Exact solutions to collisionless external gas flows

This paper presents exact density, velocity and temperature solutions for two problems of collisionless gas flows around a flat plate or a spherical object. At any point off the object, the local velocity distribution function consists of two pieces of Maxwellian distributions: one for the free stream which is characterized by free stream density, temperature and average velocity, n0, T0, U0; and the other is for the wall and it is characterized by density at wall and wall temperature, nw,Tw. Directly integrating the distribution functions leads to complex but exact flowfield solutions. To validate these solutions, we perform numerical simulations with the direct simulation Monte Carlo (DSMC) method. In general, the analytical and numerical results are virtually identical. The evaluation of these analytical solutions only requires less than one minute while the DSMC simulations require several days.

Development of an efficient particle approach for micro-scale gas flow simulations

The micro-scale gas flows are usually low-speed flows and exhibit rarefied gas effects. It is challenging to simulate these flows because traditional CFD method is unable to capture the rarefied gas effects and the direct simulation Monte Carlo (DSMC) method is very inefficient for low-speed flows. In this study we combine two techniques to improve the efficiency of the DSMC method. The information preservation technique is used to reduce the statistical noise and the cell-size relaxed technique is employed to increase the effective cell size. The new cell-size relaxed IP method is found capable of simulating micro-scale gas flows as shown by the 2D lid-driven cavity flows.

IP方法模拟硬盘读写头气膜润滑问题

本论文中采用IP方法模拟硬盘读写头的气膜润滑问题，并选择耗时但精准的DSMC方法作为该问题中IP方法的检验标准，IP方法计算得到的读写头表面压力分布及读写头所受净升力为读写头的设计提供可靠的参考数据。 首先，将读写头复杂表面简化为平面并假定尺寸为微米（远小于真实的毫米），微米尺寸时可以得到DSMC的模拟结果用于比较。IP法对该问题的模拟结果与DSMC的模拟结果完全吻合。 其次，选定一个真实的读写头表面几何，但仍然假定尺寸为微米。IP法在该问题的应用过程中遇到四点困难，我们提出或采取了相应的解决办法。在复杂表面几何的读写头气膜润滑问题中，IP法的模拟结果与DSMC的模拟结果也相符一致。 最后完成真实几何、真实尺寸（毫米）的读写头气膜润滑问题的模拟，这也是本论文的最终研究目标。IP法模拟该问题时需要采用大网格，为此我们构造二维检验模型验证了大网格的合理性。 真实几何读写头气膜润滑问题的模拟结果表明，在流场的局部区域压力沿着高度方向变化显著，这意味着读写头问题的传统计算方法（即概括性雷诺方程）在该局部区域不成立，故用它计算得到的结果值得认真检验，但文献中尚未给出过这类检验。这正是该论文的研究背景和采用新方法（IP方法）的原因。 本论文研究的问题属于过渡领域、低速流动问题，故本论文还介绍了该领域另外一种粒子模拟方法（LVDSMC方法）以及我们对该方法提出的一些改进。 关键词：硬盘读写头，气膜润滑，概括性雷诺方程，IP方法，DSMC方法，过渡领域，LVDSMC方法

Kinetic Studies of Gas Flows

Our recent studies on kinetic behaviors of gas flows are reviewed in this paper. These flows have a wide range of background, but share a common feature that the flow Knudsen number is larger than 0.01. Thus kinetic approaches such as the direct simulation Monte Carlo method are required for their description. In the past few years, we studied several micro/nano-scale flows by developing novel particle simulation approach, and investigated the flows in low-pressure chambers and at high altitude. In addition, the microscopic behaviors of a couple of classical flow problems were analyzed, which shows the potential for kinetic approaches to reveal the microscopic mechanism of gas flows.