bluuebunny/crossref_metadata_2025 huggingface dataset:
Aerodynamic Forces on a Simplified Car Body: Towards Innovative Designs for Car Drag Reduction
Authors: Khaled Mahmoud , Harambat Fabien , Yammine Anthony , Peerhossaini Hassan
Published: March 2011

The present paper exposes the study of the cooling system circulation effect on the external aerodynamic forces. We report here aerodynamic force measurements carried out on a simplified vehicle model in wind tunnel. Tests are performed for different airflow configurations in order to detect the parameters that can affect the aerodynamic torsor and to confirm others previously suspected, especially the air inlets localization, the air outlet distributions and the underhood geometry. The simplified model has flat and flexible air inlets and several types of air outlet, and includes in its body a real cooling system and a simplified engine block that can move in the longitudinal and lateral directions. The results of this research are generic and can be applied to any new car design. Results show configurations in which, with respect to the most commonly adopted underhood geometries, the overall drag coefficient can be decreased by 2%, the aerodynamic cooling drag coefficient by more than 50% and the lift coefficient by 5%. Finally, new designs of aerodynamic drag reduction, based on the combined effects of the different investigated parameters, are proposed.
The Pressure Drop and Heat Transfer of Flow in Zigzag Micro-Channel With Micro-Orifice
Authors: Wu Chengyun , Xu Hongbo , Tian Changqing
Published: March 2011

In this paper, the experimental investigation on friction factor and heat transfer of single phase liquid flow and two-phase boiling flow in single zigzag micro-channel with micro-orifice at inlet has been conducted. The dimension of the micro-orifice is 1mm×0.227mm×0.25mm. The experiment was conducted in copper rectangle zigzag micro-channel with the hydraulic diameter of 0.321mm and the length of 29 mm. The experimental results of friction factor and heat transfer coefficient were provided and the effect of the micro-orifice was discussed. It was found that the friction factor of flow in zigzag micro-channel deviated largely from the predictive value in the laminar flow, while it coincided well with the correlation for turbulent flow. In addition, the variation of local heat transfer coefficient showed that the inlet restrictor has significant effect on heat transfer of boiling flow in micro-channel.
Influence of Wall Proximity and Free Surface on Wake of Two Side-by-Side Circular Cylinders: PIV Measurement and POD Analysis
Authors: Paul S. S.
Published: March 2011

The paper reports on an experimental study of turbulent flow around a pair of circular cylinders with a pitch ratio of two. The cylinders were located in the vicinity of a plane wall, in a uniform stream, and adjacent to a free surface in an open channel. The Reynolds number based on depth of flow and fresstream velocity was 30300 while the Froude number was 0.3. A particle image velocimetry technique was used to conduct detailed velocity measurements around and in the near wake region of the cylinder pairs, from which contours and profiles of the mean velocities and turbulent statistics were obtained and discussed. The proper orthogonal decomposition was then applied to provide an insight into the structure of the flow. Reconstruction of the fluctuating velocity components for various numbers of modes were also shown to investigate the role of large-scale structures.
Study the Heat Transfer Process From Electron-Phonon Nonequilibrium in Thin Gold Film to Glass Substrate Through Transient Thermoreflectance Measurements
Authors: Ma Weigang , Wang Haidong , Zhang Xing , Wang Wei
Published: March 2011

How the energy transfers during electron-phonon nonequilibrium in thin metal films is still an open question, and how to measure the intrinsic thermal transport properties of the material under the covering layer is another challenge. In this paper, the heat transfer process from electron-phonon nonequilibrium in thin gold film to borosilicate glass substrate has been studied by resorting to different segments of the transient thermoreflectance signal, which is obtained from the rear-pump front-probe transient thermoreflectance technique. The gold film, which has a thickness of 23.1 nm, is deposited on the borosilicate glass substrate using using a physical vapor deposition (PVD) approach. Within the framework of the two-temperature model (TTM), the electron-phonon (e-ph) coupling factors of the gold film, which reflect the strength of heat flow from hot electrons to cold phonons, are derived from the signal taken after the first several picoseconds with different pump fluences, and the measured value is (1.95–2.05)×1016 W m−3 K−1. The electron-phonon coupling factor does not significantly change in response to the pump pulse fluence variation and exhibits little change compared to the bulk gold value 2.4×1016 W m−3 K−1. Furthermore, the thermal conductivity of the glass substrate is obtained through the thermoreflectance signal between 20 to 140 picoseconds and the value is W m−1 K−1.
Reformulation RELAP5-3D in FORTRAN 95 and Results
Authors: Mesina George L.
Published: March 2011

RELAP5-3D is a nuclear power plant code used worldwide for safety analysis, design, and operator training. In keeping with ongoing developments in the computing industry, we have re-architected the code in the FORTRAN 95 language [2], the current, fully-available, ANSI standard FORTRAN language. These changes include a complete reworking of the database and conversion of the source code to take advantage of new constructs. The improvements and impacts to the code are manifold. It is a completely machine-independent code that produces machine independent fluid property and plot files and expands to the exact size needed to accommodate the user’s input. Runtime is generally better for larger input models, many prior user-reported problems have been resolved, and the program is better tested. Other impacts of code reformulation are improved code readability, reduced maintenance and development time, increased adaptability to new computing platforms, and increased code longevity. Comparison between the pre- and post-conversion code are made on the basis of programming metrics and code performance.
Analytical and Experimental Characterization of Flow in Slowly-Varying Cross-Section Microchannels
Authors: Akbari M. , Bahrami M. , Sinton D.
Published: March 2011

This paper outlines a novel approximate solution for determining the pressure drop of laminar, single-phase flow in slowly-varying microchannels of arbitrary cross-section. The proposed analysis is general and applicable to symmetric and asymmetric microchannel cross-sections, as examples compact relationships are reported for elliptical and rectangular shapes for three common wall profiles of linear, sinusoidal and hyperbolic. An experimental setup is designed and pressure drop measurements are conducted to validate the proposed model for streamwised periodic microchannels with rectangular cross-section and linear wall with a range of channel geometrical parameters such as aspect ratio and channel slope. The model is also compared against the numerical and experimental data of hyperbolic contraction with rectangular cross-section collected by others. It is observed that although the proposed model is based on the solution of the elliptical cross-section, it can accurately predict the pressure drop in microchannels of rectangular cross-section.
Prediction of Residual Stress Improvement by Water Jet Peening Using Cavitating Jet Simulation With Bubble Flow Model
Authors: Fukaya Masashi , Morinaka Ren , Saitou Noboru , Hatou Hisamitsu , Tamura Yoshiaki , Matsumoto Yoichiro
Published: March 2011

We developed the new method for predicting a region of compressive residual stress on the weld surface after water jet peeing (WJP), which is a preventive maintenance technology for nuclear power plants. A cavitating jet is impinged on the weld surfaces of structures in a nuclear reactor. Bubble collapse impact causes plastic deformation of the weld surface, and changes the residual stress from tensile to compressive. Compressive residual stress prevents the occurrence of stress corrosion cracking (SCC) on the weld surface. A cavitating jet vertically injected into a submerged flat plate was investigated. Tensile stress was introduced onto the surface of the stainless steel plate by grinding before WJP in the experiment. We numerically simulated impulsive bubble pressure that varied by microseconds in the cavitating jet with the “bubble flow model”. The bubble flow model simulates the abrupt time-variations in the radius and inner pressure of bubbles based on the Rayleigh-Plesset equation in a cavitating flow. The cavitation collapse energy was estimated based on the bubble pressure. The cavitation collapse energy was compared with the measured compressive residual stress on the flat plate after WJP. The radial range of the compressive residual stress from the jet center axis is one of the most important measures of performance of WJP. The radial range of the cavitation collapse energy corresponded to that of compressive residual stress with a prediction error of +/− 20% under different conditions of jet velocity and the distance between the jet nozzle and plate surface. The results confirmed that the method we developed for predicting the region of compressive residual stress after WJP was valid.
A Semi-Empirical Method to Determine the Speed of Sound and Attenuation Factor in a Confined Fluid
Authors: Motriuk R. Wojciech Voytek
Published: March 2011

A simple semi-empirical method is presented for calculating the speed of sound and attenuation factor for an unknown medium enclosed in a pipe. This method is based on the calculation of four-pole matrix parameters. It can be used in connection with transient or steady state excitation. The material presented is limited by the fact that no ideal measurement process exists. This limitation is significant when gathering attenuation factor and speed of sound data over a wide frequency range. However, the results, which are easily obtainable, are more than sufficient in solving daily engineering problems.
Experimental Study of Nucleate Boiling Performance on Several Types of the Mixed-Wettability Pattern Surface by Micro/Milli-Sized Hydrophobic Patterns
Authors: Jo HangJin , Kang Soon Ho , Ahn Ho Seon , Kim Moo Hwan
Published: March 2011

To increase the nucleate boiling efficiency, many nucleate boiling experiments have been conducted and could get brilliant and challengeable results. A consensus was that CHF and heat transfer were affected by a modified heating surface which change the micro roughness, thermophysical properties of heating surface, or the wettability. Of the many parameters, the wettability study is regarded as the most powerful factor. For finding the optimized condition at the nucleate boiling (high heat transfer and high CHF), we design the special heaters to examine how two materials, which have different wettabilities, affect the boiling phenomena. The special heaters have several types of hydrophobic patterns which have the precise size because they were made by MEMS techniques on the silicon oxide surface. In the experiments with patterned surface, hydrophobic dots lead to an early bubble inception and induce the better heat transfer. These experiments are compared with classic and recent models for bubble inception. The all experiments are conducted under the saturated pool boiling condition with distilled water at 1 atm pressure. The peculiar Teflon (AF1600) is used as the hydrophobic material. The hydrophilic part is performed by silicon oxide through the furnace procedure. The experiments using the micro-sized patterns and milli-sized patterns are performed, and the results are compared with the reference surface. These mixed-wettability studies are expected to induce the development of the nucleate boiling condition.
A Detailed Observation of Hydrofoil Cavitation and a Proposal for Improving Cavitation Model
Authors: Nohmi Motohiko , Ochiai Naoya , Iga Yuka , Ikohagi Toshiaki
Published: March 2011

Cavitation of a hydrofoil is observed in detail by using a high speed video camera. A paint removal test is also carried out in order to evaluate cavitation aggressiveness for erosion. 2D hydrofoil profile is Clark Y 11.7% and its angle of attack is seven degrees. Cavitation number is σ = 1.08. The experimental results are compared with cavitation CFD. Numerous features of unsteady cavitation are observed such as cyclic fluctuation of the sheet cavity, existence of the glassy cones on a sheet cavity, generation of the cloud cavitation from the sheet cavity and the isolated bubbles traveling over the suction surface of the blade. The isolated traveling bubbles and their collapses are thought to be one of the main causes of the severe paint removals. The isolated traveling bubbles are derived from the flowing cavitation nucleus or from abrupt onset at the leading edge of the blade. For computing these complicated phenomena, combination of grid scale bubbles (GSB) and sub grid scale bubble model (SGSB) are proposed. GSB shall be computed by using the computational scheme for the free surface with phase change model. SGSB can be computed with conventional cavitation model. The breakup of GSB generates SGSB, and the coalescence of SGSB makes GSB. Upper limit of void fraction of SGSB is estimated in the range of five or ten percent from the simple speculation of the structure of packed spheres. The two types of cavitation bubble inception model are also discussed based on the generation of the isolated bubbles observed in the experiments. To verify the proposed concepts of cavitation model, a traveling air bubble over a hydrofoil is computed by using the free surface flow scheme of Volume of Fluid (VOF) approach. Cavitation on the hydrofoil is also computed by VOF approach with boiling model concerning the heat transfer. Both the computed results show qualitatively similar characteristics of the bubble dynamics to those in experimental results.
Control of Flow Asymmetry Around Slender Body of Revolution at High Incidence With Minute Jets
Authors: Guan Xiaorong , Xu Cheng
Published: March 2011

For controlling the flow asymmetry around slender body of revolution at high incidence, a method with skew minute jets distributed symmetrically on opposite sides before separation lines near nose apex is put forward and numerically confirmed. By configuring the jet primary parameters, an active control can be achieved along with the intensity and location variations of the concentrated vortices originated from the interaction between the jets and the flow around slender body. The control effect is realized through the regulation of the concentrated vortices on the asymmetry degree of nose vortices near nose apex. In order to achieve effective control, several rules on configuring the jet primary parameters should be followed: the concentrated vortices are intensive sufficiently and close enough to nose vortices; the concentrated vortices can keep acting on nose vortices in an adequately long axial extent.
An Effect of Swept Vane in Hydrogen-Fueled Combustion Turbine
Authors: Honda Hironori , Suzuki Masaya , Yamamoto Makoto
Published: March 2011

A lot of environmental problems such as global warning, air pollution and exhaustion of fossil fuels have been discussed frequently. Many researches have been underway in several countries to develop a propulsion system for an advanced aircraft to achieve low environmental loading. On the other hand, with the recent development of an aircraft, the propulsion system is required to have lighter weight, higher power and lower emissions. To satisfy these requirements, we have supposed a new cycle concept for advanced propulsion system, in which the combustion camber is eliminated; hydrogen gas is directly injected from turbine vane surfaces and combusted within turbine vane passages. However, to apply the cycle to practical use, there are problems of extremely high surface temperature and aerodynamic performance decrease by hydrogen combustion. It is well known that three-dimensional design approaches such as sweep, lean and twist decrease the secondary flow loss. However, there is no knowledge how these three-dimensional designs affect on the flow characteristics of the hydrogen-fueled turbine. In the present study, we focus on sweep. To clarify the sweep effect on the surface temperature and performance of the hydrogen-combustion turbine, three-dimensional numerical simulations based on RANS are carried out. We find that the swept vanes with positive sweep give the aerodynamic performance. On the other hand, the swept vanes with negative sweep suppress the vane surface temperature.
Effect of Tapering on Pulsatile Flow Through a U-Tube Model of the Human Aortic Arch
Authors: Sumida Masaru
Published: March 2011

An experimental investigation of pulsatile flow through a tapered U-tube was performed to study the blood flow in the aorta. The experiments were carried out in a U-tube with a curvature radius ratio of 3.5 and a 50% reduction in the cross-sectional area from the entrance to the exit of the curved section. Velocity measurements were conducted by a laser Doppler velocimetry for a Womersley number of 10, a mean Dean number of 400 and a flow rate ratio of 1. The velocity profiles for pulsatile flow in the tapered U-tube were compared with the corresponding results in a U-tube having a uniform cross-sectional area. The striking effects of the tapering on the flow are exhibited in the axial velocity profiles in the section from the latter half of the bend to the downstream tangent immediately behind the bend exit. A depression in the velocity profile appears at a smaller turn angle Ω in the case of tapering, although the magnitude of the depression relative to the cross-sectional average velocity decreases. The value of β, which indicates the uniformity in the velocity profile over the cross section, decreases with increasing Ω, whereas it rapidly increases immediately behind the bend exit.
Characterization of Injected Sample Plugs in Microchip Capillary Electrophoresis
Authors: Shao Zhanjie , Schneider Gerry , Ren Carolyn
Published: March 2011

Since the capillary electrophoresis was proposed to be run in the chip format, tremendous studies have been performed by covering many different aspects of this technology. One key element is the sample plug generated between the injection and separation process, because it will play a governing role on the final separation performance, i.e., the separation efficiency depends on the initial sample plug and its further dispersion development. In literature, some work has been done previously to generate various sample plugs, or optimize them by means of either channel design or operational control. However, little work has been reported to characterize the sample plug with evaluating parameters. Usually, the well-defined and reproducible sample plug is anticipated for high quality separation. By experience, thin-rectangular sample plugs are normally assumed, but not technically proved yet, to have superior performance in electrophoretic separation. Quantitative study is necessary to be performed to demonstrate the relevant qualitative estimation or analysis. All above stated are the motivation of current work.
Network Analysis of Non-Uniformly Heated Evaporative Micro-Channel Systems
Authors: Lee Hee Joon , Yao Shi-chune
Published: March 2011

A network analysis was established to model the array of evaporative micro-channels with possible non-uniformity heating as well as branching of the channels. Iterative solution of the evaporative micro-channel network can be obtained using the Hardy-Cross method together with accurate two-phase head-loss correlations. Based on the experimental evidence, cross-cutting micro-channels reduce the uneven flow distribution in parallel micro-channels at non-uniform heating. Through this network analysis, it is also evident that cross-cutting grooves on parallel micro-channels are effective in reducing non-uniform heating effects and enhancing the uniform wall temperature distribution.
Novel 4-Stage Traveling Wave Thermoacoustic Power Generator
Authors: de Blok Kees
Published: March 2011

Utilizing low temperature differences from solar vacuum tube collectors or waste heat in the range 70–200 °C seems to be the most promising and commercial interesting field of applications for thermoacoustic systems. Recently a novel 4-stage “self matching” traveling wave engine is developed and tested. Beside the low acoustic loss and compactness, due to traveling wave feedback, all components per stage are identical which is beneficial from (mass) production point of view. Based on this concept a 100 kWT thermoacoustic power (TAP) generator is under construction. This project is carried out in the framework of phase two of the Dutch SBIR program. The 100 kWT TAP will be installed at a paper manufacturing plant in the Netherlands for converting part of the flue gas at 150°C from the paper drying process into electricity. Emphasis in this project is on production and cost aspects lowering the investment per kWe to a level competitive to ORC’s. After successful completion of this pilot, commercialization and delivery of 100kW to 1 MW thermoacoustic power generators for industrial waste heat recovery and as add-on for CHP systems is planned to begin in 2012. The same concept of the 4-stage traveling wave engine is also implemented in an atmospheric pressure operated thermoacoustic cooking device for developing countries which generate beside hot water up to 50 W electricity. Details, ongoing work and experimental results of these projects will be presented.
Experimental and Numerical Study of the Three Dimensional Instabilities in the Wake of a Blunt Trailing Edge Profiled Body
Authors: Naghib Lahouti Arash , Doddipatla Lakshmana Sampat , Hangan Horia , Siddiqui Kamran
Published: March 2011

The wake of nominally two dimensional bluff bodies is dominated by von Ka´rma´n vortices, which are accompanied by three dimensional instabilities beyond a threshold Reynolds number. These three dimensional instabilities initiate as dislocations in the von Ka´rma´n vortices near the trailing edge, which evolve into pairs of counter-rotating vortices further downstream. The wavelength of the three dimensional instabilities depends on profile geometry and Reynolds number. In the present study, the three dimensional wake instabilities for a blunt trailing edge profiled body, composed of an elliptical leading edge and a rectangular trailing edge, have been studied in Reynolds numbers ranging from 500 to 1200, based on the thickness of the body. Numerical simulations, Laser Induced Fluorescence (LIF) flow visualization, and Particle Image Velocimetry (PIV) methods have been used to identify the instabilities. Proper Orthogonal Decomposition (POD) has been used to analyze the velocity field data measured using PIV. The results confirm the existence of three dimensional instabilities with an average wavelength of 2.0 to 2.5 times thickness of the body, in the near wake. The findings are in agreements with the values reported previously for different Reynolds numbers, and extend the range of Reynolds numbers in which the three dimensional instabilities are characterized.
In-Plane Gas Permeability of Proton Exchange Membrane Fuel Cell Gas Diffusion Layers
Authors: Tamayol A. , Bahrami M.
Published: March 2011

A new analytical approach is proposed for evaluating the in-plane permeability of gas diffusion layers (GDLs) of proton exchange membrane fuel cells. In this approach, the microstructure of carbon papers is modeled as a combination of equally-sized, equally-spaced fibers parallel and perpendicular to the flow direction. The permeability of the carbon paper is then estimated by a blend of the permeability of the two groups. Several blending techniques are investigated to find an optimum blend through comparisons with experimental data for GDLs. The proposed model captures the trends of experimental data over the entire range of GDL porosity. In addition, a compact relationship is reported that predicts the in-plane permeability of GDL as a function of porosity and the fiber diameter.
Refractive Index Matching With Distortion Measurements in a Bed of Irregularly Packed Spheres
Authors: Patil Vishal A. , Liburdy James A.
Published: March 2011

Experimental flow visualization in porous media is often conducted using optical techniques such as PIV and PTV for velocity field estimation and LIF for concentration field measurements. The porous bed is made optically accessible to laser light and imaging by matching refractive indices of the liquid phase to that of the solid matrix, including the channel walls. The methods most commonly employed to match the refractive indices have been to maximize the transmitted intensity through the bed or to rely on refractometers for measurement of the liquid and solid phases. Refractometers with sensitivity of 0.001 could still cause refraction problems in a porous bed, while accuracy and sensitivity of transmission based methods are limited by the camera resolution and noise scattered by impurities and stray light caused by reflections at interfaces. Both these methods fail to provide uncertainty estimates for particle position determination due to slight refractive index mismatching. This work presents a method for assessing the matching of refractive indices that relies on measuring distortion of a target when imaged through a porous bed. The target used is a grid of 250 μm dots irradiated with light at the necessary wavelength at which refractive indices are to be matched. Two principle types of distortion are quantified, distortion of the image centroid due to interface refraction and intensity distortion within the image for index mismatching as low as 0.0005.
The Effect of 3D Visualization on Optimal Design for Strut Position of Intracranial Stent
Authors: Anzai Hitomi , Nakayama Toshio , Takeshima Yuriko , Ohta Makoto
Published: March 2011

Cerebral aneurysms generally occur at arterial bifurcations and arterial curves in or near the circle of Willis. For the treatment of this disorder, stent placement has been valued as a minimal invasive therapy. The effect of stents on flow reduction in cerebral aneurysms has been examined in several computed fluid dynamics (CFD) studies, suggesting that the stent position or the strut shape may affect flow reduction. However, the position of the stent with the best effect on flow reduction is still unknown because of the flow complexity. Three-dimensional visualization may help to easily specify the inflow zone from the parent artery to the aneurysm and to find the relationship between the effective strut position and the flow pattern. However, confirmation of the ability of 3D visualization to determine the effective position of a stent has not been achieved. In this study, we simulated blood flow with several aneurysm geometries to confirm the effect of 3D visualization on determination of optimal stent position. First, flow simulation using real aneurysm geometries without a stent was performed as a “pre-stenting situation.” Meshes were generated using a commercial code (Gambit 2.3, Fluent Inc., NH). CFD was carried out using a commercial code (Fluent 6.3, Fluent Inc., NH) based on steady flow. The streamlines around an aneurysm were visualized using a 3D visualization system (EnSight Gold 8.2, Comuputational Engineering Inc., NC) in Realization Workspace (RWS) to visualize the inflow zone. Secondly, a rectangular solid as a strut model was set in the inflow zone using computer-aided design (CAD) techniques. CFD was then performed as a “post-stenting situation” under the same conditions as the pre-stenting situation using the same mesh generator and CFD code. Three-dimensional visualization showed an inflow zone in the aneurysm. A bundle of flow streamlines hit the wall of the neck of the aneurysm and entered it. The inflow zone was a narrow local part in contrast to the outflow. After setting a strut, a change of flow pattern could be observed. The flow speed and the wall shear stress (WSS) were both reduced. When the strut position was moved away from the original position, the flow speed and the WSS were not reduced. These results may suggest that 3D visualization can provide information useful for strut positioning to realize effective reduction of flow into an aneurysm, especially a side wall aneurysm.
Heat Conduction Rectification in Nanostructure With Step Change in Cross-Section
Authors: Liang Xingang , Yue Bao
Published: March 2011

Heat conduction rectifier is attracting more attention due to its potential application to process thermal currents independently and convert them into electronic signals. This work reports an investigation by molecular dynamics simulation on the heat conduction rectification effect in the nanostructure whose cross-section have step change along the heat flux. It is found that thermal resistance is different with reversed heat flux direction, which is called the heat conduction rectification. The heat conduction rectification depends on the temperature difference. By reducing temperature difference across the nanostructure, the rectification could be reversed. When the temperature difference is small enough, the thermal resistance is larger when the heat flux flows from the thick part to the thin part when the length of the structure is about 10 nm. The larger variation in the cross-section leads the larger difference in the thermal resistance with opposite heat flux. The mechanism of the rectification is discussed. If we take phonons as liquid particles and consider the case of a liquid flowing through a channel with step expansion in cross-section, the flow resistance is less with liquid flowing from the narrow part to the wide part than that in the case with contrary flow direction. In fact, the scattering of phonons at the step face reduces the mean free path of phonon when heat flux conducts from the narrow end to the wide end.
Experimental Characterization of the Unsteady Flow Over the Rear Slant of an Ahmed Body
Authors: Thacker Adrien , Aubrun Sandrine , Leroy Annie , Devinant Philippe
Published: March 2011

This study presents results of an experimental analysis of the unsteady features of the flow around the rear part of an Ahmed body with a rear slant angle of 25°. This analysis focuses on the half elliptic separation bubble that developps on the rear slanted surface and brings new information, improving the understanding of the flow unsteadiness. Flow investigations are carried out using hot wire probe measurements for velocity fluctuations in the plane of symmetry above the rear slanted surface and five unsteady flush mounted pressure taps (Kulite transducers) simultaneously acquiring static pressure fluctuations along the middle line of the slanted surface. Spectral analysis and Proper Orthogonal Decomposition of the output signal show the emergence of a low frequency unsteadiness and high frequency activities which, in accordance with bibliography about separated and reattaching flow configurations, is related to a global flapping of the separated shear layer and a large scale vortices shedding. Characteristic frequencies of both instabilities is given and physical effects of the low frequency unsteadiness is related with the flapping motion of the separated shear layer.
Numerical Investigation of Heat Transfer Enhancement in a Microchannel With Offset Micro Pin-Fins
Authors: Shafeie Haleh , Abouali Omid , Jafarpour Khosrow
Published: March 2011

This paper presents a numerical study of laminar forced convection in microchannels network heat sinks with fabricated offset pin-fins. A 3-dimensional mathematical model, for conjugate heat transfer in both solid and liquid is presented. For this aim the Navier-Stokes and energy equations for the liquid region and the energy equation for the solid region are solved simultaneously and the pressure drop together heat transfer characteristics of a single-phase microchannel heat sink were investigated. A typical microchannel was selected and it was shown that using offset pin-fins has a noticeable effect and heat removal rate can be increased using this technique. However the pressure drop is also highly increasing which leads to a low coefficient of performance for microchannel with this type of micro-structure.
Analysis of Axial Meniscus Jump-Like Transition in Rectangular Microgrooves
Authors: Guo Chaohong , Hu Xuegong , Wang Tao , Tang Dawei
Published: March 2011

The meniscus receding process was studied for the axial steady flow in open rectangular microgrooves based on experimental results. Experimental results show that the liquid film recedes remarkably as a cubic trendline from the accommodation stage to the bottom corner-flow stage, but the dead zone and the step change don’t exist. The receding process of the liquid film between the accommodation stage and the bottom corner-flow stage is named jump-like transition in the paper. Characteristics of the axial flow in rectangular microgrooves were theoretically analyzed considering the meniscus receding performance in the jump-like transition, calculation results show that radius of the meniscus curvature decreases along the groove axis, which provides drive for the axial flow; the liquid cross sectional area and the liquid height decrease evidently at the stage of the jump-like transition; the liquid velocity increases along the axis, and increases promptly at the transition stage and the corner flow stage.
Micro Fibers Motion in a Laminar Flow in a Microchannel
Authors: Dastan Alireza , Abouali Omid
Published: March 2011

In this paper the motion of micro fibers in a microchannel is studied numerically. The liquid flow regime is considered to be laminar and it is assumed that the fluid and the fibers have a one-way interaction meaning the effects of fibers on the flow are neglected. An inlet plenum of the microchannel with 15 channels considered as the physical domain. The fluid flow in the model is solved numerically by an Eulerian approach using the conventional SIMPLE algorithm. To study the motion of the micro fibers in the flow, the fibers are considered to be ellipsoids of revolution. A code is developed which uses the fluid flow results and solves the equations of ellipsoid motion in a Lagrangian reference frame. The equations of ellipsoid motion consist of three equations for the translational motion and three equations for the rotational motion. The equations are integrated numerically to find the trajectory and the orientation of the micro fibers in the microchannel.
The Jetting Behavior From Single Laser Induced Bubbles Generated Above a Solid Boundary With a Through Hole
Authors: Abboud Jack E. , Oweis Ghanem F.
Published: March 2011

An inertial bubble collapsing near a solid boundary generates a fast impulsive micro jet directed towards the boundary. The jet impact on the solid boundary can cause pitting, and this effect has been taken advantage of in surgeries such as when micro-bubbles are driven ultrasonically to cavitate in tissue and produce jets that are believed to induce the surgical effect. In this experimental investigation we are interested in the jetting from single cavitation bubbles near boundaries. By introducing a through hole in the boundary beneath a single laser-induced bubble it is hypothesized that the forming jet upon bubble implosion will proceed to penetrate through the hole to the other side and that it may be utilized in useful application such as precise surgeries. We study the cases of a bubble in an infinite medium, near a blank solid boundary, and above a hole in a solid boundary. We find in the case of the hole the unexpected formation of a counter jet that is directed away from the hole and into the bubble. These findings are contrasted to similar counter jetting behaviors from bubbles near boundaries with viscous and elastic properties.
Inlet Distortion of a Centrifugal Compressor With a Circular-Sectioned 90-Degree Bend and Its Influence on the Performance
Authors: Kang Kyung-Jun , Shin You-Hwan , Kim Kwang-Ho , Lee Yoon-Pyo
Published: March 2011

In this study, distortion due to inlet duct is experimentally analyzed and discussed in order to understand the performance characteristics of the compressor exposed to a curved inlet configuration. The flow from the bend inlet is not axisymmetric in the circumferential and radial distortion. In the combination including a centrifugal compressor, the bend inlet due to spatial limitation has been shown in a various applications such as industrial chiller. The performance of centrifugal compressors can be degraded by inlet flow distortion due to a 90 degree curved pipe. The bend have some curvature radius ratio, straight upstream and downstream pipes. The flow in bend inlet is sucked due to rotating of impeller and not axisymmetric in the circumferential and radial distortion. In general, primary flow and secondary flow are occurred in curved pipe. In this paper, flow at curved pipe was experimentally measured using by 5-hole probe and hot-wire probe. The 5-hole probe was used in observing the total pressure and velocity contour. Also, the measurement of hot-wire probe was carried out in order to obtain a streamwise velocity profile. The flow details are analyzed and discussed in order to understand the relation between inlet distortion and aerodynamic performance.
An Immersed Boundary Method for Conjugate Heat Transfer Problems
Authors: de Tullio M. D. , Latorre S. S. , De Palma P. , Napolitano M. , Pascazio G.
Published: March 2011

This paper provides an immersed boundary method using a flexible local grid refinement technique for solving conjugate-heat-transfer problems. The proposed method is used to solve the flow past a heated hollow cylinder inside a channel together with the temperature field within the cylinder and then to predict turbomachinery blade cooling.
Analysis of the Drag Reduction Using Vortex Kinematics Behind a Bluff Body
Authors: Bruneau Charles-Henri , Creuse´ Emmanuel , Depeyras Delphine , Gillie´ron Patrick , Mortazavi Iraj
Published: March 2011

The aim of this work is to analyse one of the mechanisms that contributes to the drag forces, namely the distance of the vortices to the back wall of a bluff body. The study shows the strong relationship between this distance and the pressure forces at the back. Indeed, the active control processes modify the trajectory of the vortices to accelerate their removal from the wall and consequently reduce the drag coefficient.
Optical Studies of Evaporation in Microchannel Arrays
Authors: Maikowske Stefan , Lange Ruediger , Brandner Juergen J.
Published: March 2011

Phase transition in microchannels has become an interesting filed of research within the last few years. Using arrays out of a multitude of parallel microchannels, it is possible to transfer a huge amount of thermal energy by taking advantage of the latent heat of evaporation. Another point of interest concerning this research field is the stable generation of steady vapor with homogeneous parameters such as vapor quality, mass flow, pressure or temperature. Phase transition and accompanying phenomena during evaporation of water in microchannel arrays as well as influences of microstructure geometry were observed during these research studies. Optical investigations have been done using a digital high-speed camera for visualization of transient processes, e.g. explosively and confined bubble growing behavior or phase transition fluctuation. Beside this, a novel test device for optical investigation is presented in this report. The test device enables to vary several variables like temperature, microstructure or pressure drop, to name but a few. Furthermore, results and influences of different microstructure geometries on phase transition as well as different shapes of phase transition fronts in microchannel arrays are presented. Additionally, the visualization of complete and stable phase transition in microchannel arrays with steam superheating is shown.
Production of Fluoropolymer Microchips for Droplet Microfluidics and DNA Amplification
Authors: Begolo Stefano , Colas Guillaume , Malaquin Laurent , Viovy Jean-Louis
Published: March 2011

In this paper we present a novel fabrication technique for production of monolithic microfluidic chips made from a fluoropolymer (Dyneon THV). This material retains numerous properties of commonly used fluoropolymers (low surface energy and compatibility with chemicals such as organic solvents or fluorinated oil), and is easily processable at relatively low temperatures (lower than 180°C). We used hot embossing to mold microstructures on flat sheets of this polymer. The microchips are sealed through a combination of thermal and solvent bonding by applying uniform pressure with a flexible membrane. These closed channels can be used for the production and circulation of aqueous droplets in fluorinated oil. This droplet microfluidic configuration is suitable for DNA amplification since it avoids cross contamination between adjacent droplets.
Blast-Induced Neurotrauma: Characterizing the Blast Wave Impact and Tissue Deformation
Authors: Gao Jian , Connell Sean , Shi Riyi , Chen Jun
Published: March 2011

Primary blast injury, caused by exposure to the primary pressure wave emitted from explosive ordnance, is a common trauma associated with modern warfare activities. The central nervous system is particularly vulnerable to primary blast injury, which is responsible for many of the war related casualties and mortalities. An ex vivo model system is developed to introduce a blast wave, generated from a shock tube, directly to spinal cord tissue sample. A high-speed shadowgraphy is utilized to visualize the development of the blast wave and its interaction with the tissue samples. The surface deformation of the tissues is also measured for the analysis of internal stress and possible damage occurred in the tissue sample. Understanding the temporal development of the blast-tissue interaction provides valuable input for characterizing and modeling blast-induced neurotrauma. Particularly, tracking the sample surface deformation over time provides realistic boundary conditions for numerically simulating the injury and understanding the temporal development of stress.
Flow of Moving Wall Jet Near Channel Exit at Moderate Reynolds Number
Authors: Amin Rizwana , Khayat Roger E.
Published: March 2011

The two-dimensional jet flow of a Newtonian fluid at moderate Reynolds Number emerging from a channel where the upper plate is moving is examined theoretically in this study. In this case, the equations of motion are reduced by expanding the flow field about the basic Couette flow. Inertia is assumed to be large enough, allowing asymptotic development in terms of the inverse Reynolds number. A boundary layer forms adjacent to the free surface, and a classical boundary-layer analysis is applied to find the flow in the free surface and the moving wall. The influence of this boundary layer is investigated with the aid of the method of matched asymptotic expansions. The flow and stress fields are obtained as composite expansions by matching the flow in the boundary-layer region near the free surface and the flow both in the inner (boundary-layer) region and in the outer region of the core. The influence of wall velocity on the shape of the free surface, the velocity and stress is emphasized. The formulation allows for the determination of the steady state flow and free surface profiles analytically. The present work provides the conditions near exit, with the help of Higher-order boundary-layer effects (i.e. the cubic term of the inverse Reynolds number), to determine the jet structure further downstream.
Analyzing the Performance of a Hovering Ducted Rotor in Ground/Wall Effects to Improve the Controlling Aspects of VTOL Vehicles in Confined Spaces
Authors: Hosseini Zahra , Martinuzzi Robert J. , Ramirez Serrano Alejandro
Published: March 2011

This paper examines ground/wall effects on a ducted fan, with the aim to improve the handling quality of an unmanned aerial vehicle targeted for operations in confined spaces. In order to do this, flow field at different configurations of the fan-ground-wall is visualized computationally in Fluent. Herein the standard k-ε model and the non-equilibrium wall function are used to model the turbulence while the fan is modeled using the Virtual Blade Model (VBM). It is shown that in ground effects thrust is larger at distances relatively close to the ground. Adding a wall introduces an asymmetry to the flow which produces a considerable amount of horizontal force attracting the fan towards the wall. Moreover, in presence of the wall, the total thrust reduces slightly. The paper also describes the observation that at sufficiently close distances between the fan and the wall/ground, a large portion of the downwash re-enters to the fan which causes significant performance loss.
Scale-Dependent Energy Fluxes in a Rough-Wall Turbulent Channel Flow
Authors: Hong Jiarong , Katz Joseph , Schultz Michael P.
Published: March 2011

In this paper, we investigate the characteristics of energy fluxes within rough-wall turbulent boundary layer at different scales in the context of large eddy simulations (LES) utilizing high resolution Particle Image Velocimetry (PIV) data obtained in an optically index-matched facility. The rough surface consists of closely-packed pyramidal elements, and the measurement region includes the entire roughness sublayer and lower portion of outer-layer with a vector resolution of ∼9 wall units and 14% of roughness height. Our recent study has demonstrated that the entire boundary layer is flooded with an excessive number of roughness-scale eddies that are generated near the wall and advected away from it by large scale coherent structures present in the outer layer. Following this observation, the original velocity field is spatially filtered using 2D top-hat filter of length scale Δ = 1k, 3k, 6k (k is roughness height) that represent roughness, intermediate and large scale motions, respectively. In these ranges, the subgrid scale (SGS) energy fluxes show substantial increase with scale and with decreasing distance from the wall. The latter trend persists even when fluxes are scaled with the local TKE production rate. When the fluxes are decomposed to local and non-local contributions, they show a scale-dependent near-wall increase of energy transfer which bypasses the typical cascading process. This non-local flux is even larger than the local one near the wall for the [k, 3k] range. These trends are attributed to interactions of large scale turbulence with the wall roughness and the abundant roughness scale eddies near the wall. The paper also examines the behavior of Smagorinsky model coefficients, and show scale-dependence when trends at filter scales of 1k and 3k are compared to those at 6k. Both dissipation based and dynamic model coefficients show very little variation with height as long as the filter scale is in the 1–3k range, but increase with elevation for Δ = 6k.
Predictions of Turbulence Intensity in a Combustor Model Using Neural Network Analysis
Authors: Ahmed Saad A. , El Kadi Hany
Published: March 2011

Predictions of turbulence intensity and continuous evolution of fluid flow characteristics in a combustor model are useful and essential for better and optimum design of gas turbine combustors. Many experimental techniques such as Laser Doppler Velocimetry (LDV) measurements provide only limited discrete information at given points; especially, for the cases of complex flows such as dump combustor swirling flows. For this type of flow, usual numerical interpolating schemes appear to be unsuitable. Neural Network Analysis (ANN) is proposed and the results are presented in this paper and are compared with the experimental data used for training purposes. This pilot study showed that artificial neural network is an appropriate method for predicting swirl flow characteristics in a model of a dump combustor. These techniques are proposed for better designs and optimization of dump combustors.