Characters of the Recirculation Zone of Backward Facing Step for Hypersonic in Near Space

The ambient air of backward facing step in near space is rarefied; consequently, the traditional numerical simulation methods based on continuous fluid hypothesis are not applicable in near space. For the backward facing step located in near space, the characteristic quantities of its recirculation zone, such as the reattachment point, reattachment length, and inclination angle were obtained by Direct Simulating Monte Carlo (DSMC) algorithm. A new ray tracing technique based on computational grid and direct simulation is proposed for computing the aero-optical aberration of recirculation zone. Based on the computational results obtained by employing DSMC and ray tracing method for several cases of different hypersonic Maher numbers in near space, analysis and discussion are performed and conclusions drawn from this paper are also made, as follows: (1) The length of recirculation zone of backward facing step is significantly shortened for hypersonic in near space, which indicates that the recirculation zone is severely squeezed by the shear flow induced by the separation of inflow as traversing the backward facing step. (2) The effect of Mach number on Strehl Ratio can be neglected if the altitude exceeds 40 Km. Specifically, the aero-optical distortion of the recirculation zone is significant for hypersonic if the altitude under 30 Km, whereas it is tiny and approximately invariable for hypersonic if the altitude exceeds 40 Km