bluuebunny/crossref_metadata_2025 huggingface dataset:
Flow Conditions at the Inlet of Aspirating Pipes: Part 1—Theory
Authors: Axisa Franc¸ois
Published: March 2011

Drastic changes occur in the dynamics of pipes conveying fluid when passing from the discharging flow case to the aspirating flow case. This can be attributed to modifications in the flow conditions which prevail at the aspirating tube inlet versus those prevailing at a discharging tube outlet. In the present paper the point is investigated in relation with the behavior of two elementary systems, namely a single degree of freedom system (1-DOF oscillator) made of a rigid pipe supported at one end by a rotational spring and a two degrees of freedom system (2-DOF coupled oscillators) made of a pair of articulated tubes. In the first system, flow-structure coupling reduces essentially to a Coriolis force while in the second, it arises via both centrifugal and Coriolis forces. The present paper is devoted to theoretical modeling and prediction of the dynamical behavior of these systems according to a few basic assumptions concerning the flow conditions at the aspirating inlet. It serves as an introduction to the companion paper by Debut et al. [1] where theoretical predictions are confronted to experiment. As a major result it is found that even “small changes” in the flow conditions at the aspirating inlet can lead to important changes in the tube dynamics which are amenable to measurement, even if the experiments are restricted to relatively low flow velocities because of limited capacity of the test loop.
The Dynamics of Cantilevered Flexible Plates in Axial Flow
Authors: Zhao Wensheng , Pai¨doussis Michael P. , Tang Liaosha , Liu Meiqing , Jiang Jin
Published: March 2011

A theoretical and experimental study has been conducted to investigate the dynamics of cantilevered flexible plates in axial flow. In this paper, a nonlinear equation of motion of plate based on the inextensibility assumption, coupled with an unsteady lumped vortex model for the aerodynamic part is used to analyze the dynamical behaviour of this fluid-structure system theoretically. Experiments have been conducted in a 3 ft × 2 ft wind tunnel, using polypropylene carbonate films, thin brass plates, polyester sheets, and type 304 stainless steel sheets, with maximum dimensions 22.4 cm × 16.8 cm. In the experiments, time traces, PSDs, phase-plane plots, Poincare´ maps, PDFs, and autocorrelations are used to characterize the motions of the system. Periodic, period-doubling and chaotic oscillations have been observed. In the experiments, flutter arises via a subcritical bifurcation accompanied by hysteresis for low aspect ratio plates; the hysteresis disappears for large aspect ratio plates. The hysteresis phenomenon is considered to be due to three-dimensional bending of the plates. Furthermore, for flow velocities in the hysteresis loop, the stable plate subjected to a small external disturbance will flutter with the same amplitude limit cycle oscillation as self-excited oscillation at the same flow velocity. The experimental critical velocities for flutter onset are in good qualitative and quantitative agreement with the theoretically predicted values.
Vortex Shedding and Acoustic Resonance of Single and Tandem Finned Cylinders
Authors: Eid Mohammed , Ziada Samir
Published: March 2011

The effect of fins on vortex shedding and acoustic resonance is investigated for isolated and two tandem cylinders exposed to cross-flow in a rectangular duct. Three spacing ratios between the tandem cylinders (S/De = 1.5, 2 and 3) are tested for a Reynolds number range from 1.6×104 to 1.1×105. Measurements of sound pressure and flow velocity are performed for bare and finned cylinders with three different fin densities. The effect of fins on the sound pressure generated before the onset of acoustic resonance as well as during the pre-coincidence and coincidence resonance is found to be rather complex and depends on the spacing ratio between cylinders, the fin density and the nature of the flow-sound interaction mechanism. For isolated cylinders, the fins reduce the strength of vortex shedding only slightly, but strongly attenuate the radiated sound before and during the acoustic resonance. This suggests that the impact of the fins on correlation length is stronger than on velocity fluctuations. In contrast to isolated cylinders, the fins in the tandem cylinder case enhance the vortex shedding process at off-resonant conditions, except for the large spacing case which exhibits a reversed effect at high Reynolds numbers. Regarding the acoustic resonance of the tandem cylinders, the fins promote the onset of the coincidence resonance, but increasing the fin density drastically weakens the intensity of this resonance. The fins are also found to suppress the pre-coincidence resonance for the tandem cylinders with small spacing ratios (S/De = 1.5 and 2), but for the largest spacing case (S/De = 3), they are found to have minor effects on the sound pressure and the lock-in range.
The LBM Research on Transverse-Axis Rotary Viscous Pump
Authors: Yang Fan , Wang Haimin , Chen Eryun , Liu Lianguo
Published: March 2011

In this study, the flow pattern in the novel viscous pump is analyzed using the lattice Boltzmann method (LBM) to simulate the whole flow field in the pump with three shape cross-sections of cylinders, the circular, rectangular, as well as square. The solid curved wall boundary condition based on interpolation and bounce-back on the wall of the LBM simulation is used in steady and unsteady flow, and the moving boundary condition is also used in the latter. The analyses predicted the distributions of streamlines and the average dimensionless velocities at the exit profile which change with time. The numerical results of the average dimensionless velocities at the exit profile are in agreement with previous experimental works, which indicate that the circular contour is better than the rectangular or square ones.
Effect of Chordwise Flexibility and Depth of Submergence on an Oscillating Plate Underwater Propulsion System
Authors: Barannyk Oleksandr , Buckham Brad , Oshkai Peter
Published: March 2011

This work was dedicated to the experimental study of oscillating plate propulsors undergoing a combination of heave translation and pitch rotation. The oscillation kinematics are inspired by swimming mechanisms employed by fish and other marine animals. The primary focus was on the propulsive characteristics of such oscillating plates, which were studied by means of direct force measurements in the thrust-producing regime. Experiments were performed at constant Reynolds number and constant heave amplitude. By varying the Strouhal number, the depth of submergence and the chordwise flexibility of the plate, it was possible to investigate corresponding changes in the generated thrust and the hydromechanical efficiency. It was possible to establish a set of parameters, including the driving frequency of the system, the ratio of rigid to flexible segment length of the plate, and the range of Strouhal numbers that led to a peak efficiency of approximately 80%. The experiments involving plates with various ratios of rigid to flexible segment lengths showed that greater flexibility increased the propulsive efficiency and thrust compared to an identical motion of the purely rigid plate. By submerging the plate at different depths, it was observed that the proximity of the propulsor to the bottom of the channel led to overall increase in the thrust coefficient.
Heat Flux to Fluids Within a Rock Fracture in a Geothermal System
Authors: Crandall Dustin , Ahmadi Goodarz , Bromhal Grant
Published: March 2011

Fractures in rocks enable the motion of fluids through the large, hot geologic formations of geothermal reservoirs. The heat transfer from the surrounding rock mass to the fluid flowing through a fracture depends on the geometry of the fracture, the fluid/solid properties, and the flow rate through the fracture. A numerical study was conducted to evaluate the changes in heat transfer to the fluid flowing through a rock fracture with changes in the flow rate. The aperture distribution of the rock fracture, originally created within Berea sandstone and imaged using a CT-scanner, is well described by a Gaussian distribution and has a mean aperture of approximately 0.6 mm. Water was used as the working fluid, enabling an evaluation of the efficiency of heat flux to the fluid along the flow path of a hot dry geothermal system. As the flow through the fracture was increased to a Reynolds number greater than 2300 the effect of channeling through large aperture regions within the fracture were observed to become increasingly important. For the fastest flows modeled the heat flux to the working fluids was reduced due to a shorter residence time of the fluid in the fracture. Understanding what conditions can maximize the amount of energy obtained from fractures within a hot dry geologic field can improve the operation and long-term viability of enhanced geothermal systems.
Nonlinear Localization for Electrowetting-Based Digital Microfluidic Actuation
Authors: Abolhasani Milad , Devlin Kurt D. , Najjaran Homayoun , Hoorfar Mina , Holzman Jonathan F.
Published: March 2011

The method described in this paper introduces a new multiplexing format for cross-referencing of DMF systems through the simultaneous use of threshold-based voltage actuation (which sets a minimum voltage to initiate droplet motion) and bi-polar voltage activation on the overlying and underlying electrodes. The design makes use of bi-polar voltage activation and threshold effects to eliminate inter-droplet interference and overcome addressability limitations. In the proposed DMF multiplexer structure, these two requirements must both be satisfied for 2-D multiplexed addressability. Experimental characterization of the threshold voltage associated with the first requirement is presented. With regard to requirement two, the bi-polar voltage activation scheme is applied to a fabricated DMF multiplexer, and independent microdroplet motion is shown. The technique can be applied in actuating isolated microdroplets or microdroplet groups (simultaneously) in large-scale/highly-parallel DMF devices.
Numerical Simulation of a Mini-Channel Three-Way Catalytic Converter
Authors: Akbari M. H. , Roohi R. , Asaee S. A.
Published: March 2011

A three-dimensional model is developed to simulate the behavior of a single-channel three-way catalytic converter. The flow regime is assumed to be steady and laminar, and the channel walls are considered as isothermal. A multi-step, global heterogeneous reaction mechanism with 16 reactions and 11 species is used in this investigation to enhance the accuracy of the results. The chemical reactions are assumed to occur only on the reactor walls. The developed model is validated against available experimental data for stoichiometric operating conditions. The effect of the feed temperature on the conversion efficiency of the main pollutant components is studied. The light-off temperature for the stoichiometric A/F is found to be about 530 K for CO, NO and UHC, and 425 K for H2 conversion. The model is also applied to predict the effect of reactor length and inlet mixture space velocity on the conversion efficiency at two different temperatures. By using the same kinetics a well-stirred, unsteady model is also developed to identify the sensitivity of the multi-step kinetic mechanism to the mixture composition. The effect of mole fraction variation of each species on the conversion of other mixture components is investigated.
The Effect of Sound Pressure on the Aeroacoustic Sources Around Two Ducted Tandem Cylinders
Authors: Finnegan Shane Leslie , Meskell Craig , Ziada Samir
Published: March 2011

An empirical investigation of the spatial distribution of aeroacoustic sources around two tandem cylinders subject to ducted flow and forced transverse acoustic resonance is described. The work builds on a previous investigation by the authors and utilises Howe’s theory of aerodynamic sound. The influence of the sound pressure level in the duct on the strength and location of the aeroacoustic sources in the flow was the main focus of the investigation and experiments to resolve the aeroacoustic source distribution were concentrated at a low main-stream flow velocity (before acoustic-Strouhal coincidence), at a medium mainstream flow velocity (just after acoustic-Strouhal coincidence) and at a high mainstream flow velocity (substantially higher than acoustic-Strouhal coincidence). The sound pressure level was found to have a considerable effect on the “lock-in”’ range of the cylinders which widened as the sound pressure level increased. A proposed normalisation of the net acoustic energy transfer per spanwise location appears to show good metric for the distribution of the aeroacoustic sources in the flow field. Using this, it was found that the amplitude of the sound pressure had a negligible influence on the aeroacoustic sources in the wake and the gap region for all the tested cases apart from the lowest flow velocity. This particular case showed indications that the aeroacoustic source strength and location could be altered for certain changes in sound pressure level.
Effect of Fins on Vortex Shedding Noise Generated From a Circular Cylinder in Cross Flow
Authors: Hamakawa Hiromitsu , Kouno Yuji , Nishida Eiichi
Published: March 2011

In the present paper, the effect of twist-serrated fins around a bare tube on the Aeolian tone was experimentally investigated. These fins were mounted spirally around a bare tube and had the same geometry as those actually used in boiler tubes. We measured the intensity of velocity fluctuation, spectrum of velocity fluctuation, coherence of Karman vortex in the spanwise direction, dynamic lift force, and sound pressure level of the aerodynamic noise generated from finned tubes with various fin pitches. An Aeolian tone induced by Karman vortex shedding was observed in the case of a finned tube, although the complicated fin was mounted around a bare tube. A decrease in the pitch of the fin effectively caused an increase in the equivalent diameter, which acted as the characteristic length of a cylinder with fins. The equivalent diameter depended on the Reynolds number. We modified a relation to calculate the characteristic diameter of the finned tube, which in turn was used to calculate the Strouhal number. The coherent scales in the spanwise direction for the cases with various fin pitches were slightly larger than that of a simple circular cylinder. It is known that the sound pressure level of the Aeolian tone depends on the coherent scale of the Karman vortex in the spanwise direction. However, when the pitch of the fins decreased, the peak level of the sound pressure spectrum decreased. A correlation analysis between the flow field and Aeolian tone was carried out.
An Evaluation of a Two-Phase Damping Model on Tube Bundles Subjected to Two-Phase Cross Flow
Authors: Sim W. G. , Mureithi Njuki W.
Published: March 2011

The analytical model (Sim; 2007), to predict the two-phase damping ratio for upward cross-flow through horizontal tube bundles, has been evaluated. The damping model was formulated, based on Feenstra’s model (2000) for void fraction and various models (homogeneous, Levy, Martinelli-Nelson and Marchaterre) for two-phase friction multiplier. The analytical results of drag coefficient on a cylinder and two-phase Euler number were compared with the experimental results by Sim-Mureithi (2010). The factor, a relation between frictional pressure drop and the hydraulic drag coefficients, could be determined by considering experimental results. The two-phase damping ratios, given by the analytical model, were compared with existing experimental results. It was found that the model, based on Marchaterre’s model, is suitable for air-water mixture while the Martinelli-Nelson’s model for steam-water and Freon mixtures. The two-phase damping ratio is independent on pitch mass flux for air-water mixture, but it is more or less influenced by the mass flux for steam-water/Freon(134) mixtures. The two-phase damping ratios, given by the present model, agree well with experimental results for a sufficiently wide range of pitch mass ratio, quality and p/d ratios.
Simulation of Long Microchannel Flow in Transitional Regime Using Lattice Boltzmann Method
Authors: Normohammadzadeh Mohammad , Rahnama Mohammad , Jafari Saeed , Akhgar Alireza
Published: March 2011

Microscale flow simulation is considered in this paper for a microchannel flow geometry. Lattice Boltzmann Model (LBM) was used as the numerical method for flow simulation, in which an effective mean free path was used in relaxation time appeared in LBM. The effective mean-free-path makes it possible to investigate flow characteristics in transition flow regime, for which Knudsen number varies from 0.1 to 10. Such implementation does not change the computational efficiency of LBM significantly. Results are obtained for flow configuration in a long microchannel. The slip velocity was predicted in this flow configuration with good accuracy. Good correspondence with Direct Simulation Monte Carlo (DSMC) method was observed.
3-D Numerical Simulation of Gas-Liquid Flow in a Minichannel With a Non-Uniform GDL Surface
Authors: Ding Yulong , Bi Xiaotao T. , Wilkinson David P.
Published: March 2011

Gas-liquid two-phase flow in rectangular minichannels of polymer-electrolyte membrane fuel cells (PEMFCs) has a major impact on the fuel cell performance and durability. Different from traditional two-phase flow in other applications, water in the PEMFCs is introduced into the minichannel from the gas diffusion layers (GDLs) through random pores of different sizes. Meanwhile, the four channel surfaces may have different wettabilities due to the different materials used. Thus, the microstructure of GDLs and the surface wettability should be considered in investigating the two-phase flow in PEMFC channels. One challenge in simulating PEMFCs is that, full consideration of detailed microstructure of GDL needs extremely large computational time. In this work, we simplified the microstructure of GDL to a number of representative pores on the 2D GDL surface. A 3-D minichannel with 1.0 mm × 1.0 mm square cross section and 100 mm long was used in the simulation. Operating conditions and material properties were selected according to realistic fuel cell operating conditions. Volume of fluid (VOF) method was employed to explicitly track the droplet surfaces emerging from the non-uniform GDLs. Simulation results show that, as the flow develops along the channel, the flow pattern evolves from corner flow on the bottom and side wall to corner flow on the top wall, annular flow and slug flow. The effects of liquid injection rates were studied, and it is found that the high liquid flow rate would accelerate the flow pattern development. The effect of wall surface material wettability was also studied by changing the hydrophobicity of GDL surface and side walls, separately. Simulation results show that the material wettability has a strong impact on the two-phase flow pattern, with a more hydrophilic side walls and/or a more hydrophobic GDL surface being more beneficial for expelling water out of the channel.
Effects of Rolling Angular Velocity on the Flutter of Wing-Store Under Follower Force
Authors: Mazidi A. , Fazelzadeh S. A. , Marzocca P.
Published: March 2011

The flutter analysis of a swept aircraft wing-store configuration subjected to follower force and undergoing a roll maneuver is presented. Concentrated mass, follower force, and roll angular velocity terms are combined in the governing equations which are obtained using the Hamilton’s principle. The wing is modeled from a classical beam theory and incorporates bending-torsion flexibility. Heaviside and Dirac delta functions are used to consider the location and properties of the external mass and the follower force. Also, Peter’s unsteady aerodynamic pressure loadings are considered and modified to take the wing sweep angle effect into account. The Galerkin method is applied to convert the partial differential equations into a set of ordinary differential equations. Numerical simulations are validated with available published results. In addition, simulation results are presented to show the effects of the roll angular velocity, sweep angle, follower force, and engine mass and location, on the wing flutter. Results are indicative of the significant effect of the rigid body roll angular velocity and the follower force on the wing-engine dynamic stability. Furthermore, distances between the engine center of gravity and the wing elastic axis contribute considerable effects in the wing-engine flutter speed and frequency.
Effect of Fluid Flow Nonlinearities on the Dynamic Behaviour of Cylindrical Shells Subjected to a Supersonic Flow
Authors: Ramzi R. , Lakis A. A.
Published: March 2011

An analytical model is presented to predict the influence of nonlinearities associated with supersonic fluid flow on the dynamic and stability behavior of thin isotropic cylindrical shells. The method developed is a combination between finite element method, sander’s shell theory and nonlinear aerodynamic theory (third-order piston theory). The shell is subdivided into cylindrical finite elements, the displacements functions are derived from exact solutions of Sanders equations for thin cylindrical shells and the influence of stress stiffening due to internal or external pressure and axial compression is also taken into account. Expressions for the masse and stiffness matrices are determined by exact analytical integration. With the nonlinear dynamic pressure, we develop nonlinear matrices: stiffness, damping and coupling matrices for flow. The nonlinear equation of motion is then solved using a fourth-order Runge-kutta numerical method. Frequency variations are determined with respect to the amplitude of the motion for different cases. This is a powerful model to predict linear, nonlinear vibrations and stability characteristics of cylindrical shells subjected to external supersonic flow that can be applied for the aeroelastic design of aerospace vehicles.
Analysis of Flow-Induced Vibration of the Volute of a Centrifugal Pump Based on Finite Element Method
Authors: Yuan Jianping , Liang Yun , Yuan Shouqi , Xiong Haifang , Pei Ji
Published: March 2011

During the operation of centrifugal pumps, radial hydraulic force is generated due to non-uniform flow within pumps, which is one of the main sources of the vibration of the centrifugal pump volute. In this paper, based on CFD and finite element method, it was calculated and analyzed that the volute vibration of a centrifugal pump caused by radial hydraulic force. The reason of the occurrence of radial force was analyzed, and by simplifying the theoretical formulas the force was calculated. Then the unsteady flow field of a centrifugal pump was simulated and analyzed under different running conditions by CFD method. Based on the simulation results, the radial hydraulic force of the pump was calculated. Finally, vibration response of the pump volute due to the hydraulic radial force was obtained. By analyzing the vibration response datum, vibration parameters were got such as the displacement, velocity and acceleration of vibration. It was obtained that the main vibration frequencies of the pump volute which is caused by unsteady flow are blade frequency and its harmonic frequencies. The pump volute has a minimum vibration under design flow rate condition, and it has a maximum vibration at the 1.5 times design rated flow whilst the vibration frequency is the integral multiple of the blade frequency. This study is helpful to understand the flow-induced vibration of pump volute and to improve the hydraulic design of the centrifugal pump.
Numerical Investigation on Aerodynamic Performance of Twisted Blade for Human Powered Helicopter
Authors: Hayashi Ryosuke , Suzuki Masaya , Yamamoto Makoto
Published: March 2011

It is a dream for the human to freely fly in the sky by his/her own power. A human powered airplane has been developed and now we can fly for more than 100 km, while nobody has arrived at the height more than 1 m by a human powered helicopter. Therefore, the realization of a human powered helicopter will be a next dream of the human. The weight reduction is one of the most critical problems to realize a human powered helicopter. However, blades made of light materials are twisted and bent during hover. In this work, we perform three-dimensional simulation considering with fluid-structure interaction to obtain the design guidelines. The simulation is based on a weakly coupling followed as: (1) calculate the flow field, (2) calculate the deformation of blade, (3) remesh and return to phase (1). To investigate the blade performance, some cases of initial attack angle from 2 deg. to 20 deg. every 2 deg. are conducted. The results show that the optimum initial attack angle is 8 deg. and then total lift obtained by the blade is 1.36 kN. This would be sufficient for realizing a human powered helicopter.
Fully-Coupled Fluid Solid Computation for Simulation of Flutter in Tubes and Tube Arrays
Authors: Longatte E.
Published: March 2011

This work is concerned with the modelling of the interaction of a fluid with a rigid or a flexible elastic cylinder in presence of axial or cross-flow. A partitioned procedure is involved to performe the computation of the fully-coupled fluid solid system. The fluid flow is governed by the incompressible Navier-Stokes equations and modeled by using a fractional step scheme combined with a co-located finite volume method for space discretisation. The motion of the fluid domain is accounted for by a moving mesh strategy through an Arbitrary Lagrangian-Eulerian (ALE) formulation. Solid dyncamics is modeled by descrete or beam elements in the linear elasticity framework and systems are solved through a finite element method. The resulting strongly coupled fluid solid set of non linear equations is solved by means of a partitioned solution procedure. A fixed point method combined with under-relaxation is involved to ensure the optimal convergence of the iterative procedure. In the present work two examples are presented to show the methodology robustness and efficiency. The purpose is to attempt to simulate a fluid structure interaction resulting in the development of a dynamic instability induced by a positive damping generation of the system. Both flutter of a flexible cylinder conveying an internal fluid and fluid-elastic instability of a tube array submitted to an external cross flow are investigated numerically. According to first results the partitioned procedure relies on consistant numerical methods ensuring energy conservation at the interface and describing with a sufficient accuracy the mechanical energy transfer between fluid and solid systems through the interface with a limited numerical diffusion. Therefore it seems to be qualitatively convenient for simulation of flutter. For a quantitative evaluation of the methodology further complementary simulations validating these developments from a physical point of view will be required in order to confirm these first trends.
Consequences of the Confinement on the Mass or Heat Transfer on a Spherical Particle in Non-Newtonian Fluids
Authors: Despeyroux Antoine , Ambari Abdlehak , Ben Richou Abderrahim , Champmartin Ste´phane
Published: March 2011

In non-Newtonian fluids, the mass or heat transfer on particles are of major interest in many industrial processes using suspensions such as fluidized beds or microfluidics reactors. In all these problems we often face hydrodynamic and mass or thermal interactions between a single particle and others or between a single particle and some walls. In this study, such confined configurations can be modeled by a spherical particle translating parallel to the axis of a cylindrical tube. As the suspending fluid may be non-Newtonian, and before examining any possible additional viscoelastic effect on suspension, the first step in the understanding of the consequences of the principal non-Newtonian behavior is the study of the shear thickening or shear thinning (power law model) regarding the transfer phenomena. Then, when the particle translates along the axis of the tube in symmetrical configuration, we numerically solved the momentum and mass (or heat) transfer equations using the stream/vorticity functions formulation coupled to the singularity technique in order to make a numerical conformal mapping for the mesh. For Newtonian fluids, the successful comparisons firstly between our numerical results and asymptotical solutions obtained by us in the lubrication regime, and secondly between our results and those obtained by other authors in unlimited medium, confirm the validity of our approach. Thereby we extended this method to power law fluids. As the geometrical distribution of particles in suspensions is not at all symmetric, we study the influence of some geometrical disturbance breaking the symmetry of the system. To answer this question, we numerically investigate, using the finite volume method, the simple configuration of single spherical particles translating parallel to and in the off-axis position in the tube.
An Experimental Investigation of the Skin Friction in a Plane Turbulent Wall Jet Over Smooth and Rough Surfaces
Authors: Rostamy Noorallah , Bergstrom Donald J. , Sumner David , Bugg James D.
Published: March 2011

Estimation of the skin friction in a turbulent wall jet flow over smooth and rough surfaces was studied experimentally. Wall jet flows can be found in many engineering applications in which knowledge of the skin friction behavior is essential for predicting the drag force as well as the heat transfer rate at the wall. Although there are many studies which consider a wall jet on a smooth surface, only a few experiments have examined wall jet flows on a rough surface. This paper reports on an experimental investigation which used a two-component laser Doppler anemometry (LDA) system to measure the mean velocity field in a plane turbulent wall jet on both smooth and transitionally rough surfaces. The Reynolds number based on the slot height and exit velocity of the jet was approximately Re = 7500. A glass plate was used for the smooth surface, while the rough surface consisted of a 36-grit sheet glued to the glass plate. The momentum-viscosity scaling originally introduced by Narasimha et al. (1973) and revisited by Wygnanski et al. (1992) can be used to construct a similarity profile for a wall jet on a smooth surface, which together with the momentum integral equation leads to a convenient expression for the friction velocity and hence skin friction coefficient Cf. This approach has been used to process the experimental results, which gives values of Cf which are consistent with the results of other methods and some existing empirical correlations. However, for rough wall flow, the friction at the wall is not only governed by viscosity, but also by surface roughness. Hogg et al. (1997) suggested that for a fully rough surface, the viscosity be replaced by the roughness parameter Uoke, where Uo and ke are the initial velocity and roughness length, respectively. Here, this approach is applied to our recent velocity measurements in a wall jet on a transitionally rough surface, where both viscous and roughness effects are present. The present results indicate that for an equivalent sand-grain roughness range of 40 < ks+ < 70, the momentum-viscosity scaling is able to capture the skin friction behavior compared to that obtained from the logarithmic and power laws. The results also show that the scalings proposed by Hogg et al. (1997) and Wygnanski et al. (1992) both result in similar values for the friction velocity. However, the values of Cf estimated by both scalings are considerably larger (approximately 47%) than those obtained from the logarithmic and power laws.
Numerical Analysis of Single Mode Panel Flutter in a Viscous Gas Flow
Authors: Vedeneev Vasily V.
Published: March 2011

In this paper single mode panel flutter, which occurs at low supersonic Mach numbers, is studied. Numerical analysis which does not require solution of coupled FSI problem has been conducted. Flutter boundaries obtained are compared with previously known analytical results.
Characterization of Capillary Flow Within a Homogenously Dispersed Array of Vertical Micropillars
Authors: Zhang Conan , Hidrovo Carlos H.
Published: March 2011

When considering fluidic devices at the micron length scale, surface tension forces become dominant relative to body forces. Albeit smaller than mechanical and electrical pumps, capillary forces are commonly exploited as a mechanism to drive fluid flow. Unlike pumps, capillary driven flows are passive in nature and are not dependent on auxiliary equipment to drive fluid flow. Although beneficial from an energy standpoint, the lack of a supplementary driving potential causes the flow to be limited by the wick structure dimensions that generate the capillary forces. Subsequently, investigation into the contributions of the wick structure must be performed in order to optimize the fluid flow through a capillary structure. General capillary theory states that capillary forces increase inversely proportional to the pore radius. Consequently, arrays of vertically aligned nanopillars grown on silicon substrates are considered for fluid flow optimization due to their small pores. To simulate these nanopillars, an ab initio analysis was done on a homogenously dispersed array of vertically aligned pillars. An analytical solution to predict the maximum achievable capillary flow with respect to the structure dimensions was found through this method. Subsequently, this analytical solution can be used to produce a set of optimal geometric conditions that would induce the maximum capillary flow through a wick comprised of vertically aligned pillars. Experimental results are also presented to validate the analytical solution. Homogeneously dispersed cylindrical pillars were created on silicon wafers via reactive ion etching to reconstruct the geometry assumed by the analytical solution. The capillary limit was found for structures with varying geometric dimensions. By contrasting the empirical data with the values predicted by the analytical model, the validity of the analytical model was found to be in good agreement.
An Analysis of Saturated Critical Heat Flux in Micro/Mini-Channels
Authors: Wu Zan , Li Wei , Wang Zhike
Published: March 2011

This paper verified the macro-to-micro-scale transitional criterion BoRel0.5 = 200 proposed by Li and Wu [W. Li, Z. Wu, A general criterion for evaporative heat transfer in micro/minichannels, International Journal of Heat and Mass Transfer 53 (2010) 1967–1976] because data points where BoRel0.5 ≤ 200 and BoRel0.5 > 200 show very different trends for the entire database (1,672 data points). For the 859 data points with BoRel0.5 ≤ 200, the boiling number at CHF decreases greatly with length-to-diameter ratio Lh/dhe when Lh/dhe is small, while Lh/dhe presents negligible effect on the boiling number when Lh/dhe > 150. For the region where Lh/dhe ≤ 150 and BoRel0.5 ≤ 200, a simple saturated CHF correlation was proposed by using the boiling number, length-to-diameter ratio, and exit quality. Heated length and heated equivalent diameter were adopted in the length-to-diameter ratio, considering the actual heat transfer conditions. A combined dimensionless number WemCal0.8 was introduced to correlate the micro/mini-channel database for the region: Lh/dhe > 150 and BoRel0.5 ≤ 200. The new method can predict the overall micro/mini-channel database accurately on the whole. It can predict almost 95.5% of the non-aqueous data and 93.5% of the water data within the ± 30% error band.
Application of Stochastic Optimization Methods in the Gas Turbine Projection Problems
Authors: Afanasjevska V. E. , Tronchuk A. A. , Ugryumov M. L.
Published: March 2011

When projecting the gas turbine an important problem is an ensuring the high values of gas turbine parameters and required gas turbine operating characteristics on the different operating conditions. These requirements can be reached by engine function units system perfecting on base of multicriterion stochastic optimization problems solution. Three stochastic optimization problems definitions were formulated. Each problem has own features and can be used for different application solution. These applied problems are: M-problem can be used on the technical system unit conceptual design stage; V-problem can be used for the problem solution of tolerancing during the technical system unit production; P-problem can be used for interval analysis of technical system functional unit. The multicriterion stochastic optimization problem rational decision is realized by the evolutional method. This method makes it possible to find the solution with given accuracy by attraction the less information recourses than standard methods. In the stochastic optimization problems definitions the input data random character is taken into account. It makes it possible to find the optimal values of desired parameters. These parameters ensure the maximal probability of finding the objective function in given range.
A Characterization of Limit Cycle Oscillations in Membrane Wing Micro Air Vehicles at Low Angle of Attack
Authors: Attar Peter J. , Johnston Jordan W. , Romberg William A. , Parthasarathy Ramkumar N. , Morris Brian J.
Published: March 2011

In this paper, a (three) batten-reinforced fixed wing membrane micro air vehicle is used to determine the effect of membrane pre-strain on flutter and limit cycle behavior of fixed wing membrane Micro Air Vehicles. For each configuration tested, flutter and subsequent limit cycle oscillations are measured in wind tunnel tests and predicted using an aeroelastic computational model consisting of a nonlinear finite element model coupled to a vortex lattice solution of the Laplace equation and boundary conditions. Agreement between the predicted and measured onset of limit cycle oscillation is good as is the prediction of the amplitude of the limit cycle at the trailing edge of the lower membrane. A direct correlation between levels of strain and the phase of the membranes during the limit cycle is found in the computation and thought to also occur in the experiment.
Out-of-Plane Vibration of a Curved Pipe due to Pulsating Flow (Nonlinear Interactions Between In-Plane and Out-of-Plane Vibrations)
Authors: Yamashita Kiyotaka , Nakamura Keita , Yabuno Hiroshi
Published: March 2011

A great deal of study has been done on the dynamics of straight pipes conveying fluid. In contrast, only a few studies have been devoted to the dynamics of curved pipe conveying fluid. In this paper, a theoretical and experimental investigation was conducted into out-of-plane vibration of a curved pipe for the case that the fluid flow contains a small time-dependent harmonic component. The nonlinear out-of plane vibrations of a curved pipe, which is hanging horizontally and is supported at both ends, are examined when the frequency of the pulsating fluid flow is near twice the fundamental natural frequency of out-of-plane vibration. The main purpose of this paper is to investigate the nonlinear interactions between the in-plane and the out-of-plane vibrations analytically and experimentally. The partial differential equations of out-of-plane motions are reduced into a set of ordinary differential equations, which govern the amplitude and phase of out-of-plane vibration, using the method of Lyapnov-Schmidt reduction. It is clarified that the excitation of the in-plane vibration produces significant responses in the out-of-plane vibrations. Finally, the experiments were conducted with a silicon rubber pipe conveying water. The typical features of out-of-plane vibration are confirmed qualitatively by experiment.
Advanced Instrumentation for Measuring Fluid-Structure Coupling Phenomena in the Guide Vanes Cascade of a Pump-Turbine Scale Model
Authors: Roth Steven , Hasmatuchi Vlad , Botero Francisco , Farhat Mohamed , Avellan Francois
Published: March 2011

In the present study, the fluid-structure coupling is investigated in the guide vanes cascade of a pump-turbine scale model placed in the EPFL PF3 test rig. The paper focuses on the advanced instrumentation used to get reliable and complete fluid-structure coupling results. Semi-conductor strain gages are installed on three guide vanes which are especially weakened to account for stronger fluid-structure coupling phenomena. These are statically calibrated in terms of torsion torque and bending force. A laser vibrometer is used to measure the vibrating guide vane velocity. Piezo-resistive pressure sensors are placed around the weakened guide vanes to monitor the influence of the structural vibrations on the surrounding flow. An underwater non-intrusive system is used to get an impulse excitation. The instrument set enables a reliable fluid-structure coupling investigation in hydraulic pump-turbine scale model. Finally, the results show a strong coupling between the vibrating guide vanes and the surrounding unsteady flow.
EWOD in Coplanar Electrode Configurations
Authors: Malk Rachid , Rival Arnaud , Fouillet Yves , Davoust Laurent
Published: March 2011

This paper deals with ElectroWetting-On-Dielectric using AC voltage (AC EWOD) in both open and closed configuration. The open configuration is more convenient for fundamental studies and observations of electrowetting associated phenomena whereas the closed configuration is particularly adapted for practical use in a Digital lab-on-chips (DLC). Software developed for the study of electrowetting and droplet oscillations in coplanar electrode configuration are presented. Particular features of electrowetting are outlighted. Droplet oscillations and hydrodynamic induced flows in open coplanar configuration are considered and applications for DLC are illustrated in closed configuration.
Microfluidic Formation of Hydrogel Microparticles for Drug Delivery
Authors: Betz Amy , Zhang Hanwei , Chen Weiliam , Attinger Daniel
Published: March 2011

Creating monodispersed hydrogel microparticles is advantageous for drug delivery applications. We explore microfluidic flow focusing as a method for generating such particles. Our hydrogel has a unique composition that makes it biodegradable and mechanically strong. We designed and manufactured a polymer microfluidic chip that mixes three viscous precursor solutions and generates a steady stream of microparticles from that solution. We found that microfluidic flow focusing produce particle with a coefficient of variance around 9%; this was a four times improvement over traditional methods.
A Novel Meshing Approach for Large Eddy Simulation
Authors: Zhang Bin , Wang Tong , Gu Chuan-gang , Dai Zheng-yuan
Published: March 2011

In large eddy simulation (LES), the filtering grid scale (FGS) of LES equations is calculated generally by local mesh size. Therefore, proper LES Meshing is very decisive for better results and more economical cost. An effort was made to provide an available approach for LES meshing by turbulence theory and CFD methods. The expression for proper filtering grid scale (PFGS) was proposed on the basis of −5/3 law of inertial sub-range. A new parameter named grid ratio coefficient was put forward for the mesh adjustment. The proper mesh of LES could be built directly from the adjustment of RANS mesh. Two test cases both backward facing step flow and turbulent channel flow were provided to verify the approach. There were three kinds of mesh size, including coarse mesh for RANS (RCM), adjusted mesh for LES with the novel approach (NAM) and fine mesh for LES (LFM), employed here. The grid numbers of NAM were less than those of LFM evidently, and the results of NAM were in a good agreement with those of DNS and experiments. It was also revealed that results of NAM were very close to those of LFM. The conclusions provided positive evidences in the application of the approach.
Large Eddy Simulation of Turbulent Air Flow Over a Surface Mounted Hemisphere
Authors: Tavakol M. M. , Abouali O. , Yaghoubi M.
Published: March 2011

Turbulent air flow around a surface mounted hemisphere is investigated numerically. The flow field has been simulated using Large Eddy Simulation (LES) technique. Using LES results, variations of flow velocity at various sections along the midplane are presented and compared with the wind tunnel measurements. Also the results of LES are compared with a RANS model (k-ε RNG). It is concluded that LES with the proper grid resolution is capable to capture the complex features of the flow especially in the separation bubble formed behind the hemisphere. The LES results are in a better agreement with experimental data compared with k-ε RNG model. Also various SGS models for LES technique are tested and the predictions of these models are compared.
Experimental and Computational Flow and Temperature Fields in a Heat Exchanger Crystallizer Geometry for Validation and Implementation of a Novel Process Sensor
Authors: Pascual Marcos R. , Kramer Herman J. M.
Published: March 2011

During crystallization processes the control and effective distribution of heat transfer from the heat exchanger to the solution plays an important role. The turbulent flow field and the temperature variations in the solution determine the local supersaturation profiles and the spatial particle distribution. Thus they have a strong impact on the final product quality, production capacity and efficiency of the process. In this sense, a sensor able to determine in situ the local process variables to get better insight in the process behavior, would be required. The recently proposed Smart moving Process Environment Actuators and Sensors (Smart PEAS) is a promising initiative to achieve a more efficient process control. In this PEAS system a network of floating sensors is integrated using Ultra Wide Band (UWB) wireless technology to form a monitoring and control system. As a first phase in the development of the Smart PEAS, the research is focused on the accurate measurements of the flow field and local temperature distribution in a reactor. The hydrodynamics of the Smart PEAS are very important to achieve the necessary accurate process information. In order to determine the hydrodynamic characteristics of the Smart PEAS and to validate and compare the obtained results, laternative non intrusive techniques are used to investigate them in this work. Microencapsulated liquid crystals are used as a measurement technique to study the local temperature and flow field in a heat exchanger crystallizer geometry. To measure the 3D flow and temperature fields the microencapsulated liquid crystals are recorded in a sheet of light plane inside the crystallizer by two digital color cameras in a stereoscopic position. The images of the liquid crystals are correlated to obtain the three velocity components (3C), while from the colors of the microencapsulated liquid crystals the local temperature can be deduced after appropriate calibration. Parallel experiments are done to investigate the three dimensional trajectories of different sensor geometries in the equipment by direct visualization of the sensors. Computational fluid dynamics simulations are performed to calculate the flow field, temperature distribution and sensor trajectories. The results are compared with the experimental results for validation. The validation of the computational simulation results with the experiments gave the necessary confidence to predict flow fields in new crystallizer designs. On the basis of the analysis the Smart PEAS sensor design and the reliability of the measurements obtained can be compared and implemented.