bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of the Motion and Deformation of a Bubble Rising in a Viscoelastic Fluid
Authors: Pillapakkam Shriram , Singh Pushpendra , Blackmore Denis L. , Aubry Nadine
Published: September 2008

A finite element code based on the level set method is developed for performing two and three dimensional direct numerical simulations (DNS) of viscoelastic two-phase flow problems. The Oldroyd-B constitutive equation is used to model the viscoelastic liquid and both transient and steady state shapes of bubbles in viscoelastic buoyancy driven flows are studied. The influence of the governing dimensionless parameters, namely the Capillary number (Ca), the Deborah Number (De) and the polymer concentration parameter c, on the deformation of the bubble is also analyzed. Our simulations demonstrate that the rise velocity oscillates before reaching a steady value. The shape of the bubble, the magnitude of velocity overshoot and the amount of damping depend mainly on the parameter c and the bubble radius. Simulations also show that there is a critical bubble volume at which there is a sharp increase in the bubble terminal velocity as the increasing bubble volume increases, similar to the behavior observed in experiments. The structure of the wake of a bubble rising in a Newtonian fluid is strikingly different from that of a bubble rising in a viscoelastic fluid. In addition to the two recirculation zones at the equator of the bubble rising in a Newtonian fluid, two more recirculation zones exist in the wake of a bubble rising in viscoelastic fluids which influence the shape of a rising bubble. Interestingly, the direction of motion of the fluid a short distance below the trailing edge of a bubble rising in a viscoelastic fluid is in the opposite direction to the direction of the motion of the bubble, thus creating a “negative wake”. In this paper, the velocity field in the wake of the bubble, the effect of the parameters on the velocity field and their influence on the shape of the bubble are also investigated.
Swirl Flow Effects on Heat Release/Luminous Radiation in a Vortex Engine
Authors: Saidi M. H. , Ghafourian A. , Kargar M. , Faisal M.
Published: September 2008

Radiation heat transfer as an important phenomenon in combustion applications is an interesting subject for scientists and combustion researchers. Heat release and luminous radiative transfer phenomena in an experimental vortex engine are compared with a similar axial flow type engine. A detector sensitive to emission from C2* excited radically is utilized for the measurement of chemiluminescence emission at the centerline of chamber along all axial positions. The filtered photographs of flame are used to compare total C2* emission from flame. Mixtures of Propane and Butane with air enriched by oxygen are used as fuel and oxidizer. The effects of equivalence ratio and oxidizer mass flow rate are investigated as well.
Shape Optimization of Turbomachinery Blade Using Multiple Surrogate Models
Authors: Samad Abdus , Kim Kwang-Yong , Goel Tushar , Haftka Raphael T. , Shyy Wei
Published: September 2008

Performances of multiple surrogate models are evaluated in a turbomachinery blade shape optimization. The basic models, i.e., Response Surface Approximation, Kriging and Radial Basis Neural Network models as well as weighted average models are tested for shape optimization. Global data based errors for each surrogates are used to calculate the weights. These weights are multiplied with the respective surrogates to get the final weighted average models. Sequential Quadratic Programming is used to search the optimal point from these constructed surrogates. Use of multiple surrogates via weighted averaged surrogates gives more robust approximation than individual surrogates. Three design variables are selected to enhance the performance of transonic axial compressor (NASA rotor 37) blade and the design points are selected using three level fractional factorial D-optimal designs. The performance of compressor is improved by optimization because of reduction of losses and movement of separation line towards down stream directions. The present approach can help address the multi-objective design on a rational basis with quantifiable cost-benefit analysis.
Verification and Validation Studies for Laminar Hypersonic Flow Over a Blunted Cone and a Flat Plate
Authors: Gokaltun S. , Skudarnov P. V. , Lin C. X. , Thornburg Hugh
Published: September 2008

In this paper, verification and validation analysis for laminar hypersonic flow fields is presented. The simulations include a Mach 8 flow of calorically perfect gas over a spherically blunted cone and a Mach 14 flow over a flat plate. Numerical results were obtained using the finite volume method on structured grids. The verification of the numerical solutions was performed by calculating the Grid Convergence Index (GCI) for both test cases. A set of three different grids is used to calculate the discretization uncertainty, where each grid was generated by doubling the number of cells in each direction of the coarser grid. The value of GCI allows calculating the observed order of accuracy of the numerical method for local values of surface pressure at various points and the net drag force for the blunted cone case and for the local heat transfer coefficient for the flat plate case. The error band was observed to be 2.4% for the surface pressure in the blunted cone problem and 0.5% for the heat transfer coefficient in the flat plate problem. Finally the numerical results were validated with experimental data using the local surface pressure measurements for the hypersonic cone and the local heat transfer coefficient measurements for the hypersonic flat plate.
In Search of Random Uncorrelated Particle Motion (RUM) in a Simple Random Flow Field
Authors: Reeks Michael W. , Fabbro Luca , Soldati Alfredo
Published: September 2008

DNS studies of dispersed particle motion in isotropic homogeneous turbulence [1] have revealed the existence of a component of random uncorrelated motion (RUM) dependent on the particle inertia τp (normalised particle response time or Stoke number). This paper reports the presence of RUM in a simple linear random smoothly varying flow field of counter rotating vortices where the two-particle velocity correlation was measured as a function of spatial separation. Values of the correlation less than one for zero separation indicated the presence of RUM. In terms of Stokes number, the motion of the particles in one direction corresponds to either a heavily damped (τp < 0.25) or lightly damped (τp > 0.25) harmonic oscillator. In the lightly damped case the particles overshoot the stagnation lines of the flow and are projected from one vortex to another (the so-called sling-shot effect). It is shown that RUM occurs only when τp > 0.25, increasing monotonically with increasing Stokes number. Calculations of the particle pair separation distribution function show that equilibrium of the particle concentration field is never reached, the concentration at zero separation increasing monotonically with time. This is consistent with the calculated negative values of the average Liapounov exponent (finite compressibility) of the particle velocity field.
Characterizing the Onset of Gas Entrainment in a Single Discharging Bottom Branch Through Flow Visualization
Authors: Bowden R. C. , Hassan I. G.
Published: September 2008

The incipience of two-phase flow in discharging branches, from a stratified gas-liquid region, has major implications in industrial applications where safety is of primary concern. The transition from single phase liquid to a two phase gas-liquid mixture is termed the onset of gas entrainment, and may be formed by either vortex or vortex-free mechanisms. The two phase flow quality in the discharging branch is greatly impacted by vortex-free gas entrainment, as compared with gas entrainment due to vortex formation. Presented is a preliminary experimental investigation to demonstrate quantitative measurements of the liquid flow field at the onset of gas entrainment in a single discharging bottom branch. The three dimensional flow field is resolved into three horizontal planes and a single vertical plane. Results indicate a strong dependency of the flow field on the radial component, directed toward the branch center, and where a solid wall is present a larger tangential component.
Experimental Study of Swirling Flow of a Viscoelastic Fluid With Deformed Free Surface
Authors: Li Feng-Chen , Oishi Masamichi , Kawaguchi Yasuo , Oshima Nobuyuki , Oshima Marie
Published: September 2008

An experimental investigation was performed on the swirling flow of viscoelastic fluid with deformed free surface in a cylindrical container driven by the constantly rotating bottom wall. The tested fluid was an aqueous solution of CTAC (cetyltrimethyl ammonium chloride), which is a cationic surfactant. Water, 40ppm, 60ppm and 200ppm CTAC solution flows were tested at Froude numbers ranging from 2.59 to 16.3. PIV was used to measure the secondary velocity field in the meridional plane and the deformed free-surface level was extracted from the PIV images. At a similar Froude number, the depth of the dip formed at the center region of the free surface was decreased for CTAC solution flow compared with water flow. The inertia-driven vortex at the up-right corner in the meridional plane becomes more and more weakened with increase of the solution concentration or viscoelasticity. Through analyzing the overall force balance compared with water flow, the first normal stress difference or the weak viscoelasticity was estimated for the dilute CTAC solution flows.
Simulation of Hydraulic Pipeline Pressure Transients Accompanying Cavitation and Gas Bubbles Using Matlab/Simulink
Authors: Dan Jiang , Li Songjing
Published: September 2008

In order to predict pressure transients accompanying cavitation and gas bubbles in hydraulic pipeline operating at low pressure, a mathematical model and a simulation method are studied. The mathematical model is based on the two basic equations of motion and continuity. The growing and collapsing of cavitation and gas bubbles accompanying pressure pulsations are modelled to calculate the volumes of cavitation and gas bubbles. The pipeline dynamic friction model is introduced. Meanwhile, a simulation method, using finite difference method and Matlab/Simulink platform, is developed to handle the prediction of pressure transients. Finally an example of fluid transients inside hydraulic pipeline is simulated after a downstream valve is closed rapidly. Simulation results show that, for a certain example pipeline, the mathematical model can handle the prediction of pressure transients accompanying cavitation and gas bubbles in low pressure pipeline. The use of combining finite difference method with Matlab/Simulink platform provides a relatively simple and effective tool to understand the nature of pressure transients accompanying cavitation and gas bubbles.
Active Separation Control in Circular and Transitioning S-Duct Diffusers Using Vortex Generator Jets
Authors: Pradeep A. M. , Sullerey R. K.
Published: September 2008

Performance enhancement of three-dimensional S-duct diffusers by separation control using vortex generator jets is the objective of the current experimental investigation. Two different diffuser geometries namely, a circular diffuser and a rectangular–to–circular transitioning diffuser were studied in uniform inflow conditions at a Reynolds number of 7.8 × 105 and the performance evaluation of the diffusers was carried out in terms of static pressure improvement and quality (flow uniformity) of the exit flow. Detailed measurements that included total pressure, velocity distribution, surface static pressure, skin friction and boundary layer measurements were taken and these results are presented here in terms of static pressure rise, distortion coefficient and total pressure loss coefficient at the duct exit. The mass flow rate of the air injected through the VGJ was about 0.06 percent of the main flow for separation control. The distortion coefficient reduced by over 25 percent and the total pressure loss coefficient reduced by about 30 percent in both the diffusers. The physical mechanism of the flow control devices used has been explored using smoke visualization images.
Numerical Investigation of Entropy Generation in the Case of a Finned Oval Tube With a Punched Longitudinal Vortex Generator in Form of Delta Winglet
Authors: Herpe J. , Bougeard D. , Russeil S. , Baudoin B.
Published: September 2008

In order to improve the performance of compact heat exchangers engineers and researchers explore different passive techniques of flow manipulation. Among them one can find the delta wing shaped longitudinal vortex generator. In order to assess the optimal shape of finned tube heat exchangers, the engineer has at disposal many coefficients, such as the Colburn factor j and friction factor f. In the present paper the second law of thermodynamics is introduced to explore the flow and thermal field generated by punched longitudinal vortex generators shaped as winglets around an oval tube. The winglets are in common flow up configuration near the leading edge of the fin. Not only the heat transfer and fluid flow characteristics are studied, but also the local irreversibility methodology is applied to predict the two components of entropy generation rate: the one caused by direct dissipation and the other due to heat transfer. The flow velocity and temperature are numerically determined by solving the Navier-Stokes and energy equations with a finite volume method. The local entropy production is then calculated with the use of available information from the solved flow and thermal fields. This paper is based on Chen works. He has studied the flow characteristics for such a geometrical configuration. But here the fin efficiency is supposed to be equal to the unity. The influence of the angle of attack of winglets on the entropy production is studied. Three elemental configurations are displayed. Each one corresponds respectively to an angle of attack β equal to 20°, 30° and 45°. The minimal entropy principle is adopted to evaluate a global heat exchanger build up as a pile up of elemental component.
Detached Shock Waves Analysis
Authors: Alhussan Khaled
Published: September 2008

The work to be presented herein is a Computational Fluid Dynamics investigation of the complex fluid phenomena specifically with regard to the structure of detached shock waves over a moving body. This paper will show a relationship between the geometrical data and the strength and position of the detached shock waves, over moving body. The aim of this paper is to develop a relationship between the Mach number, the geometry and the strength of the detached shock wave. A study was completed on a different number of configurations. Results including contour plots of Mach number, static pressure, and static temperature showed the structure of 2-D detached shock waves in a complex region. A CFD analysis enables one to understand the complex flow structure of the detached shock waves over a body with different configurations. The results will show that for η>1 the detached shock will stand at farther distance than η<1 for the same geometrical shape. Through this computational analysis, a better interpretation of the physical phenomenon of the two dimensional detached shock waves can be achieved and to use this knowledge to achieve a design methodology that will benefit the industrial applications.
Numerical Simulations on the Aerodynamics Drag of a Tractor With a Tandem-Trailer
Authors: Sitlani M. , Aung K.
Published: September 2008

The aerodynamic drag characteristics of a heavy duty truck with two configurations, a tractor and a single trailer, and a tractor and a tandem-trailer (two trailers), have been studied. The aerodynamic drag of a truck depends on geometry, frontal area, and the speed of the truck. The basic geometry used in the simulation is 1:8 scale Ground Transportation System (GTS). The present simulation model has a simplified geometry of GTS with a cab-over engine design with either one or two trailers. In particular, the effects of the gap between the tractor and the trailer, and the gap between the tandem trailers on the aerodynamic drag were determined. The effects of vehicle geometry, vehicle speed, and the gap size were investigated and the drag coefficients were computed. CFD software STAR-CD with an expert tool, es-aero, was used for all the analyses reported in this paper. The simulation results were validated with available experimental data and good agreements were found for vehicle speeds at highway and city limits. The results showed that closing the gaps and incorporating boat-tails at the rear of the trailer could reduce the drag by as much as 40 percent.
Dynamic Analysis of Fluid Flowing Through Micro Porous Filters Using Bondgraph Approach
Authors: Malik M. Afzaal , Rashid Badar , Khushnood Shahab
Published: September 2008

Delivery of optimized fuel injection pressure to combustion chamber of an engine assembly leads to optimum torque and horsepower. Contaminant free supply of fuel without compromising on volume flow rate is the most important design requirement. Incorporation of very fine fuel filters having less than 10 micron rating reduces volume flow rate at the injection nozzles whereas fuel filter with larger pore size stabilize the injection pressure but may result into failure of fuel injection pump assembly due to scuffing produced by the fuel contaminant between the plunger and sleeve of hydraulic head of fuel injection pump. The fuel flows from fuel tank through low-pressure injection line, primary and secondary fuel filters, fuel transfer pump, fuel injection pump, and high-pressure injection line and injector nozzles. Modeling and simulation of volume flow rate vis-a`-vis fuel injection pressure together with micro-porous fuel filter poses a formidable challenge. Bondgraph method (BGM) is ideally suited for the modeling and simulation of such a multi-domain dynamic system. The aim of this research is to apply BGM to model and simulate the optimized fuel injection pressure and analysis of filters with different micro-porosity and their effect on volume flow rate. Fuel filter porosity, inlet and outlet pressures of transfer pump, fuel injection pump and low/high pressure injection line pressure have been determined experimentally. These experimentally determined parameters are then used as input in our Bondgraph model for the dynamic analysis of fuel injection pressure incorporating micro-porous filters.
Numerical Simulation and Experiments of the Multiphase Flow in a Liquid-Liquid Cylindrical Cyclone Separator
Authors: Reyes Miguel A. , Pacheco Jorge E. , Mari´n Juan C. , Rojas Luis R. , Rinco´n Jose´
Published: September 2008

A Liquid-Liquid Cylindrical Cyclone separator (LLCC) is a device used in the petroleum industry to separate the oil-water mixture obtained from the well. The use of this device has not been widespread due to the lack of tools for predicting its separation capability. This paper presents a numerical and experimental study of the fluid dynamic performance of this type of cylindrical cyclone separators. The use of numerical simulations would reduce the time and cost necessary to obtain information for predicting the behavior of the equipment. The objective of this study is to determine if CFD (Computational Fluid Dynamics) techniques are able to reproduce the behavior of a LLCC separator. The CFD software examined was ANSYS-CFX 5.6™ and numerical simulations were carried out using the dispersed model with oil as the dispersed phase. The oil and water mixture entering the separator is divided due to centrifugal and buoyancy forces in an upper (oil rich) exit and a bottom (water rich) exit. The separation capability is determined as the maximum amount of water removed from the mixture with the minimum amount of oil content in the water rich exit. The experiments were conducted in a transparent LLCC separator that allows the visualization of the mixture and the measurement of the oil content. Experiments were conducted for three variables: mixture velocity and water content at the entrance, and the split ratio. The split ratio is defined as the bottom exit flow rate divided by the water flow rate at the entrance. The results showed that CFD tools are able to reproduce the oil content obtained from the experiments for all analyzed conditions. Additionally, the mixture distribution images from numerical and experimental data showed good agreement. This study confirms the capacity of CFD tools for the multiphase flow analysis of LLCC separators.
A Numerical Analysis on the Flow Characteristics in a Helical Screw Agitator With a Draught Tube
Authors: Kim Youn-Jea , Hwang Jung-Hoon
Published: September 2008

Mechanical agitators are widely used in several industries which can mix one or more reactants, or mix some of the high viscous fluids. In general, simple impeller type is applied to mix low viscous fluids, while a helical screw agitator (HSA) is applied to the high viscous fluids. The study of flow characteristics inside the agitator is important because the mixing efficiency is greatly affected by the existence of draught tube. The HSA with a draught tube is known more efficient than the others. In this study, three-dimensional model consisting of outer vessel, helical screw and draught tube are considered. The draught tube in the vessel disturbs radial flow, hence it makes intermixing between the screw and tube. Also, the pitch of screw is capable of influencing the flow pattern. Consequently, the shapes of vessel, helical screw and draught tube are the main parameters for improving the performance of HSA. Numerical analyses were carried out with different geometrical configurations. Using a commercial CFD code, Fluent, velocity and pressure distributions inside of the HSA are obtained under steady, laminar flow and no-slip condition. Results are graphically depicted with several parameters.
Structure of Turbulence Within a Sheared Wake of a Rotor Blade
Authors: Soranna Francesco , Chow Yi-Chih , Uzol Oguz , Katz Joseph
Published: September 2008

Stereoscopic PIV measurements examine the flow structure and turbulence within a rotor near wake located within a non-uniform field generated by a row of Inlet Guide Vanes (IGVs). The experiments are performed in a refractive index matched facility that provides unobstructed view of the entire flow field. The data are acquired at 10 closely spaced radial planes located near mid-span, enabling measurements of all the components of the phase averaged velocity and strain rate, as well as the Reynolds stress and the triple correlation tensors. The rotor wake is sheared and bent towards the pressure (inner) side by a non-uniform flow field generated by IGV wake segments that propagate along the suction and pressure sides of the rotor passage with different speeds. The axial velocity fluctuations increase along the suction/outer side of the wake, while the other components decay. On the pressure/inner part of the bent wake the circumferential velocity fluctuations are higher. The Reynolds shear stress has a complex distribution, but is higher on the suction side. The turbulent kinetic energy is also consistently higher on the outer (suction) side of the wake. This trend is fundamentally different from those observed in prior studies of curved wakes where turbulence is enhanced on the inner side of the wake due to the destabilizing effect of curvature. To explain the difference, we examine the contributors to turbulent kinetic energy production rate in a curvilinear coordinate system aligned with the wake-centerline. The contribution of streamwise curvature to the production rate of turbulent kinetic energy, although consistent with expected trends, is overwhelmed by effects of wake shearing. The primary contributor to turbulent kinetic energy production rate is the product of Reynolds shear stress with cross-stream gradients of streamwise (in a frame of reference relative to the rotor blade) velocity in the wake. The location of peak in turbulent kinetic energy is almost aligned with that of production rate. The turbulence diffusion term opposes the production rate peaks, but also has high values along the edge of the wake.
Interaction and Reflection of Shock Waves in Three-Dimensional Turbulent Flow
Authors: Alhussan Khaled
Published: September 2008

In this paper some characteristics of non-steady, compressible, flow are explored, including compression and expansion waves creation reflection and interaction shock waves. The work to be presented herein is a Computational Fluid Dynamics investigation of the complex fluid phenomena that occur inside 2-D and 3-D regions, 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 reflections and interactions. The flow is non-steady, viscous, compressible, and high-speed supersonic. This paper will show a relationship between the Mach numbers and the angles of the reflected shock waves, over a double step, opposed equal wedges, and opposed unequal wedges. The aim of this paper is to study the characteristics of the flow inside 2-D and 3-D regions where creation, reflection and interaction of multiple shock waves.
Characteristics of Non-Steady Pressure Exchange Ejectors
Authors: Alhussan Khaled , Garris Charles
Published: September 2008

The work to be presented herein is a Computational Fluid Dynamics investigation of the complex fluid mechanisms that occur inside a non-steady, three-dimensional, supersonic pressure exchange ejector, specifically with regard to the pressure exchange mechanisms and the induction processes between a driving “primary” fluid and a driven “secondary” fluid and how this is related to the pressure exchange zone. This paper is based upon utilizing a non-steady flow field resulting in the work of pressure forces acting at a fluid interface between primary and secondary flows. These interfaces are produced through the aerodynamic design of a flow field, non-steady in the laboratory frame of reference, consisting of rotating oblique shock waves and expansion fans. Minimizing the possible losses from the oblique shocks and boundary layers offer the potential of achieving adiabatic efficiencies approaching those of turbo-machines. The results will show the length of pressure exchange zone. Results will show some characteristics of flow induction mechanism, especially when a pressure recovery is needed.
Direct Numerical Simulation of Homogeneous Turbulent Shear Flow Containing Bubbles
Authors: Kawamura T. , Nakatani T.
Published: September 2008

Direct numerical simulations of homogeneous shear turbulent flows containing deformable bubbles were carried out for clarifying the mechanism of drag reduction by microbubbles. The results show that presence of bubbles can suppress or enhance the development of turbulence depending on condition. The dissipation rate of turbulent kinetic energy is always increased by bubbles, while the production rate can be either increased or decreased depending on the turbulent and shear Reynolds numbers. As a result, the growth rate of turbulent kinetic energy can be either increased or decreased by bubbles depending on conditions. It was shown that the production rate tends to decrease at smaller shear Reynolds number, at larger turbulent Reynolds number, and at larger Weber number. Based on the results, a hypothetical model to explain the dependency on the Reynolds numbers has been proposed.
Deformation of a Droplet in a Uniform Electric Field
Authors: Singh Pushpendra , Aubry Nadine
Published: September 2008

Fluid in a micro-device can be transported either as a continuous stream in a channel or in the form of droplets. The latter holds great promise because of the possibility to move, split and fuse droplets for applications ranging from biochemical assays to drug delivery. In this paper, we consider the deformation of a liquid drop immersed in a surrounding fluid under the application of a uniform electric field. Specifically, we present the first direct numerical simulation of a droplet subjected to both hydrodynamic (viscous and capillary) and electrostatic forces. Our technique is based on a finite element scheme in which the droplet and its surrounding fluid are moved and deformed using the fundamental equations of motion. The interface is tracked by the level set method and the electrostatic forces are computed using the Maxwell stress tensor. Applying our method to a droplet subjected to a uniform electric field, we show how the drop deforms under the action of non-uniform stresses on its surface before eventually rupturing. A good agreement with previous analytical results is found for small drop deformations and a small dielectric mismatch between the drop and the ambient fluid. When these two conditions are relaxed, however, the discrepancy can be non-negligible.
Direct Simulation of the Migration of Leukocytes in Pressure Driven Flow
Authors: Jin Quan , Verdier Claude , Singh Pushpendra , Aubry Nadine , Duperray Alain
Published: September 2008

We use the direct numerical simulation (DNS) approach to study the motion and deformation of leukocytes in pressure driven flows in a parallel plate channel in the case where there is an adhesion force between the leukocytes and the channel wall and when the adhesion force is absent. Two composite fluid models, consisting of a membrane, cytoplasm and a nucleus, are used to describe leukocytes. The first is the composite-drop model in which the cytoplasm and the nucleus are modeled as fluids, and the second is the drop-rigid-particle model in which the cytoplasm is modeled as a fluid and the nucleus as a rigid particle. The cytoplasm is modeled as a Newtonian fluid. The nucleus in the first model is assumed to be a viscoelastic liquid. The adhesion force is computed using two adhesion force models. In the first model, the adhesion force is given by a potential that varies as the fourth power of the distance between the cell and the adhesive wall. In the second model, the adhesion force is given by the Dembo’s kinetic adhesion model. The numerical code is based on the finite element method and the level-set method is used to track the cell membrane position. In the absence of the adhesion force, the equilibrium location of a freely suspended leukocyte in a pressure driven flow in a channel is shown to depend on the ratio of the cell to plasma viscosities. In presence of the adhesion force, the leukocyte is attracted to the layer of endothelial cells and, as it gets closer, it also deforms to get flatter under the shear forces. This deformation, in turn, further increases the adhesion force.
Analysis and Optimization of Turbulent Mixing With Large Eddy Simulation
Authors: Huai Ying , Sadiki Amsini
Published: September 2008

In this work, Large Eddy Simulation (LES) has been carried out to analyze the turbulent mixing processes in an impinging jet configuration. To characterize and quantify turbulent mixing processes, in terms of scalar structures and degree of mixing, three parameters have been basically introduced. They are “mixedness parameter”, which represents the probability of mixed fluids in computational domain, the Spatial Mixing Deficiency (SMD) and the Temporal Mixing Deficiency (TMD) parameters for characterizing the mixing at different scalar scale degrees. With help of these parameters, a general mixing optimization procedure has then been suggested and achieved in an impinging jet configuration. An optimal jet angle was estimated and the overall mixing degree with this jet angle reached around six times more than the original design. It turns out that the proposed idea and methodology can be helpful for practical engineering design processes.
Icing Analysis of Fan Rotor at Part Load Conditions
Authors: Das K. , Hamed A. , Basu D.
Published: September 2008

Numerical simulations have been performed to investigate the supercooled water droplet trajectories and ice accretion in a fan rotor blade under part load conditions. An Eulerian-Lagrangian approach is used in formulating the flow and droplet governing equations in the rotating reference frame. A one-way interaction model is used to model the effects of momentum and energy exchange effects with the flow on the droplets as they travel through the rotor. Simulation results are presented for the flow field at 60%, 70%, 80%, 90%, and 100% design rotational speeds. Results showing the droplet trajectory paths and collection efficiency contours at part load conditions highlight the influence of engine speed and blade geometry. A heat transfer analysis shows the effect of speed on droplet temperature rise. Based on the impingement statistics and computed flow characteristics, a quasi-3D analysis of the ice accretion over the rotor blade is also conducted using the code LEWICE. Results are presented for the ice shape variation along the span and rotor speeds. It was found that the accreted ice mass and shape is highly dependent on rotor speed and local flow velocity.
Investigation of Modeling Approaches for Scalar Transport and Mixing in LES
Authors: Sun Olivia S. , Su Lester K. , Burton Tristan M.
Published: September 2008

LES models for subgrid scalar transport are assessed through large eddy simulation of passive scalar mixing in a turbulent round jet. The simulation is performed on a spherical coordinate grid using semi-implicit time advancement. The models used to close the subgrid scalar flux term are varied while the same modeling approach is used for the momentum model in order to isolate the effects of the scalar modeling. The subgrid scalar flux models tested include an eddy diffusivity model and a one-parameter mixed model. Model performance is analyzed by examining mean resolved-scale quantities and PDFs of the scalar concentration field.
Meshless Local Petrov-Galerkin (MLPG) Method for Incompressible Viscous Fluid Flows
Authors: Haji Mohammadi Mohammad
Published: September 2008

In this paper, the truly Meshless Local Petrov-Galerkin (MLPG) method is extended for computation of unsteady incompressible flows, governed by the Navier–Stokes equations (NSE), in vorticity-stream function formulation. The present method is a truly meshless method based only on a number of randomly located nodes. The formulation is based on two equations including stream function Poisson equation and vorticity advection-dispersion-reaction eq. (ADRE). The meshless method is based on a local weighted residual method with the Heaviside step function and quartic spline as the test functions respectively over a local subdomain. Moving Least Square approximation (MLS) is employed in shape function construction for approximation of a gauss point. Due to dissatisfaction of kronecker delta property in MLS approximation, the penalty method is employed to enforce the essential boundary conditions. In order to overcome instability and numerical errors encountering in convection dominant flows, a new upwinding scheme is used to stabilize the convection operator in the streamline direction (as is done in SUPG). In this upwinding technic, instead of moving subdomains the weight function is shifted in the direction of flow. The efficiency, accuracy and robustness are demonstrated by some test problems, including the standard driven cavity together with the driven cavity flow in an L shaped cavity and flow in a Z shaped channel. The comparison of computational results shows that the developed method is capable of accurate resolution of flow fields in complex geometries.
Comparison of 2-D Turbulent Particle Laden Density Current and Wall Jets
Authors: Hormozi S. , Firoozabadi B. , Ghasvari Jahromi H. , Afshin H.
Published: September 2008

Dense underflows are continuous currents, which move down the slope due to the fact that, their density are heavier than ambient water. In turbidity currents the density differences arises from suspended solids. Vicinity of the wall make density currents and wall jets similar in some sense but Variation of density cause this flows more complex than wall jets. An improved form of ‘near-wall’ k-ε turbulence model is chosen which preserve all characteristics of both density and wall jet currents and a compression is made between them. Then the outcomes from low Reynolds number k-ε model is compared with v2–f model which show similarity. Also results show good agreement with experimental data.
Prediction of Vortex Shedding From a High Reynolds Number Airfoil Using LES With and Without Wall Model
Authors: Chang Peter A. , Wang Meng , Gershfeld Jonathan
Published: September 2008

ATTACHED, wall-bounded flows impose computational requirements on LES that increase drastically with Reynolds number. For that reason, even simple geometries, such as airfoils at small angles of attack, with spanwise uniform section shape, challenge the bounds of LES as chord-based Reynolds numbers increase much above 1 million. Of particular concern is the ability of LES to predict the occurrence, and strength of, weak vortex shedding from the airfoil trailing edge (by weak vortex shedding we mean that the acoustic vortex shedding signature may rise only a few decibels above that for the broadband turbulent boundary layer acoustic sources). Correct prediction of weak vortex shedding may depend on accurately predicting the flow over the entire airfoil that includes the attached, turbulent upstream flow, adverse pressure gradient and separated flow regions and finally, the turbulent wake. This paper compares results of two full-LES and two LES with wall-stress model for the flow about a modified NACA 0016 airfoil with a 41° trailing edge apex angle and a slightly convex pressure side. Comparisons of vortex shedding, as measured by the power spectral density (PSD) of wall pressure fluctuations (WPF) on the pressure side of the TE and the PSD of the vertical velocity fluctuations in the wake are made. The results indicate that vortex shedding predictions are dependent upon the stream-wise and spanwise grid resolution. In order to reduce the large computational times required for simulating the high-Reynolds number flows with fully-resolved LES, a wall-stress model that solves the turbulent boundary layer equations in the near-wall region is applied. Compared with the fully-resolved LES, the LES with wall-stress simulations require about 20 percent the number of grid points and require about 10 percent of the computational time. However, the LES with wall stress model results under-predict the vortex shedding peak in the wake and are not able to predict the vortex shedding signature in TE wall pressure spectra. These results indicate that near-wall turbulence structures need to be resolved in order to correctly predict the occurence and strength of vortex shedding.
Spray Size Evolution of Diesel Sprays
Authors: Jawad Badih A. , Riedel Chris
Published: September 2008

Variations of fuel spray characteristics during cyclic operation is directly correlated to efficient operation of an engine. Measurements of drop size distribution and drop concentration under transient conditions during injection are difficult. A very rapid and synchronized measurement technique is necessary for sectional and temporal analysis of an intermittent spray during approximately 50 ms after each injection. A pulsed-spray sizer based on Fraunhofer diffraction pattern analysis was modified so that the repeatability of each injection can be determined by an obscuration-trace measurement. The sizer with a built-in adjustable delay was synchronized to the needle-lift of the injector and the drop size data were captured over 20 microseconds. For every single injection, the attenuation of the transmitted beam monitoring the arrival, the duration, and the drop concentration in the spray was recorded and stored on a digital scope. A solenoid controlled the position of the rack on the fuel line and ensured single injection. The obscuration-trace was used as a “fingerprint” in testing the reproducibility of successive injections. Drop size measurements were statistically averaged over many reproducible cycles.
Large Eddy Simulation of Compressible Flows Using a Non-Body Conformal Grid Method
Authors: Wang Wen , Pletcher Richard H.
Published: September 2008

Large eddy simulations of turbulent flow around complex geometries are presented. A non-body conformal grid method is developed and combined into the compressible Navier-Stokes solver. Time derivative preconditioning is used to deal with the low Mach number situation. A second-order accurate finite volume formulation is chosen and the implicit LU-SGS scheme is implemented. A localized dynamic subgrid-scale (SGS) model is used to evaluate the subgrid-scale stresses and heat fluxes. A second-order bilinear (for 2D situation) or trilinear (for 3D situation) interpolation method is employed to satisfy the no-slip boundary condition on the immersed boundary. The method is validated for flow past a circular cylinder with different Reynolds numbers. Large eddy simulation of pipe flow with high Reynolds numbers using this non-body conformal grid method is conducted and the turbulent statistics are consistent with body-fitted grid method results. Furthermore, the LES simulation results with non-body conformal grids for channel flow containing a rod bundle are presented.
Composite Solution Procedure for 3-D Flow Simulations on a Multi-Box Grid
Authors: Chen Larry , Ghia Urmila
Published: September 2008

Fluid flows in biological systems are typically complex, due to factors such as non-Newtonian behavior of biochemical fluids and complex geometry, as well as the interaction of muscles and fluid. With the advent of modern computational technology, these problems are gradually resolved. The present research illustrates two such examples. Grid generation is essential for conducting numerical simulation of fluid flow. In the present research, a new grid generation technique is developed and implemented into a flow solver. This technique enables one to create a grid for complex geometry using only a single computational zone. The flow field can therefore be analyzed without iteration between zones. The numerical scheme developed for solving the grid generation equations is an extension of the traditional three-dimensional Douglass-Gunn Alternating-Direction Implicit (ADI) scheme. A unique feature of the demonstrated grid generation scheme is the concept of multi-box computational domains. In this scheme, the physical domain is mapped onto a multi-box geometry in the computational space, rather than a single box as the traditional methods do. Therefore, the numerical scheme is adjusted accordingly. Flow simulations were performed using the software INS3D, which employs the method of artificial compressibility. This method transforms the Navier-Stokes equations into a system of hyperbolic-parabolic equations, and then marches along the pseudo-time axis until the velocity field becomes divergence-free. Two biological flow problems were analyzed using the aforementioned method. The flow field in an arterial graft as well as in the Left atrium (LA) of the human heart was studied. The effect of Reynolds number and flow-division ratio is examined in the graft problem. The Reynolds number effect is demonstrated via the presence of a helical flow structure and the overall pressure drop. The flow-division ratio alters the flow field in a way that moves the stagnation points. The simulated flow field closely resembles that observed clinically. The steady-state simulation of the flow field in the left atrium of the human heart provided information about the long-term performance of the heart chamber. The simulation demonstrates the existence of low wall shear region, which is therefore susceptible to blood clot formation. This observation also agrees with the clinical findings. In summary, the present research demonstrates application of CFD techniques in the analysis of flow in a biological system. A new grid generation technique is realized, and proved to be useful in simulating these flows. The flow simulation results provide insights into the system, and may be useful for clinical reference.
Effect of Frequency and Electrode Configuration on Yeast Cells Subjected to Traveling Electric Fields
Authors: Nudurupati Sai Chaitanya , Singh Pushpendra , Aubry Nadine
Published: September 2008

There is great interest in trapping and manipulating small sized particles such as biological, glass, polymer and carbonaceous particles suspended in a liquid. One way to trap such micro/nano sized particles is by means of a microfluidic chamber equipped with electrodes at the bottom and thus generating conventional dielectrophoresis based on an electric field of spatially varying magnitude. In this work, we explore the use of traveling wave dielectrophoresis induced by an electric field of spatially varying phase, which offers both particle capturing/separation and transport capabilities (without having to pump the fluid itself). Particles are subjected to electrostatic and hydrodynamic forces and torques that are computed solving the full equations of motion for both the fluid and the particles without any modeling (from first principles) and using a finite element scheme based on the Distributed Lagrange Multiplier (DLM) method. We consider two typical microfluidic channels (MEMS devices) with electrodes embedded in the bottom wall. It is found that the motion and destination of the particles strongly depend on the frequency dependent complex Clausius-Mossotti factor (the mismatch between the particles and fluid electric properties), and that the hydrodynamic and electrostatic particle-particle interactions play a crucial role on the particles dynamics. These conclusions are demonstrated on model particles having the properties of yeast cells.
Using Chaos for Fluid Mixing in Pulsed Micro Flows
Authors: Goullet Arnaud , Aubry Nadine
Published: September 2008

Even though mixing is crucial in many microfluidic applications where biological and chemical reactions are needed, efficient mixing remains a challenge since the Reynolds number of these flows is typically low, thus excluding turbulence as a potential mechanism for stirring. While various approaches relying on clever geometries, cross-flows, miniature stirrers or external fields have been used in the past, our work has focused on generating stirring in microchannels of simple geometry by merely pulsing flow rates at the inlets through which the two fluids are brought into the device. Flow visualizations from experiments, as well as numerical simulations, have indicated that the majority of the mixing takes place in the confluence region. Even though it has been shown in previous work that good mixing can be achieved at relatively large scales using this technique, one of the challenges is to make sure that mixing occurs at small scales (i.e., particle scales) as well. To address this issue, we carefully study the dynamics of tracer particles using both computational fluid dynamics and dynamical systems theory, and explore the parameter space in terms of the Reynolds number, Strouhal number and phase difference between the two inlet flows. Specifically, we generate a bifurcation diagram in which both regular and chaotic dynamics occur. As expected, the chaotic regime exhibits stretching and folding of material lines at all (large and small) scales, and is thus promising as an effective mixing tool.