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Hypersonic vehicles travel across the atmosphere at very high speed, and the surrounding gas experiences complicated physical and chemical processes. These processes produce real gas effects at high temperature and rarefied gas effects at high altitude where the two effects are coupled through molecular collisions. In this study, we aim to identify the individual real gas and rarefied gas effects by simulating hypersonic flow over a 2D cylinder, a sphere and a blunted cone using a continuum-based CFD approach and the direct simulation Monte Carlo method. It is found that physical processes such as vibrational excitation and chemical reaction will reduce significantly the shock stand-off distance and flow temperature for flows having small Knudsen number. The calculated skin friction and surface heat flux will decrease when the real gas effects are considered in simulations. The trend, however, gets weakened as the Knudsen number increases. It is concluded that the rarefied gas effects weaken the real gas effects on hypersonic flows. [ABSTRACT FROM AUTHOR]

A hybrid continuum/particle approach is proposed for micro scale gas flows in this paper. The approach couples the DSMC-IP method and a Navier-Stokes solver with an adaptive interface. The continuum solver uses the particle cells as ghost cells because the IP method preserves the hydrodynamic information that the continuum solver uses. In order to generate particles from the continuum side, two strategies are proposed. The first one uses a condition similar to the Marshak condition in generating particles through the interface. The second strategy adopts buffer cells and reservoir cells, which avoids directly generating particles. The interface is determined by a continuum breakdown parameter that is evaluated in every time step. In order to track the interface, a mapping technique is used in the code. Numerical examples show that the hybrid approach couples the continuum solver and the particle method very smoothly. Simulated results also show the effects of the cutoff value of the continuum breakdown parameter. [ABSTRACT FROM AUTHOR]

In this work we present numerical results for the problem of 'high' (of order unity) Knudsen number gas flow past a micro-airfoil, for low flow velocity. The results are generated using an enhanced version of the transition probability matrix (TPM) method. The TPM is a non-statistical kinetic method [1] for computing neutral particle transport in high Knudsen number flows. The problem of high Knudsen number, low Mach number gas flow has been studied in the past using several computational approaches, such as the Information Preservation (IP) method [2] and the direct simulation Monte Carlo (DSMC) method [2]. For low Mach numbers, the DSMC approach suffers from statistical noise [3]. The IP method extends the range of the particle method by reducing the statistical noise of the approach. The need for a method which is capable of describing the particle distribution function for high Knudsen number flows at low flow velocities has led to an investigation of alternative kinetic approaches, such as the IP[4]. In this paper we present an altogether different approach to the problem of statistical noise, the transition probability matrix (TPM) method [1, 5, 6, 7]. We give a brief overview of the TPM method, and compare its strengths and weaknesses to those of the IP and DSMC methods. Finally, we present results for the micro-plate and compare them to the results generated by both the IP and DSMC methods. [ABSTRACT FROM AUTHOR]