Modern Physics Letters B

The complex interactions of two and three spark-generated bubbles are studied using high speed photography. The corresponding simulations are performed using a 3D Boundary Element Method (BEM) code. The bubbles generated are between 3 to 5 mm in radius, and they are either in-phase or out-of-phase with one another. The possible interaction phenomena between two identically sized bubbles are summarized. Depending on their relative distances and phase differences, they can coalesce, jet towards or away from one another, split into smaller bubbles, or 'catapult' away from one another. The 'catapult' effect can be utilized to generated high speed jet in the absence of a solid boundary or shockwave. Also three bubble interactions are highlighted. Complicated phenomena such as bubble forming an elliptical shape and bubble splitting are observed. The BEM simulations provide insight into the physics of the phenomena by providing details such as detailed bubble shape changes (experimental observations are limited by the temporal and spatial resolution), and jet velocity. It is noted that the well-tested BEM code [1,2] utilized here is computationally very efficient as compared to other full-domain methods since only the bubble surface is meshed.

The interaction of shock wave and turbulence for transonic flow over a circular cylinder is investigated using detached-eddy simulation (DES). Several typical cases are calculated for free-stream Mach number M_∞ from 0.85 to 0.95, and the physical mechanisms relevant to the shock wave and turbulence interaction are discussed. Results show that there exist two flow states. One is unsteady flow state with moving shock waves interacting with turbulent flow for M_∞ < 0.9 approximately, and the other is quasi-steady flow with stationary shocks standing over the wake of the cylinder for M_∞ > 0.9, suppressing the vortex shedding from the cylinder. Moreover, local supersonic zones are identified in the wake of the cylinder and generated by two processes, i.e., reverse flow and shock wave distortion induced the supersonic zone. Turbulent shear layer instabilities are revealed and associated with moving shock wave and traveling pressure wave.

Based on a multiple stage BGK-type collision model and the Chapman–Enskog expansion, the corresponding macroscopic gas dynamics equations in three-dimensional space will be derived. The new gas dynamic equations have the same structure as the Navier–Stokes equations, but the stress strain relationship in the Navier–Stokes equations is replaced by an algebraic equation with temperature differences. In the continuum flow regime, the new gas dynamic equations automatically recover the standard Navier–Stokes equations. The current gas dynamic equations are natural extension of the Navier–Stokes equations to the near continuum flow regime and can be used for near continuum flow study.

Recent rapid advances in microelectromechanical system have facilitated the applications of various oscillating microstructures, where fluid damping plays a critical role and becomes a main cause of energy dissipation. In the present study, we have used lattice Boltzmann method to investigate the Stokes' second problem for non-equilibrium effect.

There are two conventional boundary treatment methods in molecular dynamics (MD) simulation for particle suspension or fiber in cross flow. One is that the fiber is represented by evenly spaced atoms, the other is that the fiber is treated as one mass point. The calculations with the two boundary treatment methods have been compared, it is found that, with the same properties, both methods could generate slip boundary condition on the fiber surface, however the one mass point method could result in very large slip boundary condition on the fiber surface, and the discrepancy could be as high as 2600%.

Direct numerical simulation has been performed to study flow interactions in multiple jets in cross-flow. Configurations considered are twin jets side-by-side and triple jets in tandem. Computations are carried out at the jet to cross-flow velocity ratio of 2.5 and the Reynolds number 225 based on the free-stream quantities and the jet width D. For twin jets, results show that in the vicinity of jet exits, the merging of two counter rotating vortex pairs (CRVP) is strongly dependent on the gap of two jets. Downstream in the far-field, a large single CRVP dominates. The simulation is in qualitatively good agreement with the experimental findings by other researchers. For triple jets, more complicated flow structures are revealed, in which a total of three vortex pairs has been identified, but none of them is dominating. The observations of complex flow structure could assistant relevant industrial applications.

Hull cavitation evolution induced by an underwater explosion (UNDEX) near a deformable steel structure is numerically investigated using a multiphase compressible fluid solver^1-3 coupled with an unsteady one-fluid cavitation model^4,5. A series of computations are conducted with varying structure surface curvature. Results suggest that structure surface curvature influence the peak pressures generated from the shock impact and cavitation collapse; a concave-designed surface not only causes local shock focus but also enhances the subsequent cavitation reload.

A modeling of the turbulent flow in a complex passage with dynamical fluid-structure interaction (FSI) is established on the generalized variational principle. A monolithic coupling method on the finite element formulations (FEM) is used to realize numerical computation of the flow with dynamical FSI. The comparisons with LES show that the results on the FEM formulations suggested in this paper are favorable, and the computing effort is economical.

A new immersed boundary-lattice Boltzmann method (IB-LBM) is presented in this work. In the conventional IB-LBM, the restoring force is pre-calculated, which makes the non-slip boundary condition to be only approximately satisfied. As a result, the streamline penetration to the solid body occurs. In the present study, the velocity correction (restoring force) is considered as unknown. It is determined in such a way that the non-slip boundary condition is enforced. As compared with conventional IB-LBM, the solution procedure of current IB-LBM is almost the same except that the non-slip boundary condition is guaranteed in the present scheme while it is only approximately satisfied in the conventional scheme. Numerical results for simulation of flows over fixed circular cylinders showed that the present method can provide accurate solutions without any streamline penetration phenomenon.

The physical properties of incompressible fluids used in heat exchangers, such as viscosity and thermal conductivity, change considerably with temperature during their normal operating conditions. This study investigates the heat transfer characteristics of microchannel flows by taking these variations into account. Our results demonstrate that the temperature effects are significant and must be taken into account if accurate predictions are to be obtained.

The global stability of flow past two side-by-side circular cylinders is investigated by a linear global stability analysis method based on a spectral/element Navier-Stokes solver. The special attention is paid to the parameter regime of 1<T/D<3, where T is the spacing between two cylinder centers and D the cylinder diameter. A remarkable feature we found is that there exist two critical Reynolds numbers associated with twice bifurcations, first a pitchfork and second a Hopf one, for about 1.55<T/D<1.65. A new flow pattern namely biased steady pattern is identified. In addition, the transition from in-phase lock-in to anti-phase lock-in pattern for larger separation distance T/D>1.65 is proved to be related to the switch between two leading eigenmodes.

Based on the models of the porous-electrode, a lattice Boltzmann model of a solid oxide fuel cell is presented, which allows the computation of the local distributions of the electrical potential, current density, and concentration of the chemical species. The physics of the cell and the simplifying assumptions are presented; a sketch of the numerical procedure is also given. The numerical results obtained with hydrogen as the fuel are compared with results from other simulation codes which were developed for a planar geometry. The numerical results show that the counter-flow design as being the most efficient. Furthermore, and with increasing the percent H₂ in the fuel stream, the voltage drops profile alters from precipitous to gentle. These results are excellent.

This paper depicts a ghost cell method to solve the three dimensional compressible time-dependent Euler equations using Cartesian grids for static or moving bodies. In this method, there is no need for special treatment corresponding to cut cells, which complicate other Cartesian mesh methods, and the method avoids the small cell problem. As an application, we present some numerical results for a special moving body using this method, which demonstrates the efficiency of the proposed method.

A numerical method which is based on unstructured grids to compute high-temperature ionized air radiation is described. The multi-species N-S equations are used and the chemical model includes 11 species (O₂, N₂, O, N, NO, NO⁺, N⁺, O⁺, N⁺₂, O⁺₂, e^-) and 20 reactions. For simulating thermal non-equilibrium effect, the two-temperature model is considered. The finite volume method (FVM) is used for spatial and directional discretization for the RTE on unstructured grids. The code can deal with different kinds of species and radiative bands. Particularly, the Delta, Epsilon, Beta prime and Gamma prime bands of NO are considered in this paper. The numerical results of MESES-C for hypersonic flow with high-temperature ionized radiation are shown, and compared well with the reference data and experimental data.

Detonation process of a condensed explosive was simulated using a finite difference method. Euler equations were applied to describe the detonation flow field, an ignition and growth model for the chemical reaction and Jones-Wilkins-Lee (JWL) equations of state for the state of explosives and detonation products. Based on the simple mixture rule that assumes the reacting explosives to be a mixture of the reactant and product components, 1D and 2D codes were developed to simulate the detonation process of high explosive PBX9404. The numerical results are in good agreement with the experimental results, which demonstrates that the finite difference method, mixture rule and chemical reaction proposed in this paper are adequate and feasible.

A lattice Boltzmann method (LBM) based on nonuniform quadtree mesh is proposed. For the two steps in LBM, the collision step is a local procedure and can be implemented in the same way for both uniform and nonuniform mesh grids, while the streaming step is equivalent to solving several linear advection equations. On quadtree mesh (and some other nonuniform meshes), it is usually trivial to construct a first order interpolation scheme for the streaming step. Then the back and forth error compensation and correction (BFECC) method is employed to improve the accuracy order, such that a second order scheme is obtained in streaming step. Several numerical test cases are carried out to prove the accuracy and efficiency of our method.

This paper presents the application of an adaptive stencil diffuse interface method to the simulation of dam break problem. The adaptive stencil diffuse interface method is the combination of the diffuse interface method and a stencil adaptive algorithm, where the diffuse interface method is used as the solver, and the adaptive stencil refinement scheme is applied to improve the resolution around the interface so that the fine-scale interface behavior can be captured. In this paper, we use this method to simulate the dam break problem, study the dam height and leading edge position, and compare our results with the experiment data available in the literature. It is shown that the results using the adaptive stencil diffuse interface method agree very well with the experimental results.

Two dimensional hypersonic magnetohydrodynamics(MHD) flows with the chemical non-equilibrium effects are numerically simulated using upwind splitting scheme based on unstructured meshes. The governing equations are 2D MHD equations with the chemical components, where 5 species and 17 chemical reactions are considered. The AUSM scheme is implemented in the spatial discretization for the MHD equations, and an explicit 5-stage Runge-Kutta scheme is used for time integration. A loosely coupled approach is used to communicate between the MHD equations and the chemical reaction model. The computational model is a 2D blunt body, around which a dipole magnetic field is located. With hypersonic incoming flows, four different cases are numerically simulated to analyze the effects caused by the magnetic field and/or non-equilibrium chemical reactions. Numerical results are obtained and compared well with available data.

Large eddy simulation was applied to simulate the compressible flow past multiple wall-mounted rectangular cylinders in a channel flow. The dynamic sub-grid stress model was employed to approximate the sub-grid scale effects. For flow past single wall-mounted cylinder, our calculated results agree well with the results of both previous experimental and numerical results which showed the formation and diffusion of vortexes around the cylinder. Flow past two wall-mounted cylinders has also been simulated numerically and our numerical results disclosed the formation of the vortex street behind the second cylinder and the developing process of flow field, which is important for practical engineering application.

A numerical method for the simulation of parachute inflation process is presented in this paper. The unsteady compressible N-S equations are fully coupled with MSD (Mass Spring Damper) structure model and integrated forward in time. The CFD solver is based on an unstructured finite volume algorithm and the preconditioning technique is applied to alleviate the stiffness caused by low Mach number. The Spalart-Allmaras one-equation turbulence model is implemented to evaluate the turbulent viscosity. The whole system (fluid equations and structural model equations) is marched implicitly in time using a dual time stepping method. An overset deforming grids method is adopted in this paper to deal with the very large domain deformation during the parachute inflation process. Finally numerical test is performed to validate the robustness of this method.

In this paper, the BR2 high-order Discontinuous Galerkin (DG) method is used to discretize the 2D Navier-Stokes (N-S) equations. The nonlinear discrete system is solved using a Newton method. Both preconditioned GMRES methods and block Gauss-Seidel method can be used to solve the resulting sparse linear system at each nonlinear step in low-order cases. In order to save memory and accelerate the convergence in high-order cases, a linear p-multigrid is developed based on the Taylor basis instead of the GMRES method and the block Gauss-Seidel method. Numerical results indicate that highly accurate solutions can be obtained on very coarse grids when using high order schemes and the linear p-multigrid works well when the implicit backward Euler method is employed to improve the robustness.

This paper presents an approach, which combines the conventional finite volume method (FVM) with the lattice Boltzmann Method (LBM), to simulate compressible flows. Similar to the Godunov scheme, in the present approach, LBM is used to evaluate the flux at the interface for local Riemann problem when solving Euler/Navier-Stokes (N-S) equations by FVM. Two kinds of popular compressible Lattice Boltzmann models are applied in the new scheme, and some numerical experiments are performed to validate the proposed approach. From the sharper shock profile and higher computational efficiency, numerical results demonstrate that the proposed scheme is superior to the conventional Godunov scheme. It is expected that the proposed scheme has a potential to become an efficient flux solver in solving compressible Euler/N-S equations.

The immersed boundary method (IBM) is an innovative approach for modeling flow with complex geometries and is more efficient than traditional method. In the present investigation, the shock wave propagation over one circular cylinder is simulated numerically with the Ghost-Body Immersed Boundary Method and high-resolution Roe scheme. To validate the IBM, a plane incident shock wave passing through a square cylinder is predicted and good agreement with previous experiments was obtained. Then based on our calculation, the reflection and diffraction processes of a shock wave passing through a circular cylinder were visualized and discussed in detail.

Inviscid and viscous boundary layer stabilities in hypersonic flows, which are different from those in low speed flows, are investigated, and have actually multiple unstable modes due to inherent mathematic characters. Numerical integration with the singularity point is implemented by expanding calculation region onto the complex plane, and can avoid the difficulty in solving the inviscid stability equations. The high accurate numerical method of the parabolic stability equation (PSE) is used in spatial marching to solve the viscous stability equations. Computed results show good agreement with relevant data.

A parallel Navier-Stokes solver based on dynamic overset unstructured grids method is presented to simulate the unsteady turbulent flow field around helicopter in forward flight. The grid method has the advantages of unstructured grid and Chimera grid and is suitable to deal with multiple bodies in relatively moving. Unsteady Navier-Stokes equations are solved on overset unstructured grids by an explicit dual time-stepping, finite volume method. Preconditioning method applied to inner iteration of the dual-time stepping is used to speed up the convergence of numerical simulation. The Spalart-Allmaras one-equation turbulence model is used to evaluate the turbulent viscosity. Parallel computation is based on the dynamic domain decomposition method in overset unstructured grids system at each physical time step. A generic helicopter Robin with a four-blade rotor in forward flight is considered to validate the method presented in this paper. Numerical simulation results show that the parallel dynamic overset unstructured grids method is very efficient for the simulation of helicopter flow field and the results are reliable.

Based on volume of fluid (VOF) technique, the flapping motion of submerged turbulent plane jet in shallow water impinging vertically onto the free water surface was simulated. To study further on the flapping motion, the power spectrum density, the centerline velocity decay, as well as the mean velocity profiles of the jet were all investigated in this paper. The results are in great agreement with those of theoretical analyses, and the results show that the flapping motion is a new flow pattern of submerged turbulent plane jets.

In this paper we propose a second order hybrid reconstruction strategy for the fully adaptive multi-resolution scheme based on the second order finite volume method. The fully adaptive multi-resolution scheme is an adaptive grid method for solving the hyperbolic conservation laws. To improve its robustness, the third order central reconstruction is replaced by the second order hybrid reconstruction, i.e. the anti-ENO (essentially non-oscillatory) reconstruction and the ENO reconstruction have been adopted for the multi-resolution analysis and the inverse multi-resolution analysis, respectively. Several numerical examples indicate that this new hybrid reconstruction strategy is much less sensitive to the tolerance than the central reconstruction.

The formation and nonlinear evolution of turbulent spots in the near wall shear flow are researched by the direct numerical simulation of Navier-Stokes equations, and the initial multiple disturbance models with the pulse form are proposed. A group of three-dimensional coupling difference schemes with high accuracy and resolution are used to simulate evolution processes of turbulent spots, and the computed results show clearly basic characteristic of turbulent spots, specially, interaction properties between turbulent spots.

CFD modeling of water injection for turbine blade cleaning has been carried out to predict the water coverage on a stationary blade row, which will enable a better understanding on the interactions between hot-air flow and cold-water droplets. A generic configuration was used in a priori in-house experiment, which provides test data for CFD validation. The two-phase flow CFD model adopts the Eulerian-Lagrangian approach, in which the air-flow was treated as the continuous phase and water droplets as the dispersed phase. CFD predictions are found in fairly good agreement with test results, particularly the water coverage on the downstream blade row. Moreover, CFD modeling provides further details, including the trajectory of water droplets, which are difficult to be obtained by experiments, and yet extremely useful for understanding the flow physics.

A new numerical approach is presented in this work to simulate incompressible flows. The present approach combines the ideas of the conventional immersed boundary method (IBM) for decoupling the solution of governing equations with the solid boundary and the local domain-free discretization (DFD) method for implementation of boundary conditions. Numerical results for simulation of flows around a circular cylinder showed that the present approach can provide accurate solutions effectively.

The dynamics sub-grid scale (SGS) models and the dynamical system SGS model are introduced into the lattice Boltzmann method (LBM), and applied to numerical simulation of turbulent cavity flows. The results are compared with those of direct numerical simulation of turbulence and Smagorinsky SGS model. A new average method of eliminating the inherent unphysical oscillation of LBM is also given.

The spatio-temporal correlations in a turbulent planar mixing layer are acquired using the particle image velocimetry. Estimation of convection speed is recommended to be made with the spatio-temporal correlations of fluctuating vorticity. The spatial correlation can be deduced from the temporal correlation through the use of the Taylor's hypothesis when applied to the region without apparent dominant frequency.

In this paper, we take a fundamental approach to investigate the effect of spanwise flow on the prolonged attachment of leading edge vortex (LEV) on a flapping wing. By imposing a constant acceleration-constant velocity flow on elliptic wings of various sweep angles and angles of attack, our experimental and numerical results show that while spanwise flow per se has negligible influence on the prolong attachment of the LEV, vortex stretching can significantly delay detachment of the LEV, even for a small spanwise flow.

When a drop impacts onto a solid surface, it spreads out into a pancake shape and often forms a dry-spot at the center of the drop. We show that this dewetting at the center is sometimes produced by a small bubble of air which is entrapped on the substrate under the center of the drop. Capillary waves are generated on the surface of the pancake, during the rebounding of the lamellar edge. As these capillary waves converge at the axis of symmetry, their amplitude grows in size until a wave trough touches and merges with the entrapped bubble. This opens up a dry-patch at the center of the splash. We use state-of-the art high-speed video imaging to study this process, for a drop impacting onto a Perspex surface. The imaging is done by looking through the bottom plate to reveal the detailed motions of the capillary waves.

The atomization mechanisms and characteristics of a new air-assist micro-atomizer (AMA) fabricated via MEMS machining are described. Observations show that the breakup regimes can be described by the mechanisms involved with the laminar jet disintegration, aerodynamic disintegration and turbulent mode based on the atomizing pressure and Weber number. A new phenomenon of flow branching of the liquid jet emanating from the atomizer was also observed. The ratio of liquid-to-gas flow rate determines the spreading angle and droplets size of the branched liquid streams. It is also found that the flow focusing mechanism resulted in the mono-dispersed droplets stream. Furthermore, the micro spray with mean droplet size of 5µm can be obtained by adjusting the atomization pressure. It is very useful in the applications of sample introduction in inductively coupled plasma mass spectrometry (ICP-MS) and medical inhalation devices.

We developed an integrated microfluidic flow-through EC-PCR (EC-PCR) microdevice for the concurrent DNA amplification, PCR products EC detection and PCR products quantification instead of the current available fluorescence detection scheme. The microfluidic flow-through EC-PCR microdevice was fabricated with the state-of-the-art microfabrication technology, by bonding a bottom glass substrate having a microelectrode array to a top glass cover having the microchannels made of PDMS material. Both the amplification of the target DNA sequence and the subsequent EC detection of the PCR products were carried out concurrently on the integrated device by real-time monitoring. The underlying principle of the microfluidic flow-through EC-PCR method was based on the changes of current signal of methylene blue (MB), which worked as an electrochemically active species DNA intercalator in the PCR mixture, during the amplification process at the extension phase. The results shown in this work indicated that the nucleic acid analysis could be performed in a fast thermal cycling and true real-time quantitative electrochemical detection. The signal variation trends of the EC detection and the fluorescence detection were the same in our verification measurements for both methods, which suggested that the EC detection method was feasible for this application.

The free falling motion of round thin disks in quiescent water was investigated experimentally. The mean density of the disks were about 1.05 g/cm³, slightly larger than water, and thickness to diameter ratio is 0.1. In this letter, We reported two cases with different mass distributions, in one case the disk's center of mass coincided with the geometry center and in another case it was deviated. Two CCD cameras were used to photograph the falling disk simultaneously, the disk's six degrees of freedom were obtained via a stereoscopic vision method. In the first case fluttering motion was observed, the disk's oscillatory motion confined in a vertical plane, and the pitching motion occurred about an axis normal to the plane. In the second case, steady helical falling was observed, the disk moved in a helical path at constant speed, the attacking angle is constant.

In order to study the effect of wall on the flow pattern of a submerged turbulent water jet in narrow channels, the flow field was visualized by a laser-induced fluorescence (LIF) system at different Reynolds numbers. Those images showed that flow motion in a narrow channel is different from that of a turbulent plane jet without narrow channels. There are three flow patterns in narrow channels: stable impinging, stable jet with recirculation vortices and flapping turbulent jet.

To evaluate the performance of discharged foam agents used to protect structures from heat and fire damages, the thermal characteristics of fire-extinguishment foams were experimentally investigated. Especially, two different parameters of a spray nozzle, that is, the number of air holes and the orifice diameter, were considered. A simple repeatable test for fire-extinguishment foams subjected to fire radiation was performed. Experimental results showed that the expansion ratio of the discharged foam with the small orifice throat (d₀= 9.5 mm) and opened air hole (N_h=9) was large. Results also showed that although the temperature gradient in the foam increased as the foam expansion ratio is increased, it remained constant as the intensity of heat flux increased.

Investigation of Reynolds number effect on flow asymmetry over slender bodies at high incidence was conducted. Results show that increasing of wind velocity affects flow asymmetry by changing the intensity of the vortices; Model scale affects the distance between the main vortices, which determines the stability of the vortices and results in the discrepancy in the flow asymmetry over different scale models.

Dielectrophoretic Field-Flow Fractionation (DEP-FFF) is a technique that selects particles of interest from a mixture of many samples. Conventional DEP-FFF technique used DEP force to levitate particles to different heights according to their dielectric properties. Levitated particles are then separated by their velocity difference in a microchannel. Numerical simulation and experimental results found that particles become wavy trajectory when the ratio of levitation height (h_p) and average of electrode width and spacing (d) is below 0.6 (h_p/d<0.6). In the mean time, sorted particles disperse randomly in Y-direction, too. The wavy trajectory and random distribution cause cell separation imprecisely. A novel MEMS-fabricated DEP-FFF based micro sorters is designed to improve these problems. The experimental results show that the particles can be levitated to a constant height and focus into a single particle stream along the centerline in the new micro sorter design. One can conclude that the new design leads to better cell separation in a DEP-FFF based micro sorter.

This paper describes the development of the needle-free injection with a pulsed micro-injector. The novel injection system consists of a micro-injector, a solenoid valve with an electron-control system and a high pressure liquid supply system. The diameters of the orifice are 200µm~ 300µm. The performance of both pulsed and continuous injections is characterized. Results show that the mass flow rate is increased linearly for the liquid pressure ranging from 50bar to 125bar. The jet velocity increases from 50.6m/s to 104m/s as the liquid pressure increases from 50bar to 125bar. Penetration test to the soft solid materials shows that the penetration depth is increased from 2mm to 7mm depending on the injection power under single pulsed injection. It is believed that the technique will be useful for the drug delivery as well as the localized tumor treatment.

Period tripling of standing waves in a circular tank generated by the side-wall excitation is investigated for the first time. With the increasing of forcing acceleration after the appearance of axisymmetric standing wave, period-tripled non-breaking standing waves and violent jet eruption between the modes are observed. However, the physical mechanisms for the generation of the interesting period-tripled standing waves are still unknown and need further research.

This paper investigates the effects of external excitation on the mixing performance of the micromixer with curved bluff-body structure. The micromixer was fabricated by the MEMS process of polydimethylsiloxane (PDMS). The mixing process and mixing efficiency were evaluated with a high speed camera. Results showed that the finger-spiked type flow patterns were generated in the mixing chamber under an excitation frequency of 5Hz. It turns out that the mixing efficiency as high as 85.6% is achieved at Re=0.25 with a single bluff-body structure. It demonstrates that the new design can be used to achieve complete mixing within ultra short length at mciroscale.

In order to investigate the drag reduction characteristics of a high-speed body with supercavitation shape, four types of typical disk cavitator models with different parameters were designed and tested. By measuring the velocity decrease histories during supercavitating flow experiments, the average drag coefficients were determined, which allows analysis and comparison of the influence of cavitator diameter, projectile aspect ratio, and cavitation number on the drag reduction. Based on the experimental results, numerical simulation of the drag reduction of supercavitation body was also carried out using a commercial software FLUENT6.2, and the results obtained agree well with the experimental data. Moreover, it is shown that the drag coefficients of the four bodies are in inverse ratio to the head area of cavitator when operating under natural supercavitating flow condition, and the smaller drag coefficient can be obtained by increasing the slender ratio of the bodies. Therefore, higher aspect ratio reduces drag coefficient, with the reduction of more than 95% under certain condition of cavitation number and supercaviation shape.

As combat aircraft becomes more and more maneuverable, the need to understand the unsteady behavior of aircraft in dynamic flow fields becomes more important. Usually researchers pay more attention to the effects on the changes of AOA, but ignore the effects of velocity variations. It is known that the velocity of aircraft changes greatly when the aircraft undergoes a high angle of attack maneuver, like "cobra" maneuver. To completely simulate and study the effect of rapid changes in both free stream velocity and angle of attack, a pitching motion setup is developed in the unsteady wind tunnel of NUAA. By measuring unsteady loads, unsteady pressure distribution and flow visualization, the unsteady aerodynamic behavior of a pitching isolated delta wing and the pitching delta wing coupled with unsteady free stream are investigated. It is found that the oscillating free stream velocity affects the hysteresis characteristics of the pitching delta wing further. The pressure distribution and flow visualization measurements show that the changes in the structure of the leading-edge vortices are the main reason. These studies conclude that a good understanding of the unsteady aerodynamics is vitally important in the design of super-maneuverable aircraft.

An experimental investigation was conducted in a high speed wind tunnel to explore the effects of mass-injection on cavity flow characteristics. Detailed static-pressure and fluctuating pressure measurements were obtained at the cavity floor to enable the effects of the mass-injection at the leading edge to be determined. Results indicate that varying mass-injection hole number and the flux rate of mass-injection has no significant effect on cavity flow characteristics. However, mass-injection can reduce the cavity static pressure gradient when the cavity flow type is transitional-cavity flow. The study also indicates that Mach number can influence the effect of mass-injection on cavity fluctuating pressure distributions, and at supersonic speeds, mass-injection can suppress the cavity tones effectively.

A dual synthetic jets actuator driven by different electrical factors was investigated using particle image velocimetry (PIV). A transfer-phase and sub-frequency technique was provided to capture the arbitrary phase of the dual synthetic jets, and a transfer-phase to equal technique was provided to determine the phase of the dual synthetic jets. The results show that both the amplitude and frequency of the electrical forcing voltages vastly affect the flow-field of the dual synthetic jets actuator. Both the forcing frequency and the driving voltage amplitudes contribute to the pressure difference and the area of the lower pressure, which determines the interactions of dual synthetic jets. The dual synthetic jet actuator exits a circumscription of electrical factors in which the actuator works efficiently.

Flight vehicles experience aeroelastic problems due to the interaction between structures and aerodynamic forces. Aeroelastic instability is usually a critical problem in transonic and lower supersonic regions. In present study, the aeroelastic analyses of several flight vehicles have been performed using the coupled techniques of computational fluid dynamics (CFD) and computational structural dynamics (CSD). The aeroelastic characteristics based on several aircraft models are investigated using the developed aeroelastic analysis system. On the other hand, structural nonlinearities always exist in flight vehicles. Structural nonlinearities such as freeplay and large deformation effects are considered in the present aeroelastic analysis system. Finally, aeroelastic characteristics of several flight vehicles will be explained considering both aerodynamic and structural nonlinearities.

The paper presents an adsorption cycle for cooling using the dielectric heating method for the regeneration process. Conventional adsorption (AD) chillers employs thermally driven processes for desorption and adsorption where the thermal resistances are high, resulting in a relatively low chiller COP. In this paper, dielectric heating is used to irradiate where the microwave power vibrates the dipole structure of the molecules. Owing to the direct method of energy delivery, the heating process is thus efficient, contributing to an increase in the chiller performance and the COPs. We present the modeling and simulations of the adsorption-desorption cycle in an AD chiller, demonstrating a significant improvement in chiller performance by as much as three folds.

The effects of inflow boundary conditions, ambient stratification, and geometry on the behavior of a density-induced current in a stratified reservoir are quantified through numerical experiments with a validated two-dimensional (2D) simulation model. Model results are analyzed to evaluate the sensitivity of reservoir flow variables to changes in the parameters characterizing inflow and ambient water. Quantitative evidence is presented to show that reservoir flow is more sensitive to Froude and Reynolds numbers than to other parameters. A critical value for each parameter is identified, which divides significant and insignificant effects on flow and dilution.

To avoid the delamination of bimorph actuator and enhance the performance of the room-temperature type functionally graded (RTFG) piezoelectric bending actuator, the high temperature type FG (HTFG) piezoelectric bending actuator was designed and fabricated. The material compositions with different dielectric and piezoelectric constants were selected from the Pb(Ni_1/3N_b2/3)O₃-PbZrO₃-PbTiO₃ (PNN-PZ-PT) family, and used as the five layers in the HTFG piezoelectric actuator. Compared with the FG actuator, the HTFG actuator has advantages for applications at high temperature. The durability of the fabricated HTFG piezoelectric actuators was measured in a vibration test and compared with that of the bimorph actuator to evaluate the improvement of performance. The results show that the durability of the HTFG piezoelectric actuators is much higher than that of the bimorph actuator.

A fictitious domain method, in which the Dirichlet boundary conditions are treated using boundary supported Lagrangian multipliers, is considered. The technique of the immersed boundary method is incorporated into the framework of the fictitious domain method. Contrary to conventional methods, it does not make use of the finite element discretization. It has a simpler structure and is easily programmable. The numerical simulation of two-dimensional incompressible inviscid uniform flows over a circular cylinder validates the methodology and the numerical procedure. The numerical simulation of propagation phenomena for time harmonic electromagnetic waves by methods combining controllability and fictitious domain techniques is also presented. Using distributed Lagrangian multipliers, the propagation of the wave can be simulated on an obstacle free computational region with regular finite element meshes essentially independent of the geometry of the obstacle and by a controllability formulation which leads to algorithms with good convergence properties for time-periodic solutions. The numerical results presented are in good agreement with those in the literature using obstacle fitted meshes.

This paper investigates the flow dynamic behaviors with respect to different contact angle and frequency of the piezoelectric print head. Its geometric model is divided into three zones for easy description, i.e., channel zone, nozzle zone and ejection observing zone. The length, width and orifice diameter of the micro-channel are 2,000 µm, 400 µm and 30 µm, respectively. The moving wall is located on the top wall of the channel zone in order to obtain proper condition for single drop generation; we applied the numerical simulation by commercial CFD software – CFD-ACE+ 2004. The most important purpose of this study is to find out the optimal frequency to eject droplets periodically and control the volume of droplet ejection which may provide reference for experimental work later on. The results show that by fixing the frequency 20KHz, the nozzle contact angle is from 20 degree to 80 degree, the one droplet interval time value is less than 0.1% and the droplet size value is less than 1%. According to this result, we find that the effect of contact angle is not very important. Also for the frequency value from 50KHz to 100KHz, the first shot time is almost the same.

In this research, fluid-structure interaction problem including geometric structural nonlinearity is studied for a high-aspect-ratio wing. When a high-aspect-ratio wing structure is interacted with external airload, geometric structural nonlinearity can be caused by large deflection of a wing. For the investigation of such a fluid-structure interaction problem, the transonic small disturbance theory for the aerodynamic analysis and the large deflection beam theory for the structural analysis are used, respectively. For the coupling between fluid and structure, the transformation of a displacement from the structural mesh to the aerodynamic grid is performed by a shape function which is used for the finite element and the inverse transformation of force by work equivalent load method. Static deformations in the vertical and twist deflections caused by gravity loading are compared with experimental results. Also, static aeroelastic analysis results are compared with experimental data. From the analysis results, effects of structural nonlinearity on static aeroelastic characteristics are investigated.

Based on the experimental measurement results of fluid particle transverse accelerations in fully developed pipe turbulence published in Nature (2001) by La Porta et al, the present authors recently develop a multiscale statistical model which considers both normal diffusion in molecular scale and anomalous diffusion in vortex scale. This model gives rise to a new probability density function, called Power-Stretched Gaussian Distribution model (PSGD). In this study, we make a further comparison of this statistical distribution model with the well-known Lévy distribution, Tsallis distribution and stretched-exponential distribution. Our model is found to have the following merits: 1) fewer parameters, 2) better fitting with experimental data, 3) more explicit physical interpretation.

Gridgen is employed for static multi-block grid generation. A rapid deforming technique is employed for dynamic grids. Flutters are numerically analyzed in the time domain with a coupled solution of unsteady Euler equations and structural equations of motion. Based on variable stiffness method of transonic flutter analysis, a time-domain method of transonic flutter analysis with multi-directional coupled vibrations is develpoed. For completeness, flutter characteristics of a wing model with winglets and an aircraft model with external stores are numerically analyzed.

The energy gradient method is used to analyze the turbulent generation in the transition boundary layer flow. It is found that the maximum of the energy gradient function occurs at the wall for the Blasius boundary layer flow. At this location under a sufficiently high Reynolds number, even a low level of free-stream disturbance can cause the turbulent transition and sustain the flow to be in a state of turbulence. This is an excellent explanation of the physics of self-sustenance of wall turbulence. The mechanism of receptivity for boundary layer flow can also be understood from the energy gradient criterion. That is, the free-stream disturbance can propagate towards the wall by the "energy gradient" process to cause turbulent transition, and the transition point in boundary layer can be moved forward towards the leading edge when the level of external disturbance increases.

An aeroelastic analysis of bearingless rotor systems with trailing edge flaps was conducted using large deflection-type beam theory for forward flight conditions with a focus on reducing vibration while minimizing control effort. The aerodynamic forces of the rotor blade were calculated using two-dimensional quasi-steady strip theory. For the analysis of forward flight, the nonlinear periodic blade steady response was obtained by integrating the full finite element equation in time through a coupled trim procedure with a vehicle trim. The objective function, which includes vibratory hub loads and active flap control inputs, was minimized by an optimal control process. Numerical simulations were performed for the steady-state forward flight of various advance ratios. Numerical results of the steady blade and flap deflections as well as the vibratory hub loads were also presented for various advance ratios and were compared with previously published analysis results obtained from modal analyses based on a moderate deflection-type beam theory.

The thermal plasma conditions under a practical use could be affected by the insertion of a solid, cold probe severely. In this study, we calculated flowing thermal plasmas with two different sizes of a Langmuir probe into an argon free burning arc system and investigated the thermal and flow disturbances caused by the metallic probe. From the results, the severe disturbance of a large part of the thermal plasma, especially the axial velocity field, by the inserted probe has been found. Therefore, it might be a practical solution to use a Langmuir probe which has sufficiently small diameter for avoiding such a severe disturbance.

The supercooled droplet impingement on complicated icing surface has been investigated with the Eulerian method. A simplified air/droplet two-phase model is established, and an effective algorithm is proposed to deal with the dilute, weakly coupled air/droplet flows for aircraft icing problem. An Eulerian droplets computation code has been developed and validated with the iced NACA23012, and the results agree well with the experimental data. Meanwhile, compared with the results from the traditional Lagrangian method, it indicates that the Eulerian method gives a better prediction when the multiple impingement zones occur on complicated icing surface.

Aerodynamic solver using the transonic small-disturbance (TSD) equation has frequently been used to perform practical aeroelastic analysis for many aircraft models. In the present study, the more accurate aeroelastic analysis solver using the TSD equation was developed by considering the viscous effects of the boundary-layer. The viscous effects were considered using Green's lag-entrainment equations and an inverse boundary-layer method. Through aerodynamic analyses for several aircraft wings, the viscous-inviscid interaction approach could improve the accuracy of the aerodynamic computation using the TSD equation. Finally, the aeroelastic characteristics were investigated using comparisons of the time responses between the inviscid and viscous flows.

The Taguchi robust design concept is combined with the multi-objective deterministic optimization method to overcome single point design problems in Aerodynamics. Starting from a statistical definition of stability, the method finds, Nash equilibrium solutions for performance and its stability simultaneously.

Ice accretion on aircraft is studied by a numerical method. By solving governing equations, the flow field is obtained for analyzing the icing zone and calculating the ice quantity on different parts. Influence of the fluid viscosity and compressibility on icing characters is extensively studied. And it can be found that the results agree well with those calculated by LEWICE program. This achievement could be helpful to further research on ice accretion.

The Pseudo-Shock Wave (PSW), which appears when supersonic flow in duct decelerates to subsonic, is a complicated process due to the interaction between boundary layer and shock wave. It significantly affects the performance and efficiency of flow devices. In this paper, PSW in two kinds of variable cross-section ducts, edge-varied and corner-varied, was investigated through CFD numerical simulation. Compared to the rectangular duct, a shorter and wider separation region is appeared in the corner of the edge-varied duct while the strongest separation is laterally propagated across the entire plane of the corner-varied duct's side wall. This makes the performances of varied ducts different from traditional constant cross-section ducts.

Aerodynamic characteristics of an Ejection Seat System at different angles of attack are studied by the numerical method and the flow mechanisms for such flows are carefully analyzed. The governing equations are Reynolds-averaged Navier-Stokes equations which are solved by the unstructured finite volume method. Upwind Osher scheme is used for spatial discretization and five-stage Runge-Kutta scheme is applied for temporal discretization. The DES model based on S-A one equation turbulence model is adopted. Parallel computation is based on the domain decomposition method and multi-block is achieved by using METIS system. The experimental data is used to validate this method. This research is helpful to understand the aerodynamic characteristics and flow mechanisms of Ejection Seat System at different angles of attack and Mach numbers, and can provide reasonable reference for Ejection Seat System design.

A dynamic stall model is coupled with the blade element momentum theory to calculate the cyclic variation of the aerodynamic characteristics of the wind turbine in yawed flow. In the dynamic stall model, unsteady effects under attached flow conditions are simulated by the superposition of indicial aerodynamic responses. The movement of the unsteady flow separation point is related to the static separation based on the Kirchhoff flow model via a deficiency function, in which the unsteady boundary layer response and the leading edge pressure response are taken into consideration. The induced vortex force and the associated pitching moment are represented empirically in a time-dependent manner during dynamic stall. The required input of the inflow velocity and incidence to the dynamic stall model is calculated using the improved blade element momentum theory. The calculated results are compared well with the NREL UAE Phase VI experimental data. For completeness, possible factors causing the difference between calculated and experimental results are analyzed and discussed in detail in this paper.

This paper presents an adaptive grid subdivision algorithm based on the Youngs' interface reconstruction algorithm to treat the advection of the mixed grids in the multi-material Eulerian method. The advantage of this algorithm is that it only needs to calculate the advection between pure grids, escaping the complexity encountered in the advection of mixed grids with two materials. In addition, this algorithm is a multi-dimensional advection algorithm, and it can decrease the error encountered in the direction-split advection algorithm. In order to verify the efficiency of the proposed adaptive grid subdivision algorithm in the actual physical field, it is added to the indigenously developed EXPLOSION-2D simulation software, and some numerical tests are performed to demonstrate the utility of this algorithm. The simulation results verify that the adaptive grid subdivision algorithm is simple and efficient.

In recent years, the use of phase change materials (PCM) to improve heat and moisture transfer properties of clothing has gained considerable attention. The PCM distribution in the clothing impacts heat and moisture transfer properties of the clothing significantly. For describing the mechanisms of heat and moisture transfer in clothing with PCM and investigating the effect of the PCM distribution, a new dynamic model of coupled heat and moisture transfer in porous textiles with PCM was developed. The effect of water content on physical parameters of textiles and heat transfer with phase change in the PCM microcapsules were considered in the model. Meanwhile, the numerical predictions were compared with experimental data, and good agreement was observed between the two, indicating that the model was satisfactory. Also the effects of the PCM distribution on heat transfer in the textiles-PCM microcapsule composites were investigated by using the model.

The flow field past the rotating blade of a horizontal axial wind turbine has been modeled with a full 3–D steady–RANS approach. Flow computations have been performed using the commercial finite–volume solver Fluent. The NREL phase VI wind turbine blade sections from the 3–D rotating geometry were chosen and the corresponding 2–D flow computations have been carried out for comparison with different angles of attack and in stalled conditions. The simulation results are analyzed. The main features of the boundary layer flow are described, for both the rotating blade and the corresponding 2–D profiles. Computed pressure distributions and aerodynamic coefficients show evidence of less lift losses after separation in the 3–D rotating case, mostly for the inward sections of the blade and the highest angles of attack, which is in agreement with the literature.

To explore the rheological property in turbulent channel flow of fiber suspensions, the equation of probability distribution function for mean fiber orientation and the Reynolds averaged Navier-Stokes equation with the term of additional stress resulted from fibers were solved with numerical methods to get the distributions of the mean velocity and turbulent kinetic energy. The simulation results show that the effect of fibers on turbulent channel flow is equivalent to an additional viscosity. The turbulent velocity profiles of fiber suspension become gradually sharper by increasing the fiber concentration and/or decreasing the Reynolds number. The turbulent kinetic energy will increase with increasing Reynolds number and fiber concentration.

In this article, we investigate the energy cascade of three-dimensional turbulent flows, in which the break-up process of eddy is quasi-self-similar. Mathematically this kind of turbulence with quasi-self-similar structure eddies can be regarded as cookie-cutter system, and can be generated by self-similar iterated function system (IFS) with added nonlinear disturbance. Using Bowen's result, we can calculate the exponent of dissipative correlated function, dissipated velocity, energy spectrum supported on cookie-cutter system. The present results show that the β-model is feasible for this kind of quasi-self-similar turbulence.

In order to analyze the effect of rotation speed of impeller on the inner flow and flotation dynamic characters of mechanical flotation, a kind of flotation is investigated. The gas-solid-liquid three-phase three-dimensional turbulent flow fields are analyzed numerically with the Mixture multi-phase model, K-ε turbulent model and Reynolds-Averaged Navier-Stokes equations. The void fraction test result of physical prototype is given with different rotation speeds. The simulation results show that the rotation speed of impeller has the important influence on the circle type of fluid, void fraction and speed distribution in the flotation with the same pressure of gas inlet. The experimental results show that the gas-solid-liquid three-phase flow fields with the design rotation speed is well with the flotation dynamic environment in the flotation, which also shows the importance of rotation speed of the impeller for the performance of the mechanical and pneumatic flotation.

The pattern of provincial economic integration in a country can be well measured by the error-components model, which enables us to attribute the variation of real output growth to factors that are industry specific and those that are province specific. Usually, the error-components model equation is solved by the dummy variable regression method. This paper presents an alternative approach, that is, the least square method, which can solve the error components model equation effectively. The presented approach is validated by its application to study the pattern of provincial economic integration in China for the period 1996-2004.

As numerical simulation of unsteady flows due to moving boundaries such as flexible flapping-wings is difficult by conventional approaches, an effective strategy which combines mesh deformation based on Delaunay graph mapping and unstructured overset grids is proposed in this paper. A Delaunay graph is generated for each body-fitted grid cluster which overlaps or is embedded within an off-body background grid cluster. At each time step, the graph moves according to the wing's motion and deformation, and the grids move to new positions according to a one-to-one mapping between the graph and the grid. Then, intergrid-boundary definition is implemented automatically for computation.

The phenomena of the wake flows containing ultrafine particles such as dust and pollutant in the atmosphere are usually observed downwind of the mountains and buildings. Different from the usually heavy particles, the ultrafine particles suspended in fluid undergo the processes of nucleation, growth and coagulation. Coherent structure in typical occurrence of the stretching of the Karman vortex street can be clearly seen in above mentioned gas-particle wakes. The aim of the present study is to explore the effects of coherent structures on the coagulation and growth of ultrafine particles suspended in wake flow. The particle field is represented by employing a moment method to approximate the ultrafine particle general dynamic equation. The fluid flow is obtained by solving the continuity and momentum equations with the large eddy simulation method and the subgrid-scale stresses were parametrized using the dynamic eddy viscosity model. The calculated contours of vorticity were compared with the relevant experimental results. The instantaneous spatial-temporal distribution of the particles are given and analyzed. The effects of the coherent structure on the diffusion and distribution of particle number concentration, polydispersity are discussed in detail.

Based on the wind turbine's complex wake vortex system and combining with the wind machine's aerodynamic performances, a hybrid method to design the wind turbine rotor, which discards the constant circulation along the wind rotor blade axis and embraces the airfoil's lift-drag characteristics, is presented. And the validity of the method is also proven by designing and computing a wind rotor.

A general proton exchange membrane fuel cell model including two finite-thickness catalysts is developed in this study, allowing for an in-depth understanding of the effects of the two key electrochemical reactions taking place in the two catalysts. The model is used to predict the performances of fuel cells employing two different flow channel designs, providing insights for fuel cell design and performance optimization.

A two-dimensional mathematical model simulating the long-term build-up of salinity around the brine outfall is developed in this paper. A flat seabed and steady longshore current are assumed as a first step towards modelling the actual scenario. Transient and steady-state analytical solutions for continuous point source condition are derived. These solutions are then applied to assess shoreline/offshore discharge. Characteristic features such as the temporal buildup of salinity excess and the variation of the shoreline salinity with respect to time and outfall location are investigated. The effect of the advection-to-diffusion ratio ν in the modelling results is also discussed.

The transient deformation of red blood cells (RBCs) in a shear flow is studied by a three-dimensional numerical model proposed by the present authors. The RBCs are approximated by ghost cells consisting of Newtonian liquid drops enclosed by Skalak membranes. The RBCs have an initially biconcave discoid resting shape, and the internal liquid is assumed to be the same to the fluid outside. The simulation is based on a hybrid method, in which the immersed boundary concept is introduced into the framework of the lattice Boltzmann method, and a finite element model is incorporated to obtain the forces acting on the nodes of the cell membrane which is discretized into flat triangular elements. The dynamic motion of RBCs is investigated in simple shear flow under a broad range of shear rates. At large shear rates, the present results show that the cells carry out a swinging motion, in which periodic inclination-oscillation and shape deformation superimpose on the membrane tank treading motion. With the shear rate decreasing, the swinging amplitude of the cell increases, and finally triggers a transition to tumbling motion.