed in wide range of Peclet number and conductivity ratio. This equation shows the temperature difference between solid and fluid phases.
Examination of the Flow Separation Characteristics Around a Streamlined Axisymmetric Shape
Authors: Baker C. R. , Jeans T. L. , Gerber A. G. , Holloway A. G. L. , Watt G. D.
Published: October 2008

Using computational fluid dynamics (CFD), a study was conducted to predict the hydrodynamic forces and moments on an axisymmetric body over a range of yaw angles and Reynolds numbers. Computational results for hydrodynamic forces and moments show good agreement with experimental data, being within the experimental uncertainty range at most yaw angles. Deviations outside of the uncertainty range occurred for the lateral (Y) force values at yaw angles greater than 15 degrees. The development of the after-body vortex shows good agreement with experimental observation. Primary and secondary separation points and shear stress streamline behaviour are also compared with experiment data at a yaw angle of 24 degrees. Results are discussed with a view to identifying flow features critical to the development of new force estimation methods. The after-body vortex, at increasing yaw angles, influences the overall force and moment predictions through a complex interaction between the transport of after-body vorticity and the detachment/reattachment locations of the boundary layer. Adequate modeling of this after-body region is increasingly important at high yaw angles. One of the most important features that influences the overall forces and moments is the circumferential position of shear layer detachment and reattachment, which have a direct impact on the pressure distribution along the body.
Effect of Tip Clearance in a Low Speed Centrifugal Compressor
Authors: Usha P. , Sitaram N.
Published: October 2008

Effect of tip clearance on flow field of a low speed centrifugal compressor is presented. Computational study of centrifugal compressor is carried out using structured multi block grid with fine grid in the tip clearance region. Results are obtained with finite volume method upwind scheme using TASCflow software. Centrifugal compressor impeller with four values of clearances i.e., τ = 0%, 1%, 2% and 5% of blade height at trailing edge are examined at five flow coefficients 0.28, 0.34, 0.42 (design value), 0.48 and 0.52. The effect of tip clearance on total pressure coefficient and static pressure coefficient from inlet to outlet of the compressor is analysed at flow coefficient of 0.52. The drop in static pressure coefficient and total pressure coefficient with increase in tip clearance is found to be high at the tip of the blade due to high pressure fluid leakage at the tip of the blade. The static pressure coefficient, total pressure coefficient and tangential velocity variation at outlet are presented at flow coefficient of 0.52. Mass averaged performance graph show the reduction of performance with tip clearance. Total pressure coefficient contours are analysed in five meridional locations at φ = 0.42. Relative velocity vectors are plotted at five meridional locations for τ = 5% at φ = 0.48. Relative flow angle contours are presented at five meridional locations for all clearances at φ = 0.28.
Modeling of Particulate Pressure in the Frame of Eulerian Approach for Compressible Reactive Dispersed Two-Phase Flows
Authors: Simoes Marine , Della Pieta Patrick , Godfroy Franck , Simonin Olivier
Published: October 2008

In solid rocket motors (SRM) used for space propulsion, such as Ariane 5, aluminum is frequently used as a propellant additive to increase specific impulse. However, its combustion produces aluminum oxide residues that are carried away by the flow field. Thus, physical modeling of aerodynamic flows in SRM should account for this condensed phase. In the frame of full Eulerian approach, unsteady compressible Navier-Stokes equations are solved for both phases. However, numerical simulations performed on a simple axisymmetric motor have pointed out a flaw of the basic Eulerian approach. Indeed, the variance of the particle velocity distribution is not accounted for, leading to unrealistic accumulations of particles in some specific flow region. Hence, we have developed an advanced Eulerian model to correct this shortcoming, which is presented in this paper.
The Tensile Strength of Water as a Function of Temperature
Authors: Williams Rhodri L. , Williams P. Rhodri , Al-Hussany A.
Published: October 2008

This paper reports the results of measurements of the effective tensile strength Fc of water, in experiments involving a pulse of tension (‘negative pressure’) created by the reflection of a pressure pulse at a boundary, as a function of temperature. Using a modified ‘Bullet-Piston’ (B-P) pulse reflection apparatus, measurements presented herein show that degassed, deionised water is capable of sustaining tensions an order of magnitude greater than previously reported in B-P work. A theoretical explanation is developed indicating that the pressure records arising in B-P experiments are the result of cavitation due to a pulse of tension. Results are reported for measurements of Fc made over the temperature range 1°C ≤ T ≤ 95°C.
Lift Force Effects on the Behavior of Bubbles in Homogeneous Isotropic Turbulence
Authors: Abbad Mustapha , Oesterle´ Benoiˆt
Published: October 2008

The influence of lift forces on the dispersion of small bubbles is numerically studied in a homogeneous isotropic turbulence generated by random Fourier modes, under one-way coupling approximation. The effects of bubble Stokes number and mean relative velocity are investigated by computing the statistics from Lagrangian tracking of a large number of bubbles in many flow field realizations, and comparison is provided between the results obtained with and without taking the lift force into account. The effects of preferential concentration, which are known to reduce the terminal rise velocity of bubbles, are also investigated. The lift force is found to drastically modify the correlations and integral time scales of the fluid seen by the bubbles in their fluctuating motion, and to significantly enhance the accumulation of bubbles in high vorticity regions.
Experimental Study of Vortex Shedding From a Solid Sphere in Turbulent Freestream
Authors: Tyagi Himanshu , Liu Rui , Ting David S.-K. , Johnston Clifton R.
Published: October 2008

The study of vortex shedding from a sphere assumes an important role because of its relevance to numerous aerodynamic and hydrodynamic applications. Parameters such as coefficient of drag and static pressure distribution are largely influenced by vortex shedding, and it is found by past studies that the freestream turbulence can interact and alter the vortex formation and shedding drastically. Most of these studies, however, were conducted in the low Reynolds number regime and the vortex shedding results had been described only qualitatively. To better understand the aerodynamics of a sphere in turbulent flow, an experimental study was initiated in a low speed wind tunnel to quantify the vortex shedding characteristics. The Reynolds number of the flow, based on the diameter of the sphere (d), was set at 3.3 × 104, 5 × 104 and 6.6 × 104 by varying the mean flow velocity. The sphere was placed at 20D (= 7.5d) downstream from a perforated plate, where D = 37.5 mm is the size of the holes in the perforated plate, uniquely designed for generating near-isotropic turbulence. Hot-wire measurements were taken at 10D (= 3.75d), 20D (= 7.5d) and 30D (= 11.25d) downstream of the sphere in absence and presence of the perforated plate. The vortex shedding frequency was deduced from the instantaneous flow velocity data.
Vortex Dynamics and Mechanisms for Viscous Losses in the Tip-Clearance Flow
Authors: You Donghyun , Wang Meng , Moin Parviz , Mittal Rajat
Published: October 2008

The tip-clearance flow in axial turbomachines is studied using large-eddy simulation with particular emphasis on understanding the underlying mechanisms for viscous losses in the end-wall region and the unsteady characteristics of the tip-leakage vortical structures. Systematic and detailed analysis of the mean flow field and turbulence statistics has been made in a linear cascade with a moving end-wall. The tip-leakage jet and tip-leakage vortex are found to produce significant mean velocity gradients, leading to the production of vorticity and turbulent kinetic energy. These are the major causes for viscous losses in the cascade end-wall region. An analysis of the energy spectra and space-time correlations of the velocity fluctuations suggests that the tip-leakage vortex is subject to a pitchwise low frequency wandering motion.
Vortex Induced Vibration Tests of Smooth and Rough Flexible Cylinders at High Reynolds Numbers
Authors: Allen Don W. , Henning Dean L. , Lee Li
Published: October 2008

Vortex-induced vibration (VIV) tests have been performed on long, flexible pipes with various levels of roughness, in sheared flows in a circular towing tank at high Reynolds numbers. The test pipes, made of fiberglass composite, were mounted horizontally beneath a rotating arm that has a span of 129 ft, and a width of 25 ft. As the towing bridge rotates, it drives the cylinder in a circular path in still water. The sheared flows experienced by the cylinder excite its VIV motion. The Reynolds numbers for the tests reported herein ranged from 152,000 to 339,000 at the high-speed end of the pipe. Two surface roughness levels were tested: one comprised of the exterior surface of a filament wound fiberglass pipe; and one with carpet glued to the exterior of the pipe. The VIV responses of the test cylinders, represented by displacement time traces, spectrum, and motion trajectories, are presented in this paper. Effects of the surface roughness and Reynolds numbers on the VIV responses are discussed. The response behavior of the cylinders varied from single-mode dominance to multi-mode responses, in addition to certain traveling wave activities. These results should be of interest to researchers and engineers in the area of vortex-induced vibrations.
Conceptual Design of an Adaptive Wind Tunnel for the Generation of Unsteady Complex Flow Patterns
Authors: Kirkman Richard , Metzger Meredith
Published: October 2008

The present paper describes the conceptual design of a three-dimensional adaptive wind tunnel capable of generating complex, unsteady flow fields in a relatively compact physical domain. The design involves multiple, independently controllable vents located around the periphery of a semi-enclosed facility. Desired flow patterns at target areas within the facility are produced by actively steering the inlet flow via appropriately adjusting the magnitude and direction of the air flow entering from each vent. The present study is motivated by a desire to incorporate tactile wind sensation into CAVE-like virtual environments, thereby increasing the overall sense of immersion in the virtual reality. The present wind tunnel design concept may also have potential application to laboratory studies of such problems as unsteady aerodynamics. Results in the present study include examples of two flow patterns obtained from numerical simulations using Fluent. Results from a companion parametric study analyzing the sensitivity of the numerical solution to mesh size and tolerance are also provided. In addition, the feasibility of using a linear-based control strategy to generate prescribed flow patterns within the wind tunnel is discussed.
Visualization of Pulsed Aerated Liquid Jet in Supersonic Cross Flow
Authors: Sapmaz H. , Alkan B. , Lin C. X. , Ghenai C.
Published: October 2008

The success of supersonic air-breathing propulsion systems will be largely dependent on efficient injection, mixing, and combustion inside the supersonic combustion chamber. Fuel/air mixing enhancement inside the combustion chamber will depend on the strategies used to control the fuel jet penetration and liquid fuel droplet size, trajectory, and dispersion. We present in these paper experimental results on the mixing of pure liquid jet, aerated liquid jet and pulsed aerated liquid jet in supersonic cross flow. Transverse aerated liquid jet injection will offer relatively rapid near-field mixing, good fuel penetration and better atomization of liquid fuel. Fully modulated or pulsed fuel jet injection will introduce additional supplementary turbulent mixing. High speed imaging system is used in this study for the visualization of the injection of liquid jet in high speed cross flow. The results presented in this paper show the effect of jet/cross flow momentum ratio, the gas/liquid mass ratio and pulsing frequency on the penetration of aerated liquid jet in supersonic cross-flow. The data generated in this study will be used for the development of active control strategies to optimize the liquid fuel jet penetration and supersonic fuel/air mixing.
Measurements in High Void-Fraction Bubbly Wakes Created by Ventilated Supercavitation
Authors: Wosnik Martin , Fontecha Lucas Gomez , Arndt Roger E. A.
Published: October 2008

A detailed study of ventilated supercavitation in the reentrant jet regime is being carried out in the high-speed water tunnel at St. Anthony Falls Laboratory, as the hydrodynamics part of an interdisciplinary study on stability and control of high-speed cavity-running bodies. It is aimed at understanding the interaction between a ventilated supercavity and its turbulent bubbly wake, with the goal to provide the information needed for the development of control algorithms. Here Particle Image Velocimetry (PIV) measurements in high void fraction bubbly wakes created by the collapse of ventilated supercavities are reported. Bubble velocity fields are obtained, and shown to submit to the same high Reynolds number similarity scaling as the single-phase turbulent axisymmetric wake. A grayscale technique to measure local average void fraction is outlined. Initial results of a time-resolved PIV experiment (2000 Hz) are also presented.
Effects of Surface Roughness on Rebound Strength in Particle-Wall Collisions in Air and Liquids
Authors: Nguyen Y. Q. , Wells John C.
Published: October 2008

Experiments to examine the effects of surface roughness on the strength of rebounds from particle-wall collisions, in liquids and in air, are presented. The target walls are glass plates and have three different values of surface roughness: less than 0.3 μm, 10.5 μm, and 40.1 μm. Particles are glass beads with diameters from 1.05mm to 2.35mm. The experiments are conducted in tap water, sugar-water, and in air. Pre-impact Stokes numbers in liquids are in the range 20–170. For impacts in air, no significant effect of the surface roughness on rebound velocity is observed. In liquids, rough surfaces yield stronger rebounds than smooth surfaces. For a given ratio of surface roughness/particle’s radius, the enhancement of the surface roughness on the rebound compared to the smooth one increases when the impact Stokes number decreases toward the “rebound threshold”. These observations are in qualitative agreement with suggestions in the literature.
Numerical and Experimental Investigation of Hydrodynamic Forces Due to Non-Uniform Suction Flow to a Mixed-Flow Pump
Authors: van Esch B. P. M. , Bulten N. W. H.
Published: October 2008

This paper presents an investigation of the effect of a non-uniform suction flow on forces on the impeller of a waterjet pump. In such a pump, used for ship propulsion, the non-uniformity of the suction flow is caused by the boundary layer under the hull of the ship and the shape of the inlet duct. The paper covers both experimental and numerical studies. A model of a mixed-flow waterjet pump is built into a closed-loop test rig. In order to measure the instantaneous forces and bending moments on the impeller, a newly designed co-rotating dynamometer is used, which is built between the impeller and the shaft of the pump. The design of the dynamometer will be presented. Various entrance flow distributions to the pump are achieved by means of a device situated in the suction pipe. In this manner the axial velocity at the inlet of the pump is varied between uniform and non-uniform distributions. Results of measurements show the influence of suction flow and blade interaction on forces. Results of experiments are compared with CFD calculations of a waterjet pump installation with similar entrance flow conditions. Quasi-steady calculations are performed for the pump which is equipped with a vaned stator bowl. Calculations show a good quantitative agreement with measurements.
Study on the Performance of Pump Using CFD Tool
Authors: Song P. , Schoenbrun S. , Ray R. , Amano R. S.
Published: October 2008

In this paper, a numerical simulation of the three-dimensional fluid flow inside a centrifugal pump has been carried out. The pump has a three-vane commercial solid-handling impeller with 853 mm impeller outlet diameter, nominal flow of 0.559 m3/s and 50 m of head at the rotating speed 890 rpm. Because of the large clearance between the impeller and volute of a solids handling pump, their interaction has been neglected in the present study. The flow inside the rotating impeller is investigated without the stator volute to demonstrate the hydraulic performance of the impeller itself. The velocity and pressure distributions inside the impeller operating in the design and off-design conditions are presented. The numerical results of the overall performance attributes such as total differential head and efficiency through the impeller are compared with the test data. The governing equations including continuity, momentum, and turbulence equations are solved in the rotating frame to simulate the three-dimensional turbulent flow inside the impeller by using the commercial Computational Flow Dynamics (CFD) software STAR-CD. The turbulence is modeled by using the high Reynolds standard k-ε model.
Periodically Unsteady Flow in a Single-Blade Centrifugal Pump: Numerical and Experimental Results
Authors: Benra F.-K. , Dohmen H. J. , Sommer M.
Published: October 2008

The composition of sewage water with partially large portions of fibers and solids requires a special pump design, in order to avoid operational disturbances by clogging. In most applications for sewage water transport, single-stage pumps with single-blade impellers are used. With this special impeller geometry largest flow channels can be realized. So fibers and solids up to an appropriate size can be transported by the pump. This minimum impeller blade number however brings disadvantages for pump operation. The development of a pressure and a suction surface of the blade gives an asymmetric pressure distribution at the perimeter of the rotor outlet and a periodically unsteady flow field arises. In a numerical approach the time accurate flow in a single-blade centrifugal pump has been calculated by solving the 3-dimensional time dependent Reynolds averaged Navier-Stokes equations (URANS) in a wide range of pump operation. The investigation of the flow included all details between suction flange and pressure flange of the pump. The numerical results show a strong dependence from impeller position for all flow parameters. For the investigated operating points strong vortices have been obtained at particular impeller positions. Experimental results have been used to verify the numerical results of time dependent flow in the single-blade pump. The computed flow field has been compared to results which were obtained from optical measurements of flow velocities by Particle Image Velocimetry at different impeller positions. A very good qualitative agreement between measurements and calculations has been obtained for all investigated operating points.
Flow Field Analysis of Under-Expanded Supersonic Jets Impinging on an Inclined Flat Plate: Analysis With PSP/Schlieren Images and CFD Simulations
Authors: Fujii Kozo , Oyama Akira , Tsuboi Nobuyuki , Tsukada Moto , Ouchi Hirofumi , Ito Masato , Hayashi Koichi
Published: October 2008

Flow fields of Mach number 2.2 jet impinging on an inclined flat plate are experimentally investigated using the Pressure Sensitive Paints (PSP) and Schlieren flow visualization. The flow filed structure is mainly determined by two geometrical parameters (nozzle-plate distance and plate angle against the jet) and one flow parameter (pressure ratio). The results suggest that all the observed flow fields can actually be classified into three types of flow structure based on the three parameters above. As an extension of the authors’ earlier work, experiments are carried out for higher plate angles. The new results show the effectiveness and limitation of the classification that we proposed. To find out the flow structure, some of the flow fields are computationally simulated. Good agreement of the pressure distributions with the experiment validates the simulation. Although analysis so far is limited, the result reveals three dimensional complex flow structure that created pressure peaks over the plate surface.
Effects of Spatial Resolution and Box Size on Numerical Solutions of Turbulent Flows
Authors: Zhou Dongmei , Ball Kenneth S.
Published: October 2008

This paper has two objectives, (1) to examine the effects of spatial resolution, (2) to examine the effects of computational box size, upon turbulence statistics and the amount of drag reduction with and without the control scheme of wall oscillation. Direct numerical simulation (DNS) of the fully developed turbulent channel flow was performed at Reynolds number of 200 based on the wall-shear velocity and the channel half-width by using spectral methods. For the first objective, four different grids were applied to the same computational domain and the biggest impact was observed on the logarithmic law of mean velocity profiles and on the amount of drag reduction with 28.3% for the coarsest mesh and 35.4% for the finest mesh. Other turbulence features such as RMS velocity fluctuations, RMS vorticity fluctuations, and bursting events were either overpredicted or underpredicted through coarse grids. For the second objective, two different minimal channels and one natural full channel were studied and 3% drag reduction difference was observed between the smallest minimal channel of 39.1% and the natural full channel of 36.2%. In the near-wall region, however, the minimal channel flow did not exhibit significant difference in the mean velocity profiles and other lower-order statistics. Finally, from this systematical study, it showed that the accuracy of DNS depends more on the spanwise resolution, and it also confirmed that a minimal channel model is able to catch key structures of turbulence in the near-wall region but is much less expensive.
Turbulent Boundary Layer Control by Wall Oscillations
Authors: Zhou Dongmei , Ball Kenneth S.
Published: October 2008

The objective of this paper is to examine the effect and the effectiveness of wall oscillation as a control scheme of drag reduction in order to better understand the mechanism of drag reduction. Two flow configurations were considered: constant flow rate and constant mean pressure gradient. The Navier-Stokes equations were solved using Fourier-Chebyshev spectral methods and the oscillation in sinusoidal form was enforced on the walls through boundary conditions for the spanwise velocity component. Results to be shown included the effects of oscillation frequency, amplitude, oscillation orientation, and peak wall speed at Reynolds number of 180 based on wall-shear velocity and channel half-width as well as the Reynolds number dependency s in both flow configurations. Comparison of effectiveness made between these two flow configurations has showed similarities as well as differences. Drag reduction as a function of peak wall speed was compared with both experimental and numerical data and the agreement was good in the trend and in the quantity. Comparison between these two flow configurations in the transient response to the sudden start of wall oscillation, turbulence statistics, and instantaneous flow fields was detailed and differences were clearly shown. Analysis and comparison are allowed to shed some light on the way that oscillations interact with wall turbulence.
Numerical Simulation of Boiling Flows Using an Eulerian Multi-Fluid Model
Authors: Han J. , Wang D.-M. , Filipi D.
Published: October 2008

A mathematical model to simulate boiling flows in industrial applications is presented. Following the Eulerian multifluid framework, separate sets of mass, momentum, and energy conservation equations are solved for liquid and vapor phases, respectively. The interactions between the phases are accounted for by including relevant mass, momentum, heat exchanges and turbulent dispersion effects. Velocity-pressure coupling is achieved through a multiphase version of the SIMPLE method and the standard k-ε turbulence model is employed. In order to validate and assess the accuracy of the boiling model, subcooled nucleate boiling flows in a vertical annular pipe are simulated in the steady-state mode. The computed axial velocities, volume fractions, temperature profiles are compared with available experimental data (Roy et al., ASME J. of Heat Transfer, Vol. 119, 1997). The result obtained by assuming a constant value for the bubble diameter shows a reasonable agreement, but several limitations are observed in the details. A more advanced mathematical model incorporating separate transport equations for the bubble number density and the interfacial area is suggested.
Prediction of Sheet Cavitation in a Centrifugal Pump Impeller With the Three-Dimensional Potential-Flow Model
Authors: Dijkers R. J. H. , Fumex B. , Op de Woerd J. G. H. , Kruyt N. P. , Hoeijmakers H. W. M.
Published: October 2008

The occurrence of cavitation is one of the main limiting factors in the operation and design of centrifugal pumps. In this paper a model for the prediction of sheet cavitation is described. This model has been implemented in a three-dimensional finite-element package, employing the potential-flow approximation of the governing flow equations. At the interface between vapor and liquid, pressure equilibrium is required. The closure region of the cavity is modeled as the collapse of a bubble, whose motion is described by the Rayleigh-Plesset equation. The effect of displacement of the flow due to presence of the sheet cavity is incorporated by the transpiration technique. This is a linearised approach which is well-known from techniques for coupling inviscid-flow methods to boundary-layer methods. The model gives the location of the sheet cavity (if present); its length is thus also predicted. The model has been validated by comparing sheet cavitation at the blades of a centrifugal pump impeller, obtained from CFD-computations and from visual observations in a model test.
Gravitational and Magnetic Convection in Magnetic Colloids
Authors: Putin Gennady F. , Bozhko Alexandra A.
Published: October 2008

Experiments were performed to examine the influence of homogeneous magnetic field on convection instability and flows in a layer of magnetic fluid heated from one wide side and cooled from another. The temperature sensors were used for measurement of heat transport across the layer. Visualization of flow patterns was provided by a liquid crystal sheet. The results indicate that with the help of a magnetic field it is possible to control the convection stability, intensity of the heat transfer and the form of convection motions. The interaction of gravitational and magnetic convection mechanisms for different orientations of the layer and magnetic field were studied. The essential influence of gravitational sedimentation of magnetic particles and their aggregates on convection in magnetic colloids is shown. A number of non-linear regimes of convection, including localized states and repeated long-wave transients from convection to conduction were observed.
Pulse Jet Mixing of a Non-Newtonian Waste Slurry Simulant
Authors: Guerrero Hector N. , Eberl Kurt R.
Published: October 2008

The mixing of radioactive waste slurries by Pulse Jet Mixers (PJM) was simulated by using non-Newtonian clay suspensions. A quarter scale mockup of a 6 PJM cluster configuration was tested with different nozzle sizes of 25.4 mm, 31.8 mm, and 38.1 mm I.D., nozzle velocities of 8–18 m/sec, and yield stresses from 11 to 35 Pa. Mixing was measured by dye and ultrasonic velocity probe methods. Under PJM only operation, a well mixed region or cavern is developed which is distinct from the stagnant region. The data of cavern heights to tank diameter ratio are well correlated as a function of the product of the Yield Reynolds number to the square root power and the ratio of nozzle to tank diameter. A fully mixed tank was obtained for a nozzle diameter of 38.1 mm, nozzle velocity of 18 m/sec and yield stress of 21 Pa. An alternate mixing configuration consisting of 25.4 mm nozzles and 5 spargers at 3 scfm each, 18 m/s nozzle velocity and yield stress of 30 Pa was found also to be fully mixed using glass bead tracers which also simulated large solids.
A Global Laser Rangefinder Profilometry System for the Measurement of Three Dimensional Wave Surfaces
Authors: Carneal Jason B. , Atsavapranee Paisan , Baumann Carl W. , Hamilton John H. , Shan Jerry
Published: October 2008

A Global Laser Range Profilometry (GLRP) System has been developed at the Naval Surface Warfare Center, Carderock Division (NSWCCD) for the measurement of three dimensional wave surfaces. A laser diode array consisting of 100 diodes operating at 650nm with an energy output of 3.5 mW was used to illuminate the wave surface seeded with fluorescent dye at various points in the Miniature Water Basin (MWB) facility at NSWCCD. A CCD camera located above the water surface recorded successive images of the laser array at 30 frames per second. Image processing techniques were used to locate the centroid of the laser array diode locations in the collected images. Calibration images were taken at various calm water heights in order to convert the image centroid locations to real coordinates. Two- and three-dimensional results are presented, along with error analysis of the GLRP system and comparison to flow visualization. A discussion of possible applications and planned future work is also presented in this work.
Model of Two-Phase Heterogeneous Mixture Flow in a Microchannel With Incorporated Flow Patterns
Authors: Keska Jerry K.
Published: October 2008

In the two-phase or multiphase flow of such heterogeneous mixture like gas-liquid there is a very complicated and random phenomenon of flow patterns, which needs to be quantitatively and accurately, incorporated. Unfortunately, a model with quantitatively incorporated flow patterns in full range of concentration or method of how to measure flow patterns is not available. Recognizing these challenges this paper will present an approach to incorporate flow pattern phenomenon into the two-phase flow calculation model by (1) developing a mathematical model for pressure losses in two-phase flow based on in-situ parameters, (2) developing and defining a flow pattern coefficient, which incorporates the flow pattern phenomena, and (3) present the developed mathematical model with the incorporation of flow patterns, which demonstrated significant increase of accuracy of calculations based on conducted experimental research on air-water two-phase mixture flow in a horizontal square microchannel.
The Effect of Aspect-Ratio on the Development of the Large-Scale Structures in the Three-Dimensional Wall Jet
Authors: Hall Joseph W. , Ewing Daniel
Published: October 2008

The development of the large-scale structures in three-dimensional wall jets exiting rectangular nozzles with aspect-ratios of 1 and 4 was investigated using simultaneous measurements of the fluctuating wall pressure across the jet. The pressure fluctuations in the jets were asymmetric and caused the fluctuating wall pressure to be poorly correlated across the jet centerline. A Proper Orthogonal Decomposition analysis indicated that both the first and second modes make similar contributions to the variance of the fluctuating pressure, and were symmetric and antisymmetric, respectively, and the interplay between these modes caused the asymmetry in the instantaneous pressure fluctuations across the jet centreline. A wavelet analysis of the instantaneously reconstructed pressure fields indicated that the fluctuations were predominantly in two frequency bands near the jet centerline, but were only contained in one band on the outer lateral edges of the jet, indicating there were two different large-scale motions present. The development of large-scale structures in the two jets initially differed in the intermediate field with the antisymmetric mode being more prominent in the square jet and the symmetric mode being more prominent in the larger aspect-ratio jet. Further downstream, the symmetric mode was more prominent in both jets.
Pinch-Off Mechanism of a Free Elongated Liquid Ligament
Authors: Tong Albert Y. , Wang Zhaoyuan
Published: October 2008

The time-dependent relaxation dynamics of a moderately elongated liquid ligament has been studied numerically. The Navier-Stokes equations are solved using a finite-volume formulation with a two-step projection method on a fixed grid. The free surface of the liquid ligament is tracked by a coupled level set and volume-of-fluid (CLSVOF) method with the surface tension force determined by the continuum surface force (CSF) model. The relaxation process of a free elongated liquid ligament has been simulated and the numerical results are in agreement with findings reported in the literature. The end-pinching mechanism of the breakup process has been thoroughly examined. The determining factor for reopening of a pinching neck has been identified. The effects of several parameters on the relaxation mechanism have also been examined. It has been found that the initial end shape of the ligament and the Ohnesorge number play a vital role in the overall relaxation process.
Pipe-Wall Friction in Vertical Sand-Slurry Flows
Authors: Matousek Vaclav
Published: October 2008

Friction due to the presence of solid particles suspended in a flow is a result of processes in a relatively thin layer near the pipe wall. Pipe-wall friction generated by particles in permanent contact with pipe wall is relatively well understood. However, very little is known about the friction deriving from sporadic contact (collisions) of particles with the wall. This friction is a major contributor to the frictional pressure drop in many slurry pipeline applications. The paper describes results of extensive laboratory tests of vertical flows of different sand fractions (fine, medium and coarse sands) carried out in the Laboratory of Dredging Engineering of the Delft University. In order to identify mechanisms that govern the solid-particle friction at the pipe wall the paper analyses friction conditions in observed vertical flows. The effects of particle-particle interactions and particle-liquid interactions on the pipe-wall friction are evaluated. One of the interesting phenomena observed in the laboratory was that frictional pressure drops in highly-concentrated flows at high velocities are lower for slurries of medium sand and coarse sand than for slurries of fine sand. The observed trend is believed to be associated with the liquid–lift force acting on solid particles traveling near a pipe wall. This off-wall force seems to be the most effective for medium to coarse particles traveling in highly concentrated mixture in the near-wall region. Thus pressure drops due to the presence of solids in non-stratified flows seem to be primarily produced by the combined effect of the Bagnold collisional force (force that colliding particles exert against the pipe wall) and liquid lift force acting on solid particles in the near-wall zone of the slurry flow.