Modern Physics Letters B

An efficient lattice Boltzmann method (LBM) on non-uniform Cartesian mesh is presented in this work. In the standard LBM, the uniform mesh is used. To well capture the boundary layer and in the meantime, to save computational effort, many efforts have been made to improve the LBM so that it can be implemented on the non-uniform mesh. On the other hand, LBM has been combined with other numerical schemes to simulate complex flows recently. To solve immersed boundary (IB) problem efficiently, a new version of LBM on non-uniform Cartesian mesh is proposed in this study. A second-order local interpolation is used to calculate the distribution function at a position different from mesh points. The numerical results from the simulation of flow over a circular cylinder compare well with the data in literature.

This paper describes a numerical study of vortex shedding from a quasi-streamlined 2D body with the trailing edge made in the form of a base cavity. Direct numerical simulations (DNS) are conducted to assess the effect of Reynolds number and streamwise cavity length on the flow. It was found that the flow is governed by the formation of a vortex within the base cavity, which either serves to retard or convolute the shedding process. For a shorter cavity length, this vortex is periodically replaced, and the result is a reduction in the shedding frequency and the time-averaged form drag. As the cavity length is increased, this periodic behavior ceases and the vortex which first forms within the cavity remains there permanently throughout the observation. The origins of the deflected wake pattern are traced to the presence of a vortex which resides within the base cavity. Reductions in drag are observed for all the investigated cavity configurations and additionally it is found that the magnitude of the reduction obeys a direct relationship with the length of the cavity up to a certain asymptotic value.

In this paper, a kind of Godunov-type Lagrangian scheme is developed in the one space dimension. The Riemann problems are constructed at the interface and the velocity and pressure are evaluated using an implicit characteristic method. Two different methods are used to solve for the equation of energy conservation. Four one-dimensional numerical examples are first presented to obtain the parameter through comparison of the L₁ errors with the changing parameter values. The method having the minimal error is then extended to two dimensions and a cell-centered conservative Lagrangian scheme is proposed for the compressible multi-medium flow. The numerical results for some classical two dimensional hydrodynamic test cases show that the proposed numerical methods are effective and feasible.

A supersonic flow past a hemispherical nose with an opposing jet placed on its axis has been investigated using large eddy simulation. We find that the flow behaviors depend mainly on the jet total pressure ratio and can be classified into three typical flow regimes of unstable, stable and transition. The unstable flow regime is characterized by an oscillatory bow shock with a multi-jet-cell structure and the stable flow regime by a steady bow shock with a single jet cell. The transition regime lies between the unstable and stable ones with a complex flow evolution. Turbulence statistics are further analyzed to reveal the relevant turbulent behaviors in the three flow regimes. The results obtained in this study provide a physical insight into the understanding of the mechanisms underlying this complex flow.

In this paper, we present a coupled fluid flow and geomechanics model for simulating coalbed methane recovery. In the model, the fluid flow process is simulated with a triple porosity/dual permeability representation, and the coupling effects of effective stress and matrix swelling/shrinkage approach are simulated with a coupled fluid flow, geomechanics and gas adsorption/desorption model. The mathematical model is implemented with a fully implicit finite volume method and simulation is conducted to evaluate the effect of coupled fluid flow, geomechanics, and gas adsorption/desorption.

Numerical study of hole shape effect on blade adiabatic cooling effectiveness has been carried out on four geometry models comprising a standard cylindrical hole, a cylindrical hole with an upstream ramp, a shaped diffuser, and a double console slot. In all the cases, the hole centerline has an inclination angle of 35 degree against the mainstream gas flow. Results of the cylindrical hole model are in good agreement with available experimental and other numerical data. For the other three hole geometry variants considered, it was found that the cooling effectiveness has been considerably enhanced by a max factor of 2, compared to that from the base model. The physical mechanism for this is mainly due to the weakening of coolant flow penetration in the vicinity of the hole exit, thus reducing the level of mixing and the entrainment with the surrounding hot gas flow.

With the rapid development of the air-breathing hypersonic vehicle design, an accurate description of the combustion properties becomes more and more important, where one of the key techniques is the procedure of the liquid fuel mixing, atomizing and burning coupled with the supersonic crossflow in the combustion chamber. The movement and distribution of the liquid fuel droplets in the combustion chamber will influence greatly the combustion properties, as well as the propulsion performance of the ramjet/scramjet engine. In this paper, numerical simulation methods on unstructured hybrid meshes were carried out for liquid spray atomization in supersonic crossflows. The Kelvin-Helmholtz/Rayleigh-Taylor hybrid model was used to simulate the breakup process of the liquid spray in a supersonic crossflow with Mach number 1.94. Various spray properties, including spray penetration height, droplet size distribution, were quantitatively compared with experimental results. In addition, numerical results of the complex shock wave structure induced by the presence of liquid spray were illustrated and discussed.

Newton method is a widely used iteration method in solving nonlinear algebraic equations. In this method, a linear algebraic equations need to be solved in every step. The coefficient matrix of the algebraic equations is so-called Jacobian matrix, which needs to be determined at every step. For a complex non-linear system, usually no explicit form of Jacobian matrix can be found. Several methods are introduced to obtain an approximated matrix, which are classified as Jacobian-free method. The finite difference method is used to approximate the derivatives in Jacobian matrix, and a small parameter is needed in this process. Some problems may arise because of the interaction of this parameter and round-off errors. In the present work, we show that this kind of Newton method may encounter difficulties in solving non-linear partial differential equation (PDE) on fine mesh. To avoid this problem, the continuous Newton method is presented, which is a modification of classical Newton method for non-linear PDE.

A computational fluid dynamics (CFD) is coupled with a computational structural dynamics (CSD) to simulate the unsteady rotor flow with aeroelasticity effects. An unstructured upwind Navier-Stokes solver was developed for this simulation, with 2nd order time-accurate dual-time stepping method for temporal discretization and low Mach number preconditioning method. For turbulent flows, both the Spalart-Allmaras and Menter's SST model are available. Mesh deformation is achieved through a fast dynamic grid method called Delaunay graph map method for unsteady flow simulation. The rotor blades are modeled as Hodges & Dowell's nonlinear beams coupled flap-lag-torsion. The rotorcraft computational structural dynamics code employs the 15-dof beam finite element formulation for modeling. The structure code was validated by comparing the natural frequencies of a rotor model with UMARC. The flow and structure codes are coupled tightly with information exchange several times at every time step. A rotor blade model's unsteady flow field in the hover mode is simulated using the coupling method. Effect of blade elasticity with aerodynamic loads was compared with rigid blade.

A two-dimensional model for toxic contaminants was developed and incorporated into a laterally integrated hydrodynamics and transport model to investigate the effect of reservoir flow regime on contamination level in a reservoir after a toxic spill. The model describes the physical, chemical, and biological processes and predicts unsteady vertical and longitudinal distributions of a toxic chemical. Simulation results suggested that the persistence of a contaminant was significantly influenced by different flow regimes. It was found that the toxicant plume was more persistent in an interflow than in an overflow which moved more slowly and experienced greater volatilization and dissipation. This analysis can assist in spill control and reservoir management.

Isoparametric transformation was applied to the interpolation-supplemented lattice Boltzmann method (ISLBM). As a result, it can be applied to arbitrarily structural grids and reserve the simple procedure of ISLBM. The simulation of the lid-driven cavity flow and the flow around a cylinder were performed to validate the proposed method. The numerical results agree excellently with the data in the literature.

During the last few years, cavities have gained attention in the scramjet community as a promising flame-holding device. The present simulation study attempts to understand the reacting flow cavity characteristics for scramjet flame-holder application. The fuel was injected from upstream of cavity in reacting flow. Effects of cavity length to depth ratio were investigated. As the ratio of length-to-depth (L/D) is 3 or 4, the distribution form of the floor static-pressure of cavity in reacting flow is not consistent with that of cavity in non-reacting flow, even though open cavity flow occurs for the two cases. Open cavity flow occurs for L/D = 5, 6, 7 and 8 in non-reacting flow. Transitional cavity flow occurs for L/D = 5, 6 and 7, and closed cavity flow occurs for L/D = 8 in reacting flow. The flame holding capability of cavity with L/D = 7 is the best among all the cavities by comparison of the recirculation dimensions. Considering the drag of cavity, the cavity with ratio L/D = 5 is the best flame-holder among all cavities. The simulation results demonstrate that the cavity characteristics is different in non-reacting and reacting flows, and the conventional definition of cavity flow characteristics based on non-reacting flows needs to be revised accordingly.

The energy problem is getting increasingly serious. As such, unused energy recovery technology is crucial for environmental protection, which has been investigated extensively. Several methods have been developed to utilize scavenged energy from the environment, such as waste heat, solar energy, wind energy, and tides energy to convert into useful power. There is a new idea of piezoelectric jellyfish generator which combines the utilization of sea wave and vibration energy. When sea wave passes through the jellyfish, the wave causes the tentacles to vibrate. The tentacles is made of piezoelectric polymer which can convert the strain energy into electrical energy. This paper discusses about the piezoelectric jellyfish's tentacles being disturbed by wave in the sea. We employed the commercial CFD software CFD-ACE+ 2006 to simulate this phenomenon. The parameters including its tentacle length (L) and wave propagating function (Y) are studied which affect the piezoelectric jellyfish capacity to generate power.

An increasing amount of attention has been paid on the study of thermodiffusion effects on mass transport. This paper presents a novel mathematical model for an entire proton exchange membrane fuel cell (PEMFC) with focus placed on the modeling and assessment of the role of thermodiffusion that has been usually neglected in previous fuel cell research work. Built upon the equations of continuity, momentum, energy, species concentrations, and electric potentials in different regions of a PEMFC, a set of nonlinear partial differential equations are numerically solved using finite element methods. The simulation results demonstrate that the thermodiffusion has a noticeable impact on transport of species in an operational PEMFC.

The convection and diffusion processes of free vortex in compressible flows are simulated by using high precision numerical method to solve for the Navier–Stokes equations. Accurate treatment of the boundary condition is extremely important for simulation of vortex flows. The developed numerical methods are well presented by combining six-order non-dissipation compact schemes with Navier–Stokes characteristic boundary condition having transverse and viscous terms, and can accurately simulate the movement of free vortex. The numerical reflecting waves at the boundaries are well controlled.

In this paper the two-dimensional Euler equations, with a simple chemical reaction model, are used as the governing equations for the detonation problem. The spatial derivatives are evaluated using the fifth-order WENO scheme, and the third-order TVD Runge-Kutta method is employed for the temporal derivative. The characteristics of the two-dimensional detonation in an argon-diluted mixture of hydrogen and oxygen are investigated using Adaptive Mesh Refinement (AMR) method. From computational accuracy point of view, AMR enables the detonation front to be clearer than the method with basic meshes. From the other point of computational time, AMR also saves about half the time as compared with the case of refining the entire field. It is obvious that AMR not only increases the resolution of local field, but also improves the efficiency of numerical simulation.

In this study, we predicted the thermal breakdown of high-voltage interrupter with the characteristics of thermal plasmas such as temperature, pressure and concentration of the ablated material by using a commercial CFD program. The results showed that the pressure build-up inside the chamber was proportional to the magnitude of applied current because the quantities of heat energy and ablated mass also increase together with the current during the compression process. During the decompression process, the reverse flow found did not coincide with the magnitude of current due to the compressibility of the gas through backflow channel. The present method is expected to be useful for the design of guideline and interruption capacity on the thermal breakdown of a PASB chamber.

Accurate description of the aerodynamic and aerothermal environment is crucial to the integrated design and optimization for high performance hypersonic vehicles. In the simulation of aerothermal environment, the effect of viscosity is crucial. The turbulence modeling remains a major source of uncertainty in the computational prediction of aerodynamic forces and heating. In this paper, three turbulent models were studied: the one-equation eddy viscosity transport model of Spalart-Allmaras, the Wilcox k-ω model and the Menter SST model. For the k-ω model and SST model, the compressibility correction, press dilatation and low Reynolds number correction were considered. The influence of these corrections for flow properties were discussed by comparing with the results without corrections. In this paper the emphasis is on the assessment and evaluation of the turbulence models in prediction of heat transfer as applied to a range of hypersonic flows with comparison to experimental data. This will enable establishing factor of safety for the design of thermal protection systems of hypersonic vehicle.

In this paper, a parallel Eulerian hydrocode for the simulation of large scale complicated explosion and impact problem is developed. The data dependency in the parallel algorithm is studied in particular. As a test, the three dimensional numerical simulation of the explosion field in an unlimited atmosphere is performed. The numerical results are in good agreement with the empirical results, indicating that the proposed parallel algorithm in this paper is valid. Finally, the parallel speedup and parallel efficiency under different dividing domain areas are analyzed.

In this paper, numerical investigation was conducted for an oscillating NACA0012 foil combining the pitching and plunging motions. The plunging follows a sinusoidal motion while the pitching trajectory is controlled by achieving the resultant effective angle of attack (AOA) to be a harmonic cosine form. Computations were conducted over a range of the Strouhal number (St), different maximum effective AOA and different phase difference between pitching and plunging (ψ). Results show that, at higher St, significant improvement on propulsion performance has been achieved when the effective AOA profile maintains a harmonic cosine form by controlling the pitching motion of the foil.

Cavitating flows around a hydrofoil are simulated in this work by using a transport equation-based model. It is shown that the original Launder-Spalding k-ε model significantly over-predicts the viscosities. The viscosity-corrected models can simulate better the cavitation characteristics including the shedding process.

The velocity autocorrelation coefficient correlates the velocity in the time domain but at the same spatial position. Turbulent planar mixing layer consists of two types of turbulence, that is, shear turbulence in the central shear layer and nearly homogeneous turbulence in both the high- and low-speed free stream sides. It is interesting to know what kind of function forms can be used to represent faithfully the experimental observations of the velocity autocorrelation coefficients in the mixing layer. Various velocity autocorrelation functions are tested with the measured data. It is found that the Frenkiel function family is the most proper form to represent the measured velocity autocorrelation coefficients in both the shear layer and free stream regimes.

The simulation of an oscillating bubble near various bio-materials (brain and cornea) using the Boundary Element Method (BEM) is presented. The bio-materials are modeled as an elastic fluid with different Young's modulus and density. It is found that the bubble tends to split into two or more bubbles when it is oscillating near soft bio-materials (such as brain). The bubble will collapse with a high speed jet (about 100 m/s) near hard bio-materials (such as cornea). It is found from simulation that under certain conditions (light and soft elastic material), the oscillating bubble will jet away from the elastic boundary similar to a bubble near a free surface. An experiment using a laser generated bubble is carried out to confirm this phenomenon.

In this work we address the frontal instability of gravity currents. The planar laser-induced fluorescence (PLiF) flow visualization is utilized to analyze the detailed dynamics of the current, which are generated in a lock-exchange Perspex tank. We believe that two dominant modes of instability determine the complex structures at the head of the flow. The first one resembles Kelvin–Helmholtz instability, which results in Kelvin–Helmholtz billows rolling up in the shear zone above the head. The other, categorized as convective instability known as "lobes and clefts", which stems from ground friction as well as unstable inverse density stratification, and is considered to be the cause for the disruption of the span-wise symmetry of Kelvin–Helmholtz billows. Moreover, our observations indicate that the convective instability also contributes to a secondary instability associated with Kelvin–Helmholtz vortex breakdown. These instabilities not only play a central role in shaping the three-dimension characteristics of the currents, but also govern the mixing and entrainment mechanisms. Therefore, more precise measurement of the positions of the frontal instability and the flow structures, especially the turbulent structures is indeed necessary.

This paper investigates the characteristics of a piezoelectric micro-injector for drop-on-demand (DOD) applications. The micro-injector is designed with an inner block inside the chamber to enhance the instability energy for the production of mono-size droplet. The micro-nozzle was fabricated by MEMS processes. The upper chip is a silicon chip with two holes as the inlet and outlet of the liquid matter. A diaphragm is mounted on the center of the upper chip. The lower chip has an orifice of 50µm in diameter. The flow through the chamber is used to promote the refilling mechanism for droplet generation. A piezoelectric actuator operated in push mode (D₃₃) was mounted on the upper chip to drive the liquid through the nozzle. An inner block is designed on the inner side of the upper chip. Results show that the micro-injector with inner block could generate mono-size droplet under the driving voltage ranging from 62.5 to 150 volt at frequency of 3.2 kHz. The droplets size was 60µm with velocity ranging from 3.3 to 4.7 m/s which is higher than the case without inner block. As a comparison, the injection of the micro-injector without inner block needs a much higher driving voltage of 112.5 volt at driving frequency of 9.7kHz. It is concluded that the micro-injector with the inner block performs better than the one without the inner block.

This research paper presents a study of boiling heat transfer from longitudinal rectangular and square pin finned surfaces immersed in saturated water at low vapor pressures of 2 and 9 kPa. Conventional boiling analysis, which is based on the nominal surface area of the heater, was compared with a boiling analysis that considers the total "wetted" surface area.

When saturated vapor passes over a colder substrate, liquid drops nucleate and grow by coalescence with surrounding drops. Typically speaking, nucleation and growth rates of water droplets are faster on a hydrophilic surface than on a hydrophobic surface. However, heat transfer efficiency degrades once surface becomes filmwise condensation. In this paper, vapor condensing on a gradient surface to prevent filmwise condensation is studied. New gradient surfaces are fabricated. It is demonstrated that 10% increase of condensation heat flux can be achieved on a silicon wafer with C = 1 mm gradient surface. The main mechanism for heat transfer enhancement is found to be that drops condensing on C = 1 mm gradient surface begin to move at a much smaller size compared with those on silicon wafer without modification.

An experimental investigation of the stability in roll of a square section missile at high incidence was conducted in FL-23 wind tunnel. Dynamic motions were obtained on a square section missile that is free to rotate about its longitudinal axis. Different dynamic rolling motions were observed depending on the incidence of the model sting. These dynamic regimes include damped oscillations, quasi-limit-cycle wing-rock motion, and constant rolling. A coupling numerical method was established by solving the fluid dynamics equations and the rigid-body dynamics equations synchronously in order to predict the onset and the development of uncommented motions and then explore the unsteady movement characteristics of the aircraft. The study indicates that the aircraft loss stability at high incidence is caused by the asymmetric vertex on the level fin tip liftoff and attach alternately. The computation results are in line with the experiment results.

Cai et al.⁵ developed a vortex stability theory for slender conical bodies and analyzed the stability of vortex pairs over slender conical wing-body combinations under small perturbations. An experimental study is presented in this paper to verify the validity of the theoretical predictions. A sharp-edged flat-plate delta wing is tested in a low-speed wind tunnel. A smoke-laser-sheet visualization technique is used to visualize and measure the positions of the vortex pair, which are found to be symmetric and conical over the wing. The same tests are performed on an identical delta wing model but with a flat-plate dorsal fin mounted vertically in the incidence plane of the wing. Two fin heights are tested. The ratios of the local fin height to the local wing semi-span are 0.75 and 1.50. The test results clearly indicate that the vortices become asymmetric and non-conical over the model with the fin height ratio of 0.75 and recover symmetry and conicity over the model with the fin height ratio of 1.50, providing direct experimental evidence of the theoretical predictions.

The evolutions and mechanisms of micro-jet penetration into the solid soft material are described in this paper. The diameter of the micro injector is 300µm. We use the gelatin gel as the solid soft material and water as the injection fluid. Experimental results show that the penetration processes can be described by three phases. I. Induction phase: this phase covers the penetration process of the skin layer. This period lasts only a very short time depending on the power of the micro jet. II. Break phase: This phase covers the penetration process of the inner layer. The volume of jet penetration grows dramatically in this period. III. Saturation phase: The saturation condition was attained when the penetration depth was found to be fixed with time. It is also found that the penetration rate increased linearly with the injection power. It can be fitted in a line with slope of 0.04. It indicates that the stress work of the soft solid is proportional to the injection power. The characteristics can be used to determine the strength of the soft solid materials by jet injection. These data are useful in the needle-free injection for the drug delivery with the endoscope system.

A bi-fluid micro-flow system is proposed for separating particles from its original solvent and re-diluting them into another solvent simultaneously. In this micro-flow system, two different miscible solvents flow parallel to each other through a 2-inlet-2-outlet micro-channel, where an acoustic standing wave is set up. Due to the differences in acoustic properties of these solvents, the pressure node of the acoustic wave is shifted from the middle line of the channel. Under the action of the acoustic radiation force, particles with positive ϕ-factors are extracted from their original solvent and re-suspended into the other solvent, wherein the pressure node resides. Particles suspended in the new solvent are collected at one of the two outlets downstream. Experiments were conducted on a prototype using two aqueous solutions: deionized water and 40% glycerin aqueous solution with polystyrene micro-particles. The results show that under the action of the acoustic standing wave, most of the particles were successfully transported from its original solvent to the other solvent and collected at the outlet.

A pair of plasma actuators with horseshoe shape is proposed for dynamic manipulation of forebody aerodynamic load at high angles of attack. Preliminary wind tunnel pressure measurements show that asymmetric force over a conical forebody with semi-apex angle 10° can be manipulated by activating the plasma actuator mounted on one side of the cone tip. Further work is suggested.

Forward blowing from a pair of plasma actuators on the leeward surface and near the apex is used to switch the asymmetric vortex pair over a cone of semi-apex angle 10° at high angles of attack. Wind tunnel pressure measurements show that by appropriate design of the actuators and appropriate choice of the AC voltage and frequency, side forces and yawing moments of opposite signs can be obtained at a given angle of attack by activating one of the plasma actuators. Further work is suggested.

In order to obtain the flow field characteristics and the influence of boundary layer, numerical simulations and wind tunnel tests are conducted for two streamline traced Jaws inlets at Mach number 7. The inlets are designed based on a flow field with 8-7 planar shock wave (the ramp in pitch plane is inclined at 8° to the free stream and in yaw plane is inclined at 7° to the free stream, yielding planar shocks). In the study, the static pressure distributions were measured and analyzed along the plane-symmetric centerline of the inlet with and without the boundary layer correction, respectively. Results show that boundary layer correction can obviously weaken the viscous influence to the inlet, increasing the mass flow coefficient and improving total pressure recovery.

This study investigates side force of a slender body with slenderness from 4.4 to 8.0. The experimental results show that flow over a slender body experiences a significant side force at angle-of-attack (AOA) higher than 30°. The side force reaches its maximum at AOA ≈ 50°. The present study demonstrates that slenderness (L/D) produces obvious influence on sectional side force distribution at high AOA. To understand the mechanism, evolution of near-wall vortex structure is investigated via hot wire and surface pressure measurements. It was found that one strong vortex is situated close to body surface and the other weak vortex away from the body, inducing a significant side force. Because the weak vortex lifts off early, a new vortex forms in near-wall region. Formation and evolution of the new vortex is the major mechanism that causes local sectional side force distribution exhibiting a wavy form with an alternating sign along the body. Therefore, overall side force does not necessarily increase with increasing slenderness. Reducing overall side force by canceling the alternating vortex-induced forces over the body surface is found if the slenderness L/D > 6.8 at AOA > 40°.

An experiment was conducted in a high speed wind tunnel to study the effects of dynamic pressure waves in a cavity from subsonic speeds to supersonic speeds. The effects on dynamic pressure of various parameters such as the Mach number, cavity length to depth ratio and the number of generic store were demonstrated. Detailed static-pressure and fluctuating pressure were measured on the cavity floor to determine the variation of the steady and unsteady pressures. Results show that cavity flow aero-acoustic characteristics are closely related to the Mach number and the cavity length to depth ratio. It is also found that installing store/stores in cavity can effectively improve the aero-acoustic environment in the cavity.

This paper presents the adsorption isotherm experiment of Methane and Maxsorb III activated carbon that was conducted and analyzed at temperatures from 5 to 55 °C and pressures up to 2.2 MPa in a volumetric apparatus. It is a complementary to the previous efforts by ways of analyzing the importance between charging and discharging rate, temperature increase and reduce of the bed, the heat transfer process, and the cylinder cooling and heating requirements during the charging and discharging of adsorbed natural gas.

Reynolds Equation is the most commonly used governing equation in turbulence. However, its application in wall-bounded turbulent shear flows may involve a defect. In general, Reynolds averaging should be ensemble averaging and Reynolds stresses are supposed to express all the actions of turbulence on the mean field. In statistically steady three-dimensional flows, Reynolds stresses are usually defined as correlations of temporal velocity fluctuations so that they cannot contain the influences of steady components of streamwise vortices. This is believed to be one of the reasons why many closure models in RANS meet problems in flows where streamwise vortices play significant roles. In this paper, Spatial-Temporal (S-T) averaged Reynolds stresses were defined, which separates the turbulence actions caused by temporal or spatial velocity fluctuations. DNS data for a fully developed channel flow were then used to check balancing of equations. Comparison showed that the balancing errors in the S-T averaged Reynolds equations were obviously smaller than those in the temporal averaged one, in particular, in the near wall region where the streamwise vortices located. Thus, a combination of traditional model with a supplemental model expressing influences of streamwise vortices might be a way out to improve the turbulence modeling.

A size-dependent gradient method is used to study the Mie scattering visualization of the compressible turbulent mixing layers at two convective Mach numbers M_c = 0.107 and 0.474 in order to reveal multi-scale structures. Coherent vortices and inclined wavy structures are clearly visualized by applying the method. Similarity between the grey-level and the velocity fluctuations is discovered with the spectral analysis. These results suggest a way to capture turbulent structures from scalar fields.

The focus of this paper is on physics of coherent structures in boundary layer flow in near-wall region of a vibrating plate. A dynamical model is developed based on Galerkin projection of the governing equation of the wall layer flow onto a subspace spanned by the orthogonal divergence-free Fourier basis functions. The interactive physics of the coherent structures with the wall vibration is studied with the established model truncated at any order. The compared results show that the prevailing coherent structures in the layer flow near a vibrating wall region are captured.

Based on the energy gradient method, criteria for turbulent transition are proposed for pressure driven flow and shear driven flow, respectively. For pressure driven flow, the necessary and sufficient condition for turbulent transition is the presence of the velocity inflection point in the averaged flow. For shear driven flow, the necessary and sufficient condition for turbulent transition is the existence of zero velocity gradient in the averaged flow profile. It is shown that turbulent transition can be effected via a singularity of the energy gradient function which may be associated with the chaotic attractor in dynamic system. The role of disturbance in the transition is also clarified in causing the energy gradient function to approach the singularity. Finally, it is interesting that turbulence can be controlled by modulating the distribution of the energy gradient function.

Periodically unsteady shedding of partial cavity and forming of cavitation cloud have a great influence on hydraulic performances and cavitation erosion for ship propellers and hydro machines. In the present study, the unsteady cavitating flow around a hydrofoil has been calculated by using the single fluid approach with a developed cavitation mass transfer expression based on the vaporization and condensation of the fluid. The numerical simulation depicted the unsteady shedding of partial cavity, such as the process of cavity developing, breaking off and collapsing in the downstream under the steady incoming flow condition. It is noted that good agreement between the numerical results and that of experiment conducted at a cavitation tunnel is achieved. The cavitating flow field indicates that the cavity shedding was mainly caused by the re-entrant jet near cavity trailing edge, which was also clearly recorded by high-speed photographing.

Turbulence structures are classified by defining the structure parameters. It is verified that the probabilities of these structure classes keep invariant at different scales within the inertial range. It is also shown that this scale-invariant property of turbulence leads to the existence of scaling laws for a series of physical quantities. From them we deduce the variation principle and the physical constraints to optimize the subgrid-scale stress model coefficients for large eddy simulation.

The resonance phenomenon for nonmodal perturbation of Batchelor vortex is studied. For azimuthal wavenumber n = - 1, two resonant peaks appear and the left one is always dominant. For n = 1, the resonant character becomes very complicated. There is a resonant mode switch from right peak to left peak as swirl parameter q increases from 2 to infinity. The resonant wavenumber k is the largest when q approaches to infinity for n = - 1 while it is the smallest for n = 1. The maximum value of the optimal energy growth for n = 1 is at q approaches to infinity, whereas it decreases monotonically as q increases for n = - 1. The resonance for n = - 1 is the more important one.

In the previous study, possible applications of metal core piezoelectric fibers with a diameter of 200 to 250 µm as bionic airflow sensors mimicking the flow sensitive receptor hairs of crickets have been proposed. This study aims to investigate the dynamic responses of the metal core piezoelectric fibers to unsteady airflow. The metal core piezoelectric fiber is half coated on the outer surface and is used in the bending mode. Wind tunnel tests were carried out and the output voltage of the fiber under the excitation of the unsteady aerodynamic force during flow acceleration and deceleration was measured when the wind tunnel was suddenly closed or opened by a shutter. The relationship between the maximum voltage and the steady-state velocity and that between the voltage and the acceleration of flow were also obtained.

The large eddy simulation (LES) of compressible turbulent channel flow controlled by active spanwise wall fluctuations (ASWF) is carried out in this paper. Heat transport and its relation with the momentum transport are paid with great attentions. In all the flow cases of present study, the changes of temperature fluctuation, heat transport and temperature streaks are in consistence with streamwise velocity fluctuation, momentum transport and velocity streaks respectively. This demonstrates the existence of same mechanism in heat transport and momentum transport in turbulent boundary layer, which gives the theoretical support for using the drag reduction technology to control wall heat flux.

The stability analysis in a stratified liquid film flowing down an inclined heated plate is investigated. The boundary value problem of the stability differential equation on small perturbation for general density distribution is derived. Then, the boundary value problem is solved and the solution to the problem is obtained for a special case. The result for non-stratified is agreement with the known one. And the effects of stratification and the other factors such as Re, Ma, We, Bi, Pr, the inclined angle β on the stability of the film are analyzed.

A new transport equation model is proposed for simulating cavitating flows in miniature machines. In the developed model, the surface tension, viscous force, and thermal effect of cavitation are considered to reflect their influence on the cavitation bubble growth. The cavitating flow in a miniature pump is calculated by applying the proposed cavitation model. The comparison between numerical results and experimental data indicates that the new cavitation model is applicable for simulating the cavitating flow in miniature machines.

The organic thin film transistor (OTFT) on flexible substrate electroplated electrodes has many advantages as in the fabrication of low cost sensors, e-paper, smart cards, and flexible displays. In this study, we simulated the mechanical and electrical characteristics of the OTFT with various voltage conditions by using COMSOL. The model consisting of a channel, source and drain was employed to investigate the temperature distribution and thermal stress concentration. The channel length is 40 µm and the voltage ranged between -20V and -40V. The OTFT was fabricated using pentacene as a semiconducting layer and electroplated Ni as a gate electrode. Mechanical properties of the fabricated OTFT were characterized by thermal stress which was predicted with the result of stress distribution.

Aeroelastic analysis of hingeless rotor blades in hover was performed. Large deflection beam theory was applied to analyze blade motions with effects of geometric structural nonlinearity. Aerodynamic loads for aeroelastic analysis were calculated through a three-dimensional aerodynamic model which is based on the unsteady vortex lattice method. Wake geometry was described using a time-marching free-wake method. Lead-lag damping ratio and frequency were calculated to evaluate aeroelastic stability of hingeless rotor system. Numerical results of aeroelastic analysis for hingeless rotor blades were presented and compared with results based on experimental data and two-dimensional quasi-steady strip theory in which uniform inflow model was used. It was shown that wakes significantly affect the steady-state deflections and aeroelastic stability.

Flutter phenomenon is one of the most dangerous problems in aeroelasticity. When it occurs, the aircraft structure can fail in a few second. In recent aeroelastic research, computational fluid dynamics (CFD) techniques become important means to predict the aeroelastic unstable responses accurately. Among various flow equations like Navier-Stokes, Euler, full potential and so forth, the transonic small disturbance (TSD) theory is widely recognized as one of the most efficient theories. However, the small disturbance assumption limits the applicable range of the TSD theory to the thin wings. For a missile which usually has small aspect ratio wings, the influence of body aerodynamics on the wing surface may be significant. Thus, the flutter stability including the body effect should be verified. In this research an inverse design method is used to complement the aerodynamic deficiency derived from the fuselage. MGM (modified Garabedian-McFadden) inverse design method is used to optimize the aerodynamic field of a full aircraft model. Furthermore, the present TSD aeroelastic analyses do not require the grid regeneration process. The MGM inverse design method converges faster than other conventional aerodynamic theories. Consequently, the inverse designed aeroelastic analyses show that the flutter stability has been lowered by the body effect.

The necessity of the vacuum interrupters (VIs) has been widely recognized on switching and controlling the fault currents in medium voltage level. An axial magnetic field (AMF) electrode has more advantages of the switching capability than other contact designs such as securing higher current value for transferring from the constriction arc to the diffuse. The heat flux and the local temperature on the electrode are increased by arc constriction, which is influenced by Lorentz force. It has undesirable influence on the characteristics of vacuum arc. In this study, we simulated the influence of Lorentz force on vacuum arc behaviors with an AMF electrode by using a commercial FEM package, ANSYS. The vacuum arc has been modeled with the sequential coupling method of two different fields, which are on the electromagnetic and thermal-flow. Arc constriction with various applied currents could be predicted with the results of temperature distribution.

The field of ionic liquid has gained rapid growth in recent years and occupied a forefront in green chemistry. As a useful instrument to the research and development of novel ionic liquids, the physical properties are of utmost importance. Thus, great efforts have been made to obtain these important data, as it is far from getting sufficient physiochemical information for intensive and extensive investigation which consequently becomes a bottleneck for the theoretical and applicable research of ionic liquids. Additionally, with the given immeasurable possible ionic liquids by various cation and anion combinations, it is an impossible task to find an ideal ionic liquid with desired physical properties using conventionally "try and error" process. For these reasons, exploration of novel prediction models for physical properties of ionic liquid is imperative. This paper gives an overview of recent progress of various prediction models.

The scaled boundary finite element method is used to determine the seepage free surface of a dam with unbounded base. Two models for the bounded and the unbounded domains are established by using the mentioned method, and the numerical solutions are compared with the analytical results. Also, the free surface seepage of a dam with unbounded base is solved by combining the substructure and scaled boundary finite-element methods. The method has satisfactory accuracy and is quite efficient.

This paper presents a method for thermal simulation of a plate fin heat sink using an analytical solution of the three-dimensional heat equation resulting from an appropriate plate fin heat sink thermal model. The steady-state temperature map on the surface of the plate fin heat sink is computed analytically employing the Green's function method. The entire solution methodology is illustrated for the particular examples of the plate fin heat sink subjected to non-uniform heat density on the surfaces. The simulation results are validated with experimental measurements.

The present paper studies the cavitation characteristics in thermodynamic condition. The present work modifies thermodynamic cavitation mass transfer expression based on the Rayleigh-Plesset equation. The pressure difference, surface tension and thermodynamic effects are considered in new mass transfer expression on the basics of the evaporation and condensation mechanics according to the micro-kinetic theory. The hydrofoil NACA0015's thermal cavitation characteristic is calculated at 25°C, 50°C, 70°C with the improved model. The shear stress transport model is adopted as turbulence kinetic energy transport equation. The pressure coefficient is compared with experiment data at different temperatures to validate the model. The temperature difference between local temperature and ambient temperature is obtained and the cavitation volume fraction is calculated.

In a turbine flow meter, cavitation will take place when local pressure falls below the vapor pressure of liquid products and it usually speeds up the rotor at the given high flow rate. In order to study its effects on meter factor, numerical simulation on transient unsteady turbulent flow is carried out based on the mixture homogeneous two phase cavitation model which is deduced from the theory of evaporation and condensation on a plane. The momentum source terms from the variation rotating speed of transient processes and the cavitation mass transport source terms are introduced into the transient unsteady governing equations. The results show that the meter factor grows with the increase of the cavitation number and meter factor will be affected by cavitation.