bluuebunny/crossref_metadata_2025 huggingface dataset:
A Framework to Assess the Quality and Robustness of LES Codes
Authors: Meyers J. , Sagaut P. , Geurts B. J.
Published: September 2008

We present a framework which can be used to rigorously assess and compare large-eddy simulation methods. We apply this to LES of homogeneous isotropic turbulence using a Smagorin-sky model and three different discretizations. By systematically varying the simulation resolution and the Smagorinsky coefficient, one can determine parameter regions for which one, two or multiple flow predictions are simultaneously predicted with approximately minimal error. To this end errors on the predicted longitudinal integral length scale, the resolved kinetic energy and the resolved enstrophy are considered. Parameter regions where all considered errors are simultaneously (nearly) minimal are entitled ‘multi-objective optimal’ parameter regions. Surprisingly, we find that a standard second-order method has a larger ‘multi-objective optimal’ parameter region than two considered fourth order methods. Moreover, the errors in the respective ‘multi-objective optimal’ regions are also lowest for the second-order scheme.
Aerodynamic Vectoring Particle Sorting
Authors: Smith Barton L. , Humes Zachary E. , Minichiello Angela
Published: September 2008

An experimental and numerical demonstration of a new, non-contact particle sorting technique called Aerodynamic Vectoring Particle Sorting (AVPS) is presented. AVPS uses secondary blowing and suction control flows to sharply turn a 2D, particle-laden jet. As the jet is turned, the inertia of particles present in the flow is balanced by a resultant force that is dependent upon their size and due to the combined effects of pressure, and drag. Since the balance of these forces determines the particle’s trajectory, turning the flow leads to a separation of particles downstream. This simple, low-pressure-drop sorting technique classifies particles with less risk of damage or contamination than currently available sorting devices. AVPS is experimentally demonstrated using a rectangular air jet. A single blower is used for the blowing and suction, thus requiring that their mass fluxes are equal. The unladen jet flow is examined using Particle Image Velocime-try. The experimental results demonstrate that while large blowing slot momentum flux is important (meaning that a smaller slot can be more effective for fixed mass flux), as the aspect ratio of the blowing slot becomes very large, the jet tends to suffer from 3-D instabilities resulting in large fluctuations in the vectoring angle (and thus a detrimental effect on sorting performance). The vectoring angle can be substantially improved introducing a modest angle in the blowing slot (toward the vectoring direction). Particle size and trajectory are measured using the Particle Shadow Imaging method. Numerical simulations are performed using a commercial CFD solver.
CFD Performance Analysis of a Flowmaker
Authors: Rosendahl Lasse A. , Wang Xiaopeng , Jacobsen Christian B.
Published: September 2008

In the present work, the mean flow field in a stirred tank equipped with a scale model of a commercially available Grundfos AFG.40.230.35 flowmaker is investigated using CFD simulation and Laser Doppler Anemometry (LDA), in order to provide information on the interaction between flow, propeller and wall proximity. The propeller is placed at a specified location in the tank, and measurements are taken at various locations in the tank to provide as detailed a representation of the resulting flow as possible as well as insight into the near-field of the flowmaker. The simulation, carried out with Ansys CFX 10, used a multiple frame of reference (MFR) approach to include a full representation of the flowmaker blade and motor geometry, to fully include the effects of the blade shape and variable pitch. The reported results are based on a k-e model using a second order discretization scheme. The results show good agreement on downstream axial velocities immediately after the flowmaker, although the numerical results exhibit symmetry to a greater extent than the experimental data, which is believed to be due to a combination of wall proximity effects in the latter and the turbulence model in the latter. However, the results provide valuable insight into the performance of CFD analysis on this type of flow maker, and highlight aspects for future work.
Influence of Pipe Rotation and Reciprocation in Flow of Non-Newtonian Fluids in Eccentric Annular Spaces
Authors: Lugo David J. , Blanco Armando J.
Published: September 2008

Some industrials processes are associated with flow of non-Newtonian fluids in annular spaces created between parallel pipes. Examples are found in oil industry and food industrial processing. Depending on relative position of both axes, a concentric or eccentric annular space is created. In some typical applications the fluid rheology non-Newtonian and models such as Bingham Plastic or Power Law are required for adequate representation of internal deformations of fluid elements when shear stresses are applied. Depending on annulus eccentricity high resistance can be opposed to flow on narrowest section, including the possibility of having static or quasi-static fluid close to the internal annulus walls. In order to remove this static fluid, two different operations are usually proposed: pipe rotation and pipe reciprocation. In this way, less mobile fluid can be put in motion increasing shear stress. These operations are justified by experimental evidence exists. Scale experiments have been done and predictions for flow behavior in large facilities are extrapolated. However, in large facilities, as oil wells are highly pressurized and they are very deep, it is almost impossible to verify if the whole fluid is mobile and no by-pass fluid remains in the narrowest section of annular space. So, Computational Fluid Dynamics constitutes an ideal technique for analyzing this kind of problem. In this paper, though a Computational Fluid Dynamics study we aim to evaluate the efficiency of pipe rotation and pipe reciprocation in static or quasi-static fluids for Bingham Plastic or Power Law fluid. In order to consider realistic scenarios, oil industry typical conditions are considered for fluid density, rheological parameters, flow rates, casing and hole sizes, and annulus eccentricity. The influence of the variables eccentricity and rotation speed, and the use of reciprocation in shear stress at walls, were used as a measure to evaluate efficiency in static fluid removal. The flow regime was considered laminar. Numerical model capability to reproduce accurately flow patterns in these conditions was assured by comparing it with others analytical-numerical solutions for concentric systems. Results show that both operations are effective for helping in static fluid remotion. However, notable increment for efficiency is observed for eccentricities below 60%. In particular, pipe rotation is effective when rotation speed is greater than 20 RPM for eccentricity greater than 40%. Below this limit, pipe reciprocation is more effective than pipe rotation, independently of the rheological model used to represent the fluid.
Large Eddy Simulation of Film Cooling Flow Above a Flat Plate From Inclined Cylindrical Holes
Authors: Iourokina Ioulia V. , Lele Sanjiva K.
Published: September 2008

Large Eddy Simulation of a realistic film cooling configuration is performed, consisting of a large plenum feeding a periodic array of short film cooling holes with length to diameter ratio L/d = 3.5. Film cooling jets are issued at 35 degrees into the turbulent crossflow boundary layer above the flat surface. Time-averaged flowfield is analyzed to reveal steady and unsteady structures occurring as a result of plenum-jet-crossflow interactions. Among these structures are the flow separation inside the film-hole, reverse flow zone right behind the jet injection and the counter-rotating vortex pair in the wake of the jet. All of these structures influence the wall temperature distribution and have a negative effect on film cooling effectiveness.
Meshless Solution of 2D Fluid Flow Problems by Subdomain Variational Method Using MLPG Method With Radial Basis Functions (RBFs)
Authors: Haji Mohammadi Mohammad , Shamsai A.
Published: September 2008

This paper deals with the solution of two-dimensional fluid flow problems using the truly meshless Local Petrov-Galerkin (MLPG) method. The present method is a truly meshless method based only on a number of randomly located nodes. Radial basis functions (RBF) are employed for constructing trial functions in the local weighted meshless local Petrov-Galerkin method for two-dimensional transient viscous fluid flow problems. No boundary integration is needed, no element matrix assembly is required and no special treatment is needed to impose the essential boundary conditions due to satisfaction of kronecker delta property in RBFs. Three different radial basis functions (RBFs), i.e. Multiquadrics (MQ), Gaussian (EXP) and Thin Plate Splines (TPS) are examined and the selection of their shape parameters is studied based on closed-form solutions. The effect of quadrature domain size is also studied. The variational method is used for the development of discrete equations. The results are obtained for a two-dimensional model problem using three RBFs and compared with the results of finite element and exact methods. Results show that the proposed method is highly accurate and possesses no numerical difficulties.
Numerical Simulation of a Contaminated Droplet by Front-Tracking Method Taking the Effect of Surfactant Transport on the Interface
Authors: Yamamoto Yasufumi , Yamauchi Makoto , Uemura Tomomasa
Published: September 2008

In this study, a front-tracking (FT) method combined with a solver of interfacial transport of surfactant was proposed in order to resolve interfacial flows affected by contamination. In the FT method, because the interfaces are presented explicitly, advection-diffusion equation on the interface can be easily treated and can be solved highly accurately. In this study, a scheme which conserves the total amount of surfactant completely was constructed. Numerical simulations of a water drop sinking in silicone oil were performed taking the Marangoni effect into account. The effects of three parameters, a damping coefficient of interfacial tension, a diffusion coefficient and a total amount of surfactant, were evaluated. Calculated results were compared with PTV measurement results and were in very good agreement with them on the points of stagnant cap size, flow separation point, peak of shear stress and so on. So, we can expect that our simulations can estimate the conditions of surfactant on the interfaces, which is very difficult to be measured.
Numerical Simulation of Turbulent Airflow and Particle Deposition in Human Upper Oral Airways
Authors: Zahmatkesh Iman , Abouali Omid , Ahmadi Goodarz
Published: September 2008

The objectives of this work were twofold. Firstly, a second-order closure turbulence model (LRR model) was used for capturing the anisotropic turbulence intensities within the human upper oral airways. Secondly, dispersion and deposition of particles were analyzed by a Lagrangian particle tracking approach. The computational model was validated with experimental data in terms of particle deposition fractions. The results showed that the airflow in oral airways were three dimensional and included separated flow regions, and contains strongly anisotropic turbulence fluctuation fields. Taking into account the turbulence anisotropy provided important information for particle dispersion analysis; in particular, particle deposition in the human oral airways was noticeably affected by the presence of anisotropic turbulence fluctuation fields.
Flow Characterization of an Electrostatic Resonant Plate Micropump-Mixer by a Scaled Model
Authors: Nilsen Oyvind , Mohseni Kamran
Published: September 2008

Flow characterization of an electrostatically activated resonant-plate micropump-mixer was investigated. Detailed visualization of the mixing process at the tip of the resonant plate, which is almost impossible due to the high actuation frequency (10–30 kHz) and small scale of the resonant plate (250 micron) under normal conditions, was realized with a macro scale flow visualization experiment within the range of common visualization equipment such as a SLR camera. Flow phenomena such as distinct circulative regions, observed at the micro scale by Linderman et. al [1,2], were observed in this study. In addition, the transition between two different flow regimes was observed, corresponding to vortex shearing and vortex shedding respectively. This transition took place in a gradual manner over a range of Reynolds numbers between 20 and 98. Below this regime the resonant plate will only generate limited deformation of the interface between the two fluids. However, for larger Reynolds numbers, equivalent to higher plate frequencies, organized vortex roll-up is observed. Vortex roll-up indicates significant fluid entrainment, and consequently mixing. The visualization of the flow, generated by the resonating fan shed new light on the detailed flow phenomena involved, and may help guide future design and optimization of micro scale fans/mixers based on this principle.
Heat Transfer in Laminar Tube Flow of a Class of Non-Affine Viscoelastic Fluids
Authors: Siginer Dennis A. , Letelier Mario F.
Published: September 2008

The fully developed thermal field in constant pressure gradient driven laminar flow of viscoelastic fluids in straight pipes of arbitrary contour ∂D is investigated. The nonlinear fluids considered are constitutively represented by a class of single mode, non-affine constitutive equations. The driving forces can be large and inertial effects are accounted for. Asymptotic series in terms of the Weissenberg number Wi are employed to represent the field variables. Heat transfer enhancement due to shear-thinning is identified together with the enhancement due to the inherent elasticity of the fluid. The latter is the result of secondary flows in the cross-section. Increasingly large enhancements are computed with increasing elasticity of the fluid as compared to its Newtonian counterpart. Large enhancements are possible even with dilute fluids. Isotherms for the temperature field are presented and discussed for several non-circular contours such as the ellipse and the equilateral triangle together with heat transfer behavior in terms of the Nusselt number Nu.
Turbulent Impinging Jets Into Permeable Media
Authors: de Lemos Marcelo J. S. , Graminho Daniel R.
Published: September 2008

Impinging jets are widely used in industry to modify local heat transfer coefficients. The addition of a porous substrate covering the surface contributes to better flow distribution, which favors many engineering applications. Motivated by that, this work shows numerical results for a turbulent jet impinging against a cylindrical enclosure with a porous substrate at the bottom. Macroscopic time-averaged equations for mass and momentum are obtained based on a concept called double decomposition, which considers spatial deviations and temporal fluctuations of flow properties. The numerical technique employed for discretizing the governing equations is the control volume method in conjunction with a boundary-fitted coordinate system. The SIMPLE algorithm is used to handle the pressure-velocity coupling. The influence of the cylinder height on the mean and statistical flow fields within the entire cavity is presented.
Tube Transportation of Highly Concentrated Emulsions
Authors: Masalova Irina , Malkin Alexander Ya.
Published: September 2008

Tube transportation of highly concentrated emulsions is an important technological process in mining works. Emulsions used for this particular type of application are so-called “liquid explosives” — highly concentrated dispersions of aqueous droplets in a continuous oil phase. The concentration of droplets reaches 96w. %. The width of the inter-phase layers in such a multi-phase system is of the order of nano-level. The length of tube transportation in a real manufacturing process can be of the order of several miles. Hence, the design of the transportation line is of primary technical interest. The practical calculations are based upon comprehensive studies of the rheological properties of highly concentrated emulsions, including an understanding of the role of droplet size, concentration of disperse phase, temperature and time effects (stability of emulsions). Direct measurements were carried out in a wide shear rate range. The results of the measurements indicated that the emulsions under study are rheopectic liquids (viscosity increases over time at a constant shear rate). Their steady flow curve is typical for a visco-plastic medium and is well fitted by the Hershel-Bulkley model. The yield stress is of the order of several tens Pa. The choice of a rheological model is however not crucial for application, since transportation in real technological regimes takes place at high flow rates where the power-type model of flow curves dominates. Systematic studies demonstrated that wall slip is absent over the entire range of the shear stresses under study. This type of rheological behavior was then used for tube transportation design. A more careful examination (based on rheological as well as direct optical observations) also showed that inflation could be observed on the flow curve. It was proven that this type of rheological behavior is related to the two-step mechanism of the flow of a multi-phase liquid. Measurements of normal stresses in shear flows are in accordance with this model of flow. Aqueous droplets in the emulsions under study are super-cooled water solutions of nitrate salts, with the concentration of the latter being of the order of 85%. This system is thermodynamically unstable. The study of time effects (“aging”) showed that slow crystallization in dispersed droplets takes place. This leads to the evolution of the rheological properties of emulsions that can be treated as an emulsion-to-suspension transition. The work was carried out in the Flow Process Research Center, Engineering Faculty, Cape Peninsula University of Technology, Cape Town, Republic of South Africa.
Capillary Flow of Yield-Stress Fluids in Microchannels
Authors: Bertola V.
Published: September 2008

The wicking of a model yield-stress fluid (hair-gel solution in water) in a capillary tube is studied experimentally. By changing the hair-gel concentration in the solution, the yield stress varied from 5 to 20 Pa. A simple force balance between capillary and viscous forces suggests that the fluid should stop flowing as soon as the wall shear stress reaches the yield value, at a critical distance from the inlet which is independent of the tube diameter. However, this theoretical argument is not confirmed by experiments, which show that the fluid moves well beyond the critical distance determined theoretically, and that there is a well-defined effect of the tube diameter. It is proposed that such behavior may be determined by wall slip, which causes the flow to switch from the Poiseuille flow regime to the plug flow regime.
Fixed Valve Piezoelectric Micropump for Miniature Thermal Management Module
Authors: Faulkner D. , Ward C. , Gilbuena D. , Shekarriz R. , Forster F. K.
Published: September 2008

In this paper, we discuss implementation of a micropump with fixed-geometry Tesla-type valves in a closed-loop forced convection thermal management system. The micropump was integrated with a heat sink in a stacked array and fabricated using a photochemical etching process. Two different micropump cavity diameters of 10-mm and 15-mm were fabricated and tested. For each cavity diameter, there were three valve sizes ranging from 140-μm to 340-μm in width. For the best-performing micropump we also evaluated the impact of varying the aspect ratio by adding and removing layers within the micropump. Our results indicated that as the diameter or cavity depth increased the performance of the pump in terms of block load pressure and flow rate degraded. Also, decreasing the valve width for each cavity diameter and height tended to increase the block load pressure and the resulting flow rate. For a pump with 140-μm valve width and an optimal cavity height of 550-μm (11-layers), the maximum flow and pressures obtained for a single pump subassembly were nearly 1.1-mL/min and 0.8-psi. A stack of 4 micropump subassemblies provided more than 5.5 mL/min flow rate and 0.5-psi pressure. The micropump power consumption was less than 50-mW per subassembly, and including the driving electronics power conversion, it consumed less than 0.5-W of power under these conditions. The thermal performance of an integrated liquid cooling module on standard Pentium P4 microprocessor running at up to 40-W was comparable to an off-the-shelf heatsink, but in a package less than 1/10 the size. This unit is currently being considered for blade server applications.
CFD Modeling of Gas-Liquid Flow and Heat Transfer in a High Pressure Water Electrolysis System
Authors: Agranat V. , Zhubrin S. , Maria A. , Hinatsu J. , Stemp M. , Kawaji M.
Published: September 2008

A high-pressure water electrolysis system has been investigated numerically and experimentally. The advanced CFD model of two-phase flow, which calculated the 3D distributions of pressure, gas and liquid velocities and gas and liquid volume fractions, has been developed to account for all the major components in the system, and appropriate constitutive equations for two-phase flow parameters were selected for various parts of the system, such as the cell stack, riser, separator and downcomer. Heat transfer between the two phases, and between the gas-liquid mixture and cooling coils located in the gas-liquid separator was also accounted for. The model was validated using comparisons of predicted liquid flow rate with the liquid flow rate measured in the downcomer, where a single-phase liquid flow existed. The effects of pressure, current density, number of cells, and bubble size were investigated with the numerical model. The numerical predictions matched the general trends obtained from the experimental results with regard to the effects of pressure and current density on the liquid flow rate. The validated CFD model is being used as a cell design tool at Hydrogenics Corporation.
Oil Recovery by In-Situ Gas Generation: Volume and Pressure Measurements
Authors: Bakhtiyarov Sayavur I. , Shakhverdiyev Azizaga Kh. , Panakhov Geilani M. , Abbasov Eldar M.
Published: September 2008

The experimental studies were conducted to measure the volume and the pressure of the CO2 gas generated according to the new oil recovery technology. An experimental device was designed, built and used for these purposes. The designed setup allowed initiating and controlling the reaction between the “gas-yielding” (GY) and “gas-forming” (GF) agents proposed by this technology. The temperature was controlled, and the generated gas pressure and volume were recorded during the reaction process. The effect of water salinity on generated CO2 gas pressure and volume was investigated.
Enhancement of Turbulent Mixing by Embedded Longitudinal Vorticity: A Numerical Study and Experimental Comparison
Authors: Mohand Kaci Hakim , Lemenand Thierry , Della Valle Dominique , Peerhossaini Hassan
Published: September 2008

This work concerns the characterization of turbulent flow underlying the mixing phenomenon in a static mixer-reactor HEV (high-efficiency vortex). An experimental test section made of a cylindrical tube equipped with seven rows of vortex generators was designed and constructed for this purpose. Each row has four vortex generators fixed symmetrically on the tube wall. This new type of mixer generates coherent structures in the form of longitudinal counter-rotative vortices. The resulting flow enhances radial mass transfer and thus facilitates the dispersion and mixing of the particles. The energy cost of this mixer is 1000 times less than that of other mixers for a given interface area [1, 2]. The aim of this work is to study numerically and experimentally the turbulence structure of the flow generated by the mixer, in particular the more energetic structures present in the base flow. Numerical simulations of the velocity distribution and turbulence levels inside the static mixer were conducted for various turbulence models by using the commercial mesh-generator code Gambit coupled with the CFD package Fluent. Attention was focused on the evolution and distribution of the rate of turbulent kinetic energy dissipation as the underlying mechanism for turbulent mixing. Experiments were carried out on the test section in a flow loop by using LDA. Mean and turbulent quantities were measured and numerical results were compared with experimental results. This study provides a basis for understanding the physical mechanisms in the mixing and homogenising of the flow and therefore the efficiency of the mixer.
Fundamental Issues Related to the Numerical Simulation of Two-Phase Flows With Phase-Change
Authors: Jamet Didier
Published: September 2008

In direct numerical simulation (DNS) of two-phase flows, all the interfaces of the two-phase system are tracked individually. If this technique is computationally expensive, it is also very powerful, especially to study basic phenomena. In particular, it helped to better understand fundamental issues such as the forces acting on a single bubble (e.g. [1]) or the interaction of a couple of bubbles in a bubbly flow (e.g. [2,3]) and it now begins to be used to assess average models in detail ([4]). Currently, most of the basic phenomena studied involve non-miscible fluids, where no mass transfer between the phases occurs (air and water for instance). However, in many applications of industrial interest, phase-change phenomena are very important because high heat flux can be achieved with moderate temperature gradients (since the energy exchange through latent heat occurs at a constant temperature). It is thus widely used in the energy industry (nuclear energy in particular) and it is used to design compact heat exchangers (e.g. heat pipes for space or electronic devices). Moreover, basic phenomena related to phase-change are, to a large extent, still misunderstood, which make phase-change phenomena of fundamental interest as well. For instance, despite several decades of valuable scientific studies, the boiling crisis, which is an instability of the nucleate boiling regime, is still misunderstood from a fundamental point of view. It is one of the very few fundamental issues that are still open in fluid mechanics. Since DNS has already been successful to study fundamental issues in two-phase flows of non-miscible fluids, it should be successful to study these issues as well. However, the DNS of two-phase flows with phase-change is more difficult than that of two-phase flows involving non-miscible phases. These issues are both numerical and physical and some of them are discussed in this paper.
Effect of Nozzle-Rotor Clearance on Turbine Performance
Authors: Jeong Eunhwan , Park Pyun Goo , Kang Sang Hun , Kim Jinhan
Published: September 2008

This paper presents the results of performance test and numerical analysis of a supersonic impulse turbine. The test has been conducted using high pressure cold air. The overall turbine performance and turbine nozzle behavior for various operating conditions have been investigated. Experiment and numerical analysis also have been conducted to investigate the nozzle-rotor clearance effect on the turbine performance. It has been found that turbine performance degrades with increasing the axial clearance and this phenomenon is mainly due to the increased stagnation pressure loss in the axial clearance region.
Numerical Simulation of the Transient Three-Dimensional Gas-Liquid Flow in a Cylindrical Bubble Column
Authors: Lai´n Beatove Santiago , Sommerfeld Martin
Published: September 2008

In this paper the transient three-dimensional flow developing in a cylindrical laboratory bubble column is addressed from a numerical point of view. The simulation scheme combines a Large Eddy Simulation (LES) for describing the liquid phase and a Lagragian approach for the gas (discrete) phase. The bubble equation of motion considers all the relevant forces, i.e., buoyancy, pressure, drag, added mass and transverse lift. From the calculations, the transverse lift in combination with the drag is identified as the main mechanism allowing the bubbles to spread over the column cross-section. The liquid and gas velocity profiles obtained are compared with the experimental data and k–ε results presented in Lai´n et al. (2001). As a matter of fact, the dynamic structure of the liquid flow induced by the rising bubbles is well reproduced and also good quantitative results for all measured variables of both phases, gas and liquid, are obtained.
A Review on Fluid Structure Interaction Modeling Using Bondgraph Method
Authors: Malik M. Afzaal , Rashid Badar , Khushnood Shahab
Published: September 2008

Modeling of complex multi-dynamic systems is a formidable challenge. Extensive research has already been carried out focusing on modeling of fluid flow using conventional techniques. This research paper aims at carrying out review on fluid structure interaction specifically adopting bondgraph approach. A comprehensive review, on modeling of fluid dynamic systems, osmosis and hydraulics, rotary piston hydraulic motor, with the help of bondgraph method is carried out. This will facilitate to explore important areas of original research using bondgraph method.
Study of High Speed Turbulent Flow Over Rotating Two Stage Three Dimensional Double Wedge
Authors: Alhussan Khaled
Published: September 2008

In this paper some characteristics of non-steady, compressible, flow are explored, including compression and expansion wave interactions and creation. The work to be presented herein is a Computational Fluid Dynamics investigation of the complex fluid phenomena that occur inside three-dimensional region, specifically with regard to the structure of the oblique shock waves, the reflected shock waves and the interactions of the shock waves. The flow is so complex that there exist oblique shock waves, expansion fans, slip surfaces, and shock wave interactions and reflections. The flow is non-steady, turbulent, viscous, compressible, and high-speed supersonic. The work to be presented herein is a Computational Fluid Dynamics analysis of flow over a 15-degree angle double wedge for a compressible air, with spin angle of 10-degree and Mach number of 2.5. The problem to be solved involves formation of shock waves, expansion fans and slip surfaces, so that the general characteristics of supersonic flow are explored through this problem. Shock waves and slip surfaces are discontinuities in fluid mechanics problems. It is essential to evaluate the ability of numerical technique that can solve problems in which shocks and contact surfaces occur. In particular it is necessary to understand the details of developing a mesh that will allow resolution of these discontinuities. Results including contour plots of pressure, temperature, and Mach number will show that CFD is capable of predicting accurate results and is also able to capture the discontinuities in the flow, e.g., the oblique shock waves and the slip surfaces. Through this computational analysis, a better interpretation of the physical phenomenon of the three-dimensional shock waves interaction and reflection can be achieved.
Fibre Orientation Control Related to Papermaking
Authors: Carlsson A. , Lundell F. , So¨derberg L. D.
Published: September 2008

The wall effect on the orientation of fibres suspended in a shear flow has been studied experimentally. A fibre suspension, driven by gravity down an inclined glass plate, constitutes the shear flow field. A CCD-camera was mounted underneath the flow in order to visualize the flow. The orientation of fibres in the plane perpendicular to the plate was determined, by using the concept of steerable filters. In a region close to the smooth plate surface the fibres oriented themselves perpendicular to the flow direction. This did not occur when the surface structure was modified with ridges.
Turbulent Drag Reduction Effect by Hydrogen and Oxygen Microbubbles Made by Electrolysis
Authors: Lu Xinlin , Kato Hiroharu , Kawamura Takafumi
Published: September 2008

Turbulent drag reduction by very small hydrogen microbubbles was investigated experimentally. The method for generating microbubbles of 10–60 μm by water electrolysis was established firstly. Experiments were carried out using a circulating water tunnel, and it was observed that the small microbubbles generated by electrolysis can achieve the same drag reduction as the injected air bubbles at much lower void fraction. The distribution of microbubble was examined using the microscope photography. The peak of local void fraction was found to be very close to the wall, while no correlation was found between the average bubble diameter and the distance from the channel wall. The present experimental results suggest that the very small microbubbles produced by electrolysis are 10∼100 times more effective in terms of the drag reduction than large bubbles made by air injection. So it is considered that the diameters of microbubbles play an important role to drag reduction.
3-D Simulation of Turbulent Density
Authors: Hormozi S. , Firoozabadi B. , Ghasvari Jahromi H. , Afshin H.
Published: September 2008

Density current is a dense fluid, which is continuously released from a source and spreads down a sloping surface inside a lighter, motionless fluid. A low-Reynolds number k-ε model (Launder and Sharma, 1974) has been used to simulate the behavior of 3-D density currents. Density current with a uniform velocity and concentration enters the channel via a sluice gate into a lighter ambient fluid and moves forward down-slope. The model has been verified with the experimental data sets. Although the k-ε Launder and Sharma model is applied here to a conservative density current, it seems the analysis is valid in general for turbidity current laden with fine particles.
Two-Dimensional Imaging of Soot Volume Fraction in Pulsed Diffusion Flames by Laser-Induced Incandescence
Authors: Sapmaz H. , Ghenai C.
Published: September 2008

Laser-Induced Incandescence (LII) is used in this study to measure soot volume fractions in steady and flickering ethylene diffusion flames burning at atmospheric pressure. Better understanding of flickering flame behavior also promises to improve understanding of turbulent combustion systems. A very-high-speed solenoid valve is used to force the fuel flow rate with frequencies between 10 Hz and 200 Hz with the same mean fuel flow rate of steady flame. Periodic flame flickers are captured by two-dimensional phase-locked emission and LII images for eight phases (0°–360°) covering each period. LII spectra scan for minimizing C2 swan band emission and broadband molecular florescence, a calibration procedure using extinction measurements, and corrections for laser extinction and LII signal trapping are carried out towards developing reliable LII for quantitative applications. A comparison between the steady and pulsed flames results and the effect of the oscillation frequency on soot volume fraction for the pulsed flames are presented.
Analytical Solution of Turbulent Couette Flow by Cosserat Continuum Model and Gradient Theory
Authors: Ghasvari Jahromi H. , Atefi G. , Moosaie A. , Hormozi S. , Afshin H.
Published: September 2008

In present paper the theory of the micropolar fluid based on a Cosserat continuum model has been applied for analysis of Couette flow. The obtained results for the velocity field have been compared with known results from experiments done by Reichardt at Max Plank institute for fluids in Gottingen [1,2] and analytical solution of the problem from Gradient theory by Alizadeh [3]. The boundary condition used here was the no slip one and Trostel’s slip boundary condition [4]. A good agreement between experimental results and the results of the problem for Reynolds near 18000 has beeen found. A new dimensionless number introduced that indicates the theoretical relation between cosserat theory and slip theory and their interaction.
Numerical Modeling and Analysis of Drainage Patterns of Saltcake in a Column
Authors: Ye Chunmiao , Tachiev Georgio , Lin Chengxian
Published: September 2008

Saltcake, previously dissolved salts, such as sodium nitrate and sodium nitrite that crystallize out of solution, is a kind of porous media and an important component in many nuclear waste tanks. In this paper both one-dimensional single-phase and multi-phase flow model was employed to simulate unsaturated drainage of saltcake in a column caused by gravity or external forces under given initial and boundary conditions. Porous media properties used in the modeling was determined from our previous experimental data by inverse modeling method. Good agreement was found between the drainage result from available experiments and that from the numerical simulation using multiphase flow model. The impact on drainage by external pressure applying at the top of column and the influence of the height of column were also be investigated and presented in this study.
Large-Eddy Simulation of a Turbulent Diffusion Flame Laden With Evaporating Droplets
Authors: Xia Jun , Luo Kai H. , Kumar Suresh
Published: September 2008

To investigate the effect of water dilution on turbulent flames, large eddy simulations (LES) of a turbulent reactive jet laden with evaporating droplets are performed under various conditions. A hybrid Eulerian-Lagrangian formulation is adopted for the liquid-gas reactive system described by finite-rate chemistry. Fully dynamic subgrid-scale (SGS) models are developed for the filtered momentum, energy and chemical species equations while a scale similarity SGS model is used for the filtered reaction rate. The developed LES methodology is capable of capturing highly unsteady and localized features and is applied to studying the interaction among turbulence, combustion and evaporating droplets at various droplet size and mass loading ratio (MLR). It is found that smaller droplets can seamlessly follow the flow structures into the reaction zones while larger droplets are insensitive to flow fields, leading to different outcome of combustion suppression. With the same MLR, smaller droplets have a larger total surface area, which increases the evaporation rate and leads to more effective separation of the reaction zones from the oxidizer. For the same droplet size, increasing the MLR initially leads to stronger suppression of combustion. However, further increase in the MLR beyond a critical value would inhibit evaporation and thus limit its effect on combustion.
A Lattice Boltzmann Equation Method Without Parasitic Currents and Its Application in Droplet Coalescence
Authors: Lee Taehun , Fischer Paul F.
Published: September 2008

A formulation of the intermolecular force in the nonideal gas lattice Boltzmann equation (LBE) method is examined. Discretization errors in the computation of the intermolecular force cause parasitic currents. The parasitic currents can be eliminated to round-off if the potential form of the intermolecular force is used with compact isotropic discretization. Numerical tests confirm the elimination of the parasitic currents. In order to demonstrate the applicability of the present model, inertial coalescence of droplets at high density ratio is studied.
Application of CFD to Study Performance of Hydrofoil-Based Ship-Stabilization Systems
Authors: Mahalatkar Kartikeya , Litzler Jeff , Ghia Urmila , Ghia Karman , Ramchandani Arun
Published: September 2008

Computational Fluid Dynamics is used for simulating the flow over an oscillating hydrofoil found in a typical active-fin ship stabilization system. The hydrofoils oscillate through large angles of attack well above the static stall angle to generate large lift forces. Lift forces are calculated on the hydrofoil by using a simple sinusoidal motion at mean frequency of fin oscillation. The lift data is used in a linear system model created to simulate the active-fin ship-stabilization system. A Proportional Integral Derivative (PID) control system has also been developed for the purpose of controlling the fin motion. The simulation provides the typical motion experienced by a hydrofoil used in a ship stabilization system. This motion is fed back to a CFD solver to understand the effect of non-sinusoidal oscillation on Lift, Drag and Moment of the hydrofoil. The aerodynamics of the non-sinusoidally oscillating hydrofoil is analyzed so as to find the effect of pitch rate of the hydrofoil on the lift forces and hence on the performance of the ship stabilizer.
Flow Behavior in an Experimental Facility for Airspeed Metrology Application
Authors: Tolentino Eslava Rene´ , Tolentino Eslava G. , Sa´nchez Silva F. , Toledo Vela´zquez M.
Published: September 2008

An experimental characterization of flow behavior in an experimental facility for type S Pitot tube calibration by means of velocity profiles and turbulence is present. The measurements were carried out with a Pitot static tube and a hot wire anemometer at 4, 8 and 12 diameters from fan outlet for maximum velocities from 5.8 m/s to 35 m/s. The velocity and turbulence profiles were measurement at the discharge of the duct, in order to evaluate the possibility to use this installation for anemometer calibration too. Results showed that plane located at 12 D from fan outlet is adequate for establish the test section for type S Pitot tube calibration. Measurement in the exit of ducted outlet showed a good flow quality for anemometers calibration.
2D and 3D Unsteady Flow in Squirrel-Cage Centrifugal Fan and Aeroacoustic Behavior
Authors: Younsi M. , Bakir F. , Kouidri S. , Rey R.
Published: September 2008

The objective of this paper is the study and the analysis of the complex phenomena related to the internal flow in a centrifugal fan, using Computational Fluid Dynamics (CFD) tools, completed with experimental investigation in order to validate the used numerical models. The CFD analysis concerns 2D and 3D unsteady flow. The studied phenomena are the interactions and unsteadiness induced by the motion of the rotating blades relatively to the volute and their impact on the aeroacoustic behavior of the fan. Thus, 3D and 2D unsteady calculations using Unsteady Reynolds Averaged Navier Stokes (URANS) approach has been applied on a hybrid mesh grid whose refinement has been studied and adapted to the flow morphology. Turbulence has been modeled with the k-ω-Shear Stress Model (SST) model. The computational domain has been divided into two zones, a rotating zone including the impeller and stationary zone including the volute. A sliding mesh technique has been applied to the interfaces in order to allow the unsteady interactions between the two zones. The overall performances predicted by the computations have been validated at different flow rate. For each geometry modeling (2D and 3D), the unsteady part of the study is illustrated by analyzing the pressure fluctuations on different points from the lateral surface of the volute. The analysis of the wake generated by the rotation of the blower shows that the volute tongue is the main zone of unsteadiness and flow perturbations. In order to predict the acoustic pressures, the unsteady flow field variables provided by the CFD calculations have been used as inputs in the Ffowks Williams-Hawkings equations.
Two-Point Correlations of the Near and Far-Field Pressure in a Transonic Jet
Authors: Hall Joseph W. , Pinier Jeremy , Hall Andre´ , Glauser Mark
Published: September 2008

To better understand the relationship between the near-field pressure and the acoustic far-field, multi-point measurements of the near-field pressure around the periphery of a cold Mach 0.85 round jet are compared to simultaneous multi-point far-field acoustic pressure measurements. The results indicate that the near-field pressure is low dimensional and the instantaneous contribution from both azimuthal mode 0 and 1 is sufficient to accurately recover the dynamics of the near-field pressure. Correlations of the acoustic far-field with the contribution of each azimuthal mode to the near field pressure, however, indicate that only azimuthal mode 0 is well correlated with the far-field pressure, suggesting that the acoustic source in the jet is predominantly axisymmetric. The correlation of the higher azimuthal pressure modes with the far-field acoustic pressure is extremely poor suggesting that the axisymmetric source is weakened by the presence of higher azimuthal modes in the near field of the jet.
Spatial Velocity Correlations and Near Field Pressure Measurements From a Heated Transonic Axisymmetric Jet
Authors: Mann Richa , Ukeiley Lawrence , Seiner John
Published: September 2008

Turbulence properties and the near field pressure of a Mach 0.85 jet at two different static temperature ratios (0.87 and 2.4) are studied. The turbulence properties were studied using PIV measurements which were performed with the light sheet aligned in the radial and the streamwise directions at locations between 3.3 and 8.2 jet diameters downstream from the nozzle exit. Measurements of the near field pressure fluctuations were acquired using a linear array of 6 pressure transducers evenly spaced from 3.45 to 8.45 jet exit diameters. From the PIV measurements it was observed that the heated jet is narrower than the unheated jet and has a higher peak normalized TKE. The highly correlated area of the 2 point spatial velocity correlations is larger for the unheated jet than the heated jet. The measurements from the pressure transducers reveals that the relative amplitude of the mean fluctuating pressure is slightly higher for the heated jet after the end of the potential core. The convection speed of the waves propagating along the pressure sensor array was calculated to be larger for the heated jet.
Experimental Measurements of Wind-Resistant Support in a Circular Stack Design
Authors: Mitra Debojyoti , Chakraborty Ashish , Bhattacharya Sujata , Dey Subrata , Bhattacharya Sukritranjan
Published: September 2008

Higher capacities of power plants together with increased awareness on environmental considerations have led to taller height of stacks. It is seen that strong wind can result in falling of stacks. So aerodynamic considerations of stacks is very important in order to save the falling of stacks. One stack is not enough in industries and power sectors and two or three stacks are required for proper operation of the unit. It is very important to arrange the stacks in proper way to resist their downfall. The present experimental study concentrates on the mutual effect of three nearby stacks on each other at three different arrangements, viz. linear, side-by-side and triangular. The experiments find out the directions of resultant forces acting on the stacks in different configurations so that proper arrangement of supports can be made with respect to the wind directionality obtained from local meteorological data. The other conclusions that could be drawn from these experiments are: (a) the second stack in along wind linear arrangement experiences the most drag and hence is much more susceptible to falling down with high gusts, whereas the last stack experiences the least drag and hence less prone to the downfall. (b) In side-by-side arrangement, although the middle one experiences maximum drag, the lift forces on the right and left stacks are far greater and hence the resultant force on them are much more in comparison to the middle one. (c) In the triangular arrangement also, the lift force being so pronounced on the right and left corner stacks, the resultant force on them will be much higher in comparison to that on the front stack sitting at the apex of the triangle. Thus, this study is important in studying the effect of wind force on three stacks in different arrangements and is very helpful in placing the supports in proper places in order to avoid failing of stack-like structures due to wind.