Specific heat ratio effects of compressible Rayleigh–Taylor instability studied by discrete Boltzmann method

Rayleigh–Taylor (RT) instability widely exists in nature and engineering fields. How to better understand the physical mechanism of RT instability is of great theoretical significance and practical value. At present, abundant results of RT instability have been obtained by traditional macroscopic methods. However, research on the thermodynamic non-equilibrium (TNE) effects in the process of system evolution is relatively scarce. In this paper, the discrete Boltzmann method based on non-equilibrium statistical physics is utilized to study the effects of the specific heat ratio on compressible RT instability. The evolution process of the compressible RT system with different specific heat ratios can be analyzed by the temperature gradient and the proportion of the non-equilibrium region. Firstly, as a result of the competition between the macroscopic magnitude gradient and the non-equilibrium region, the average TNE intensity first increases and then reduces, and it increases with the specific heat ratio decreasing; the specific heat ratio has the same effect on the global strength of the viscous stress tensor. Secondly, the moment when the total temperature gradient in y direction deviates from the fixed value can be regarded as a physical criterion for judging the formation of the vortex structure. Thirdly, under the competition between the temperature gradients and the contact area of the two fluids, the average intensity of the non-equilibrium quantity related to the heat flux shows diversity, and the influence of the specific heat ratio is also quite remarkable.     

Nonequilibrium and morphological characterizations of Kelvin–Helmholtz instability in compressible flows

We investigate the effects of viscosity and heat conduction on the onset and growth of Kelvin–Helmholtz instability (KHI) via an efficient discrete Boltzmann model. Technically, two effective approaches are presented to quantitatively analyze and understand the configurations and kinetic processes. One is to determine the thickness of mixing layers through tracking the distributions and evolutions of the thermodynamic nonequilibrium (TNE) measures; the other is to evaluate the growth rate of KHI from the slopes of morphological functionals. Physically, it is found that the time histories of width of mixing layer, TNE intensity, and boundary length show high correlation and attain their maxima simultaneously. The viscosity effects are twofold, stabilize the KHI, and enhance both the local and global TNE intensities. Contrary to the monotonically inhibiting effects of viscosity, the heat conduction effects firstly refrain then enhance the evolution afterwards. The physical reasons are analyzed and presented.     

Kinetic Simulation of Nonequilibrium Kelvin-Helmholtz Instability

The recently developed discrete Boltzmann method(DBM), which is based on a set of uniform linear evolution equations and has high parallel efficiency, is employed to investigate the dynamic nonequilibrium process of Kelvin-Helmholtz instability(KHI). It is found that, the relaxation time always strengthens the global nonequilibrium(GNE), entropy of mixing, and free enthalpy of mixing. Specifically, as a combined effect of physical gradients and nonequilibrium area, the GNE intensity first increases but decreases during the whole life-cycle of KHI. The growth rate of entropy of mixing shows firstly reducing, then increasing, and finally decreasing trends during the KHI process. The trend of the free enthalpy of mixing is opposite to that of the entropy of mixing. Detailed explanations are:(i) Initially,binary diffusion smooths quickly the sharp gradient in the mole fraction, which results in a steeply decreasing mixing rate.(ii) Afterwards, the mixing process is significantly promoted by the increasing length of material interface in the evolution of the KHI.(iii) As physical gradients are smoothed due to the binary diffusion and dissipation, the mixing rate reduces and approaches zero in the final stage. Moreover, with the increasing Atwood number, the global strength of viscous stresses on the heavy(light) medium reduces(increases), because the heavy(light) medium has a relatively small(large) velocity change. Furthermore, for a smaller Atwood number, the peaks of nonequilibrium manifestations emerge earlier, the entropy of mixing and free enthalpy of mixing change faster, because the KHI initiates a higher growth rate.     

Hydrodynamic and Thermodynamic Nonequilibrium Effects around Shock Waves: Based on a Discrete Boltzmann Method

A shock wave that is characterized by sharp physical gradients always draws the medium out of equilibrium. In this work, both hydrodynamic and thermodynamic nonequilibrium effects around the shock wave are investigated using a discrete Boltzmann model. Via Chapman–Enskog analysis, the local equilibrium and nonequilibrium velocity distribution functions in one-, two-, and three-dimensional velocity space are recovered across the shock wave. Besides, the absolute and relative deviation degrees are defined in order to describe the departure of the fluid system from the equilibrium state. The local and global nonequilibrium effects, nonorganized energy, and nonorganized energy flux are also investigated. Moreover, the impacts of the relaxation frequency, Mach number, thermal conductivity, viscosity, and the specific heat ratio on the nonequilibrium behaviours around shock waves are studied. This work is helpful for a deeper understanding of the fine structures of shock wave and nonequilibrium statistical mechanics.     

Thermodynamic Nonequilibrium Features in Binary Diffusion

Diffusion is a ubiquitous physical phenomenon where thermodynamic nonequilibrium effects(TNEs) are outstanding issues. In this work, we employ the discrete Boltzmann method to investigate the TNEs in the dynamic process of binary diffusion. The main features of the distribution function in velocity space are recovered and discussed.It is found that, with the decreasing gradients of macroscopic quantities(such as density, concentration, velocity, etc.),both the local and global TNEs decrease with the time but increase with the relaxation time in a power law, respectively.     

Anisotropic origin of the bending instability of the flux-antiflux interface in type-II superconductors

The physical nature of the macroturbulence in vortex matter in YBCO superconductors is investigated by means of a magneto-optic study of the instability in a single crystal prepared especially for this purpose. The instability develops near those sample edges where the oppositely directed flow of vortices and antivortices, guided by twin boundaries, is characterized by the discontinuity of the tangential component of the hydrodynamic velocity. This fact indicates that the macroturbulence is analogous to the instability of fluid flow at a surface of a tangential velocity discontinuity in classical hydrodynamics and is related to the anisotropic flux motion in the superconductor.     

燃烧系统的离散Boltzmann建模与模拟研究进展

燃烧系统的诸多模拟依托于流体建模, 离散Boltzmann方法(discrete Boltzmann method, DBM) 是近年来发展起来的一种新的流体介观建模方法. 本文简要评述DBM发展的两个方向——Navier-Stokes等偏微分方程的数值逼近解法和复杂系统的微介观动理学建模. 主要介绍在燃烧系统模拟方面DBM已有的工作、新近的思路、与传统流体建模的异同以及近期的研究成果. 本文重点传递的信息为: 作为复杂系统微介观动理学建模出现的DBM在模拟过程中同时给出“流动”及其相伴随的、关系最密切的那部分“热动”非平衡效应; 它为燃烧等复杂系统中各类非平衡行为的描述、非平衡信息的提取、非平衡程度的度量提供了一种简洁、有效的方法; 它所提供的热动非平衡测量量有两类: 一类是直接比较分布函数和平衡态分布函数的动理学矩关系得到的, 一类是来自于Chapman-Enskog多尺度分析给出的热传导和黏性项. 基于第二类DBM, 可以实现(燃烧等)一大类复杂流体系统的多尺度物理建模.     

超声速流体Kelvin-Helmholtz不稳定性速度梯度效应研究

利用加权本质上无振荡（WENO）方法模拟超声速流体Kelvin-Helmholtz（KH）不稳定性,研究速度梯度对KH不稳定性线性增长率和后期非线性演化的影响.模拟发现超声速流体中的速度梯度对KH不稳定性具有较强的致稳作用,给出了包含速度梯度致稳的线性增长率经验公式.数值模拟和经验公式符合得很好.模拟给出了清晰的流场密度等值线,这说明WENO方法模拟超声速流体KH不稳定性具有较好的界面变形捕捉能力.模拟结果表明速度梯度影响KH涡的演化,在给定密度梯度的情况下速度梯度越大KH涡的横向尺度越小. 关键词： Kelvin-Helmholtz不稳定性 超声速流体 速度梯度     

微通道内给定壁面热流的气体换热研究

本文利用实施给定热流边界条件的DSMC方法,对短通道内给定壁面热流边界条件下的气体换热情况进行了模拟.结果表明,壁面热流密度增大导致通道内压力分布非线性程度增加.随着热流密度的增大,截面速度分布趋于平缓,滑移速度增大.给定热流密度的通道壁面温度与气流截面平均温度的差值沿程增大,温度梯度沿程下降,气体稀薄性增大时,通道换热减弱.     

Viscosity,heat conductivity,and Prandtl number effects in the Rayleigh–Taylor Instability

The two-dimensional Rayleigh–Taylor instability problem is simulated with a multiple-relaxation-time discrete Boltzmann model with a gravity term. Viscosity, heat conductivity, and Prandtl number effects are probed from macroscopic and nonequilibrium viewpoints. In the macro sense, both viscosity and heat conduction show a significant inhibitory effect in the reacceleration stage, which is mainly achieved by inhibiting the development of the Kelvin–Helmholtz instability. Before this, the Prandtl number effect is not sensitive. Viscosity, heat conductivity, and Prandtl number effects on nonequilibrium manifestations and the degree of correlation between the nonuniformity and the nonequilibrium strength in the complex flow are systematically investigated.     

On the stability of capillary waves on the surface of a space-charged dielectric liquid jet moving in a material medium

We have derived and analyzed the dispersion equation for capillary waves with an arbitrary symmetry (with arbitrary azimuthal numbers) on the surface of a space-charged cylindrical jet of an ideal incompressible dielectric liquid moving relative to an ideal incompressible dielectric medium. It has been proved that the existence of a tangential jump of the velocity field on the jet surface leads to a periodic Kelvin–Helmholtz- type instability at the interface between the media and plays a destabilizing role. The wavenumber ranges of unstable waves and the instability increments depend on the squared velocity of the relative motion and increase with the velocity. With increasing volume charge density, the critical value of the velocity for the emergence of instability decreases. The reduction of the permittivity of the liquid in the jet or an increase in the permittivity of the medium narrows the regions of instability and leads to an increase in the increments. The wavenumber of the most unstable wave increases in accordance with a power law upon an increase in the volume charge density and velocity of the jet. The variations in the permittivities of the jet and the medium produce opposite effects on the wavenumber of the most unstable wave.     

On the construction of an extended thermodynamic framework for irreversible processes

It is shown that the practice in extended irreversible thermodynamics of raising the physical fluxes, such asq, the heat flux density, etc. to the status of independent thermodynamic variables does not meet the basic thermodynamic requirements nor in a real situation by keeping them constant one can vary the other thermodynamic independent variables. To elucidate this point the phenomena of rigid body heat conduction is used as an illustrative expample. It is argued that the thermometric temperature of systems whether in equilibrium or in nonequilibrium is the same physical entity.     

并联管两相流不稳定性的研究

在高压汽水回路上对垂直并联管中汽液两相流不稳定性进行了试验研究．确定了压力、质量流速、入口过冷度、热负荷及热负荷不对称分布、进口及出口节流、可压缩容积等对不稳定性的影响．得出了压力降型和密度波型不稳定性的界限，给出了并联管中计算不稳定性起始条件的无因次方程，为大型直流锅炉和蒸汽发生器的设计提供依据．     

HL-2A边缘等离子体中捕获电子模的线性数值模拟

利用GTC程序和HL-2A装置的实验数据,对托卡马克边缘等离子体中的漂移波微观不稳定性进行了线性数值模拟。模拟结果表明,在HL-2A边缘等离子体中捕获电子模(TEM)是不稳定的,它均匀且规则地分布在托卡马克的弱磁场区域,而且其增长率随着等离子体的温度梯度和密度梯度的增加而增加。另外,模拟结果还表明,TEM的实频率远远小于离子的漂移频率,这一点与理论研究结果是一致的。     

The entropy flux and the evolution equations for the fluxes including nonlocal terms from the point of view of extended irreversible thermodynamics

The expression for the entropy flux is analysed from the point of view of irreversible thermodynamics. In connection with this problem the evolution equations for the heat flux and for the electric current density including nonlocal terms are derived and discussed. The relation for the entropy flux is compared with that obtained by the statistical nonequilibrium thermodynamics on the basis founded on a generalized Gibbs' ensemble method for nonequilibrium systems.     

筒形溅射阴极的磁场优化及其高功率放电特性研究

基于高功率脉冲磁控溅射(HiPIMS)技术开发的筒形溅射阴极,配合电磁系统可有效地提升等离子体的输运效率.然而电磁系统的引入反作用于筒内放电特性,从而使靶面放电面积和放电强度无法同时维持.鉴于此,本文通过调整磁场布局,研究了靶面切向(横向)磁场和法向(纵向)磁场对靶面放电的作用规律,优化后靶面切向磁场分布更加均匀,磁场强度高于40 mT的靶面区域占比由51%增至67%,同时法向峰值强度外移,强度由73 mT增至96 mT.采用Ar/Cr体系放电发现:相同工艺条件下,优化后的溅射阴极辉光变亮,靶电流增大,放电面积变宽,放电特性得到显著提升.利用等离子体整体模型仿真和发射光谱仪检测发现优化后离子电流和光谱强度得到明显提升,Cr粒子密度提高一倍,增至2.6×10^20 m^–3,且离化率上升至92.1%,同时输出离子通量提高近一倍,实现了靶面放电与离子输出的双促进.     

Free Convective Nonaligned Non-Newtonian Flow with Non-linear Thermal Radiation

The present study explores the free convective oblique Casson fluid over a stretching surface with non-linear thermal radiation effects. The governing physical problem is modelled and transformed into a set of coupled non-linear ordinary differential equations by suitable similarity transformation, which are solved numerically with the help of shooting method keeping the convergence control of 10~(-5) in computations. Influence of pertinent physical parameters on normal, tangential velocity profiles and temperature are expressed through graphs. Physical quantities of interest such as skin friction coefficients and local heat flux are investigated numerically.     

Experimental studies on isentropic efficiency of a nonequilibriumMHD disk generator

Isentropic efficiency of the nonequilibrium MHD power generator was studied by a shock tube driven disk generator. Cesium seeded helium was used as a working gas. From the measurements of Faraday current density distribution, it was possible to estimate the general tendency of Joule dissipation in the generator. The Joule dissipation did not decrease due to the occurrence of nonuniformity of the plasma when external load resistance was low, although it decreased with the decrease in the load resistance when the load resistance was high. The electrical efficiency increased with the increase in applied magnetic flux density. This fact is thought to be caused by high Hall parameter and the stabilization of the plasma due to high degree of seed ionization     

Nonclassical transport in a nonequilibrium laser plasma

The onset of a nonequilibrium distribution function in a plasma produced by laser radiation at fluxes I - 10¹⁴ W/cm² is considered. The model employed takes consistent account of the nonlocality of the electron distribution at large density and temperature gradients, the demaxwellizing effect of the external hf field of the laser, and the nondiffusion of the transport in the high-energy part of the electron-distribution spectrum. Numerical solution of the corresponding kinetic equations shows that establishment of a nonequilibrium electron distribution in the corona suppresses the heat transport substantially. Good agreement is obtained between the kinetic value of the heat flux and the phenomenological value needed to reconcile the hydrodynamicsimulation results with the experimental data.Quantum Radiophysics Division, Lebedev Physics Institute. Translated from Preprint No. 179 of the Ledebev Physics Institute, Academy of Sciences of the USSR, Moscow, 1989.     

离散发热器件基于(火积)耗散率最小和最高温度最小的构形优化比较

建立了导热基座上圆柱体离散发热器件的三维湍流散热模型,基于构形理论,考虑空气变物性及可压缩性和黏性耗散,研究了器件材料的热导率、热源强度和流体流速对器件最高温度、基于(火积)耗散定义的当量热阻和平均Nu数的影响.结果表明:在总发热功率一定的条件下,以器件最高温度和当量热阻为性能指标进行热设计,均存在最优热源强度分布使得散热性能最优.当各热源强度相同且热源热导率小于基座热导率时,提高热源热导率可明显改善散热性能;将热源热导率沿流动方向从低到高布置可降低器件最高温度,而将热源热导率均匀布置可使当量热阻最小.所得结果可为实际热设计中不同材质和不同发热率的电子器件最优布置提供理论支撑.