bluuebunny/crossref_metadata_2025 huggingface dataset:
Directed Particle-Laden Micro-Jet for Dental Caries Evacuation (Keynote Paper)
Authors: Amitay Michael , Kondor Shayne , Herdic Scott , Anderson Steven L.
Published: February 2009

Active and passive approaches to control the velocity and concentration of a high speed round particle-laden jet are investigated experimentally using a stereo PIV system. Active control of the flow field and the particles’ velocity and concentration fields, via the addition of swirl to the carrier jet, has shown to have a significant effect in altering both phases. Control is also affected by placing passive pins at the jet exit plane, which results in alteration of the velocity in planes across and normal to the pins. Furthermore, the mixing is increased and the spreading rate is modified. Depending on the number of pins used and their azimuthal location, their interaction with the carrier jet flow lead to the modification of the cross-flow shape of the jet and the direction of the flow.
The Role of Streamwise Vorticity in the Control of Impinging Jets
Authors: Alvi F. S. , Lou H. , Shih C.
Published: February 2009

Supersonic impinging jets produce a highly unsteady flowfield leading to very high dynamic pressure loads on nearby surfaces. In earlier studies, we conclusively demonstrated that arrays of supersonic microjet, 400 μm in diameter, effectively disrupted the feedback loop inherent in high-speed impinging jet flows. This feedback disruption results in significant reductions in the adverse effects associated with such flows. In this paper, by primarily using detailed velocity field measurements, we examine the role of streamwise vorticity in order to better understand the mechanisms behind this control scheme. The velocity field measurements clearly reveal the presence of well-organized, streamwise vortices with the activation of microjets. This increase in streamwise vorticity is concomitant with a reduction in the azimuthal vorticity of the primary jet. We propose that the streamwise vorticity is mainly a result of the redirection of the azimuthal vorticity, which leads to a weakening of the large-scale structures in the primary jet. The appearance of strong vortices in the shear layer near the nozzle exit due to microjets further weakens the spatial coherence of the coupling between the acoustic waves and shear layer instability, while thickening the jet shear layer. All these effects are thought to be collectively responsible for the efficient disruption of the feedback loop using microjets.
Design and Analysis of a Shear Stress Sensor for Microcirculation Investigations (Invited Paper)
Authors: Robinson Risa , Fuller Lynn , Palmer Harvey , Frame Mary
Published: February 2009

Blood flow regulation in the microvascular network has been investigated by means of computational fluid dynamics, in vivo particle tracking and microchannel models. It is evident from these studies that shear stress along the wall is a key factor in the communication network that results in blood flow modification, yet current methods for shear stress determination are acknowledged to be imprecise. Micromachining technology allows for the development of implantable shear stress sensors that will enable us to monitor wall shear stress at multiple locations in arteriole bifurcations. In this study, a microchannel was employed as an in vitro model of a microvessel. Thermal shear stress sensors were used to mimic the endothelial cells that line the vessel wall. A three dimensional computational model was created to simulate the system’s thermal response to the constant temperature control circuit and related wall shear stress. The model geometry included a silicon wafer section with all the fabrication layers — silicon dioxide, poly silicon resistor, silicon nitride — and a microchannel with cross section 17 μm × 17 μm. This computational technique was used to optimize the dimensions of the system for a 0.01 Reynolds number flow at room temperature in order to reduce the amount of heat lost to the substrate and to predict and maximize the signal response. Results of the design optimization are presented and the fabrication process discussed.
Basic Study for Micro Pump Using Electroosmotic Flow
Authors: Fujiwara Tamio , Kamiya Akinori , Kitoh Osami , Ushijima Tatsuo
Published: February 2009

The characteristic features of electroosmotic flow have been studied to obtain important information for applying the flow to a micro pump as a driving device. Here, an electroosmotic flow of water was generated in a donut channel constructed by a gap between two parallel donut-shaped glass plates. The flow rate was measured in relation to the applied voltage, the gap width of the channel, the pressure gradient and the properties of the fluid. The experimental results were compared with a theoretical equation of electroosmosis, the Helmholtz-Smoluchowski equation, which predicts that the electroosmotic flow rate is proportional to the applied voltage as well as to the gap width. The Electroosmotic flow rate increased linearly with the applied voltage. however, there appeared some particular voltage ranges of nonlinear relation unlike the Helmholtz-Smoluchowski equation. The water properties (the conductivity and the kinds of impurities included) had a great influence on the electroosmotic flow rate characteristics including the degree of non-linearity and even the flow direction. The cause of these phenomena has not been clarified. It is conjectured that the zeta potential of the glass-water interface was altered by the applied electric field. It is confirmed that the electroosmotic flow rate is proportional to the gap size of the channel. When a pressure gradient existed in the direction of the electroosmotic flow, the total flow rate was given as a sum of the flow rates of the electroosmotic and Poiseuille flows. These findings provide important information for micro pumps. For measuring the electroosmotic flow rate, we developed a micro flow meter consisting of a capillary and two fine wires. This flow meter makes it possible to measure a flow rate the order of 1×10−3 mm3/sec (= 1 nl/sec) and has a potential ability to measure even much smaller flow rates.
CFD Simulations of an Axisymmetric Mixed-Compresssion Inlet With Bleed Through Discrete Holes
Authors: Benson David B. , Shih Tom I.-P. , Davis David O.
Published: February 2009

CFD simulations were performed to investigate boundary-layer control through bleed patches in an axisymmetric mixed-compression inlet in which the bleed patches are modeled by two global bleed boundary conditions (BCs). In one bleed BC, the locations of the bleed holes are discerned. In the other bleed BC, each row of bleed holes is modeled as a porous surface, where the number of bleed holes in each row is accounted for by adjusting the discharge coefficient to give the correct bleed rate. Results are presented for the predicted bleed rates, pressure on the cowl and centerbody surfaces, and the flow field. Comparisons were made with available experimental data. Also presented is a method based on one-dimensional isentropic and normal shock solutions to get the flow “started” in CFD simulations of critical flow in mixed-compression inlets. This computational study is based on the ensemble-averaged conservation equations of mass (continuity), momentum (compressible Navier-Stokes), and total energy closed by shear-stress transport (SST) turbulence model, where integration is to the wall. Solutions were generated by a cell-centered finite-volume method that uses third-order accurate flux-difference splitting of Roe with limiters, multigrid acceleration of a diagonalized ADI scheme with local time stepping, and patched/overlapped structured grids.
LES of Turbulent Separated and Reattached Flow Around a Surface-Mounted Plate
Authors: Yoshikawa Hiroyuki , Konno Masakazu , Okamoto Ryuta , Ota Terukazu
Published: February 2009

LES method is applied to simulate numerically a turbulent separated and reattached flow around a surface-mounted plate. Smagorinsky model is used in the analysis and fundamental equations are discretized by means of the finite volume method, and their resulting finite difference equations are solved using SMAC method. The calculations are conducted for Re = 105. It is found that the present numerical results, in general, agree well with the previous experimental ones. The complicated vortical flow structures around the plate and their time variations are visualized through various fields of flow quantities.
Experimental Investigation of Free Stream Turbulence Effects on Flow Separation
Authors: Ardebili Mahmoud , Andreopoulos Yiannis
Published: February 2009

An experimental investigation of a separated boundary layer flow has been attempted which has been created by perturbing a flat plate flow with a favorable pressure gradient immediately followed by an adverse pressure gradient. The aim of the research program is possible control of flow separation by means of free stream turbulence. The flow is configured in a large-scale low speed wind tunnel where measurements of turbulence can be obtained with high spatial and temporal resolution. A model has been designed by using CFD analysis. Mean wall pressure and vorticity flux measurements are reported in this paper. Twelve experiments with three different mesh size grids at three different Reynolds numbers have been carried out. Three bulk flow parameters seem to characterize the flow: The Reynolds number of the boundary layer, Re+, the Reynolds number of the flow through the grid, ReM, and the solidity of the grid. It was found that the pressure coefficient depends weakly on the solidity of the grids. Vorticity flux also depends on the grid used to generate free stream turbulence. The location of maximum or minimum vorticity flux moves upstream at higher ReM.
Numerical Simulation of Three-Dimensional Separated Flow and Heat Transfer Around a Surface-Mounted Square Plate
Authors: Yoshikawa Hiroyuki , Shimizu Keisuke , Ota Terukazu
Published: February 2009

Direct Numerical Simulation results of three-dimensional laminar separated and reattached flow and heat transfer around a surface-mounted square plate are presented in this paper. Numerical calculations of Navier-Stokes equations and energy one are carried out using the finite difference method with SMAC method. A square plate is presumed to be mounted in a laminar boundary layer developing on a flat surface and to be heated under a constant heat flux. Numerical calculations are made on two boundary layer thicknesses at the plate, and the Reynolds number is varied from 300 to 1000. Details of the separated and reattached flow and the thermal field therein are clarified.
Flow Visualization, Transport Mechanism, and Temperature Distributions in an Enclosure With Two Side Walls Each Differentially Heated: Upper Half, Cold; Lower Half, Hot
Authors: Li H. , Braun M. J. , Evans E. , Wang G. , Paudal G. , Miller J.
Published: February 2009

Due to the experimental difficulties brought about by the high pressure and temperature growth conditions flow and heat transport in a hydrothermal autoclave for the growth of single quartz crystals has been studied mostly numerically. To date, most of the numerical models and associated results are not validated, or only qualitatively validated through results derived from crystal growth production. In this study, the authors used a simulated model reactor represented by an enclosure with the two lower half sidewalls uniformly heated while the upper halves are uniformly cooled. Flow in the reactor is qualitatively visualized using a full field flow lighting and seeding technique and quantitatively evaluated using particle image velocimetry. Finally, based on the physical setup and experimentally determined boundary conditions, flow is numerically simulated and compared to the experimental results. The agreement between the experimental and the numerical data was used to validate the numerical model. The ensuing parametric study shows the changing of flow pattern and velocity magnitudes for two differential temperature cases: (ΔT = 10°C and ΔT = 1°C) and a variety of enclosure aspect ratios.
Performance Characteristics of a Shear Pump Operating in High Viscous Solutions
Authors: Chiba Katsumi , Ito Yukio , Kobayashi Kazunori , Kawamorita Atsushi
Published: February 2009

In many manufacture fields, such as foods, medical supplies, cosmetics, lubricants and paints of present in used, a Shear pump successively takes advantage of such cavitation effects as a large amount of cavitation bubbles and the associated impulsive high shock pressures, to the mixing, to the stirring and to the emulsion. In this study, to estimate the behavior of the cavitation effects in the shear pump, we precisely evaluate the performance. Although the performance of the shear pump, that is the total head Hp, the shaft power Ps, the pump efficiency η — the flow rate Q curves, is very similar to those of the centrifugal pump tested in tap water, they are much different from that in higher viscosity solutions of 500 cP.
Nozzle Flow of a Ship Propulsion Equipment Driven by High-Pressure Gas
Authors: Tsutahara Michihisa , Ogawa Kazuhiko , Sakamoto Masahiko , Matsui Takahiro
Published: February 2009

A two-dimensional open type nozzle has been investigated for ship propulsion equipment. In this nozzle, a high-pressure gas (air) is ejected from below into water flow and accelerates the water towards downstream. Two cases, continuously and intermittently ejection of gas, are studied. In the latter case, the thrust itself is smaller than that in the former case, but thrust based on the ejection duration is larger. Even in the continuously ejected case, it was found that the gas phase and the water phase are separated. The water waves of large amplitude appear on the interface inside the nozzle and these waves propagate towards downstream. A numerical study by the lattice Boltzmann method was also performed to clarify the flow inside the nozzle.
Effect of Tip Clearance on the Flow and Performance of a Centrifugal Compressor
Authors: Hong Soon-Sam , Schleer Matthias , Abhari Reza S.
Published: February 2009

To see the tip clearance effect on a centrifugal compressor, time-resolved flow measurements were carried out at the discharge of a centrifugal compressor impeller, which is 30 deg back swept from radial and operated at the blade tip speed of 260 m/s. Tested were six cases of tip clearance ratio ranged from 0.06 to 0.21. A fast response aerodynamic probe equipped with a single pressure sensor was used for the flow measurements. From the distribution of time-resolved relative velocity, the jet-wake structure was observed. The wake downstream of the splitter blade was located in the shroud-suction corner, but that downstream of the full blade was in the mid-shroud. By increasing the tip clearance, the wake region was increased and the deficit of the relative total pressure governed the wake region, therefore, the loss was enlarged.
Development of a Direct Drive Servo Valve With Flow Force Compensated Spool
Authors: Shin Weongyu , Choi Hyunyoung , Shin Hyopil , Moon Euijoon
Published: February 2009

This paper is on the development of a Direct Drive servo Valve (DDV) with flow force compensated spool. In valves with spool, flow force is caused by the unbalance of pressure exerted on each land of spool when high-speed flow passes through very narrow orifices. A simple method for flow force compensation using a stepped spool is presented in this paper. It is easy to manufacture the stepped spool of the presented method because the shape of it is simple. The method has another merit that the size of valve need not be increased. Actuating force required for driving the spool can be decreased through the compensation of flow force. The effectiveness of the proposed method is predicted through CFD analysis. The results of the CFD analysis are also utilized for the optimization of step shape. Prototypes of flow force compensated DDV are manufactured and the measurements of flow force are carried out. The measured effectiveness of flow force compensation is very similar to that from the CFD analysis. Performance test results of prototype DDV satisfy required specifications.
Secondary Flows and Separation in S-Duct Diffusers: Their Detection and Control
Authors: Sullerey R. K. , Pradeep A. M.
Published: February 2009

An experimental investigation was undertaken to detect and control secondary flows and separation in two-dimensional Sduct diffusers. Active control by vortex generator jets was applied to control secondary flows. The experiments were carried out with and without inflow distortion. Detailed measurements including total pressure and velocity distribution, surface static pressure, skin friction and boundary layer measurements were taken at an inlet Reynolds number of 7.8 × 105. These measurements are presented here along with static pressure rise, distortion coefficient, total pressure loss coefficients and the transverse velocity vectors at the duct exit determined from the measured data. Substantial improvement in static pressure rise and flow quality was achieved by the use of vortex generator jets.
The Use of Flow Straightening Baskets in Submerged Pump Remediation
Authors: Havice Matthew , Werth David
Published: February 2009

This paper presents guidelines for the applicability and development of a flow-straightening basket, which can be used to alleviate hydraulic problems in existing pump stations as well as in new pump stations. In addition, limitations and applications where the use of this device is not recommended are also discussed. This paper also compares model study results to actual field performance for a variety of different basket configurations and applications.
Numerical Study on Fluid-Structure Interaction in VFP Artificial Heart With Jelly-Fish Valve
Authors: Kudriavtsev Vladimir , Kawano Satoyuki , Isoyama T. , Arai H. , Yambe T. , Abe Y. , Imachi K. , Nitta S. , Hashimoto H.
Published: February 2009

We analyze sinusoidal pulsating flow that develops in the vibrating flow pump (VFP) artificial heart casing. In such system flow is induced by the axial movements of the vibrating pipe. Pipe is capped with the flexible thin disk that is called jelly-fish valve (JFV). Valve is opened during the downward pipe motion and is closed during the upward motion. Valve movement is very similar with the movement of falcon wings. It is due to the pipe motion and happens under the influence of fluid inertial, JFV spring, fluid shear and pressure forces. Authors utilized industrial strength CFD-ACE+/FEMSTRESS software package from CFDRC to analyze dynamic fluid-structure interaction, flow velocity field and time-dependent vorticity distribution. It was shown in the previous studies that blood hemolysis is closely correlated with the maximum values of vorticity fianction ω. In the paper we analyzed valve deformation, related flowfield and vorticity at different transient flow conditions. We can clearly conclude that dynamic formulation allows us to estimate and pinpoint with much greater accuracy the local maxima in vorticity. Vorticity peaks in two areas. First zone is at valve/pipe throat and second zone is at the casing throat. Vorticity is highest at the casing wall, thus pointing the direction for design improvements. Reduction in JFV stiffness helps to open valve wider and to reduce flow vorticity in its vicinity. These are work-in-progress results and additional studies will follow.
Parametic Study of a Fan Blade Cascade Using a New Parametric Flow Solver Turb’Opty
Authors: Moreau S. , Aubert S. , N’Diaye M. , Ferrand P.
Published: February 2009

The newly developed parameterized CFD solver Turb’Opty™, based on a Taylor series expansion to high order derivatives of the solutions of the discretized Navier-Stokes equations, has been successfully applied to the turbulent incompressible flow field of an engine cooling fan blade cascade. Comparisons with the classical CFD results have validated the accuracy of the parameterized solutions obtained by a simple polynomial reconstruction around a reference solution with respect to two different flow parameters for two different cases: a fifth order expansion with respect to these coupled parameters for a frozen turbulence and a first order expansion with respect to each parameter for a variable turbulence. The latter is found to have a better accuracy and a larger range of application. Starting from a reference solution obtained with another commercial code has also been successfully tested. Finally, further industrial perspectives of turbomachinery global optimization are finally demonstrated by coupling this method with a simple genetic algorithm.
Detailed Study of an Efficient Small Diameter Automotive Engine Cooling Fan System
Authors: Moreau S. , Bakir F.
Published: February 2009

This paper deals with stator efficiency improvement meant for automotive engine cooling fan systems. A new rapid prototype of a long stator designed for a small diameter Valeo rotor (320 mm) has been manufactured and tested on a recently designed Valeo-LEMFI test rig. The following points are presented: • Overall performances of the 320 mm rotor alone. • Overall performances of the 320 mm rotor combined with the new long stator vanes: this study confirms the gain of efficiency foreseen previously with the simplified radial equilibrium code VENTAX. • Steady velocities measured 41 mm downstream of the stage configuration: These measurements obtained by using a 5-hole probe show high deflection carried out by the stators with long chord lengths. Comparisons with preliminary CFD simulations are also shown for both the overall performances and the local velocities.
A Numerical Investigation on the Centrifugal Compressor Stage for a Small Turboshaft Engine
Authors: Kim Jinhan , Lee Eunseok , Yang Sooseok
Published: February 2009

A prototype centrifugal compressor was designed for applying to a small turboshaft engine, and tested [1, 2]. As a component of the small turboshaft engine, the impeller has rather high value of inlet hub-tip ratio, 0.64. Also, for the compactness of engine, the compressor was designed with restricted dimensions in diameter. The absolute Mach number at the exit of the impeller reaches to sonic velocity to attain design pressure ratio, 4.89. Thus, the effective pressure recovery is the one of the main concern within the given dimension. The compressor in this study employs two-row tandem diffusers aiming high efficiencies. 3D viscous calculations are carried out to understand the flow structures in detail. Numerically predicted flow fields for off design conditions as well as for on design condition are discussed. Effects on the relative circumferential positions of the diffuser rows are studied for the cases of isolated diffusers to quantify the preferable configurations. According to the flow fields, it would be preferable to have some twist angles at leading edges for both rows of the diffusers to adapt incoming flows. On choking condition, recirculating flows are observed at the shroud region of the 2nd row diffuser.
Aero-Acoustic Design of Fan Using Inverse Method
Authors: Lee S. , Kim H.-J. , Song W.-S. , Ahn K.-W.
Published: February 2009

A multi-dimensional inverse algorithm was developed to design low noise fan. In this approach, mean-line analysis, 3-D CAD, automatic grid generation method, and CFD solver can be integrated with aero-acoustic inverse design method. The overall sound pressure level distributions obtained from specified sets of unsteady blade loading distribution serve input data to the multi-dimensional acoustic inverse module, and may be used to design optimal geometry to satisfy the aerodynamic and aero-acoustic performance. To execute an effective inversion of non-minimum phase system, SVD is introduced. With the inverse design concept used in the developed software, the design phase of an optimal blade geometry, which yields a required aerodynamic performance with minimal acoustic noise, may be significantly shortened. The experimental result of prototype fans, designed by using this software, showed that the aerodynamic and aero-acoustic performance is reasonably well predicted.
Electroosmotic Flow and Mass Species Transport in a Microcapillary Under Influences of Joule Heating
Authors: Tang Gongyue , Yang Chun , Chai Cheekiong , Gong Haiqing
Published: February 2009

This study presents a numerical analysis of Joule heating effect on the electroosmotic flow and species transport, which has a direct application in the capillary electrophoresis based BioChip technology. A rigorous mathematic model for describing the Joule heating in an electroosmotic flow including Poisson-Boltzmann equation, modified Navier-Stokers equations and energy equation is developed. All these equations are coupled together through the temperature-dependent parameters. By numerically solving aforementioned equations simultaneously, the electroosmotic flow field and the temperature distributions in a cylindrical microcapillary are obtained. A systematic study is carried out under influences of different geometry sizes, buffer solution concentrations, applied electric field strengths, and heat transfer coefficients. In addition, sample species transport in a microcapillary is also investigated by numerically solving the mass transfer equation with consideration of temperature-dependant diffusion coefficient and electrophoresis mobility. The characteristics of the Joule heating, electroosmotic flow, and sample species transport in microcapillaries are discussed. The simulations reveal that the presence of the Joule heating could have a great impact on the electroosmotic flow and sample species transport.
On Oscillation of a Confined Jet With a Downstream Obstacle
Authors: Hirata Katsuya , Funaki Jiro , Yamada Katsuya , Wakisaka Hirohisa
Published: February 2009

In the present study, the authors investigate an oscillatory phenomenon of a two-dimensional confined jet with a square prism, by experiments and computations. In experiments in an air duct, jet’s frequencies were measured near the target by a hot-wire anemometer, in the Reynolds-number range of 300–5000. In computations, the numerical method adopted here is a finite difference method formulated in terms of vorticity and stream function. As a result, we can see the Reynolds-number effect, the target-distance effect and the target-size effect upon Strouhal number. Regarding the Reynolds-number effect, it was found that there is less influence, which guarantees widerange workability as a flowmeter or a mixer. Regarding geometrical parameters, the results show information for optimal configurations. The results can be surnmarised in an empirical formula describing the relation between Strouhal number and geometrical parameters, with a specified unstable range. Computed jet’s frequencies were confirrned to be in good agreement with experimental ones, which indicates that the phenomenon is intrinsically two-dimensional. Numerical results reveal details of flow field and possibility for applications.
Applicability of the Vortex Methods for Automotive Flows
Authors: Ojima Akira , Kamemoto Kyoji
Published: February 2009

This paper describes recent works of practical applications of vortex element methods to study of automotive flows, carried by the authors’ group, explaining the mathematical basis of the method based on the Biot-Savart law. It is pointed as one of the most attractive features of the vortex method that the numerical simulation using the method is considered to be a new and simple technique of large eddy simulation, because they consist of simple algorithm based on physics of flow and it provides a completely grid-free Lagrangian calculation. As typical results of automotive flows, unsteady flows around a simplified vehicle model such as a wedge model, unsteady flows around a heavy vehicle model as a tractor-trailer, and unsteady aerodynamic characteristics of a tractor-trailer with meandering motion are explained.
Numerical Simulation of Partial Oxidization Processing of Diesel for Fuel Cells
Authors: Shi Shaoping , Rogers William A. , Berry David A. , Shekhawat Dushyant , Gardner Todd H. , Goldin Graham
Published: February 2009

The application of a reduced, fuel-rich chemistry mechanism for n-Heptane partial oxidation was valid over the temperature range from 900 K to 1200 K and at the O/C ratio of 1.57. In this work, n-Heptane was utilized as a single fuel representative for diesel fuel to quantify CFD – Chemistry interation. The turbulence-reaction model is one of the standard combustion models developed in FLUENT, a commercial CFD (Computational Fluid Dynamic) software package. In this model, n-Heptane reduced chemistry mechanisms were coupled with the multi-dimensional CFD solver. The interaction between the chemical reactions and turbulence has been considered. In this paper, some preliminary results are presented and comparisons between the experiments and the simulations are made.
Dimensional Effects in Interfacial Tension Micro-Scale Measurements
Authors: Alteraifi Abdullatif M.
Published: February 2009

Three different experimental configurations to measure oilwater interfacial tension (IFT) are developed using micropipet manipulation technique. By applying the minimum suction pressure required for the interface to flow in a micro-size pipet, IFT is calculated using Young-Laplace equation by achieving mechanical equilibrium condition for the two liquids separated by an interface. The three different configurations produced similar results for critical pressure measurements versus pipet diameter. IFT measurements using micropipet technique found to fall within two regimes: geometry-dependent regime and geometry-independent one. The notion of geometry-dependent IFT seemingly rises from the presence of a layer of an “ultra-structured” liquid exists near the solid surface. The layering effect appears to vanish at a pipet diameter of 40 μm, which is only applicable only to the present system comprised of crude oil, water, and glass. The IFT value obtained in the geometric-dependent regime is found to be in a good agreement with the value obtained from spinning droplet technique.
Existence Limits for Straight and Curved Interfaces Exposed to Temperature Gradients
Authors: Floryan J. M. , El-Gammal M.
Published: February 2009

Distortion of capillary interfaces exposed to external heating is analyzed. This problem is of interest in material processing under zero gravity conditions and in thermal management of spacecrafts. The analysis is focused on a model problem where thermocapillary effect is the dominant driving force. Flow response to an external heating consists of convection in the liquid and interface deformation, with both effects being strongly coupled. Detailed results are presented in the case of cavity with a free upper surface. It is demonstrated that large interface deformation and rupture represent the dominant response of such dynamical system if the interface is sufficiently long. Flow conditions corresponding to the limits points have been identified. Limit points identify extreme values of flow parameters that guarantee the existence of steady continuous interface. It is shown that rupture can be delayed by changing the mass of the fluid, i.e., changing the initial shape of the interface.
Dynamic Break-Up and Drop Formation From a Liquid Jet Spun From a Rotating Orifice: Part II — Theoretical
Authors: Parau Emilian , Decent Stephen , King Andrew , Simmons Mark , Wong David
Published: February 2009

We examine the dynamics of a spiralling slender liquid jet which emerges from a rotating cylindrical drum. Such jets arise in the manufacture of fertiliser and magnesium pellts in the prilling process. Exploiting the slenderness of the jet we determine the steady trajectory of the jet, and find that at leading-order it is a function of the rotation rate of the drum, the surface tension and density of the liquid, the exit speed and exit radius of the jet, the radius of the cylinder, but not of the viscosity of the liquid. We carry out a linear stability analysis of the steady solution, using both inviscid and viscous perturbations, and considering both temporal and spatial stability. We extend this approach by considering finite amplitude unsteady disturbances of a steady jet. We derive nonlinear partial differential equations for these disturbances. Solving these equations gives us a better approximation to the break up length of the jet, information on droplet size and distribution, as well as details of the dynamics of the break up of a spiralling jet. The numerical approximations were computed using a finite difference scheme and the time-integration method is a fully implicite scheme. A Lax-Wendroff method is used to solve the nonlinear finite difference equations.
Sharp Interface Capturing Methods Based on Level-Set Method for Incompressible Flow With Surface Tension
Authors: Couderc Frederic , Estivalezes Jean-Luc
Published: February 2009

In this paper, we present the development of a code for two-phase incompressible flow simulation with interface. Interface between non miscible fluid is captured by a level-set method. However, unlike classical approachs, jump conditions throught the interface are explicitely taken into account without any smoothing. Different tests cases are then simulated and show drastic improvement compared to existing methods like VOF or others.
Electrohydrodynamics and Surface Force Analysis in AFM Imaging of a Charged, Deformable Biological Membrane in a Dilute Electrolyte Solution
Authors: Fan Tai-Hsi , Fedorov Andrei G.
Published: February 2009

Surface forces arising in AFM imaging of a deformable, negatively charged biological membrane in an electrolyte solution are investigated in the limit of continuous electrohydrodynamics. Specifically, we extend our previous analysis [1] of purely hydrodynamic interactions between an AFM tip and the elastic cell membrane by accounting for electric double-layer forces under the assumption of a dilute electrolyte solution and local electrochemical equilibrium. The solution of the problem is obtained by integrating the quasisteady, electrically-forced Stokes equation for the electrohydrodynamic field, the linearized Poisson-Boltzmann equation for the electrostatic field in the electrolyte inside and outside of the cell, and the Laplace equation for the electrostatic field within a dielectric AFM tip. The Helfrich and Zhongcan’s equation for an equilibrium shape of the cell membrane is employed as a quasi-steady, nonlinear boundary condition linking the stress fields on both sides of the cell membrane augmented by the local membrane incompressibility condition in order to find the local tension/compression force acting on the membrane. For the first time, an integrated framework for the dynamic coupling of the membrane double-layer effects and the AFM tip-electrolyte-membrane motion is established that allows for characterizing of the local electrolyte flow field, the electrostatic field, the elastic deformation of the membrane, and the electrohydrodynamic surface force acting on the AFM tip in great detail. The results of the analysis provide information on the motion of the membrane and the surface forces induced by both an electrolyte motion and the Maxwell stresses resulting from the charge double-layer screening effect for a full cycle motion of the AFM tip in a non-contact mode imaging of the cell membrane.
Visualization and Measurements of Microdroplet Impact Dynamics on a Curved Substrate
Authors: Chow Chu Kei , Attinger Daniel
Published: February 2009

The basic problem of the impact of a microdroplet on a curved substrate is of central importance to micromanufacturing and targeted spray cooling of microdevices. The transient fluid dynamics involved in the droplet spreading and eventual splashing drastically influence the heat transfer involved in these processes. Although the fluid dynamics and heat transfer related to microdroplet impact on flat substrates is well documented, there is a lack of systematic study concerning microdroplet impact on a substrate of comparable curvature. Such a study is reported in this paper. Droplets of typical diameter and velocity of respectively 80 micrometer and 9 m/s were produced with a piezoelectrically-driven generator. The fluids used were filtered water and isopropanol. The size, velocity and temperature of the droplets were maintained constant. The primary variable is the substrate curvature, which was varied using a collection of spheres with diameters ranging from 60 micrometer to 10 millimeter. The dynamic behavior of the microdroplet during impact is visualized and quantified in this paper. Based on the experimental results, an existing analytical correlation that predicts the spreading temporal evolution is extended to account for substrate curvature.
Detection and Characterization of Microscale Breaking Waves
Authors: Siddiqui M. H. Kamran , Loewen Mark R.
Published: February 2009

Microscale breaking waves are short wind-generated waves that break without air entrainment. At low to moderate wind speeds microscale breaking waves play an important role in enhancing air-water heat and gas transfer. We report on a series of experiments conducted in a wind-wave flume at Harris Hydraulics Laboratory (University of Washington, Seattle) designed to investigate the importance of microscale breaking waves in generating near-surface turbulence and in enhancing air-sea gas and heat transfer rates. Non-invasive experiments were performed at wind speeds ranging from 4.5 m/s to 11 m/s and at a fetch of 5.5 m. The skin-layer or water surface temperature was measured using an infrared (IR) imager and digital particle image velocimetry (DPIV) was used to obtain simultaneous measurements of the two-dimensional velocities immediately below the water surface. Analysis of the simultaneous DPIV and infrared datasets revealed that microscale breaking waves generate strong vortices in their crests that disrupt the cool skin layer at the water surface and create thermal wakes that are visible in the infrared images. While non-breaking waves do not generate strong vortices and hence do not disrupt the skin layer. We developed a scheme based on the magnitude of vorticity in the wave crest to identify microscale breaking waves. The results show that at a wind speed of 4.5 m/s, 11% of the waves broke. The percentage of breaking waves increased with wind speed and at a wind speed of 11 m/s, 91% of the waves were microscale breaking waves. Comparison of different geometric and flow properties of microscale breaking and non-breaking waves revealed that microscale breaking waves are steeper, larger in amplitude and generate more turbulent kinetic energy compared to non-breaking waves.
Slip of Newtonian Fluid in the Polymer Hydrogel Tube Wall
Authors: Islam Mohammad Hamidul , Morikawa Hirohisa , Shirai Hirofusa , Hirai Toshihiro
Published: February 2009

Poly(vinyl alcohol) (PVA) hydrogel tube was prepared from PVA-dimethylsulfoxide/water solution by repetitive cooling method. The inner surface of the PVA hydrogel tube was chemically modified with poly(acrylic acid) (PAA). Slip of Newtonian fluid in intact and modified PVA hydrogel tubes (sub-millimeter inner diameter) was studied. The slip was inferred by observing the motion of micron-sized spherical monodispersed polystyrene particles suspended in the flowing water by using an optical microscope from very low to medium flow rate. The particle velocity was estimated by both particle tracking (PT) and particle image deformation (PID) method and the slip velocity were measured by extrapolating the experimental data of the velocity profiles on the gel surfaces. Microscopic measurements reveal that the slip velocities on the hydrogel surfaces depended on the chemical nature and the surface roughness of the hydrogel surfaces and it was proportional to Reynolds number. Slip velocity of modified PVA hydrogel tube was higher than intact PVA hydrogel tube.
Interfacial Area Transport of Bubbly Flow Under Microgravity Environment
Authors: Fukamachi Norihiro , Hazuku Tatsuya , Takamasa Tomoji , Hibiki Takashi , Ishii Mamoru
Published: February 2009

In relation to the development of the interfacial area transport equation, axial developments of one-dimensional void fraction, bubble number density, interfacial area concentration, and Sauter mean diameter of adiabatic nitrogen-water bubbly flows in a 9 mm-diameter pipe were measured by using an image-processing method under microgravity environment. The flow measurements were performed at four axial locations (axial distance from the inlet normalized by the pipe diameter = 7, 30, 45 and 60) under various flow conditions of superficial gas velocity (0.0083 m/s ∼ 0.022 m/s) and superficial liquid velocity (0.073 m/s ∼ 0.22 m/s). The interfacial area transport mechanism under microgravity environment was discussed in detail based on the obtained data and the visual observation. These data can be used for the development of reliable constitutive relations which reflect the true transfer mechanisms in two-phase flow under microgravity environment.
Lattice Boltzmann Simulation of Nucleation
Authors: Seta Takeshi , Okui Kenichi , Takegoshi Eisyun
Published: February 2009

We propose a lattice Boltzmann model capable of simulating nucleation. This LBM modifies a pseudo-potential so that it recovers a full set of hydrodynamic equations for two-phase flows based on the van der Waals-Cahn-Hilliard free energy theory through the Chapman-Enskog expansion procedure. Numerical measurements of thermal conductivity and of surface tension agree well with theoretical predictions. Simulations of phase transition, nucleation, pool boiling are carried out. They demonstrate that the model is applicable to two-phase flows with thermal effects. Using finite difference Lattice Boltzmann method ensures numerical stability of the scheme.
Numerical Simulation of Drop Formation From a Nozzle in Liquid-Liquid Systems
Authors: Homma Shunji , Honda Haruhisa , Koga Jiro , Matsumoto Shiro , Song Museok , Tryggvason Gre´tar
Published: February 2009

Numerical simulation code is developed to study the formation of liquid drops from a nozzle into another quiescent liquid. The Navier-Stokes equations for two immiscible, incompressible, Newtonian fluids are solved on a fixed, staggered grid of cylindrical axisymmetric coordinates. Interfacial motion is captured using a Front-Tracking Method. The time variation of interfacial shape simulated by the code is in excellent agreement with experiments. Simulation results show that the viscosity ratio affects the size of the satellite drops.
DNS of Particle-Turbulence Interaction
Authors: Balachandar S. , Bagchi P.
Published: February 2009

The effect of a freestream isotropic turbulence on the drag of a particle and its wake structure is studied through direct numerical simulations. We present here six different cases where the particle diameter is varied from 1.5 to 10 times the Kolmogorov scale, and the particle Reynolds number from about 50 to 600. The DNS results show that turbulence has no substantial effect on the time-averaged mean drag which can be reasonably well predicted by standard drag correlations, such as the Schiller-Neumann law. The accuracy of prediction of the instantaneous drag however decreases with increasing particle size. The inclusion of the added-mass force may worsen the prediction, while the Basset history force is observed to be very small. The mean wake in a turbulent flow shows a shortened wakelength compared to the uniform flow result at the same Reynolds number. The instantaneous wake structures for the different cases are studied and the salient features are presented. For the smallest particle, the wake is simply oscillating without any shedding. For the particle of intermediate size, freestream turbulence creates unsteady wake, and Λ-shaped vortices are formed. For the largest particle, the wake is characterized by strong unsteady shedding but unlike in a uniform flow the vortices do not maintain their structural integrity and they quickly dissipate downstream.
Computations of Boiling
Authors: Esmaeeli Asghar , Tryggvason Gre´tar
Published: February 2009

A numerical method for boiling flows, based on a finite volume/front tracking approach, is described. The method is used to examine film boiling and results from two simulations are discussed briefly. In one case the system is assumed to be two-dimensional and the breakup of many bubbles from the film is followed for a sufficiently long time so that it is possible to compute the average heat transfer rate. The other simulation is a fully three-dimensional system, but only one mode is followed for a relatively short time. In both cases the heat transfer rate is in reasonably good agreement with experimental correlations.
Computational Modeling and Analysis of Airflow in a Tritium Storage Room
Authors: Chen Z. , Konecni S. , Whicker J. J.
Published: February 2009

In this study, a commercial computational fluid dynamics (CFD) code, CFX-5.5, was utilized to assess flow field characteristics, and to simulate tritium gas releases and subsequent transport in a storage room in the tritium handling facility at Los Alamos. This study was done with mesh refinement and results compared. The results show a complex, ventilation-induced flow field with vortices, velocity gradients, and stagnant air pockets. This paper also explains the time-dependent gas dispersion results. The numerical analysis method used in this study provides important information that is possible to be validated with an experimental technique of aerosol tracer measurement method frequently used at Los Alamos. Application of CFD can have a favorable impact on the design of ventilation systems and worker safety with consideration to facility costs.
The Effect of Turbulence and Scalar Transport Models on Prediction of Worker Exposure to Aerosols
Authors: Li Jun , Celik Ismail B. , Yavuz Ibrahim , Guffey Steven E. , Bird Aaron J.
Published: February 2009

The present study is concerned with the relative concentrations of contaminants in the breathing-zone (BZ) when toxic airborne contaminants are released within an armlength in front of a worker, a common location that often leads to high exposures. Three-dimensional turbulent flow around a modeled human body in a wind tunnel has been simulated numerically and the results from various models are compared with each other. A model was built using a mesh generator to represent a real human body as closely as possible. Four different turbulence models, namely, standard k-ε turbulence model, RNG k-ε turbulence model, Reynolds Stress turbulence model (RSM) and Large Eddy Simulation (LES), were used to evaluate their effect on the predicted concentration levels used in assessing the exposure of the worker. Results from Eulerian scalar transport method and Lagrangian particle tracking method are also compared. Concentration levels are calculated at various sampling locations in the vicinity of the human face. It is found that the predicted concentration varies significantly from model to model: at the breathing zone the coefficient of variation for predicted concentration is 30%.
Automatic Shape Optimisation of a Hydropower Draft Tube
Authors: Marjavaara B. D. , Lundstro¨m T. S.
Published: February 2009

Draft tube designs have to a large extend been based on intuition and on the experience of the design engineer. In a close future, CFD simulations coupled with optimisation algorithms will assist in the search for an optimal technical solution. Such a shape optimisation technique to redesign an existing draft tube is presented in this paper. By this method, a design can be predicted in terms of a predefined objective function, here the pressure recovery factor. The optimisation is performed with the Response Surface Method (RSM) implemented on the commercial code iSIGHT7.0, while the CFD simulations are carried out with the commercial code CFX4.4. The boundary conditions are based on detailed experimental data and the turbulence is modelled with the standard k-ε turbulence model. With this set-up and with two different parameterisations, Adapted and Profile Design respectively, an optimal geometry of the ERCOFTAC Turbine-99 draft tube can be predicted.