bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical Simulation of the Unsteady Flow in a High-Head Pump Turbine and the Runner Improvement
Authors: Ran Hongjuan , Luo Xianwu , Zhang Yao , Zhuang Baotang , Xu Hongyuan
Published: July 2009

The unsteady flow in a high-head pump-turbine whose head-discharge curve has the positive slopes at high-partial-load operation condition was investigated. It is noted that the numerical methods is very important for predicting this kind of head-discharge curve with positive slopes, and better agreement between calculation results and experimental data was achieved by using Spalart-Allmaras turbulence model and mesh strategy with y+ controlling for numerical simulation. From the analysis of hydraulic losses at different parts in the pump turbine, it is found that the head loss at the flow passage of the guide vane and stay vane was not small at pump mode. In order to make clear the reason why the positive slopes at head-discharge curve occur, the flow between the impeller exit and the inlet of spiral casing was checked carefully. Much intensive vortex was observed near the impeller shroud, and there was strong rotor stator interaction for those operation conditions with positive slope. It is suspected the instability such as positive slope at head-discharge curve was resulted from the vortex formation near the flow channel wall. Based on the flow analysis, the runner optimization was conducted so as to mitigate the intensive rotor stator interaction. It is noted that the pressure fluctuation as well as the flow pattern was improved by applying the optimized impeller.
A Condensed Phase Computational Fluid Dynamics Model for Polymer Melt Flow
Authors: Tang Qing , Bockelie Michael
Published: July 2009

This paper presents a condensed phase computational fluid dynamics (CFD) based tool for modeling the processes of melting, flow and gasification of thermoplastic materials exposed to a high heat flux. Potential applications of the tool include investigating the behavior of polymer materials commonly used in personal computers and computer monitors if exposed to an intense heat flux, such as occurs during a fire. The finite-volume based model uses a three-dimensional body-fitted time dependent grid formulation to solve the unsteady Navier Stokes equations. A multi-grid method is used to accelerate convergence at each time step. Sub-models are included to describe the temperature dependent viscosity relationship and in-depth gasification and absorption of thermoplastic materials, free surface flows and surface tension. A series of test cases have been performed and the model results are compared to experimental data to investigate the impacts of different sub-models, boundary conditions, material properties and problem configurations on the accuracy, efficiency and applicability of the modeling tool.
Analytical Determination of Viscous Permeability of Fibrous Porous Media
Authors: Tamayol A. , Bahrami M.
Published: July 2009

In this study, the permeability of ordered fibrous media towards normal and parallel flow is determined analytically. In this approach, porous material is represented by a “unit cell” which is assumed to be repeated throughout the media. Several fiber arrangements including: touching and non-touching arrays are considered. Modeling 1D touching fibers as a combination of Channel-like conduits, a compact relationship is proposed to predict permeability. Employing an integral technique and assuming a parabolic velocity profile within the unit cells, analytical relationships are developed for pressure drop for rectangular arrangements. The developed models are successfully compared with existing experimental data collected by others for square arrangement over a wide range of porosity. Due to the random nature of the porous micro structures, determination of exact permeability of real fibrous media is impossible. However, the analyses developed for ordered unit cells enable one to predict the trends observed in experimental data. Moreover, it is shown that the proposed normal flow permeability of square unit cell serves as a lower bound for the permeability of fibrous media. The effects of unit cell aspect ratio and fibers diameter on the permeability are also investigated. It is noted that with an increase in the aspect ratio the normal permeability decreases while, the parallel permeability remains constant. It is also shown that the permeability of fibrous media is related to the diameter of fibers squared.
Experimental Study on the Bursting Phenomenon in the Turbulent Boundary Layer on the Compliant Wall
Authors: Owaku Yui , Misu Isao , Fujimatsu Nobuyoshi
Published: July 2009

This paper describes an experimental study on the friction drag reduction on the compliant wall. In the fluid dynamics, the research on the turbulent friction drag reduction is one of the important subjects. The use of the compliant wall has the possibilities of the friction drag reduction. Many researchers have investigated the effects of the compliant wall since 1960’s. However, the past studies are focused on those effects such as dependency of the material properties and flow conditions. Those effects are intimately related with the turbulent flow fields. In order to clarify the mechanism of the drag reduction, the coherent structures near the wall surface must be investigated. The purpose of the present study is to investigate the vortical semi-periodic motions over the solid and compliant walls in detail. The experiments are conducted at Re = 5.0 × 105 based on unit length and the free stream velocity using the NPL type wind tunnel. The compliant wall is made of silicone resin with Young’s modulus E = 2.83 MPa. The boundary layer flows are measured using the single and X-array hot wire probe. The VITA and four quadrant analyses are applied to investigate the bursting phenomena in the turbulent flow structures. The experimental results show that the skin friction drag over the compliant wall is smaller than that on the solid wall about 15%. Further the bursting frequencies over the compliant wall are greater than those over the solid wall. The four quadrant analysis is also applied to investigate the turbulent coherent motions. These results show that the ejection event over the compliant wall is increased. The contribution rate of the ejection event to the Reynolds stress over the compliant wall is greater than that of the sweep event. The increment of the bursting frequency is consistent with that of the ejection event. The friction drag decreases because the velocity near the wall is decelerated when the contribution rates in second and third quadrants to the Reynolds stress increase. These tendencies are also confirmed over the compliant wall with the various thicknesses. The present results show that the friction drag reduction over the compliant wall is related with the increment of contribution rate of the ejection event in the bursting phenomena.
Numerical Simulation of Bubble Formation in Co-Flowing Mercury
Authors: Ibrahim Ashraf , Wendel Mark , Felde David , Riemer Bernard
Published: July 2009

In this work, we present computational fluid dynamics (CFD) simulations of helium bubble formation and detachment at a submerged needle in stagnant and co-flowing mercury. Since mercury is opaque, visualization of internal gas bubbles was done with proton radiography (pRad) at the Los Alamos Neutron Science Center (LANSCE2). The acoustic waves emitted at the time of detachment and during subsequent oscillations of the bubble were recorded with a microphone. The Volume of Fluid (VOF) model was used to simulate the unsteady two-phase flow of gas injection in mercury. The VOF model is validated by comparing detailed bubble sizes and shapes at various stages of the bubble growth and detachment, with the experimental measurements at 1.66 mg/min helium gas flow rate and different mercury velocities. The experimental and computational results show a two-stage bubble formation in stagnant mercury. The first stage involves growing bubble around the needle, and the second follows as the buoyancy overcomes wall adhesion. The comparison of predicted and measured bubble sizes and shapes at various stages of the bubble growth and detachment is in good agreement.
A Preliminary Study of the Transition of an In-Line Pipe Vortex to Slug Flow Using Particle Image Velocimetry
Authors: de Witt Benjamin J. , Hugo Ronald J.
Published: July 2009

In this preliminary work, the performance of a Slug Flow Expander (SFE) was investigated. Multiphase flows consisting of air and water were injected into the SFE, a device designed to generate quality slug flow at regular intervals. It was observed that for water flow rates of 1.25 L/s & 1.50 L/s combined with air flow rates above 1.0 L/s, the flow regime within the SFE transitioned to slug flow. A LaVision Flowmaster3 stereo digital PIV system was used to perform two-component velocity measurements on the SFE design prototype. The current investigation focused on discerning the velocity profile of the liquid component within the SFE. Radial and azimuthal velocity components were extracted from the mean velocity field data. Circulation and vorticity were then computed from the azimuthal velocity component by Stokes theorem, and were used to characterize the flow physics of the liquid component as a function of increasing air flow rate. It was found that the velocity profile may be closely approximated with an Oseen vortex.
Stability of Concave Boundary Layers: Overview of Stability Mechanism and Recent Findings
Authors: Momayez Ladan , Dupont Pascal , Delacourt Guillaume , Peerhossaini Hassan
Published: July 2009

A series of experimental measurements of flow and heat transfer under streamwise Go¨rtler vortices shows conclusively that the local surface heat transfer rates can exceed that of the turbulent boundary layer even in the absence of turbulence. We have observed unexpected behavior of heat transfer in a laminar boundary layer on a concave wall at low nominal velocity, a configuration ignored in the literature. In this situation, precise measurements of the wall heat flux show that the heat transfer enhancement is extremely elevated, above that corresponding to the case of a turbulent boundary layer on a flat plate. The nonlinearly developing steady streamwise vortex (primary instability) heat transfer can already bridge the local laminar to turbulent heat transfer values in the absence of turbulence. The analysis shows that for a range of velocities less than a certain critical velocity, the transitional boundary layer is dominated by centrifugal instability. However, the steady streamwise vortices, like steady Taylor vortices between coaxial rotating cylinders, are susceptible to secondary instabilities of the varicose and sinuous modes. In experiments both modes appear to coexist and cause waviness of the primary streamwise vortices. Other results confirm this discussion based on analysis of the influence of a forcing upstream disturbance.
The Impact of a Vortex Induced by a Baffle on the Turbulent Structure
Authors: Gajusingh Shivani T. , Shaikh Nasiruddin , Siddiqui Kamran
Published: July 2009

An experimental study was conducted to investigate the impact of a rectangular baffle inside a square channel. PIV was used to measure the two-dimensional velocity fields. The measurements were conducted for two Reynolds numbers (Reτ) equal to 134 and 152 in the turbulent regime. Results show that the turbulent intensities and the Reynolds Stress are enhanced by more than an order of magnitude in the presence of a baffle. Significant enhancement was observed in a region up to two times the baffle height.
The Effect of Casing Shape on Performance of Sirocco Fan Using Contra-Rotating Rotors
Authors: Shigemitsu Toru , Fukutomi Junichiro , Shirai Yutarou
Published: July 2009

A sirocco fan using contra-rotating rotors in which an inner rotor is settled inside the sirocco fan rotor and each rotor rotates in an opposite direction was proposed for the purpose of getting the higher pressure and making the structure of a sirocco fan compact more. If the high discharge pressure is obtained with the adoption of the contra-rotating rotors, it could be used for various purposes. Pressure coefficient of a sirocco fan with contra-rotating rotors is 2.4 times as high as the conventional sirocco fan and the maximum efficiency point of contra-rotating rotors shifts larger flow rate than a conventional sirocco fan. On the other hand, it was clarified from the flow measurement results that the larger back flow region than conventional one occurred at the shroud of the casing in the case of contra-rotating type and this would induce decrease of the efficiency. In the present paper, the performance of a conventional sirocco fan and a sirocco fan with contra-rotating rotors are shown and the internal flow field at the outlet of the outer rotor of both cases is clarified. Then, the effect of casing width on the performance and internal flow field is investigated and the casing shape which would be suitable for the contra-rotating type would be discussed.
The Dynamics of Accumulating Ferrofluid Aggregates
Authors: Williams Alicia M. , Vlachos Pavlos P.
Published: July 2009

The physics of steady and pulsatile flows laden with superparamagnetic nanoparticles in a square channel accumulating under the influence of a 0.5 Tesla permanent magnet are studied by means of focused shadowgraphs. The accumulation physics of these nanoscale particles is explored as functions of the flow type (steady and unsteady) and accumulation type (injected from the top of channel versus bottom of channel). Ferrofluid is accumulated by the steady injection of a streakline that enters the test section upstream of the magnet, where an aggregate forms. The interfacial phenomena resulting from the interaction of the ferrofluid with the bulk flow is resolved using shadowgraph imaging, where a digital camera captures the side view of the aggregate. Ferrofluid aggregate physics is examined both visually in the raw frames as well as by post-processing to determine the aggregate size evolution in time and couple that bulk information with interfacial behavior using the Proper Orthogonal Decomposition (POD). The shadowgraphs show that the aggregate exhibits different regimes based on bulk flow Reynolds number, which is varied between 100 and 1000, based on the mean flow rate. The aggregate exhibits stable behavior at low Reynolds numbers, where it stretches as it grows and minimal decay of the aggregate occurs. At moderate Reynolds numbers above 400, inertial forces dominate the dynamics, and aggregates do not attain the same size and height as in low Reynolds number cases. Therefore, the interaction of the aggregate with the bulk flow is diminished. The accumulation of ferrofluids is positively impacted by increased magnetic field gradients for some Reynolds numbers, while very high or low magnetic field gradients result in smaller, unstable aggregates. This work is the first to study the accumulation of ferrofluid aggregates over such a large parameter space, which reveals many physics that were previously unexplored in ferrohydrodynamics.
3D Computational Modeling and Simulation of Cell Motion on Adhesive Surfaces in Shear Flow
Authors: Pappu Vijay , Bagchi Prosenjit
Published: July 2009

A three-dimensional computational fluid dynamic (CFD) model is presented to simulate transient rolling adhesion and deformation of leukocytes over a P-selectin coated surface in shear flow. The computational model is based on immersed boundary method for cell deformation, and stochastic Monte Carlo simulation for receptor/ligand interaction. The model is shown to predict the characteristic ‘stop-and-go’ motion of rolling leukocytes. The objective here is to understand the coupling between external shear flow, cell deformation, microvilli deformation and various biophysical parameters that govern the formation of selectin bonds. We observe that compliant cells roll more stably with lesser fluctuations. Adhesion is seen to occur via multiple tethers, but often one tether is sufficient to support rolling. The force loading on individual microvillus is not continuous, rather occurs in steps. Further, it is also shown that only the microvilli whose undeformed length is above a certain cut off length, participate in bond formation and the cutoff length reduces with increasing cell rigidity.
Characteristics of Micro Pump Driven by Conducting Polymer Soft Actuator
Authors: Naka Yoshitaka , Fuchiwaki Masaki , Tanaka Kazuhiro
Published: July 2009

The development of micro pumps are actively conducted in recent years. A micro pump used for μ-TAS transports at a micro flow rate with a high precision. Moreover, it is also needed to transport high-viscosity fluid since there are various types of drugs to be transported. Micro pumps with various driving systems have been developed so far. In this study, the authors propose a micro pump with soft actuators by conducting polymers as a driving source. We have realized the conducting polymer soft actuator with opening and closing movement. The opening and closing movement of the soft actuator in which the cation driving layer is arranged inside becomes large since the anion driving layer arranged outside drives predominantly. The opening and closing movement is realized by the characteristic that three dimensional deformations are suppressed by setting slits in a sheet-shaped soft actuator and straight-shaped soft actuators in the slits synchronize and deform. It is possible to build a micro pump that transports fluid in one direction by a micro pump with two soft actuators with opening and closing movement and that it can transport fluid even with the viscosity that is 140 times as large as that of water in addition. The micro pump with two soft actuators with opening and closing movement proposed in this paper transports fluid with an energy consumption rate less than half those of others.
Stretching Fields and Flow Mixing Enhancement of Rarified Gases in Micro-Grooved Channels by the Lattice-Boltzmann Method
Authors: Guzma´n Amador M. , Sanhueza Luis E. , Di´az Andre´s J. , Escobar Rodrigo A.
Published: July 2009

The Eulerian and Lagrangian characteristics of a rarified gas flow have been investigated and related to flow mixing enhancement in micro grooved channels. The governing Boltzmann Transport Equation (BTE) is solved by the Lattice-Boltzmann method (LBM) for the Knudsen number range of 0.01−0.1. First, the Eulerian flow characteristics are determined by performing numerical simulations for different Knudsen numbers, pressure ratio and accommodation coefficients with the objective of obtaining reliable velocity characteristics and flow patterns, and determining the transition characteristics from the macro to microscale. The numerical predictions are compared to existing analytical and numerical results. Then, the Lagrangian characteristics are obtained by integrating the Eulerian velocity field by a 4t order Runge-Kutta scheme. Ten thousands (10,000) pairs of fluid particles are used to determine fluid particle Lagrangian trajectories, future stretching fields, and Lagrangian Lyapunov exponents, which are used for determining the grooved channel regions with high and low flow mixing enhancement. Our results demonstrate that rarified gas flows develop a future stretching field leading to the existence of Lagrangian chaos and flow mixing enhancement for very low, stable and time independent Reynolds number flow regime. This flow behavior departure from the well accepted concept that Lagrangian chaos and flow mixing enhancement need a time dependent 2D flow.
Study on the Hydraulic Performance of Elbow-Inlet Passage by CFD
Authors: Cheng Li , Liu Chao , Zhou Jiren , Tang Fangping , Jin Yan
Published: July 2009

The Elbow-inlet passage with low loss and little width is a typical passage of pumping station. A new optimization criterion for hydraulic design is put forward. The incompressible N-S equations are solved by the finite volume method. Base on the RNG k-ε model with Wall-Function Law, the SIMPLEC algorithm is applied for the solution of the discretization governing equation. Using CFD, the flow pattern in passage is revealed. The flow patterns in the elbow-inlet passage are classified as three patterns phases: horizontal contraction, elbow bend and vertical phase. According to axial velocity of ten sections, the new criteria of named height of pump (NHP) and other control parameters for Elbow-inlet passage is suggested. A good agreement is achieved between the numerical simulation and experimental results. The computational model is much more effective for hydraulic performance study. The optimum hydraulic design method was established: First, the shape of passage is designed on the optimum control parameters. Second, the sections of passage are designed according to the smooth average velocity. Finally, flow patterns are studied in order to find its hydrodynamic characteristics. This method could simply the design of Elbow-inlet Passage.
Meniscus Shape and Optical Performance of a MEMS-Based Liquid Micro-Lens System
Authors: Lee Shong-Leih , Yang Chao-Fu
Published: July 2009

It is very difficult to fabricate tunable optical systems having an aperture below 1000 micrometers with the conventional means on macroscopic scale. Krogmann et al. (J. Opt. A: Pure Appl. Opt. vol. 8, 2006, pp. S330-S336) presented a MEMS-based tunable liquid micro-lens system with an aperture of 300 micrometers. The system exhibited a tuning range of back focal length between 2.3 mm and infinity by using the electrowetting effect to change the contact angle of the meniscus shape on silicon with a voltage of 0−45V. However, a serious optical aberration was found in their lens system. In the present study, a numerical simulation is performed for this same physical configuration by solving the Young-Laplace equation on the interface of the lens liquid and the surrounding liquid. The resulting meniscus shape produces a back focal length that agrees with the experimental observation excellently. To eliminate the optical aberration, an electric field is applied on the system. The electric field alters the Young-Laplace equation and thus changes the meniscus shape and the quality of the lens system. The numerical result shows that the optical aberration of the lens system can be essentially eliminated when a proper electric field is applied.
Numerical Prediction Method of Cavitation Erosion
Authors: Nohmi Motohiko , Ikohagi Toshiaki , Iga Yuka
Published: July 2009

Bubble behavior in cavitating flow is analyzed for the development of practical erosion prediction method. CFD analysis with cavitation model is carried out for the flow field around a hydrofoil. Afterwards computation of bubble dynamics is carried out coupled with flow field CFD results by one way approach. For the bubble dynamic calculation, Rayleigh-Plesset equation is adopted. Bubble behaviors in the collapse of cloud cavitaion and in the break off of sheet cavity are analyzed. Bubble behavior at the trailing edge of sheet cavity is also calculated. It is observed that steep pressure change in the flow causes oscillation of the bubbles. Based on this qualitative information of bubble behaviors, numerical cavitation aggressiveness is simply defined. This numerical cavitation aggressiveness is a function of local void fraction and pressure over the solid surface and can be calculated directly from the cavitating flow field CFD results without concerning bubble dynamics.
Pressure Loss of Gaseous Flow at Microtube Outlet
Authors: Horii Y. , Asako Y.
Published: July 2009

The pressure loss of gaseous flow at a micro tube outlet was investigated numerically. The numerical methodology is based on the Arbitrary-Lagrangian-Eulerian (ALE) method. Axis-symmetric compressible momentum and energy equations are solved to obtain the pressure loss coefficient of gaseous flow at a micro tube outlet. Computed tube radiuses are 25, 50 and 75 μm. The stagnation pressure of upper stream of the tube is chosen in such a way that the Mach number at the tube outlet ranges from 0.1 to 1.3. The ambient (back) pressure is fixed at the atmospheric pressure. The pressure loss coefficients are compared with available experimental data for a conventional sized tube. The effects of the Ma number and the tube radius on the pressure loss coefficient are discussed and the correlation for the pressure loss coefficient is proposed.
Numerical Study of Droplet Motion in a Hydrophilic/Hydrophobic Microchannel
Authors: Choi Jiyoung , Son Gihun
Published: July 2009

The droplet dynamics in a hydrophilic/hydrophobic microchannel, which is applicable to a typical proton exchange membrane fuel cell (PEMFC), is studied numerically by solving the equations governing the conservation of mass and momentum. The liquid-gas interface or droplet shape is determined by a level set method which is modified to treat the contact angles. The matching conditions at the interface are accurately imposed by incorporating the ghost fluid approach based on a sharp-interface representation. Based on the numerical results, the droplet dynamics including the sliding and detachment of droplets is found to depend significantly on the contact angle. The effects of inlet flow velocity, droplet size and side wall on the droplet motion are quantified. Also, a droplet removal process is demonstrated on the combination of hydrophilic and hydrophobic surfaces.
Prediction on Meter Factor of the Turbine Flowmeter With Unsteady Numerical Simulation
Authors: Chen Gang , Wu Yulin , Cao Guangjun , Li Mingjie , Fu Suhong
Published: July 2009

The turbine flow meter is widely used in the flow rate measuring for its high accuracy and good repeatability. The flow rate will be calculated based on its meter factor, which is the most important factor of the turbine flow meter. The meter factor means pulses or revolution of the impeller per unit volume, and it can only be got from the calibration experiment. At the given flow rate, the driving torque on the impeller is equal to the drag torque, as many paper have pointed out. Based on the torque balancing equations, unsteady numerical simulation is carried out with RNG turbulence model and UDFs (User Defined Functions) in Fluent Code. The meter factor under different flow rate is calculated with the unsteady simulation. The prediction results based on the numerical simulation showed the same trends as the calibration experiment. At the most flow rate, the meter factor keeps constant, but at the lower flow rate, the meter factor higher than the constant. Because of neglecting the bearing friction drag in the process, the meter factor by numerical simulation is larger than experiment.
Fourier Decomposed Turbulence Measurements Downstream of a High-Pressure Turbine Stage
Authors: Axelsson Lars-Uno , George William K. , Johansson T. Gunnar
Published: July 2009

This forum paper discusses phase-resolved turbulence measurements of the flow downstream of an axial turbine, and especially how phase-resolved measurements compare to time-resolved measurements. The time-resolved spectra produced higher velocity fluctuations than the corresponding phase-resolved spectra even when the periodic component was filtered out from the time-resolved measurements. The phase-resolved spectrum effectively removes all the peaks in the spectrum except for the ones associated with the blade-passage frequency.
Improved DPIV Accuracy Using Advanced Windowing Techniques
Authors: Eckstein Adric , Vlachos Pavlos
Published: July 2009

The Fourier based cross-correlation is the most common evaluation technique for DPIV estimation, due to its computational simplicity. However, because Fourier transforms are taken over discrete finite size regions, several systematic errors are introduced due to the improper windowing of the input signals. This study explores the potential of advanced windowing techniques to attenuate these Fourier based errors. The choice of window is shown to impact the spatial resolution, the measurement accuracy, and the peak detection process. Error analysis using artificial image simulations is able to characterize a set of optimal windows onto a single performance characteristic. Using this analysis, a set of criteria is defined for an optimal window, from the 50% Gaussian window is selected. The Gaussian window is further compared against standard evaluation techniques in both shear and vortex simulations, which indicate substantial performance benefits with this advanced technique. Further simulations reveal that background noise greatly amplifies the loss of correlation errors, which affect the peak detection process for non-rectangular windows. However, these effects are easily overcome through the use of image preprocessing or the Robust Phase Correlation (RPC). Images from the 2003 PIV challenge are used to validate the Gaussian window technique, which is able to remove nearly all of the erroneous vectors in comparison to standard windows.
Pressure and Acceleration Determination Methods Using PIV Velocity Data
Authors: Gharali Kobra , Johnson David A.
Published: July 2009

Simultaneous knowledge of the entire pressure, acceleration and velocity fields in a region of a flow is a major factor in understanding and modeling a case under study, especially for fluid dynamic and engineering applications. At present, the accuracy of the velocity map coming from particle image velocimetry (PIV) using higher order cross-correlation algorithms with advanced post processing including filters, removing and replacing the odd data, and smoothing functions is in an acceptable range. Using PIV velocity data to determine the acceleration and pressure distribution causes a kind of error accumulation; thus, the inaccuracy of the acceleration and pressure data is several times greater than that of the velocity data; therefore, the need for accurate algorithms cannot be ignored. In this paper, a synthetic image generation code is used to create benchmark images for an unsteady forced vortex flow with known velocity, acceleration and pressure data. These known data are necessary to investigate the accuracy of the results. Different acceleration methods including pseudo-tracing, regression and central finite difference are introduced and compared. In addition, the influence of some involved parameters, the time interval between the velocity fields (Δt), cell size and overlapping is studied synthetically. The results show that the methods strongly depend on the time interval Δt, and increasing it improves the accuracy until a critical Δt is reached. In steady flows, the methods are time independent, but for the tracing method, a time step is introduced. The tracing method among all methods represents the most accurate acceleration results for both steady and unsteady flows. Navier-Stokes equations are used as the pressure-estimation method since they show more details of the flow field. Pressure gradients are integrated by using a numerical integration method that shows high accuracy for images with no bluff body inside them.
Numerical Study of a Hemi-Spherical Cup in Steady, Unsteady, Laminar and Turbulent Flow Conditions With a Vent of Air at the Top
Authors: Izadi Mohammad J. , Mohammadizadeh Mehdi
Published: July 2009

In this paper, a 3D flow field around a bluff body (Hemi-spherical cup) in an incompressible fluid is modeled. On the top of the bluff body at its apex a vent of air is inserted. The vent ratio (Ratio of vent diameter to hemi-spherical cup diameter) at the apex of the body is varied. Four flow conditions are considered in this study. At first, the flow is taken to be steady and laminar, then unsteady and laminar, and later steady and turbulent, and finally unsteady and turbulent. Coefficients of drag, lift and pressure, and also the drag force with static pressure around the body are determined. From the four conditions investigate and comparing with the work done by Strickland, it can be seen that the steady and unsteady in turbulent condition gave better results. Also as the vent ratio increased, the drag coefficient decreased, and the highest drag obtained around 10 percent vent ratio.
Effect of Blade Angle of Attack and Hub to Tip Ratio on Mass Flow Rate in an Axial Fan at a Fixed Rotational Speed
Authors: Izadi Mohammad J. , Falahat Alireza
Published: July 2009

In this investigation an attempt is made to find the best hub to tip ratio, the maximum number of blades, and the best angle of attack of an axial fan with flat blades at a fixed rotational speed for a maximum mass flow rate in a steady and turbulent conditions. In this study the blade angles are varied from 30 to 70 degrees, the hub to tip ratio is varied from 0.2 to 0.4 and the number of blades are varied from 2 to 6 at a fixed hub rotational speed. The results show that, the maximum flow rate is achieved at a blade angle of attack of about 45 degrees for when the number of blades is set equal to 4 at most rotational velocities. The numerical results show that as the hub to tip ratio is decreased, the mass flow rate is increased. For a hub to tip ratio of 0.2, and an angle of attack around 45 degrees with 4 blades, a maximum mass flow rate is achieved.
Analysis and Optimization of Flow Distribution Channels for Uniform Flow in Fuel Cells
Authors: Li Peiwen , Coopamah Devasubramaniam , Dhar Nikesh
Published: July 2009

The paper presents the results of investigation about flow distribution and uniformity of flow fields in three types of flow distributors with novel design. The flow distributors have a number of bifurcations to split one flow into a number of sub-streams of equal flow rates on a bipolar plate which may accommodate the electrochemical reaction in a fuel cell. There are three types of bifurcation structures studied, which are the 90° tee-type, rounded-type, and slanted-type. Numerical analysis and experimental test was conducted to investigate the flow uniformity in a multiple channels that come from several stages of bifurcations. Pressure drops of the whole flow field from inlet to outlet of the bipolar plates were also studied in the numerical simulation. Overall evaluation of flow uniformity and pressure drops in the three designs was also conducted.
Explorations of Improving Flow Uniformity in the Bipolar Plate of a PEM Electrolysis Cell Using Different Designs
Authors: Nie J. H. , Chen Y. T. , Wu J. F. , Veepuri K. M.
Published: July 2009

Optimization of electrolysis cell for producing hydrogen is dependent of a set of complex physical and chemical processes occurring simultaneously. Similar to fuel cells, it has been demonstrated that these processes are strongly dependent on the fluid dynamics inside the fuel cell. Bipolar plates are important components of PEM electrolysis cells because they are the first stage of the flow distribution system. A non-uniform flow distribution across the bipolar plate surface area will probably lead to an unbalanced use of the precious catalyst, and an overall efficiency of the device lower than expected. In the present work various concepts were tested for the purpose of improving flow uniformity in the bipolar plate of a PEM electrolysis cell for hydrogen generation. Numerical results including pressure distributions and velocity profiles are reported. It is shown that the flow uniformity within the designed bipolar plate is greatly improved compared with the baseline bipolar plate for water electrolysis.
Numerical Simulations of Coupled Flow and Heat Transfer Distributions in a Bipolar Plate of the PEM Electrolysis Cell
Authors: Nie Jianhu , Wu Jianfei , Cohen Steve , Carter Blake , Chen Yitung
Published: July 2009

Hydrogen is expected to play an important role as an energy carrier of the future. Hydrogen may be used as fuel in almost every application where fossil fuels are being used today, but without harmful emissions. However, hydrogen is not an energy source, and it does not occur in nature in its elemental or molecular form. Three-dimensional CFD modeling and experimental measurements of a simplified bipolar plate of the PEM electrolysis cell were performed. The computed pressure drop and temperature distribution agree very well with the measurements. The results show that the maximum temperature appears in the fluid channels and near the exit header section, but not in the exit port. The velocity distribution in the fluid channels is very non-uniform over the test plate. A minimum of the peak values of mainstream velocity component in the channels develops in the middle of the plate. The maximum of these peak values appears in the channel near the exit tube.
LES Sub-Grid Diffusion for Lagrangian Particles
Authors: Rybalko Michael , Loth Eric , Lankford Dennis
Published: July 2009

A continuous random walk (CRW) turbulent diffusion model was developed for Lagrangian particles within flow fields simulated by hybrid RANS/LES methodologies. For RANS flow-fields, the conventional time-scale and length-scale constants were determined by the turbulence intensity and dissipation values computed by the single-phase solver with a k-ω (Menter SST) model and subsequent comparison with turbulent particle diffusion experimental results of Snyder & Lumley (1971). This allowed validation against data for four particle types ranging from hollow glass to copper shot in grid-generated turbulence. The stochastic diffusion model was then extended to utilize the Nichols-Nelson k-ω hybrid RANS-LES turbulence model in a more complex turbulent flow resulting from the unsteady, three dimensional wake of a cylinder at Mach number of 0.1 and Reynolds number (ReD) of 800. The gas flow was computed with a 5th-order upwind-biased scheme. Throughout the wake, the sub-grid random walk model yielded good predictions of particle diffusion as compared with DNS. Also, these results indicate that crossing trajectory effects and inertia-based drift corrections are critical to handling a variety of particle Stokes numbers as well as regions of non-homogeneous turbulence, even when most of the kinetic energy is captured with the resolved-scales of an LES approach.
Fluid-Power Harvesting by Pneumatic Bellow During Human Gait
Authors: Chin Robin , Hsiao-Wecksler Elizabeth T. , Loth Eric
Published: July 2009

In this paper, we describe power and pressure characteristics of bellows designed for under-foot power harvesting during human walking for a single cycle. This single cycle corresponds to the typical human gait cycle (heel-strike and stance on the floor to toe-off and free swing to subsequent re-contact with the floor, with a total duration of about one second). A bellow can be placed in a shoe insole and compressed during initial heel strike or during the mid-stance portion of the gait cycle. The compressed fluid can then be used for power applications during the remaining portion of the cycle. A collapsible spring was placed inside the bellow to extend it when the foot is off the ground, yet allow the bellow to be compressed. Air is drawn into the bellow through a one-way valve allowing the bellow to recharge as it expands during the swing phase of the gait cycle. Thus, body weight is used as the power source for a self-contained pneumatic circuit. Experimental studies were conducted on two circular bellows with outside diameters of 1.625 in and 2.5 in (4.13 cm and 6.35 cm) and stroke lengths of approximately 1.4 cm. The pressure dynamics of the bellows placed under the heel of the foot or under the ball of the foot were investigated while walking on a treadmill. These pressure profiles were then reproduced on a compression testing machine to investigate the power generated per gait cycle. The results indicate that the bellows generated a peak power during normal walking of approximately 25–30 W and a maximum pressure of 450 kPa. The average power available over a single gait cycle is on the order of 1 W. This novel use of bellows demonstrates the ability to use these devices for regenerative fluid power harvesting capabilities during walking.
Natural Convection Heat Transfer in Nanofluids: A Numerical Study
Authors: Rashmi W. , Ismail A. F. , Asrar W. , Khalid M. , Faridah Y.
Published: July 2009

Natural convection heat transfer in nanofluids has been investigated numerically using computational fluid dynamics (CFD) approach. Analytical models that describe molecular viscosity, density, specific heat, thermal conductivity and coefficient of thermal expansion have been considered in terms of volume fraction and size of nanoparticles, size of base fluid molecule and temperature. The uniform suspensions of different concentrations of Al2O3 in base fluid (water) are considered as nanofluids. Thermal conductivity of the nanofluids has been obtained by solving the governing equations in conjunction with Kinetic model and interfacial layer model using FLUNET 6.3. Numerical simulations have been carried out in a closed pipe for L/D = 1.0. The numerical values of k have also been compared with the experimental values available in the literature. Both the models gave similar predictions with experimentally compared values of k.
Effect of Dissipative Terms on the Quality of Two and Three-Dimensional Euler Flow Solutions
Authors: Bahrainian Seyed Saied
Published: July 2009

The Euler equations are a set of non-dissipative hyperbolic conservation laws that can become unstable near regions of severe pressure variation. To prevent oscillations near shockwaves, these equations require artificial dissipation terms to be added to the discretized equations. A combination of first-order and third-order dissipative terms control the stability of the flow solutions. The assigned magnitude of these dissipative terms can have a direct effect on the quality of the flow solution. To examine these effects, subsonic and transonic solutions of the Euler equations for a flow passed a circular cylinder has been investigated. Triangular and tetrahedral unstructured grids were employed to discretize the computational domain. Unsteady Euler equations are then marched through time to reach a steady solution using a modified Runge-Kutta scheme. Optimal values of the dissipative terms were investigated for several flow conditions. For example, at a free stream Mach number of 0.45 strong shock waves were captured on the cylinder by using values of 0.25 and 0.0039 for the first-order and third-order dissipative terms. In addition to the shock capturing effect, it has been shown that smooth pressure coefficients can be obtained with the proper values for the dissipative terms.
Removal of Particles From the Surface of a Droplet
Authors: Aubry N. , Singh P. , Nudurupati S. , Janjua M.
Published: July 2009

We present a technique to concentrate particles on the surface of a drop, separate different types of particles, and remove them from the drop by subjecting the drop to a uniform electric field. The particles are moved under the action of the dielectrophoretic force which arises due to the non-uniformity of the electric field on the surface of the drop. Experiments show that depending on the dielectric constants of the fluids and the particles, particles aggregate either near the poles or near the equator of the drop. When particles aggregate near the poles and the dielectric constant of the drop is greater than that of the ambient fluid, the drop deformation is larger than that of a clean drop. In this case, under a sufficiently strong electric field the drop develops conical ends and particles concentrated at the poles eject out by a tip streaming mechanism, thus leaving the drop free of particles. On the other hand, when particles aggregate near the equator, it is shown that the drop can be broken into three major droplets, with the middle droplet carrying all particles and the two larger sized droplets on the sides being free of particles. The method also allows us to separate particles for which the sign of the Clausius-Mossotti factor is different, making particles of one type aggregate at the poles and of the second type aggregate at the equator. The former are removed from the drop by increasing the electric field strength, leaving only the latter inside the drop.
Numerical Investigation on Impeller-Volute Interaction in the Centrifugal Pump With Radial Gap and Tongue Profile Variation
Authors: Guo Pengcheng , Luo Xingqi , Liao Weili , Zhu Guojun
Published: July 2009

Numerical simulations on impeller-volute interactions in a low specific speed centrifugal pump equipped with three different outlet diameter impellers and variable tongue profiles were carried out using a commercial code CFX-10. The numerical results indicated that the maximum efficiency point is shifted along the lower flow rate direction with the increasing of impeller-volute radial gap. There is no linear relation between the radial gap variation and the pressure amplitude and it is possible to get an optimum value of the radial gap, which can improve notably the pressure pulsating characteristic while the head dropping of the pump is very small. The effects of three different tongue profiles on the flow field indicated that the influence of the different tongue profiles on the performance and operation stability of the centrifugal pump is very remarkable. The high-efficiency range of the centrifugal pump can be widened to some extent while the profile of the tongue is replaced from sharp tongue to middle tongue, and the maximum efficiency point is shifted along the higher flow rate direction. The operation stability of the centrifugal pump may be influenced while the fluid flows in the diffuser of the centrifugal pump with short tongue. The amplitude of pressure fluctuation in the tongue zone drops notably while the pump equipping with the middle tongue or the short one is in running.
Numerical and Experimental Investigations on Hood Capture and Protection Efficiencies
Authors: Karaismail Ertan , Celik Ismail , Guffey Steven E. , Dodrill William H.
Published: July 2009

Effective hoods are critical for the protection of workers from airborne contaminants. The aim of this study is to determine the effects of cross-draft and presence of a worker on the effectiveness of the bench-top enclosing hoods, which, unlike laboratory fume hoods have no sash and have a constant cross-sectional area. The computational fluid dynamics (CFD) software, FLUENT, with shear-stress transport (SST) k-ω turbulence model was used to investigate the effect of orientation of the manikin and hood with respect to cross-draft on the flow and contaminant concentration field. Experiments were conducted using a manikin and a fume hood in a wind tunnel and the flow around the manikin and inside the hood was investigated via particle image velocimetry (PIV) measurements. The correlation of contaminant leakage with the turbulent intensity and recirculating vortices at the hood face were observed. The hood and manikin orientation was found to change the exposure level significantly. Maximum contaminant leakage from the hood to the working environment has been found when the hood and manikin were placed 30 degrees to the cross draft. Maximum containment was observed when the manikin and hood were oriented in the same direction as the cross draft and perpendicular to the cross draft.