A criteron for the existence of local rarefaction effect in a hypersonic flow field and the corresponding flow characteristics

Computational Fluid Dynamics (CFD) plays a very important role in the development of near space flying vehicles. However, at places where the rarefaction effect of the gas has to be considered, the conventional CFD has to be modified accordingly. Then the first question one has to answer is what would be the appropriate criterion for assessing if the rarefaction effect should be taken into consideration. And then what is the main manifestation of the rarefaction effect and how to consider this effect in CFD? In this paper, a typical hypersonic shear flow is studied by Direct Simulation Monte Carlo (DSMC). The probability density function and shear stress are investigated in details for different degrees of rarefaction and different shear strengths. A parameter Zh which can characterize the rarefaction effect is proposed, and the difference between the shear stresses obtained by the conventional continuum model and those obtained by DSMC is found to be increasing as the parameter Zh is increasing but follows a definite rule.