bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental Investigation of the Wake of a Discontinuous Cylinder
Authors: Mandava V. S. R. , Kopp Gregory A. , Herrero Joan , Giralt Francesc
Published: March 2011

The effects of a discontinuous cylinder geometry on the near wake structures was investigated experimentally. This ‘discontinuous’ circular cylinder has gaps so that solid segments 5D long are followed by gaps 2.5D long, in a repeating pattern, where D is the diameter of the cylinder. A thin steel plate was used to hold all of the cylinder pieces together. Thus, a three-dimensional (3D) wake was created at the origin with the intent to force the near wake flow to have similar structural characteristics as the far wake behind an ‘infinite/continuous’ cylinder, i.e., a near wake flow with horseshoes or double rollers formed by rapid kinking of Ka´rma´n-like vortices. Since the kinetic energy associated with the fluctuations of these near-wake 3D vortical structures is high, the flow system is considered suitable to clarify the role of these velocity patterns in the entrainment process of wake flows, which is still the subject of controversy. Particle Image Velocimetry (PIV) and Hot-Wire Anemometry (HWA) techniques were used to analyze the flow at two Reynolds numbers, Re = 10000 and 4000, in the wake of the discontinuous cylinder up to x/D = 190 downstream. The development of double rollers resulting from the interaction between the high momentum flow through the gaps and the Ka´rma´n-like vortices formed behind the solid cylindrical segments was confirmed. The Strouhal number of the double rollers in the wake is 0.14. These vortices have a dominant role in the initial wake growth. Thus, the overall flow dynamics are similar to the momentum transfer that takes place at the scale of the intermittent turbulent bulges that protrude from the wake in the far region and that were reported to be associated with double rollers.
Yield Stress Fluid Method to Measure the Pore Size Distribution of a Porous Medium
Authors: Oukhlef Aimad , Ambari Abdlehak , Champmartin Ste´phane , Despeyroux Antoine
Published: March 2011

In this paper a new method is presented in order to determine the pore size distribution in a porous media. This original technique uses the non Newtonian yield-pseudo-plastic rheological properties of some fluid flowing through the porous sample. In a first approximation, the very well-known and simple Carman-Kozeny model for porous media is considered. However, despite the use of such a huge simplification, the analysis of the geometry still remains an interesting problem. Then, the pore size distribution can be obtained from the measurement of the total flow rate as a function of the imposed pressure gradient. Using some yield-pseudo-plastic fluid, the mathematical processing of experimental data should give an insight of the pore-size distribution of the studied porous material. The present technique was successfully tested analytically and numerically for classical pore size distributions such as the Gaussian and the bimodal distributions using Bingham or Casson fluids (the technique was also successfully extended to Herschel-Bulkley fluids but the results are not presented in this paper). The simplicity and the cheapness of this method are also its assets.
An Efficient Finite Element Procedure for the Analysis of Cross-Flow Micro Heat Exchangers
Authors: Nonino C. , Savino S. , Del Giudice S.
Published: March 2011

As an alternative to massive CFD, a hybrid technique, which has the advantage of accounting for all of the three-dimensional features of the flow field, but with a limited computational effort, is used for the solution of conjugate convection-conduction heat transfer problems in cross-flow micro heat exchangers. The key feature of the proposed method is represented by the separate computation of the velocity fields in single microchannels and on the subsequent mapping of such velocity fields onto the three-dimensional grid used to solve the thermal problem. The cross-flow micro heat exchangers considered in the paper consist of a number of layers of rectangular microchannels. A parametric study is carried out on the combined effect on cross-flow micro heat exchanger thermal performances due to the variation of the microchannel cross-section and of the ratio of solid to fluid thermal conductivity.
Transport of Resolved Turbulent Stresses in a Spanwise Rotating Channel Flow
Authors: Xun Qian-Qiu , Wang Bing-Chen , Yan Gang
Published: March 2011

In this paper, we investigate the transport of resolved turbulent stresses in a plane channel flow subjected to spanwise rotation using the method of large-eddy simulation (LES). We present both the general and simplified transport equations for the resolved turbulent stresses, which are essential for understanding the unique pattern of turbulent kinetic energy production in a rotating system. Numerical simulations are performed using a dynamic nonlinear model (DNM) for closure of the filtered momentum equation. The turbulent flow field studied in this research is characterized by a Reynolds number Reτ = 150 and various rotation numbers Roτ ranging from 0 to 7.5. In order to validate the LES approach, turbulent statistics obtained from the simulations are thoroughly compared with the available experimental and direct numerical simulation results.
Numerical Simulation of Start-Up Jets in a Mixing Chamber
Authors: Lian Chenzhou , Voytovych Dmytro M. , Xia Guoping , Merkle Charles L.
Published: March 2011

Numerical simulations of a transient flow of helium injected into an established background flow of nitrogen were carried out to identify the dominant features of the transient mixing process between these two dissimilar gases. The geometry of interest is composed of two helium slots on either side of a central nitrogen channel feeding into a ‘two-dimensional’ mixing chamber. Simulations were accomplished on both two- and three-dimensional grids using an unsteady DES approach. Results are compared with experimental measurements of species distributions. Unsteady 2-D solutions give a reasonable qualitative picture of the transient mixing process in the middle of the chamber and enable cost-effective parametric analyses and grid refinement studies. The 2-D solutions also provide quantitative estimates of representative characteristic times to guide the 3-D calculations. The 3-D solutions give a reasonable approximation to span-wise events.
Development of Stent Strut Pattern for Cerebral Aneurysm
Authors: Nakayama Toshio , Jeong Shinkyu , Karkenahalli Srinivas , Ohta Makoto
Published: March 2011

Background and purpose: Stent implantation (stenting) in intracranial arteries is termed as endovascular treatment. The number of such cases has been increasing worldwide because the surgical damage resulting from stenting seem to be less than that of other treatments. The role of stenting for cerebral aneurysms is to reduce the blood flow speed in cerebral aneurysms. We have developed a computational fluid dynamics (CFD) system using a realistic stent and blood vessel and have studied the effect of the stent. Results of our study showed the stent strut pattern and stenting position to be very effective for reducing the blood flow speed in cerebral aneurysms. We have in describe the designing method used to design the stent strut pattern which reduces both the blood flow speed and the wall shear stress (WSS). Methods: An idealized aneurysm, a parent artery, and various stent shapes were used. The shape of the parent artery was a straight pipe and the aneurysm was a sphere. The stent was implanted in the neck of the aneurysm. The porosity remained of 80%, and the width of the stent strut ranged from 90 to 160[μm]. The stent strut height was fixed at a constant 150 [μm]. For the constructed shape data, a tetrahedron numerical mesh was generated. Calculation using the finite volume method was performed by a commercial solver. The optimization method was applied to the CFD results, and the stent strut patterns that reduced the blood flow speed and the WSS most were determined. Conclusion: The development method of stent strut pattern was proposed. Various stent strut patterns to reduce blood flow speed and WSS in/on cerebral aneurysm were tested. The stent strut pattern that reduced the blood flow speed and that reduced the WSS were determined. In the future works, the number of CFD cases should be increased and the optimal stent strut pattern determined.
Numerical Assessment of the Quasi-Steady Approximation in Glottal Air Flow
Authors: Bakhshaei Hani , Mongeau Luc , Mongrain Rosaire
Published: March 2011

Voice production involves air flow through the glottis and its interaction with the deformable vocal folds. The quasi-steady approximation involves modeling the complex unsteady flow through the glottis as a sequence of steady flows through rigid orifices, which is numerically less expensive. Theoretical and experimental assessments of the quasi-steady approximation have been attempted, but contradictions in previously reported results prompt further analysis. To investigate the validity of the quasi-steady approximation, a two-dimensional dynamic simulation of air flow through an idealized glottal orifice with moving walls was performed for different pressure gradients and oscillation frequencies. A series of steady flow simulations for configurations of the vocal folds and flow boundary conditions that instantaneously coincide with data from the dynamic simulations were performed. Dynamic and static simulations were performed using the COMSOL multiphysics® software. Both stationary and non-stationary geometries were created based on the M5 model with the orifice profile alternating between convergent and divergent included angles (−40° to 40°). The distance between the vocal cord tip and the centerline was established to maintain a constant vocal fold volume, thereby eliminating spurious monopole sources. The results include the fluid flow rate, the pressure drop across the glottis, the shear stress on the glottis walls, and the orifice coefficient. Comparison between these variables for both dynamic and static sets of data allowed the assessment of the accuracy of the quasi-steady approximation to predict the fluid flow in the glottis. The importance of time-dependent terms over short intervals during glottal opening and closure was scrutinized. The results may contribute to the general goal of creating flow models that are optimal for laryngeal orifice coefficient and sound pressure determination.
A Numerical Investigation for Nano-Particles Deposition in Realistic Geometry of Deviant Human Nasal Airways
Authors: Moghadas Hajar , Abouali Omid , Faramarzi Abolhasan , Tavakoli Behtash , Ahmadi Goodarz
Published: March 2011

3-D models of both sides of human nasal passages were developed to investigate the effect of septal deviation on the flow patterns and nano particles deposition in the realistic human nasal airways. 3-D computational domain was constructed by a series of coronal CT scan image before and after septoplasty from a live 25-year old nonsmoking male with septal deviation in his right side nasal passage. For several breathing rates corresponding to low or moderate activities, the steady state flow in the nasal passages was simulated numerically. Eulerian approach was employed to find the nano particles concentrations in the nasal channels. The flow field and particles depositions depend on the passage geometry. The abnormal passage has more particles deposition comparing with the normal side and post-operative passages for nano particles because of rapid change in geometry. However, regional depositions have the same behavior for the nano particles in the three different studied passages. Despite the anatomical differences of the human subjects used in the experiments and computer model, the simulation results are in qualitative agreement with the experimental data.
Carbon-Based Electric Double Layer Capacitors for Water Desalination
Authors: Fellman Batya A. , Atieh Muataz , Wang Evelyn N.
Published: March 2011

In capacitive deionization (CDI), salt water is passed through two polarized electrodes, whereby the salt is adsorbed onto the electrode surface and removed from the water stream. This approach has received renewed interest for water desalination due to the development of new high-surface area carbon-based nanomaterials. However, there is currently limited understanding as to how electrode geometry, surface properties, and capacitance affect ion capture. In this work, we experimentally investigate various standard carbon-based electrode materials, including activated carbon and carbon cloths, as well as microfabricated silicon structures for CDI. Electrochemical characterization through cyclic voltammetry was used to determine the electrochemical properties of each material. In addition, a mini-channel test cell was fabricated to perform parametric studies on ion capture. By controlling electrode geometry and chemistry in these studies, the work helps elucidate transport mechanisms and provide insight into the design of optimal materials for capacitive deionization.
Modeling of Liquid Bridge and Surface Wetting on Two Rigid Spherical Particles
Authors: Patel Raivat , Wang Dawei , Zhu Chao
Published: March 2011

Spreading and drainage of liquid over particle surface in a liquid bridge between two particles has been modeled to study the influence of temperature of particle and initial kinetic energy of liquid mass remain attached between two particles on impingement of droplet to form liquid bridge on liquid spreading. The spreading and drainage of liquid bridge is a key factor for particle agglomeration, which is commonly encountered in many industrial processes such as petroleum refinery, spray coating and flocculation which involve the collision of droplet and solids particles. A model based on mass and energy conservation of liquid trapped between two particles has been proposed to estimate the surface area of particle wetted by the liquid drainage and the thickness of liquid film under the influence of heat transfer to the liquid film from hot solid particles and initial kinetic energy of spreading liquid. The Laplace-Young model of liquid bridge is solved with discrete method to obtain the volume and surface change of liquid bridge due to spreading and drainage effect. The proposed model is capable of determining transient surface coverage of spreading liquid, liquid film thickness and its temperature. The influence of some key parameters, such as initial kinetic energy of spreading liquid, initial temperature of particle on the wetting thickness and wetting area is also investigated.
Analytical and Numerical Study of a Swirling Submerged Jet Flow
Authors: Alighanbari H. , Amiralaei M. , Savtchenko S.
Published: March 2011

The present study is conducted to investigate the details and characteristics of swirling submerged jets when transferred into a system of helical vortices downstream in a bathtub-like flow. Both analytical and numerical results are presented. In the analytical solution, upstream flow is considered to be two-dimensional with piecewise-constant vorticity profile. The instability of such a flow leads to the formation of two-dimensional dipolar or tripolar vortical structures. It is shown that the size of the vortexless annular area inside the initial vortex is a critical parameter in the two dipolar unstable or tripolar stable structure formations, and that such tripolar flow transforms downstream to a three-dimensional steady helical vortex system, which rotates as a whole and propagates in the downstream direction. The mechanism of screwing vortex filaments into a steady system of helical vortices is also presented. The numerical simulations also confirm the initiation and generation of dipolar vortex structures.
Experiments on Air Entrainment due to Free Falling- and Wall-Jets
Authors: Lopez Jose M. , Danciu Dana V. , Da Silva Marco J. , Hampel Uwe , Mohan Ram
Published: March 2011

In this article, air entrainment as a result of an impinging round water jet and a wall-jet was experimentally studied by means of videometry and image processing methods and also by means of a measurement technique based on a wire-mesh sensor. Therefore, two different experimental setups were utilized. For the first setup, a series of experiments at different conditions was performed and evaluated for both round jets and wall-jets. Jet lengths ranged between 0.01 and 0.2 m and jet exit velocities between 0.9 and 3.5 m/s. Image processing algorithms were applied to extract information about jet penetration depth, width of the bubble plume and bubble size distribution. The second facility was used to create a falling film in a square pipe (5 cm × 5 cm). Downstream of the impact point, a wire-mesh sensor was used to measure the gas entrainment characteristics at one axial location. Video image processing was also used in this experiment to gather more qualitative information about the gas entrainment process. Video images are compared with the images obtained by the wire-mesh sensor showing good qualitative agreement. The induction trumpet and a thin sheet of gas that forms around the jet and penetrates into the pool causing the entrainment were clearly identified. Results indicate that the gas void fraction increases and the bubble size decreases as the superficial liquid velocity increases.
Experimental Study on Single Phase Flow in Microchannels at High Mass Flow Rates
Authors: O¨zdemir Mehmed Rafet , Kos¸ar Ali
Published: March 2011

With the increasing speed and decreasing size of current microprocessors and microchips the dimensions of their heat sinks are continuously shrinking from mini size to micro size. The most extensively used and practical micro heat sinks are plain microchannels which find applications in many areas besides electronics cooling such as in microreactors, fuel cells, drug delivery, micropropulsion and automotive industry. Because of their widespread usage, they attracted the attention of many researchers, which gave rise to many studies on single-phase as well as on flow boiling. The proposed study aims at filling the gap in heat and fluid flow in microchannels at high mass velocities in the literature. For this purpose single-phase fluid (de-ionized water) flow was investigated over a broad range of mass velocity (1300 kg/m2s-7200 kg/m2s) in a microtube with an inner diameter of ∼ 250 μm. Besides comparing the experimental results in fully developed flow to the theory, the focus of this study is on thermally developing flows. Wall temperatures and pressure drops were measured and processed to obtain heat transfer coefficients, Nusselt numbers and friction factors. It was found that the existing theory about developing flows could fairly predict experimental data on developing flows in microscale for both laminar and turbulent conditions.
Experimental Characterization of Aerosol Flow Through a Micro-Capillary
Authors: Mahmud Zakaria , Hoey Justin M. , Lutfurakhmanov Artur , Daugs Jason , Swenson Orven F. , Schulz Douglas L. , Akhatov Iskander S.
Published: March 2011

Aerosol flow through a long and tapered micro-capillary (MC) for direct write (DW) technology is typically used with small particles of sizes ranging from 0.2 μm to 10 μm at velocities up to 100 m/s. Earlier research showed that the particles coming through a long MC experience Saffman force that moves the particles towards the center of the beam other than the geometric convergence (due to Stokes drag); thus creating a collimated aerosol beam. It was also established that the additional Saffman force becomes more effective with certain particle diameters and velocities. Therefore, for experimental validation, it is important to accurately measure the particle size distribution and velocities coming out of the long MC. However, the current sizing methods are incapable of measuring particles less than 5 μm due to optical limitations. The current paper presents results using a micro-shadowgraphy system from LaVision Inc. to characterize the flow field. A modification of the particle-sizing algorithm is proposed to measure particles of sub-micron sizes. The modified algorithm can be used to accurately size particles of 1μm diameter.
Study of the Impact of Change of Clearance on the Flow Field and Rotordynamic Coefficients for a Smooth, Whirling Annular Seal
Authors: Sekaran Aarthi , Morrison G. L.
Published: March 2011

The flow field and the rotordynamic coefficients for a smooth, whirling annular seal were investigated by means of a CFD study involving a full 3D model. The preliminary model (of clearance 1.27mm) was validated by comparison to existing experimental flow field data after which CFD simulations were made for a smaller clearance (0.127mm). The flow field changed significantly with the change in clearance and it was seen that the larger clearance showed an inertia dominated flow regime as opposed to the viscous flow regime for the small clearance. Upon the implementation of Childs’ theory for the computation of rotordynamic coefficients, it was observed that forces for the larger clearance did not exhibit the whirl ratio dependence assumed in this theory. The smaller clearance however showed the expected trend with values of the coefficients in the range predicted.
Validation of Heat Transfer Performance of Electrical Submersible Motor Using CFD
Authors: O’Bryan Roshani , Sheth Ketan , Brookbank Bruce
Published: March 2011

In oil field applications, the Electrical Submersible Pumping (ESP) unit (comprised of multistage pump, seal and motor) is placed inside a wellbore to provide necessary energy to lift reservoir fluids from the formation to the surface when the energy in the reservoir is not sufficient to lift the fluid to the surface. ESP motors produce heat while operating. The motors are cooled by the well fluid that passes the motor while being pumped. Many well fluids have very limited heat carrying capacity, resulting in higher operating temperature within the motor. Only a limited number of studies have been conducted that have analyzed the inside temperature rise in the motor. A parametric study has been conducted using the computational fluid dynamic software Ansys CFX to examine the profile of the temperature rise in the motor. The computational model is validated by experimental data which showed that the computational model predicts the temperature with 95% accuracy. Therefore, this computational model effectively represents the experimentally determined temperature distribution of the motor.
Effects of Viscosity and Two Phase Liquid–Gas Fluids on the Performance of Multi-Stage Centrifugal Pumps
Authors: Wilson Brown ‘Lyle’ , Sheth Ketan , Brown Donn
Published: March 2011

The paper reports on developmental research on the effects of viscosity and two phases, liquid–gas fluids on ESPs which are multi stage centrifugal pumps for deep bore holes. The test facility work was performed using pumps with ten or more stages moving fluids with viscosity from 2 to 2500 cP at various speed, intake pressure and Gas Void Fractions (GVF). For safety considerations the injected gas was restricted to nitrogen or air. The results are a series of curves representing the performance degradation of the pump. Note that in some cases the pump performances actually improved with increasing viscosity. The resulting information will allow a better understanding and more accurate prediction of performance than has been previously available. The data indicates a significant difference in performance correction when compared to the information available from the Hydraulics Institute.
Large Eddy Simulations of Jet Flow Interaction Within Staggered Rod Bundles
Authors: Salpeter Nathaniel , Hassan Yassin
Published: March 2011

The present work investigates the turbulent jet flow mixing of downward impinging jets within a staggered rod bundle based on previous experimental work. Two inlet jets had Reynold’s numbers of 11,160 and 6,250 and were chosen to coincide with available data [Amini and Hassan 2009]. Steady state simulations were initially carried out on a semi-structured polyhedral mesh of roughly 13.2 million cells following a sensitivity study over six different discretized meshes. Very large eddy simulations were carried out over the most refined mesh and continuous 1D wavelet transforms were used to analyze the dominant instabilities and how they propagate through the system in an effort to provide some insight into potential problems relating to structural vibrations due to turbulent instabilities. The presence of strong standing horseshoe vorticies near the base of each cylinder adjacent to an inlet jet was noted and is of potential importance in the abrasion wear of the graphite support columns of the VHTR if sufficient wear particles are present in the gas flow.
A Comparative Study of Interface Schemes in the Immersed Boundary Method for a Moving Solid Boundary Problem Using the Lattice Boltzmann Method
Authors: Kang Shin K. , Hassan Yassin A.
Published: March 2011

For moving boundary problems, previous body-conformal grid methods require frequent re-meshing as the boundary moves, thus increasing computational cost. An immersed boundary method (IBM) is an attractive method to resolve the problem since it is based on the fixed, non-body-conformal grids. In the IBM, force density terms are used so that no-slip boundary condition is satisfied on the boundary. On the other hand, lattice Boltzmann methods (LBMs) have been used as an alternative of Navier-Stokes equation method due to their efficiency to parallelize and simplicity to implement. The common feature of the IBM and the LBM of using non-body-conformal grids motivated the use of the IBM in the lattice Boltzmann method frame, which is usually called an immersed boundary-lattice Boltzmann method (IB-LBM). Besides, a split-forcing property in the LBM, due to its kinetic nature, facilitates the use of direct-forcing IBM. For the evaluation of boundary force density term, we need to adopt an interpolation scheme because the boundary, in general, does not match computational nodes. The interpolation schemes can be classified into diffuse and sharp interface schemes. The former usually uses the discrete delta function to evaluate the boundary force on the prescribed boundary points, while the latter uses interpolation from neighboring fluid nodes to evaluate the boundary force on the computation node either inside or outside closest to the boundary. In the diffuse scheme, the boundary force density terms evaluated on the boundary points should be distributed onto neighboring computational nodes using the discrete delta functions so that the boundary effect may exert on computational process. The objective of this study is to compare two interface schemes simultaneously for a moving boundary problem under the IB-LBM and to understand advantages and disadvantages of each scheme. We considered a problem of flow induced by inline oscillation of a circular cylinder since both experimental and body-conformal grid method results are available for this problem. Velocity results from both schemes showed overall good agreement with experimental data. However, the sharp interface scheme showed spurious oscillations in the surface force coefficient and pressure fields, although after filtering or smoothing, the force coefficients showed good agreement with the body-fitted results. In contrast, the diffuse interface scheme produced smooth variations in the surface force coefficient but over-predicted the absolute values especially at phase angles with the high magnitude of accelerations. These results can be attributed to the use of discrete delta functions. We could reduce the over-prediction by considering the effect of the diffuse area.
Simulation of Liquid Fuel Atomization in an Industrial Spray Nozzle of a Powerplant Boiler
Authors: Mirzaii Iman , Sabahi Hasan , Passandideh-Fard Mohammad , Shale Nasser
Published: March 2011

In this study, the liquid fuel atomization in the injector nozzle of the combustion chamber of a powerplant boiler is numerically simulated. The atomization of a liquid fuel injector is characterized by drop size distribution of the nozzle. This phenomenon plays an important role in the performance of the combustion chamber such as the combustion efficiency, and the amount of soot and NOx formation inside the boiler. The injector nozzle, considered in this study, belongs to a powerplant boiler where the liquid fuel is atomized using a high pressure steam. First, the geometric characteristics of the injector are carefully analyzed using a wire-cut process and a CAD model of the nozzle is created. Next, one of the nozzle orifices and the atomization zone where the high pressure steam meets the liquid fuel is recognized. The computational domain is extended long enough to cover the whole atomization zone up to the end of the orifice. The flow governing equations are the continuity and Navier-Stokes equations. For tracking the liquid/gas interface, the Volume-of-Fluid (VOF) method along with Youngs’ algorithm for geometric reconstruction of the free surface is used. The simulation results show the details of the liquid and steam flow inside the nozzle including velocity distribution and shape of the liquid/gas interface. It is found that the liquid breakup to ligaments and the atomization of liquid to droplets do not occur inside the nozzle orifice. A liquid jet with certain cross sectional shape leaves the orifice surrounded by a high speed steam. The numerical model provides the shape of the liquid jet, and the steam and fuel velocity distributions at the exit of the nozzle orifice. These parameters are then correlated to the final drop size distribution using analytical/experimental correlations available in literature.
CFD Modeling of Mixing Process in Pump for Two Liquids With Different Temperatures
Authors: Strongin Mikhail P.
Published: March 2011

The mixing of two or more liquids is very common in many industrial applications. In some cases the liquids are set on the pump inlet. The mixing process of two non-isothermal fluids in a pump is investigated in the presented work. Different pump geometries have been studied with comparison steady state and averaged transient results. Presented work considers the influence of the difference in temperatures of two mixing liquids on the mixing process. The pump model consists of suction, impeller and discharge parts which were meshed and calculated together. This, for instance, naturally permits the effects of non-uniformity of velocity and liquids concentrations distribution on the impeller eye and on the inlet of the discharge segment to be taken into account. Dependence of the density of liquids on the temperature is taken into account. Results of previous work with isothermal liquids demonstrated significant change in the mixing uniformity coefficient γ depending on where on the inlet the injected fluid is located and effect of very fast fading oscillations of concentration with blade passing frequency. For injection with angular symmetry on the pump inlet, γ is close to 1 (ideal mixture) on the outlet compared with γ less than 0.9 for strong angular asymmetry injection on the pump inlet, which is not sufficient for some applications. Results of presented work show the small, but visible, difference in the mixing uniformity coefficient for isothermal and non-isothermal liquids with the same flow rates on the inlet. Temperature uniformity coefficient is very close to the mixing uniformity coefficient, which is obvious, because of closeness of turbulent diffusivity and thermal conductivity coefficients.
The Fluid Mechanics of the Eye and the Role of the Mucus Layer
Authors: Platt Douglas , Vaidya Ashwin
Published: March 2011

We discuss the role of the mucus layer in the eye. The tear film in the eye is composed of a mucus layer, an aqueous layer and a lipid layer. While the aqueous and lipid layers are Newtonian, the mucus layer is non-Newtonian. It is commonly believed that the mucus layer serves as a lubricant for the cornea. However, we hypothesize that it serves a greater purpose as a protective layer from foreign particles; the normal stress effects of a viscoelastic fluid under the blinking motion of the eyelid would act towards pushing out any particle embedded in this layer. To prove this hypothesis, we mathematically study the fluid mechanics of a viscoelastic, shear thinning fluid modeled by a generalized second grade fluid. As a first step, we investigate the flow and stresses induced by a shearing motion (part of a blink cycle) and its effect upon an embedded particle, which is modeled by the Wiberg-Smith equation.
Entropy Generation in Thermally Developing Laminar Forced Convection Through a Slit Microchannel
Authors: Sadeghi Arman , Baghani Mostafa , Saidi Mohammad Hassan
Published: March 2011

The issue of entropy generation in laminar forced convection of a Newtonian fluid through a slit microchannel is analytically investigated by taking the viscous dissipation effect, the slip velocity and the temperature jump at the wall into account. Flow is considered to be hydrodynamically fully developed but thermally developing. The energy equation is solved by means of integral transform. The results demonstrate that to increase Knudsen number is to decrease entropy generation, while the effect of increasing values of Brinkman number and the group parameter is to increase entropy generation. Also it is disclosed that in the thermal entrance region the average entropy generation number over the cross section of channel is an increasing function of axial coordinate.
Mixing Process in T-Shaped Micro-Mixers With Chaotic Advection: A Numerical Approach
Authors: Esmaeelpanah J. , Kazemi S. A. , Passandideh-Fard M.
Published: March 2011

In this paper, a transient numerical model is presented to investigate the mixing phenomena in passive T-shaped barrier embedded micro-mixer (BEM) with rectangular cross-sections. The simulations are performed for two non-reactive miscible gases (i.e. oxygen and methanol). The compressibility and slip effects of the flow in the micro-channel are neglected. The model presented in this paper is used to numerically solve physical governing equations namely the continuity, momentum and specious transport equations. The equations are discritized using control volume numerical techniques. The distribution of the specious concentration within the domain is calculated. The Intensity factor is used as a criterion for mixing length. Also, the effects of the baffles’ height and span on mixing efficiency and reducing the mixing length are studied. Having baffles in the channel can substantially decrease the mixing length.
Computational Modeling of Thermal Energy Storage in Rock Beds
Authors: Sivov Tihomir G. , Palacios-Gamez Raul , Baliga B. Rabi
Published: March 2011

Thermal energy storage (TES) systems are commonly employed for enhancing the efficiency of commercial and residential heating and cooling systems, by matching thermal energy supply and demand during summer-winter, day-night, and peak-off-peak periods. TES in these systems is usually achieved by changing the temperature of materials (sensible systems) and/or inducing solid-liquid phase change (latent heat systems). Such systems are also categorized as seasonal (long-term) and diurnal (short-term). In this work, the focus is on sensible diurnal TES systems consisting of rock beds, with air as the working fluid. They are relatively simple, easy to construct, inexpensive, and quite effective for many solar energy and building engineering applications. Numerous publications on rock-bed TES systems are available, but there is an urgent need for efficient computational methods for designing and optimizing them. The contributions of this work are the following: proposal of cost-effective mathematical models of fluid flow and heat transfer in rock beds; adaptation of a finite volume method (FVM) for the solution of this model; applications of this FVM to two test problems (with analytical solutions) and one demonstration problem; proposal of suitable thermofluid performance evaluation criteria for the rock-bed TES systems of interest; and presentation and discussions of the results.
Pressure-Sensitive Paint Measurement on Co-Rotating Disks
Authors: Kameya Tomohiro , Matsuda Yu , Yamaguchi Hiroki , Egami Yasuhiro , Niimi Tomohide
Published: March 2011

There appears fluttering phenomena in a hard disk drive system with high-speed disks rotating inside a closed space, leading to degrade of reading and writing performance. The precise pressure distribution on the disk may improve the performance, but there has been no report because it is very hard to measure the surface pressure using conventional techniques, such as pressure taps. While pressure sensitive paint (PSP) seems to be suitable for the pressure measurement on the disk, we have to compensate its highly temperature-sensitive characteristics of PSP, because the temperature distribution on the disk is not assumed to be uniform. We employed PySO3H based PSP, which has small temperature sensitivity, and have obtained the pressure distribution on the disk rotated at various speeds (10000–20000 rpm) successfully. The result showed that the pressure is higher at the disk outside than at the center, and forms a concentric circle distribution. Moreover, we found that the pressure difference between the inner and outer region of the disk increases as a square of disk rotation speed.
Comparison of Different Heat Exchanger Models in a Thermoacoustic Engine Simulation
Authors: Weisman Catherine , Baltean-Carle`s Diana , Le Que´re´ Patrick , Bauwens Luc
Published: March 2011

The role of unavoidable space between heat exchangers and stack inside a thermoacoustic standing wave generator is investigated. A two-dimensional Low Mach number viscous and heat conducting flow model of the active thermoacoustic cell, comprising heater and cooler separated by a stack made of parallel conducting plates, is described. Three different models of heat exchangers are implemented and compared. Ideal heat exchangers consist of a fluid zone with imposed temperature. The other two models are made up of stacks of horizontal plates, either with specified wall temperatures, in one model and with constant prescribed heat fluxes in the other. A multiple scale analysis allows for coupling the active thermoacoustic cell model with the flow inside the resonator, obtained as a solution to a linear acoustic formulation. When a large enough temperature difference is applied between the heat exchangers, initial pressure perturbations grow. Different resonant modes are amplified for different configurations, in the same way as in experimental observations.
The Effect of Different Monolithically Integrated Check Valve Designs on the Efficiency of Disposable PZT-PDMS-Micropumps
Authors: Demming Stefanie , Fadl Ahmed , Schleßmann Mareike , Hoxhold Bjo¨rn , Zhang Zongqin , Bu¨ttgenbach Stephanus
Published: March 2011

In the presented study, check valve micropumps with three different valve designs have been developed, fabricated, and successfully tested. These check valves — in the form of differently shaped flaps — were integrated monolithically within the microchannel inlet and outlet of the pump chamber which allows for rapid and inexpensive fabrication of the device. The pump is made of Polydimethylsiloxane (PDMS) with a fully integrated circular piezo-electric transducer (PZT) as a micropump actuator. The performance of the micropumps was characterized under different actuator frequencies, depending on the excitation signal (square, sinusoidal or saw tooth), the implemented PZT diameter (10 mm or 15 mm), as well as the applied offset voltage (positive or negative). Ethanol was used as the working fluid in all experiments. The fabrication technology of the monolithically designed micropump is described, and the results are presented in terms of flow rates. The presented work suggests that the check valve design has an apparent effect on the micropump performance under different operational conditions. All in all, the first results show promising characteristics for easy and inexpensive integration of the proposed micropump in disposable lab-on-a-chip systems.
Effects of Electrode Switching Sequence on EWOD Droplet Manipulation: A Simulation Study
Authors: SadAbadi Hamid , Packirisamy Muthukumaran , Dolatabadi A. , Wuthrich Rolf
Published: March 2011

Electrowetting-on-dielectric (EWOD) is a new method for handling droplets on the microfluidic chips. By applying electrical potential, the interfacial energy of liquid-solid interface changes, results altering of droplet contact lines. To increase the flow rate of such a digital microfluidic system one way is to raise the droplet velocity. One important factor for enhancing droplet velocity in EWOD systems is the proper switching the electrodes or “switching sequence”. To examine the effect of switching in EWOD, the EDEW 1.0 simulation tool is used in this paper. By simulating the motion of a 1μL water droplet in a 1D electrode array, the resultant surface energy curves during the motion of droplet in different electrode switching sequences are obtained. The results show proper electrode switching has a remarkable effect on increasing of droplet velocity. To enhance the droplet velocity, the electrode, which is placed next to the droplet at forward direction, should be powered after droplet passed over it. In addition, it would be more efficient to first turn on the next electrode, and then turn off the previous one.
Convective Stability of Hadley Circulations in Finite Domains
Authors: Kaloni P. N. , Lou J. X.
Published: March 2011

We present numerical solutions of natural convection in a cavity of finite domain when the flow is driven by a horizontal temperature gradient between isothermal vertical walls. Only the transverse roll structure of convection is calculated by employing the pseudo spectral Chebyshev collocation method. A numerical solution of the steady flow is first obtained and, based upon it, the critical Rayleigh number (related to Grashof number) of linear transverse mode is determined. The influence of aspect ratio, on the flow and temperature patterns, is also investigated.
Analysis Methods for Vessel Generated Spray
Authors: Fullerton Anne M. , Fu Thomas C. , Drazen David A. , Walker Don C.
Published: March 2011

The droplet sizes and velocities contained in vessel generated spray are difficult to quantify. This paper describes three different methods to quantify velocity and size distributions from high speed video of spray from a planing boat. These methods include feature tracking, displacement tracking and video inversion. For the feature tracking method, the images were preprocessed using contrast limited adaptive histogram equalization, and then converted to binary images with a specific intensity cutoff level. Image statistics were then generated from this image, including droplet area and effective diameter. These images were processed using commercial PIV software to obtain velocities. For the displacement tracking method, the images were also converted to binary images with a specific intensity cutoff level. Image statistics were again compiled from this binary image. A droplet filter was then applied using a binary erosion image processing technique, where large droplets were removed because the entire droplet may not be in frame, and small droplets were removed because they might not overlap between frames. Droplets were then tracked by comparing the bounding boxes of two droplets between time frames. The video inversion method consisted of the manipulating the original high speed videos from spatial x-y frames in time space to time-y frames in x-space, where the x-axis is longitudinally along the ship and the y axis is vertical to the ship. From this orientation, the speed of the general spray mass could be determined by summing the pixels in time columns for each × frame. Comparisons of droplet size distribution between the feature and displacement tracking method yield qualitatively similar results, with some disagreement likely due to the different threshold levels. The trend of the distribution curve suggests that both methods are unable to resolve the smallest droplet sizes, due to the processing filters applied as well as the field of view of the camera. The three analysis methods compare well in their spray velocity computation, and are also similar to spray speed predictions found in the literature for a given geometry and vessel speed.
Concentration Distribution During Pulse Jet Mixing
Authors: Bamberger Judith Ann , Meyer Perry A.
Published: March 2011

Obtaining real-time, in situ slurry concentration measurements during unsteady mixing can provide increased understanding into mixer performance. During recent tests an ultrasonic attenuation sensor was inserted into a mixing vessel to measure the slurry concentration during unsteady mixing in real time during pulse jet mixer operation. These pulse jet mixing tests to suspend noncohesive solids in Newtonian liquid were conducted at three geometric scales. To understand the solids suspension process and resulting solids distribution, the concentration of solids in the cloud was measured at various elevations and radial positions during the pulse jet mixer cycle. In the largest scale vessel, concentration profiles were measured at three radial locations: r = 0, 0.5 and 0.9 R where R is the vessel radius. These radial concentration data are being analyzed to provide a model for predicting concentration as a function of elevation. This paper describes pulse jet mixer operation, provides a description of the concentration probe, and presents transient concentration data obtained at three radial positions: in the vessel center (O R), midway between the center and the wall (0.5 R) and near the vessel wall (0.9 R) through out the pulse to provide insight into pulse jet mixer performance.
Effects of gallium doping on properties of a-plane ZnO films on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy
Authors: Han Seok Kyu , Lee Hyo Sung , Lim Dong Seok , Hong Soon-Ku , Yoon Nara , Oh Dong-Cheol , Ahn Byung Jun , Song Jung-Hoon , Yao Takafumi
Published: March 2011

The authors report on the structural, optical, and electrical properties of Ga-doped a-plane (112¯0) ZnO films grown by plasma-assisted molecular beam epitaxy. Ga doping level was controlled by changing the Ga cell temperatures from 350 to 470 °C with an interval of 30 °C. With up to Ga cell temperatures of 440 °C, single crystalline Ga-doped a-plane ZnO films were grown; however, the sample with a Ga cell temperature of 470 °C showed polycrystalline features. The typical striated surface morphology normally observed from undoped ZnO films disappeared with Ga doping. ZnO films doped with Ga cell temperatures up to 440 °C did not show a significant change in full width at half maximum (FWHM) values of (112¯0) x-ray rocking curves by doping. The smallest FWHM values were 0.433° (ϕ=90°) and 0.522° (ϕ=0°) for the sample with a Ga cell temperature of 350 °C. The polycrystalline ZnO film with excessive Ga doping at the Ga cell temperature of 470 °C showed significantly increased FWHM values. Hall measurements at room temperature (RT) revealed that electron concentration began to be saturated at the Ga cell temperature of 440 °C and electron mobility was drastically reduced at the Ga cell temperature of 470 °C. The carrier concentration of Ga-doped ZnO films were controlled from 7.2×1018 to 3.6×1020 cm−3. Anisotropic electrical properties (carrier concentration and Hall mobility) were observed in measurements by the van der Pauw method depending on the direction (c- or m-direction) for the undoped sample but not observed for the doped samples. RT photoluminescence (PL) spectra from the Ga-doped single crystalline ZnO films showed dominant near band edge (NBE) emissions with negligibly deep level emission. The NBE intensity in PL spectra increases with Ga doping.
A Midzone-Based Ruler Adjusts Chromosome Compaction to Anaphase Spindle Length
Authors: Neurohr Gabriel , Naegeli Andreas , Titos Iris , Theler Dominik , Greber Basil , Díez Javier , Gabaldón Toni , Mendoza Manuel , Barral Yves
Published: March 2011

The degree of chromosome condensation can be modulated within cells to ensure proper segregation.