On the loss of smoothness of the solutions of the Euler equations and the inherent instability of flows of an ideal fluid

Certain classes of flows of an ideal incompressible liquid which with time gradually lose their smoothness are studied. The loss of smoothness is expressed as infinite growth of the vorticity as t--> infinity for three-dimensional flows and an increase of the gradient of the vorticity for planar and axisymmetric flows. Examples of such flows in the planar and axisymmetric cases are flows with a rectilinear streamline; this can be established using a special local Lyapunov function. Incompressible flows of a dusty medium are another example (it turns out that collapse is impossible for such flows, but the vorticity and the rate of deformation, as a rule, grow with no limit). Other examples can be constructed by composition of shear flows. It is shown that in the vorticity metric almost all stationary planar flows are unstable with respect to three-dimensional disturbances and in the vorticity gradient metric planar and axisymmetric flows with a rectilinear streamline are unstable. (c) 2000 American Institute of Physics.     

Alleviation of cancellation problem of preconditioned Navier–Stokes equations

An approach to alleviate the cancellation problem of preconditioned Navier-Stokes equations is proposed. Adiabatic laminar viscous flows around a circular cylinder are calculated at different Mach numbers. It is shown that a redefinition of total enthalpy to reduce the magnitude of total enthalpy can alleviate the cancellation problem for adiabatic laminar flows at low Mach numbers.     

Depinning Dynamics of Fluid Monolayer on a Quenched Substrate

Langevin simulations are preformed on the depinning dynamics of fluidmonolayer on a quenched substrate. With increase in the strength of thesubstrate, we find for the first time a crossover from elastic crystal tosmectic flows as well as a crossover from smectic to plastic flows above thedepinning. A power-law scaling relationship can be derived between the driftvelocity and the driving force for both the elastic crystal and smecticflows, but fails to be obtained for the plastic flow. The power-lawexponents are found to be no larger than 1 for the elastic crystal flow andlarger than 1 for the smectic flow. The critical driving force and theaveraged intensity of Bragg peaks remain invariant basically in the regimeof smectic flow. A sudden increase in the critical driving force is observedwithin the crossover from the smectic to plastic flows, and the averagedintensity of Bragg peaks shows sudden decreases within the crossovers bothfrom the elastic crystal to smectic flows and from the smectic to plastic flows.The results are helpful for understanding the slip dynamics of fluids on a molecular level.     

An unconditionally stable fully conservative semi-Lagrangian method

Semi-Lagrangian methods have been around for some time, dating back at least to [3]. Researchers have worked to increase their accuracy, and these schemes have gained newfound interest with the recent widespread use of adaptive grids where the CFL-based time step restriction of the smallest cell can be overwhelming. Since these schemes are based on characteristic tracing and interpolation, they do not readily lend themselves to a fully conservative implementation. However, we propose a novel technique that applies a conservative limiter to the typical semi-Lagrangian interpolation step in order to guarantee that the amount of the conservative quantity does not increase during this advection. In addition, we propose a new second step that forward advects any of the conserved quantity that was not accounted for in the typical semi-Lagrangian advection. We show that this new scheme can be used to conserve both mass and momentum for incompressible flows. For incompressible flows, we further explore properly conserving kinetic energy during the advection step, but note that the divergence free projection results in a velocity field which is inconsistent with conservation of kinetic energy (even for inviscid flows where it should be conserved). For compressible flows, we rely on a recently proposed splitting technique that eliminates the acoustic CFL time step restriction via an incompressible-style pressure solve. Then our new method can be applied to conservatively advect mass, momentum and total energy in order to exactly conserve these quantities, and remove the remaining time step restriction based on fluid velocity that the original scheme still had.     

Depinning Dynamics of Two-Dimensional Charged Colloids on a Random Laser-Optical Substrate

Using Langevin simulations, we. investigate the depinning dynamics of two-dimensional charged colloids on a random laser-optical substrate. With an increase in the strength of the substrate, we find a transition from crystal to smectic flows above the depinning. A power-law scaling relationship between average velocity and applied driving force could be obtained for both flows, and we find that the scaling exponents are no bigger than 1 for the crystal and are bigger than 1 for the smectic flows.     

涡发生器对高超声速轴对称进气道外部流动的影响

采用三维CFD黏性模拟考察涡发生器对高超声速轴对称进气道外部流动的影响.针对前缘钝化半径0.8 mm和3.2 mm的轴对称进气道外部流场,以涡发生器高度与当地位移边界层厚度比值为影响参数,考察流场结构与性能参数的影响规律.结果表明,涡发生器产生的干扰波系使得前缘激波向外偏移,下游近壁面流动与主流区出现明显的交换,下游流动出现明显的展向非均匀性.涡发生器对流动的影响沿流向逐渐减弱.在气流压缩性能方面,涡发生器下游压比、动压比沿流向开始增大,随后逐渐恢复到无涡发生器工况;Mach数、总压恢复系数开始降低,随后逐渐向无涡发生器工况趋近.涡发生器高度与当地位移边界层厚度的比值h可作为衡量其影响的重要参数.当h≤1.5时,进气道流场结构、性能参数的变化几乎可忽略,h≤3.0时进气道入口处性能参数几乎能够恢复到无涡发生器工况.      

变截面超-超引射器启动特性数值研究

为研究不同结构参数与来流参数下变截面超-超引射器的启动特性,用于指导超-超引射器设计与工况调试,采用二维雷诺平均Naiver-Stokes方程,数值研究了引射器混合室不同收缩比、一次流和二次流的不同总压比、总温比下超-超引射器的启动规律,并定义了“启动系数”来判别超-超引射流场是否建立。研究结果表明:随着收缩比（范围0.7～0.9）的增加,超-超引射器启动的临界总压比、总温比均先降低后升高,收缩比0.8时,存在最佳总压比5.88,最佳总温比0.21。结构参数一定,超-超引射器随总温比升高启动难度增加。当超-超引射器处于启动状态下,室压不随总温比、总压比变化而变化,引入的启动系数较引射系数可不依赖具体工况而直接判别超-超引射器是否启动。     

Sufficient conditions for linear long-wave instability of steady-state axisymmetric flows of an ideal liquid with a free boundary in an azimuthal magnetic field

The problem of linear stability of steady-state axisymmetric shear jet flows of an inviscid ideally conducting incompressible liquid with a free surface and “frozen-in” azimuthal magnetic field is analyzed. The sufficient conditions for theoretical (on semi-infinite time intervals) and practical (on finite time intervals) instability of these flows relative to small axisymmetric long-wave perturbations are obtained by the direct Lyapunov method. An a priori lower estimate indicating (at least) an exponential increase with time of small perturbations under investigation is constructed in the case when these conditions are valid for theoretical as well as practical instability. In addition, an illustrative analytic example of steady-state flows under investigation and small axisymmetric long-wave perturbations superimposed on them is constructed (according to our estimate, these perturbations increase with time).     

方腔内微细颗粒物运动特性的格子Boltzmann方法模拟

对Masselot and Chopard提出的模拟气固两相流动的格子Boltzmann-格子气(LBE-LGA)方法进行推广,能够反映两相间拖曳作用.利用该方法研究封闭方腔内的气固两相流运动特性,分析斯托克斯数St和模拟颗粒数目对颗粒群运动的影响,并与文献结果进行比较,表明LBE-LGA方法模拟颗粒运动是可行的.     

Three-dimensional coupled double-distribution-function lattice Boltzmann models for compressible Navier–Stokes equations

Two three-dimensional (3D) lattice Boltzmann models in the framework of coupled double-distribution-function approach for compressible flows, in which specific-heat ratio and Prandtl number can be adjustable, are developed in this paper. The main differences between the two models are discrete equilibrium density and total energy distribution function. One is the D3Q25 model obtained from spherical function, and the other is the D3Q27 standard lattice model obtained from Hermite expansions of the corresponding continuous equilibrium distribution functions. The two models are tested by numerical simulations of some typical compressible flows, and their numerical stability and precision are also analysed. The results indicate that the two models are capable for supersonic flows, while the one from Hermite expansions is not suitable for compressible flows with shock waves.     

有限远来流非对称碰撞形成射流研究

研究距离交汇点有限远的两股平面金属来流非对称碰撞的射流形成规律,给出出流厚度和流动方向的理论公式,并采用带有YOUNGS界面处理的高精度欧拉程序进行仿真验证,与KS喷管实验结果比较,符合较好.研究表明:当碰撞角确定时,来流初始位形对杵体的流动状态有重要影响,对细射流的影响不明显.     

Experimental investigation of dynamical coupling between turbulent transport and parallel flows in the JET plasma-boundary region

The dynamical coupling between turbulent transport and parallel flows has been investigated in the plasma boundary region of the Joint European Torus tokamak. Experimental results show that there is a dynamical relationship between transport and parallel flows. As the size of transport events increases, parallel flows also increase. These results show that turbulent transport can drive parallel flows in the plasma boundary of fusion plasmas. This new type of measurement is an important element to unravel the overall picture connecting radial transport and flows in fusion plasmas.     

Three-dimensional finite-difference lattice Boltzmann model and its application to inviscid compressible flows with shock waves

In this paper, a three-dimensional (3D) finite-difference lattice Boltzmann model for simulating compressible flows with shock waves is developed in the framework of the double-distribution-function approach. In the model, a density distribution function is adopted to model the flow field, while a total energy distribution function is adopted to model the temperature field. The discrete equilibrium density and total energy distribution functions are derived from the Hermite expansions of the continuous equilibrium distribution functions. The discrete velocity set is obtained by choosing the abscissae of a suitable Gauss–Hermite quadrature with sufficient accuracy. In order to capture the shock waves in compressible flows and improve the numerical accuracy and stability, an implicit–explicit finite-difference numerical technique based on the total variation diminishing flux limitation is introduced to solve the discrete kinetic equations. The model is tested by numerical simulations of some typical compressible flows with shock waves ranging from 1D to 3D. The numerical results are found to be in good agreement with the analytical solutions and/or other numerical results reported in the literature.     

Predicting the progress of diffusively limited chemical reactions in the presence of chaotic advection

The effects of chaotic advection and diffusion on fast chemical reactions in two-dimensional fluid flows are investigated using experimentally measured stretching fields and fluorescent monitoring of the local concentration. Flow symmetry, Reynolds number, and mean path length affect the spatial distribution and time dependence of the reaction product. A single parameter lambdaN , where lambda is the mean Lyapunov exponent N and is the number of mixing cycles, can be used to predict the time-dependent total product for flows having different dynamical features.     

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采用流体模型理论推导了等熵平衡条件下环向转动托卡马克等离子体中带状流的色散关系。从理论上分析了环向转动对测地声模、低频带状流和声波的频率、压力和密度扰动量的影响。结果表明,环向转动对低频带状流的频率没有影响,但会使测地声模的频率逐渐增大。此外,存在环向转动时,低频带状流会具有驻波形式的压力和密度扰动量,且测地声模和声波可以沿着极向传播。而且还发现,等熵平衡可以看成是等温平衡的一种特殊情况。     

托卡马克等熵平衡条件下等离子体环向转动对带状流的影响

采用流体模型理论推导了等熵平衡条件下环向转动托卡马克等离子体中带状流的色散关系。从理论上分析了环向转动对测地声模、低频带状流和声波的频率、压力和密度扰动量的影响。结果表明,环向转动对低频带状流的频率没有影响,但会使测地声模的频率逐渐增大。此外,存在环向转动时,低频带状流会具有驻波形式的压力和密度扰动量,且测地声模和声波可以沿着极向传播。而且还发现,等熵平衡可以看成是等温平衡的一种特殊情况。     

Change of zonal flow spectra in the JIPP T-IIU tokamak plasmas

When Ohmically heated low-density plasmas are additionally heated by higher-harmonics ion-cyclotron-range-of frequency heating, heated by neutral beam injection, or strongly gas puffed, the intensity of zonal flows in the geodesic acoustic mode frequency range in the tokamak core plasma decreases sharply and that of low-frequency zonal flow grows drastically. This is accompanied by a damping of the drift wave propagating in the electron diamagnetic drift direction, turbulence by trapped electron mode (TEM), and the increase of the mode propagating to ion diamagnetic drift direction (ITG). In the half-radius region, TEM and high-frequency zonal flows remain intense in both OH and heated phases. ITG and low-frequency zonal flows grow in heated plasmas, suggesting a strong coupling between ITG and low-frequency zonal flow.     

Subject index to volume 37

A number of new effects is predicted, which appear under resonant interaction between light and gas. The velocity distribution of the total number of particles (excited and unexcited) varies due to the increase in the cross section under excitation. This leads to uneven energy distribution between the translational degrees of freedom and, as a result, to anisotropy of pressure, temperature, and to thermal flows which result in the appearance of a density and temperature gradient in the stationary state.     

Filamentation instability in two counter-streaming laser plasmas

The filamentation instability was observed in the interaction of two counter-streaming laser ablated plasma flows,which were supersonic, collisionless, and also closely relevant to astrophysical conditions. The plasma flows were created by irradiating a pair of oppositely standing plastic(CH) foils with 1ns-pulsed laser beams of total energy of 1.7 k J in two laser spots. With characteristics diagnosed in experiments, the calculated features of Weibel-type filaments are in good agreement with measurements.