bluuebunny/crossref_metadata_2025 huggingface dataset:
Parameter Optimization of a Micro Heat Sink With Circular Pin-Fins
Authors: Koz Mustafa , Kosar Ali
Published: March 2011

Micro heat sinks have a broad applicability in many fields such as aerospace applications, micro turbine cooling, micro reactors electronics cooling and micro biological applications. Among different types of micro heat sinks, those with micro pin-fins are becoming popular due to their enhanced heat removal performance. However, relevant experimental data is still scarce and few optimization studies are present in the literature. In order to effectively optimize their performance an extensive parametric study is necessary and should be based on a realistic model. Moreover, micro pin fin heat sinks should be optimized according to appropriate performance criteria depending on the application. The objective of this paper is to fill the research gap in micro pin fin heat sink optimization based on realistic configurations. In this paper, the parameters for micro pin optimization are the pin-fin height over diameter ratio (0.5<H/D<5) and the longitudinal and transverse pitch ratios (1.5<(SL, ST)/D<5), while Reynolds number and heat flux provided from the base of the micro heat sink are in the range of (1<Re<100) and (20<q(W/cm2)<500) respectively. In this research micro pin fin heat sinks are three dimensionally modeled on a one-to-one scale with the use of commercially available software COMSOL Multiphasics 3.5a. Full Navier-Stokes equations subjected to continuity and energy equations are solved for stationary conditions. To have increased computational efficiency, half of the heat sink is modeled with the use of a symmetry plane. In order to validate the use of numerical models parametric values from previous experimental data available in the literature are exactly taken and simulated. The numerical and experimental results show a good agreement. After this validation optimization study is performed using the three dimensional numerical models.
Active Flow Control of the 3D Separation on a Ahmed Body Using Steady Microjet Arrays
Authors: Aubrun S. , Alvi F. , Leroy A. , Kourta A.
Published: March 2011

A model of a generic vehicle shape, the Ahmed body with a slant angle of 25°, is equipped with an array of blowing steady microjets 6mm downstream of the separation line between the roof and the slanted rear window. The goal of the present study is to evaluate the effectiveness of this actuation method in reducing the aerodynamic drag, by reducing or suppressing the 3D closed separation bubble located on the slanted surface. The efficiency of this control approach is quantified with the help of aerodynamic load measurements. The changes in the flow field when control is applied are examined using PIV measurements and skin friction visualizations. By activating the steady microjet array, the drag coefficient was reduced by 9 to 11%, depending on the Reynolds number. The modification of the flow topology under progressive flow control is particularly studied.
CFD Simulation of Proposed Validation Data for a Flow Problem Reconfigured to Eliminate an Undesirable Flow Instability
Authors: Johnson Richard W. , McIlroy Hugh M.
Published: March 2011

The U. S. Department of Energy (DOE) is supporting the development of a next generation nuclear plant (NGNP), which will be based on a very high temperature reactor (VHTR) design. The VHTR is a single-phase helium-cooled reactor wherein the helium will be heated initially to 750 °C and later to temperatures approaching 1000 °C. The high temperatures are desired to increase reactor efficiency and to provide a heat source for the manufacture of hydrogen and other applications. While computational fluid dynamics (CFD) has not been used in the past to design or license nuclear reactors in the U. S., it is expected that CFD will be used in the design and safety analysis of forthcoming designs. This is partly because of the maturity of CFD and partly because detailed information is desired of the flow and heat transfer inside the reactor to avoid hot spots and other conditions that might compromise reactor safety. Numerical computations of turbulent flow should be validated against experimental data for flow conditions that contain some or all of the physics expected in the thermal fluid machinery of interest. To this end, a scaled model of a narrow slice of the lower plenum of the prismatic VHTR was constructed and installed in the Idaho National Laboratory’s (INL) matched index of refraction (MIR) test facility and data were taken. The data were then studied and compared to CFD calculations to help determine their suitability for validation data. One of the main findings was that the inlet data, which were measured and controlled by calibrated mass flow rotameters and were also measured using detailed stereo particle image velocimetry (PIV) showed considerable discrepancies in mass flow rate between the two methods. The other finding was that a randomly unstable recirculation zone occurs in the flow. This instability has a very significant effect on the flow field in the vicinity of the inlet jets. Because its time scale is long and because it is apparently a random instability, it was deemed undesirable for a validation data set. It was predicted using CFD that by eliminating the first of the four jets, the recirculation zone could be stabilized. The present paper reports detailed results for the three-jet case with comparisons to the four-jet data inasmuch as three-jet data are still unavailable. Hence, the present simulations are true or blind predictions.
Analytical Velocity and Temperature Distributions for Flow in Microchannels of Various Cross-Sections
Authors: Shahsavari S. , Kjeang E. , Bahrami M.
Published: March 2011

Analytical solutions are presented for velocity and temperature distributions of laminar fully developed flow of Newtonian, constant property fluids in micro/minichannels for a wide variety of cross-sections. The considered geometries include hyper-elliptical channels and regular polygon ducts, which covers several common shapes. The analysis is carried out under the conditions of constant axial wall heat flux with uniform peripheral heat flux at a given cross section. The boundary conditions are applied using a linear least-squares point matching technique to minimize the residual between the actual and the modeled values on the boundary of the channel. Hydrodynamic and thermal characteristics of the flow are derived; these include pressure drop and local and average Nusselt numbers. The proposed results are successfully verified with existing analytical solutions from literature for a variety of channel cross-sections. The present study provides analytical-based compact solutions for velocity and temperature fields that are essential for basic designs, parametric studies, and optimization analyses required for many thermofluidic applications.
Numerical Investigation of the Effect of Geometric and Physiochemical Parameters on Biomolecule Capture Efficiency
Authors: Jomeh Sina , Hoorfar Mina
Published: March 2011

This paper presents and compares three different designs including open channel, circular pillar and screen-plate microreactors for capturing and detection of biomolecules in a buffer liquid. In general, these capturing/detection devices consist of a flow cell containing one or several reactive surfaces loaded with ligand molecules. The critical issue in the design of an efficient device is the proximity of the biomolecules to the ligands in the capturing stage since the latter is immobilized on the reactive surface and the former is freely moving in the flow. The flow pattern and the geometry of the device are the key factors in this regard. The presented designs are numerically modeled and compared in terms of capture efficiency. Immersed biomolecules are assumed to behave like a continuum medium. The Navier-Stokes and advection-diffusion equations are solved in two dimensions and the concentration profile is found after a certain sampling period. The chemical reaction between the ligand and the biomolecule is included in the model through solving the first order kinetic equation at the boundaries. The average surface concentrations of the adsorbed molecules are plotted and compared for all the geometries to determine the most efficient one. Considering the performance, ease of fabrication, and detection, the screen plates are found to be the best option for the purpose of biomolecule removal. The effects of the change in the geometric parameters (e.g., the flow path width in the microchannels) and physicochemical parameters (e.g., the diffusion constant, ligand surface density, and forward and backward reaction rates) involved in the problem on the adsorbed concentration are thoroughly inspected and the corresponding results are plotted.
A Study on Flow Through a Periodic Array of Porous Medium Cylinders by Immersed-Boundary Methods
Authors: Yang Xiaofan , Zheng Zhongquan Charlie , Xu Ying
Published: March 2011

Numerical simulations with an immersed-boundary method are presented for the incompressible flow past a periodic array of porous-medium cylinders. Fluid/porous-medium interactions are greatly influenced by the accuracy on the interface between the surface of the porous cylinder and the flow around it, because of the sudden change in the governing equations for the fluid and for the porous material. In order to retain the smoothness on the interface, momentum fluxes near the interface are discretized using several schemes, including the 2nd- and 3rd-order upwind schemes and the 5th-order Weighted Essentially Non-Oscillatory (WENO) scheme. These schemes are combined with a direct-forcing immersed-boundary method to remove the discontinuity between the fluid and the porous material, and thus accuracy near the interface can be improved. Low and moderate Reynolds number flows, both outside and inside the porous cylinders, are computed simultaneously by solving a combined governing equation set for incompressible flow. The simulation is first validated using flow over an array of impermeable cylinders. The advantage of high-order schemes is then investigated by looking at the flow parameters near the interfaces between the porous cylinders and the outside flow. Species transport in flow with the porous-cylinder-array configuration is also studied.
Capillary Instability of Liquid Jets Issuing From Elliptic Nozzles
Authors: Amini Ghobad , Dolatabadi Ali
Published: March 2011

Breakup of a liquid jet issuing from an orifice is one of the classical problems in fluid dynamics due to its theoretical and practical importance. The main application of the process is in spray and droplet formation, which is of interest in the combustion in liquid-fuelled engines, ink-jet printers, coating systems, medical equipment, and irrigation device. The complexity of the breakup mechanism is due to the large number of parameters involved such as the design of injection nozzle, and thermodynamic states of both liquid and gas. In addition, different combinations of surface tension, inertia, and aerodynamic forces acting on the jet, define main breakup regimes. Effects of nozzle geometry on the behavior of liquid jets have been overlooked in the literature. Elliptic jets have never been investigated theoretically since mostly circular jets or liquid sheets have been analyzed; while experiments have shown that by using elliptical nozzles, entrainment and air mixing of fuel in combustion will be increased. In this article, instability of an elliptic liquid jet under the effect of inertia, viscous, and surface tension forces has been studied using temporal linear analyses. The effects of the gravity and the surrounding gas have been neglected. 1-D Cosserat equation (directed curve) has been used which can be considered as simplified form of Navier-Stokes equations. Results are comparable with classical Rayleigh mode of circular jet when the aspect ratio (ratio of major to minor axis) is one. Growth rate of instability on an elliptic liquid jet under various conditions has been compared with those of a circular jet. Results show that in comparison with a circular jet, the elliptic jet is more unstable and by increasing the aspect ratio the instability grows faster. In addition, similar to the circular case, the effect of viscosity is diminishing the growth rate for the elliptic jet.
LES and RANS Simulation of Turbulent Airflow and Tracer Gas Injection in a Generic Aircraft Cabin Model
Authors: Ebrahimi Khosrow , Zheng Zhongquan C. , Hosni Mohammad H.
Published: March 2011

This study is a continuation of a previous research in numerical simulation of a turbulent airflow in a generic aircraft cabin model. Specifically, the primary objective of this project is to use Computational Fluid Dynamics (CFD) to simulate transport of a tracer gas injected into the generic aircraft cabin. The research work reported herein is composed of three parts. First, both Large Eddy Simulation (LES) and Reynolds averaged Navier Stokes (RANS) methods are used to simulate airflow from a full-height nozzle and corresponding airflow characteristics within the mockup aircraft cabin. The computational results are validated by comparing them with Particle Image Velocity (PIV) data and published CFD predictions available in the literature. Through these comparisons, the potential for using the CFD methods to predict unsteady as well as time-averaged velocity for a generic aircraft cabin model is examined. Second, airflow characteristics are studied by reducing the inlet nozzle height to one-half of its original size but keeping the total volumetric airflow rate the same as that of the full-height nozzle. Accuracy of the LES approach in predicting airflow in the half-height nozzle is evaluated by comparing prediction results with the PIV measurement data for the mockup cabin. Third, simulation of a tracer gas injection through the injecting tube placed in different locations in the half-height nozzle cabin is investigated. In this part, carbon dioxide (CO2) is chosen as the tracer gas. The LES method is used to solve the equations of motion and the unsteady species transport equation for tracer gas concentration. The predictions are compared with the average measurement data for CO2-concentration in various locations in the cabin.
Pressure Drop in Microchannels Filled With Porous Media
Authors: Tamayol A. , Khosla A. , Gray B. , Bahrami M.
Published: March 2011

The pressure drop in microchannels filled with porous media formed by square arrays of cylinders (micro-porous channels) is investigated. Combining the Brinkman equation and the existing models for permeability of regular arrays of cylinders, the pressure drop in the considered micro-porous channels is calculated theoretically. Soft lithography method is used to fabricate several Polydimethylsiloxane (PDMS) microporous-channels with porosities in the range of 0.35 to 0.95, fiber diameters varying from 50 to 400 μm, and channel depth of approximately 100 μm. Distilled water is pushed through the samples using a syringe pump with steady flow rate and the resulting pressure drops are measured for several flow rates. The developed model captures the trends of experimental data for all of the samples. Our analysis indicates that a competing behaviour exists between the permeability and the channels dimensions for controlling the pressure drop. Therefore, the Darcy number should be used to determine the dominating parameter.
Large-Eddy Simulation on the Effect of Ambient Perturbation on Vehicle Aerodynamics
Authors: Tsubokura Makoto , Ikawa Yuki , Kitayama Masashi , Nakashima Takuji
Published: March 2011

A large eddy simulation method based on a fully unstructured finite volume method was developed, and the unsteady aerodynamic response of a road vehicle subjected to transient crosswinds was investigated. The method was first validated on the 1/20-scale wind tunnel model at a static aerodynamic condition and showed good agreement in the surface pressure distributions. The method was then applied to two transient crosswind situations: a sinusoidal perturbation representing the typical length scale of atmospheric turbulence, and a stepwise crosswind velocity corresponding to a wind gust. Typical transient responses of the aerodynamic forces and moments such as phase shifting and undershoot or overshoot were observed, and their dependence on the frequency and amplitude of the input perturbation is discussed. As a result, the utility and validity of LES is demonstrated in the context that such transient aerodynamic forces are difficult to measure in the conventional wind tunnel measurements.
A Two-Way Coupled Polydispersed Simulation of Bubbly Flow Beneath a Plunging Liquid Jet
Authors: Ma Jingsen , Oberai Assad A. , Drew Donald A. , Lahey Richard T.
Published: March 2011

Plunging liquid jets are commonly encountered in nature and are widely used in industrial applications (e.g., in waterfalls, waste-water treatment, the oxygenation of chemical liquids, etc.). Despite numerous experimental studies that have been devoted to this interesting problem, there have been very few two-phase flow simulations. The main difficulty is the lack of a quantitative model to simulate the air entrainment process, which plays a critical role in this problem. In this paper, we present a computational multiphase fluid dynamics (CMFD) approach for solving this problem. The main ingredients of this approach are a comprehensive subgrid air entrainment model that predicts the rate and location of the air entrainment and a two-fluid transport model in which bubbles of different sizes are modeled as a continuum fluid. Using this approach, a Reynolds-averaged Navier Stokes (RaNS) two-way coupled two-phase flow simulation of a plunging liquid jet with a diameter of 24mm and a liquid jet velocity around 3.5m/s was performed. We analyzed the simulated void fraction and bubble count rate profiles at three different depths beneath the average free surface, and compared them with experimental data. We observed good agreement at all locations.
Variable Property Effects in Simultaneously Developing Gaseous Slip-Flow in Rectangular Microchannels With Prescribed Wall Heat Flux
Authors: Qazi Zade Azad , Renksizbulut Metin , Friedman Jacob
Published: March 2011

The effects of variable physical properties on the flow and heat transfer characteristics of simultaneously developing slip-flow in rectangular microchannels with constant wall heat flux are numerically investigated. A co-located finite-volume method is used in order to solve the mass, momentum and energy equations in their most general form. Thermophysical properties of the flowing gas are functions of temperature, while density and Knudsen number are allowed to change with both pressure and temperature. Different Knudsen numbers are considered in order to study the effects of slip-flow. Simulations indicate that the constant physical property assumption can result in under/over-prediction of the local friction and heat transfer coefficients depending on the problem configuration. Density and thermophysical property variations have significant effects on predicting flow and heat transfer characteristics since the gas temperature constantly changes as a result of the applied wall heat flux. Heat transfer coefficient is affected both due to the change in the velocity field and change in thermophysical properties. Also temperature dependence of the local Knudsen number can significantly alter the friction coefficients due to its strong dependence on slip conditions. The degree of discrepancy varies for different cases depending on the Knudsen number, and the applied heat flux strength and direction (cooling versus heating).
Numerical Investigation of Fluid Flow Inside the Porous Medium of GDL
Authors: Shahraeeni Mehdi , Hoorfar Mina
Published: March 2011

A pore-network model is developed to study numerically the transient flow of fluid through the gas diffusion layer (GDL) of the PEM fuel cell. It is shown that the agglomeration of water droplet on the interface of the GDL and catalyst layer occurs faster for the samples with smaller pore diameters and lower contents of the hydrophobic agent. The study suggests that analysis of the temporal response of the GDL is a useful tool to evaluate its performance against transporting liquids.
High Strain Rate Induced Phenomenon in Thin Nickel Films
Authors: Khan Hanif Muhammad , Kim Sung-Gaun
Published: March 2011

Molecular dynamics study has been performed to investigate the effect of extreme dynamic loading condition on thin Nickel films. Due to the novel aspects of thin films, it has attracted attention over the years for Nano-electro-mechanical systems design. This would be helpful for the analysis of ultra short pulse laser induced fabrication or machining of thin films. Uniaxial high tensile strain namely 109 and 1010 s−1 has been used on a Nickel thin film. The Embedded-atom method (EAM) potential function for Nickel has been used for the interaction of the atoms. We have observed ductile failure mechanism on both the cases. With the increase of strain rate, not only high strength has been found but also the elastic modulus has been affected less. Random non spherical void growth and their coalescence have been also observed during higher loading condition which leads to a ductile failure mechanism.
A Combined Soft-Sphere Collision/Immersed Boundary Method for Resolved Simulations of Particulate Flows
Authors: Breugem Wim-Paul
Published: March 2011

A second-order accurate and efficient Immersed Boundary Method (IBM) has been developed for simulating particle-laden flows. Recently, this method has been combined with a soft-sphere collision model to accommodate inter-particle and particle-wall collisions. Details of the collision model are given. Results are shown from a lubrication study of non-touching particles at close distance from each other. The numerical results for the drag force acting on the particles agree well with exact solutions, except when the gap width between the particles becomes significantly smaller than the numerical grid spacing. For very small gap width, lubrication force corrections are proposed for the normal approach between particles based on asymptotic analytical solutions. Results are presented from a numerical study of sphere-wall collisions in a viscous fluid. The simulated behavior of the coefficient of restitution as function of the Stokes number based on the particle impact velocity, is in good agreement with experimental data.
Semi-Analytical Determination of the Onset Conditions of Thermoacoustic Engines
Authors: Guedra Matthieu , Penelet Guillaume , Lotton Pierrick
Published: March 2011

Several analytical or numerical models available in the litterature allow to predict the onset of thermoacoustic engines [5, 6, 11]. However, most of these models rely on strong assumptions concerning for instance the stack geometry or the shape of the temperature field in the thermoacoustic core. The purpose of the work presented here is to put forward a new method allowing the prediction of the onset of the thermoacoustic instability while being exempted from a certain number of these assumptions. This method consists in measuring the transfer matrix of the thermoacoustic core by means of an appropriate experimental setup developed in our laboratory. The results are then introduced into an analytical modeling leading to the prediction of the onset conditions (in terms of heating power supply and acoustic frequency) of the thermoacoustic instability in an engine of specified geometry (straight duct or closed loop, coupled or not with an acoustic load like a secondary resonator or an electrodynamic alternator). The results of measurements will be presented, and the predictions of the onset obtained from these measurements will be compared with those actually observed in a standing-wave thermoacoustic engine.
Coupled Analysis of Unsteady Aerodynamics and Vehicle Motion of a Heavy Duty Truck in Wind Gusts
Authors: Nakashima Takuji , Tsubokura Makoto , Ikenaga Takeshi , Kitoh Kozo , Doi Yasuaki
Published: March 2011

In the present study, unsteady aerodynamic forces acting on a simplified heavy duty truck in strong wind gust and their effects on the truck’s motion were investigated by using a coupled analysis. Unsteady fluid dynamics simulation was applied to numerically reproduce unsteady aerodynamic forces acting on the truck under sudden crosswind condition. Taking account of vehicle’s motion, moving boundary techniques were introduced. Motions of the truck were simulated by a vehicle dynamics simulation including a driver’s reaction. The equations of motion of the truck in longitudinal, lateral, and yaw-rotational directions were numerically solved. These aerodynamics and vehicle dynamics simulations were coupled by exchanging the aerodynamic forces and the vehicle’s motion. In order to investigate effects of the unsteady vehicle aerodynamics on the vehicle’s motion, conventional analysis of the vehicle’s motion using quasi-steady aerodynamic forces and one-way coupled analysis with fixed vehicle attitude were also conducted. The numerical results of these simulations were compared with each other, and the effects of the two kinds of unsteady aerodynamics were discussed separately and totally. In the sudden crosswind condition, the unsteady aerodynamics effected significantly on the truck’s motion. An effect of transient aerodynamics as the truck ran into a sudden crosswind was greater than an effect of unsteady aerodynamics caused by unsteady vehicle’s motion, while both of the effects showed significance.
Aerodynamic and Thermal Measurements in a Standing Wave Thermoacoustic Refrigerator
Authors: Poignand Gaelle , Jondeau Emmanuel , Blanc-Benon Philippe
Published: March 2011

Thermoacoustic refrigerators produce a cooling power from an acoustic energy. Over the last decades, these devices have been extensively studied since they are environment-friendly, robust and miniaturizable. Despite all these advantages, their commercialization is limited by their low efficiency. One reason for this limitation comes from the complex thermo-fluid process between the stack and the two heat exchangers which is still not sufficiently understood to allow for optimization. In particular, at high acoustic pressure level, vortex shedding can occur behind the stack as highlight by [Berson & al., Heat Mass Trans, 44, 10151023 (2008)]. The created vortex can affect heat transfer between the stack and the heat exchangers and thus, they can reduce the system performance. In this work, aerodynamic and thermal measurements are both conducted in a standing wave thermoacoustic refrigerator allowing investigation of vortex influence on the system performance. The proposed device consists on a resonator operated at frequency of 200 Hz, with hot and cold heat exchangers placed at the stack extremities. The working fluid is air at ambient temperature and atmospheric pressure. The aerodynamic field behind the stack is described using high-speed Particle Image Velocimetry. This technique allows the acoustic velocity field measurement at a frequency up to 3000 Hz. Thermal measurements consist on the acquisition of both the temperature evolution along the stack and the heat fluxes extracted at the cold heat exchanger. These measurements are performed by specific micro-sensors developed by MEMS technology. The combination of these two measurements should be helpful for the further understanding of the heat transfer between the stack and the heat exchangers.
Efficient Estimation of the Friction Factor for Forced Laminar Flow in Axially Symmetric Corrugated Pipes
Authors: Rosen Esquivel Patricio I. , ten Thije Boonkkamp Jan H. M. , Dam Jacques A. M. , Mattheij Robert M. M.
Published: March 2011

In this paper we present an efficient method for calculating the friction factor for forced laminar flow in arbitrary axially symmetric pipes. The approach is based on an analytic expression for the friction factor, obtained after integrating the Navier-Stokes equations over a segment of the pipe. The friction factor is expressed in terms of surface integrals over the pipe wall, these integrals are then estimated by means of approximate velocity and pressure profiles computed via the method of slow variations. Our method for computing the friction factor is validated by comparing the results, to those obtained using CFD techniques for a set of examples featuring pipes with sinusoidal walls. The amplitude and wavelength parameters are used for describing their influence on the flow, as well as for characterizing the cases in which the method is applicable. Since the approach requires only numerical integration in one dimension, the method proves to be much faster than general CFD simulations, while predicting the friction factor with adequate accuracy.
Numerical Simulation of Flow Around a Generic Pickup With ISIS-CFD
Authors: Guilmineau Emmanuel
Published: March 2011

Computational Fluid Dynamics (CFD) is used to simulate the flow over a pickup truck. The flow solver used is ISIS-CFD developed by the CFD Department of the Fluid Mechanics Laboratory of Ecole Centrale de Nantes. CFD simulations are carried out with the Explicit Algebraic Reynolds Stress Model (EARSM) turbulence model and the Detached Eddy Simulation (DES). The focus of the simulation is to assess the capabilities of ISIS-CFD for vehicle aerodynamic development for pickup trucks. Detailed comparisons are made between the CFD simulations and the existing experiments for a generic pickup truck. The comparisons between the simulation results and the time-averaged measurements reveals that the CFD calculations are able to track the flow trends.
An Experimental Investigation of Crossflow Jet Influence on Dilute Gas-Solid Flow in a Rectangular Duct
Authors: Fu Yao , Wang Tong , Gu Chuangang
Published: March 2011

In this paper an experimental setup was built to investigate jet flow normal to a confined rectangular multiphase crossflow. In order to achieve uniform particle distribution, the inlet and transition channel were carefully designed and adjusted. The experiments were carried out with 2 test conditions, with Rec/Rejet of 7.9 × 104/3.1 × 104 and 7.0 × 104/1.8 × 104. Four classes of particles were used in both of the test conditions. The planar gas flow field and particle distribution on the symmetric cross section were measured by DPIV system. Mean fluid velocity results and transient flow visualization images were used to analyze the jet influence on the gas flow field. In the transient images of some classes of particles, vortex structures were found to be similar but not exactly the same to the free jet structure. A new set of arithmetic was developed to distinguish particle spots from noisy raw images. The results getting from this arithmetic reveals the time-average particle concentration on the jet influence. The analysis proved that the jet control method may set a gas barrier in the flow field to influence the particle distribution. Such an effect is diameter-selective. The experimental results showed a good prospect on the application of wall jet control method in inertia separator.
High Order Slip and Thermal Creep Effects in Micro Channel Natural Convection
Authors: Niazmand Hamid , Rahimi Behnam
Published: March 2011

Developing natural convection gaseous flows in an open-ended parallel plate vertical microchannel with isothermal wall conditions are numerically investigated to analyze the rarefaction effects on heat transfer and flow characteristics in slip flow regime. The Navier-Stokes and energy equations are solve by a control volume technique subject to higher-order temperature jump and velocity slip conditions including thermal creep effects. The flow and thermal fields in the entrance and fully developed regions along with the axial variations of velocity slip, temperature jump, and heat transfer rates are examined in detail. It is found that rarefaction effects significantly influence the flow and thermal fields such that mass flow and heat transfer rates are increased considerably as compared to the continuum regime. Furthermore, thermal creep contribution to the velocity slip is found to be dominant close to the channel inlet and vanishes in the fully developed region, while velocity slip approaches a finite value there. Both Mass flow rate and thermal entrance length increase with increasing Knudsen number in slip flow regime.
DNS Study of Collision and Coalescence Over a Wide Range of Volume Fraction
Authors: Luret G. , Me´nard T. , Re´veillon J. , Berlemont A. , Demoulin F. X.
Published: March 2011

Among the different processes that play a role during the atomization process, collisions are addressed in this work. Collisions can be very important in dense two-phase flows. Recently, the Eulerian Lagrangian Spray Atomization (ELSA) model has been developed. It represents the atomization by taking into account the dense zone of the spray. Thus in this context, collisions modeling are of the utmost importance. In this model results of collisions are controlled by the value of an equilibrium Weber number, We*. It is defined as the ratio between the kinetic energy to the surface energy. Such a value of We* has been studied in the past using Lagrangian collision models with various complexity. These models are based on analysis of collisions between droplets that have surface at rest. This ideal situation can be obtained only if droplet agitation created during a collision has enough time to vanish before the next collision. For a spray, this requirement is not always fulfill depending for instance on the mean liquid volume fraction. If there is not enough time, collisions will occur between agitated droplets changing the issue of the collision with respect to the ideal case. To study this effect, a DNS simulation with a stationary turbulence levels has been conducted for different liquid volume fractions in a cubic box with periodic condition in all directions. For liquid volume fraction close to zero the spray is diluted and collisions between spherical droplets can be identified. For a volume fraction close to one, collisions between bubbles are found. For a middle value of the volume fraction no discrete phase can be observed, instead a strong interaction between both liquid and gas phases is taking place. In all this case the equilibrium value of the Weber number We* can be determined. First propositions to determine We* as a function of the kinetic energy, density ratio, surface tension coefficient and the volume fraction will be proposed.
Validation of Unsteady CFD in a Confined Row of Cylinders
Authors: Wilson Brandon , Harris Jeff , Smith Barton L. , Spall Robert
Published: March 2011

A validation study for two CFD models of the time-varying flow through a confined bank of cylinders is presented. The facility mimics the lower plenum of a high temperature reactor and is arranged with the cylinders on equilateral triangles with pitch to diameter ratio of 1.7. Time-resolved Particle Image Velocimetry (PIV) measurement coupled with pressure measurements along the facilities walls are compared to both the Unsteady Reynolds Averaged Navier Stokes (URANS) k–ω model and the Detached Eddy Simulation (DES) models. Spatial (i.e. time-averaged bulk velocity and pressure losses and local velocity distributions) and temporal (i.e. dominant frequencies and correlations) validation parameters on both the local and global scale are used for validation. It is found the CFD models accurately predict frequencies present in the pressure along the walls next to the cylinders in the first and the last cylinder, yet predicts other dominant frequencies in the remaining cylinders that are not found in the experiment. The temporal behavior of the DES was generally far superior to that of the URANS model.
Automatic Particle Image Velocimetry Uncertainty Quantification
Authors: Timmins Benjamin H. , Smith Barton L. , Vlachos Pavlos P.
Published: March 2011

A method to estimate the uncertainty of each vector in Particle Image Velocimetry measurements by estimating the parameters which contribute to errors in the computed velocity field is discussed. These parameters include particle image diameter, particle density, particle displacement, and velocity gradient. After PIV processing, our code “measures” these parameters and an estimate of the velocity uncertainty is made for each vector in the flow field.
Magnetic Actuation of Nanofluids With Ferromagnetic Particles
Authors: Sesen Muhsincan , Bahceci Beste , Kosar Ali , Ulun Soner , Su Caglar Huseyin , Acar H. Yagci
Published: March 2011

Electromagnetically actuated microflows are generated by using ferromagnetic nanofluids containing Fe2O3 based nanoparticles. Because of their magnetic properties these nanoparticles are able to response to a magnetic field imposed along a microchannel so that a microflow could be driven. Nanofluid samples were located inside a minichannel and were directed with a magnetic field, which was induced by a solenoid wrapped around the minichannel, to drive the flow inside the minichannel, where its flow rate was also recorded. The flow rate was measured as a function of the imposed magnetic field. The corresponding pressure drop to deliver the same flow rate with an ordinary pump along the same minichannel was estimated so that the potential of this system for acting as a micropump in microfluidic applications was revealed.
Numerical Simulation of Electrostatic Atomization in Spindle Mode
Authors: Passandideh Fard Mohammad , Mahpeykar Mohammad Reza , Pooyan Sajad , Rahimzadeh Mortaza
Published: March 2011

The behavior of a liquid jet in an electrostatic field is numerically simulated. The simulations performed correspond to a transient liquid jet leaving a capillary tube maintained at a high electric potential. The surface profile of the deforming jet is defined using the VOF scheme and the advection of the liquid free surface is performed using Youngs’ algorithm. Surface tension force is treated as a body force acting on the free surface using continuum surface force (CSF) method. To calculate the effect of the electric field on the shape of the free surface, the electrostatic potential is solved first. Next, the surface density of the electric charge and the electric field intensity are computed, and then the electric force is calculated. Liquid is assumed to be a perfect conductor, thus the electric force only acts on the liquid free surface and is treated similar to surface tension using the CSF method. To verify the simulation results, a simplified case of electrowetting phenomenon is simulated and free surface shape in stable state is compared with experimental results. Then the electrostatic atomization in spindle mode is simulated and the ability of the developed code to simulate this process is demonstrated.
Self-Adaptive Upwinding for Large Eddy Simulation of Cross-Flow Jets
Authors: Kumar Vimal , Paraschivoiu Marius
Published: March 2011

A self-adaptive upwinding method for large eddy simulation (LES) has been describe in cross-flow in jets. The method is an extension of an existing Reynolds-averaged Navier–Stokes (RANS) code to an LES code by adjusting the contribution of the upwinding term to the convective flux. This adjustment is essentially controlled by reducing the upwind contribution in Roe MUSCL scheme. For the comparison of flow profiles obtained from the self-adaptive upwind LES code the experimental measurements of Andreopoulos and Rodi [1] are considered. The jet-to-cross-flow velocity ratio considered is 0.5 at a Reynolds number of 20,500 based on the jet bulk velocity and the jet diameter. In general, a reasonable agreement with the measurements is obtained. However, an intense backflow near the flat wall is observed. Further a vortex formation is observed behind at a distance of 0.6D from the jet axis.
The Importance of Mean Free Path in Determining Gas Micro Flow Behaviour
Authors: Dongari Nishanth , Zhang Yonghao , Reese Jason M.
Published: March 2011

We investigate whether a power-law form of probability distribution function better describes the free paths of dilute gas molecules in a confined system. An effective molecular mean free path model is derived, which allows the mean free path to vary close to bounding surfaces. Our model is compared with molecular dynamics simulation data, and also other classical mean free path models. As gas transport properties can be related to the mean free path through kinetic theory, the Navier-Stokes constitutive relations are then modified and applied to various benchmark test cases. Results for isothermal pressure-driven Poiseuille flows in micro-channels are reported, and we compare our results with conventional hydrodynamic models, solutions of the Boltzmann equation, and experimental data. Our new approach provides good results for mean free path and cross-sectional flow velocity profiles up to Knudsen numbers around 1, and for integral flow parameters such as flow rate and friction factor up to Knudsen number of 10. We discuss some limitations of our power-law model, and point to the way forward for further development.
Development of Unsteady Gas Flow Generator for Evaluating the Dynamic Characteristic of Respiratory Gaseous Flow Meter
Authors: Funaki Tatsuya
Published: March 2011

Respiratory gaseous flow measurement is one of an unsteady gas flow measurement and becoming very important. It has a wide field of application, for example, a measurement of lung function, an evaluation of respiratory gas exchange, a grasp of medical condition and so on. Especially, the evaluation of the absolute quantity and the analysis of the breathing waveform pattern are very important in the respiratory gaseous flow measurement. However, the dynamic characteristics of the respiratory gaseous flow meter has not been quantitatively measured and evaluated in the actual unsteady flows. There is substantial literature dealing with the measurement of unsteady gas flow. Most of these studies generated unsteady mass flows by using piston cylinders. Clearly, in these studies, substantial efforts must have been required in order to minimize the sensitivity dependence of density fluctuation on pressure and temperature variations. On the other hand, the dynamic characteristic evaluation of the gaseous flow meter which reproduced the sinusoidal waveform with only a single frequency component in the measurement frequency band was typically enough. However, the respiratory airflow waveform with the various frequency components and the shapes is complicated. Moreover, we already know that the respiratory waveform pattern changes by a state of health and activities. To solve these problems, this paper deals with the development of unsteady gas flow generator for the various breathing waveform reproduction. At first, we carry out the survey on the respiratory gaseous flow. Based on the research background and the above mentioned survey, we develop and introduce the unsteady gas flow generator which can generate the various respiratory flows. And we show the effectiveness of the developed unsteady gas flow generator. Moreover, we conduct the performance evaluation of the developed unsteady gas flow generator and the uncertainty analysis.
Inverse Method Used for the Determination of the Wall Shear Stress in a Sliding Rheometer Using Sandwich Probes
Authors: Berrich Emna , Aloui Fethi , Legrand Jack
Published: March 2011

The inverse method, based on a numerical sequential estimation, has been applied for the determination of the wall shear stress of a liquid single phase flow in a sliding rheometer using multi-segment probe. This method requires the inversion of the convection diffusion equation in order to apply it to instantaneous mass transfer measurements. Polarography technique, known as the limiting diffusion current method, has been used. This requires the use of Electro-Diffusion ED probe which allows the determination of the local mass transfer rate for known flow kinematics. In addition, two-segment platinum probe was mounted flush to the inert surface of the upper disk of the sliding rheometer. Hydrodynamic oscillations have been imposed to the torsional flow (type sinusoidal), in order to study the frequency response of the sandwich probe for a fixed polarization voltage. Possible error sources which are likely to affect the interpretation of the results e.g. the directional angle effect, the inertial effect, the diffusion effect and the frequencies of oscillations effect have been studied in order to test the robustness of the inverse method within the presence of such impacts. Furthermore, to demonstrate the possible effect of non-negligible inertia and diffusion, we refer to ED results for both modified Reynolds number defined by [1] and Peclet number ranges as well as for different directional angles. An algorithm has been developed for the numerically filtering of the mass transfer signals, and therefore the wall shear stress signals. It permits to eliminate any possible noise effect due to the imposed vibrations to the torsional flow. The analysis shown that the inverse method is in a good agreement with the ED experimental results for the different cases of study, i.e. for different dimensionless Reynolds numbers, for high and low oscillation frequencies, as well as for different directional angles. The little difference is probably caused by the sensitivity of the double probe to such directional angles or to the neglecting of the insulating gap effect on the inverse method solution as a first step of the study of the inverse method for double probes signals.
Experimental Investigation of Turbulent Boundary Layers Under Favorable Pressure Gradient
Authors: Joshi Pranav , Katz Joseph
Published: March 2011

The goal of this research is to study the effect of favorable pressure gradient (FPG) on the near wall structures of a turbulent boundary layer on a smooth wall. 2D-PIV measurements have been performed in a sink flow, initially at a coarse resolution, to characterize the development of the mean flow and (under resolved) Reynolds stresses. Lack of self-similarity of mean velocity profiles shows that the boundary layer does not attain the sink flow equilibrium. In the initial phase of acceleration, the acceleration parameter, K = v/U2dU/dx, increases from zero to 0.575×10−6, skin friction coefficient decreases and mean velocity profiles show a log region, but lack universality. Further downstream, K remains constant, skin friction coefficient increases and the mean velocity profiles show a second log region away from the wall. In the initial part of the FPG region, all the Reynolds stress components decrease over the entire boundary layer. In the latter phase, they continue to decrease in the middle of the boundary layer, and increase significantly close to the wall (below y∼0.15δ), where they collapse when normalized with the local freestream velocity. Turbulence production and wallnormal transport, scaled with outer units, show self-similar profiles close to the wall in the constant K region. Spanwise-streamwise plane data shows evidence of low speed streaks in the log layer, with widths scaling with the boundary layer thickness.