微尺度气体流动

了解微尺度气体流动特点是微机电系统设计和优化的基础.有关的研究可以上溯到20世纪初Knudsen的平面槽道流动质量流量的测量和Millikan的小球阻力系数的测量,实验结果揭示了稀薄气体效应即尺度效应对气体运动的重要影响.由于流动特征长度很小,微尺度气流经常处于滑流区甚至过渡领域,流动的相似参数为Knudsen数和Mach数.因此可以考虑利用相似准则,通过增大几何尺寸、减小压力的途径,解决微机电系统实验观测遇到的困难.为解决直接模拟Monte Carlo方法分析微机电系统中低速稀薄气流遇到的统计涨落困难,我们提出了信息保存法(IP),该方法能够有效克服统计散布,并已成功用于多种微尺度气流.      

稀薄气体动力学的现状

自从1946年钱学森的开创性文章发表以来,对稀薄气体动力学的兴趣主要在于高速稀薄气体对物体绕流的空气动力学问题。二十余年来这门学科发展很快,直到最近其发展势头仍未减弱,这点从历届稀薄气体     

第24届国际稀薄气体动力学会议

第24届国际稀薄气体动力学会议于2004年7月11日至16日在意大利巴里市召开.会议主席为巴里大学化学系主任马里奥·卡彼特利教授.会议讨论的专题有:(1)Boltz-mann和相关方程;(2)内部流动和真空系统;(3)动理学与输运理论;(4)Monte-Carlo方法与数值解;(5)稀薄射流和羽流;(6)多粒子动力学;(7)态间动理学;(8)粒状结构气体;(9)等离子体中的输运;(10)稀薄等离子体;(11)微流动问题;(12)稀薄气体动力学中的实验;(13)气相分子碰撞     

稀薄气体动力学现状

自从1946年钱学森的开创性文章发表以来,对稀薄气体动力学的兴趣主要在于高速稀薄气体对物体绕流的空气动力学问题。二十余年来这门学科发展很快,直到最近其发展势头仍未减弱,这点从历届稀薄气体动力学国际会议的情况也可以看出来(其中第七次会议文集未到,仅见会议较详介绍,第八次1972年夏刚刚开过)。会议文集      

有攻角组合旋成体过渡区气动特性的统计模拟

对于象卫星、载人宇宙飞船及航天飞机等长时间在外层空间飞行的物体,稀薄气体气动特性的研究具有相当重要的意义,几十年来,从事稀薄气体动力学研究的学者们一直在力图求解Boltzmann方程,但至今尚未获得重大突破.对于紧迫的实际问题,只能求助于实验和工程方法,不过,由Bird所发展的统计模拟方法几乎能在整个过渡区为     

认识稀薄气体动力学

以通俗易懂的方式介绍了空气动力学当气体间断分子效应显著时发展起来的特殊分文——稀薄气体动力学、讨论了非平衡现象与稀薄气体动力学的关系．通过与8速度气体模型的间断Boltzmann方程的对比，解释了Boltzmann方程碰撞项的物理意义和数学困难，简要综述了其一般解法、讨论了分子在物体表面的反射和问题的边界条件，着重介绍了直接模拟Monte Carlo(DSMC)方法和为克服低速稀薄流动(如MEMS中流动)中模拟困难的信息保存(IP)方法。     

关于稀薄气体动力学的发展方向

一般的流体力学、空气动力学、弹性力学等都是建立在连续介质概念的基础上,不研究构成连续介质的微观粒子的运动状态。随着高空高速飞行、宇航事业的发展,对空间科学的研究以及真空技术的发展,稀薄气体动力学得到了迅速发展。在数百公里以上的高层大气中,气体分子的平均自由程比飞行器的尺寸大得多。在广阔的日地空间里,在无限的宇宙空间中,大多数物质呈稀薄等离子体状态。有时,在一立方公里的空间内只有几十个分子在运动。用一般连续介质概念已不能描述这种运动状态,而必须从微观分析出发,也就是说,要用分子运动论的的方法研究气体的运动状态。      

高空高速飞行器气动特性研究

高空高速飞行中的黏性干扰效应、真实气体效应和稀薄气体效应成为决定未来空天飞行器能否实现安全飞行、精确控制和制导的重大基础科学问题, 必须发展相应的基础理论和研究方法.该文从计算方法、流动拓扑分析以及典型飞行器应用等方面介绍了作者所在研究团队近年来部分工作进展.      

基于稀薄效应的微气体径向轴承稳态性能

针对微气体轴承给出参考努森数的定义,根据空气不同温度时的黏度,得到参考努森数的分布范围;考虑气体稀薄效应,给出基于Burgdorfer一阶滑移速度边界的微气体径向轴承润滑Reynolds方程的修正形式; 采用有限差分法求解修正的Reynolds方程,得到不同参考努森数$Kn_0$, 轴承数以及轴颈偏心率情况下轴承的压力分布、无量纲承载能力及偏位角. 数值分析表明:随气体稀薄程度的增强,气体径向轴承的压力明显降低,无量纲承载力降低,而轴承偏位角增大. 当偏心率小于0.6时,轴承偏位角变化平缓,受$Kn_0$数的影响不明显. 当轴承数较小时,气体稀薄程度对轴承的无量纲承载力、偏位角影响较小.      

中国科学院高温气体动力学重点实验室研究进展

在钱学森先生的倡导下,中国科学院力学研究所自20世纪50年代末期就开始了高温气体动力学的基础研究,相关研究进展曾经为发展我国``两弹一星'和神州飞船做出了重要贡献.在钱学森先生创立的气动研究队伍和科研积累的基础上,中国科学院于1994年组建了中国科学院高温气体动力学重点实验室.实验室成立以来一直坚持钱学森先生倡导的技术科学理念,按照中国科学院基础性、前瞻性和战略性的办院方针,面向航空航天和国民经济的重大战略需求,研究高温、高超声速条件下,具有分子内态变化介质流动的高温气体动力学前沿学科问题.通过不断提出新方法、新概念、新技术,支撑国家重大战略需求的关键技术攻关,推进高温气体动力学的学科发展.近年来,在国家973项目,国家自然科学基金委重大、重点项目和创新研究群体,科学院国际合作伙伴团队,863项目和中国科学院方向性创新项目的支持下,实验室在高焓高超声速气体流动规律、高超声速推进方法、稀薄气体非平衡流动、高超声速飞行器构型理论与优化等主要研究方向取得了重要研究成果,在促进高超声速流动模拟实验、高超声速推进、气动热预测与防护、高超声速飞行器气动布局优化设计等重大关键技术的发展方面发挥了重要作用.      

Use of the Monte Carlo method to calculate the flow of a highly rarefied gas in systems with arbitrary wall configuration

A method is described for modeling collisions of gas molecules with the walls of a system whose geometry is given numerically in the computer memory from a graphical representation of the walls (for example, from their working drawings). When changing from the calculation of one system to another only the information on the wall changes; the computational program remains the same. The method is applicable to problems of rarefied gasdynamics which are solvable by the Monte Carlo method; in the following it is used to calculate the conductance of elements of high vacuum lines and the compression ratio created by molecular vacuum pumps, and also to calculate the forces acting on a body which rotates in a cavity filled with a highly rarefied gas.In conclusion the authors wish to thank Yu. I. Neimark for formulating the problem and discussions of the results obtained.     

Some methods of calculating the steady motion of a rarefied gas

Vacuum molecular pumps have been long known and have several advantages [1–3].Several studies have been devoted to the design of vacuum molecular pumps [7–10]. The methods developed in these studies have been based either on the formulas for gas diffusion in long pipes or on the integral equations of material balance. However, these theories do not permit obtaining design data for real designs of molecular pumps which are close to the experimental data, and, moreover, do not permit solving the practically important problem of optimizing the parameters and geometry of the molecular and turbomolecular pumps with respect to output and compression ratio. The calculations made in [8–10] are valid only for rotor speeds which are much less than the average velocities of the gas molecules. However, the studies in the second direction cannot be continued to a final result in view of the extreme complexity of the solution of the resulting system of integral equations.In the following we describe the calculation of vacuum molecular pumps, based on the Monte-Carlo method (the Monte-Carlo method has been used to calculate the conductance of the elements of vacuum lines in the free molecular regime in [4, 5, 6] and to calculate using the method of sequential approximations the flow of a rarefied gas with account for the collisions between molecules in [11]).We shall apply this method not only to systems with a high vacuum, when the collisions between molecules may be neglected, but also to systems in which in addition to the molecule collisions with the wall it is necessary to consider the possibility of a small number of mutual collisions.     

End effects in rarefied gas outflow into vacuum through a long tube

The problem of steady outflow of rarefied gas from a reservoir into vacuum through a long cylindrical tube of circular cross-section at a constant temperature is considered on the basis of the kinetic S-model. The kinetic equation is solved numerically using a conservative second-order method. The basic calculated quantity is the gas flow rate through the channel; the flowfields are also studied. The solutions obtained are compared with the known results.     

Rarefied planar jets into vacuum

This paper presents a fundamental gaskinetic study on high speed rarefied jets expanding into vacuum from a cluster of planar exits. Based on the corresponding exact expressions for one planar jet, this paper straightforwardly derives the combined multiple jet flowfield solutions of density and velocity components, however, for the combined temperature and pressure solutions, extra attention shall be practiced. Several direct simulation Monte Carlo simulation results are provided and they validate these analytical solutions of rarefied planar jet flows.     

Asymptotic solutions of the two-dimensional equations for a thin viscous shock layer in a rarefied gas

Two-dimensional hypersonic rarefied gas flow around blunt bodies is investigated for the continuum to free-molecular transition regime. In [1], as a result of an asymptotic analysis, three rarefied gas flow regimes, depending on the relationship between the problem parameters, were detected and one of these regimes was investigated. In the present study, asymptotic solutions of the thin viscous shock layer equations at small Reynolds numbers are obtained for the other two flow regimes. Analytical expressions for the heat transfer, friction and pressure coefficients are obtained as functions of the incident flow parameters and the body geometry and temperature. As the Reynolds number tends to zero, the values of these coefficients approach their values in free-molecular flow. The scaling parameters of hypersonic rarefied gas flow around bodies are determined for different regimes. The asymptotic solutions are compared with the results of direct Monte Carlo simulation.     

Asymptotic solutions of the thin viscous shock layer equations near the symmetry plane of blunt bodies in hypersonic rarefied gas flow

The hypersonic rarefied gas flow over blunt bodies near the symmetry plane is investigated for the regime transitional from continuum to free-molecular. Three rarefied gas flow regimes are considered depending on the relationship between the determining parameters of the problem. For all regimes, at small Reynolds numbers, asymptotic solutions of the thin viscous shock layer equations near the symmetry plane of blunt bodies are obtained in the form of simple analytical expressions for the heat transfer, skin friction and pressure coefficients as functions of the gas-dynamic parameters of the free-stream flow and the geometric parameters and temperature of the body. With decrease in the Reynolds number these coefficients approach their values in free-molecular flow (with the accommodation coefficient equal to unity). From comparison with the data calculated using the direct simulation Monte Carlo method, the accuracy and applicability limits of the analytical solution are estimated.     

Asymptotic solution of the two-dimensional equations for a thin viscous rarefied shock layer on a cold surface

Hypersonic rarefied flow past blunt bodies is studied in the continuum-free-molecular transition regime. On the basis of an asymptotic analysis three rarefied gas flow patterns are established depending on the relation between the relevant parameters of the problem. In the first regime corresponding to a cold surface asymptotic solutions of the equations of a thin viscous shock layer are derived at low Reynolds numbers in the axisymmetric and plane cases. Simple analytical expressions for the pressure and the heat transfer and friction coefficients are obtained as functions of the freestream parameters and the body geometry. With decrease in the Reynolds number the coefficients approach the values corresponding to free-molecular flow. In this regime a similarity parameter for the hypersonic rarefied flow past bodies is determined. The asymptotic solutions are compared with numerical solutions and the results of direct statistical simulation by the Monte Carlo method.     

Mass flow rate prediction of pressure–temperature-driven gas flows through micro/nanoscale channels

In this paper, we study mass flow rate of rarefied gas flow through micro/nanoscale channels under simultaneous thermal and pressure gradients using the direct simulation Monte Carlo (DSMC) method. We first compare our DSMC solutions for mass flow rate of pure temperature-driven flow with those of Boltzmann-Krook-Walender equation and Bhatnagar-Gross-Krook solutions. Then, we focus on pressure–temperature-driven flows. The effects of different parameters such as flow rarefaction, channel pressure ratio, wall temperature gradient and flow bulk temperature on the thermal mass flow rate of the pressure–temperature-driven flow are examined. Based on our analysis, we propose a correlated relation that expresses normalized mass flow rate increment due to thermal creep as a function of flow rarefaction, normalized wall temperature gradient and pressure ratio over a wide range of Knudsen number. We examine our predictive relation by simulation of pressure-driven flows under uniform wall heat flux (UWH) boundary condition. Walls under UWH condition have non-uniform temperature distribution, that is, thermal creep effects exist. Our investigation shows that developed analytical relation could predict mass flow rate of rarefied pressure-driven gas flows under UWH condition at early transition regime, that is, up to Knudsen numbers of 0.5.     

A parallel cell‐based DSMC method on unstructured adaptive meshes

A parallel DSMC method based on a cell‐based data structure is developed for the efficient simulation of rarefied gas flows on PC‐clusters. Parallel computation is made by decomposing the computational domain into several subdomains. Dynamic load balancing between processors is achieved based on the number of simulation particles and the number of cells allocated in each subdomain. Adjustment of cell size is also made through mesh adaptation for the improvement of solution accuracy and the efficient usage of meshes. Applications were made for a two‐dimensional supersonic leading‐edge flow, the axi‐symmetric Rothe's nozzle, and the open hollow cylinder flare flow for validation. It was found that the present method is an efficient tool for the simulation of rarefied gas flows on PC‐based parallel machines. Copyright © 2004 John Wiley & Sons, Ltd.