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

摘要

The ambient air of backward facing step in near space is rarefied;rnconsequently, the traditional numerical simulation methods based onrncontinuous fluid hypothesis are not applicable in near space. For thernbackward facing step located in near space, the characteristic quantities ofrnits recirculation zone, such as the reattachment point, reattachment length,rnand inclination angle were obtained by Direct Simulating Monte Carlorn(DSMC) algorithm. A new ray tracing technique based on computationalrngrid and direct simulation is proposed for computing the aero-opticalrnaberration of recirculation zone. Based on the computational resultsrnobtained by employing DSMC and ray tracing method for several cases ofrndifferent hypersonic Maher numbers in near space, analysis and discussionrnare performed and conclusions drawn from this paper are also made, asrnfollows: (1) The length of recirculation zone of backward facing step isrnsignificantly shortened for hypersonic in near space, which indicates thatrnthe recirculation zone is severely squeezed by the shear flow induced by thernseparation of inflow as traversing the backward facing step. (2) The effectrnof Mach number on Strehl Ratio can be neglected if the altitude exceeds 40rnKm. Specifically, the aero-optical distortion of the recirculation zone isrnsignificant for hypersonic if the altitude under 30 Km, whereas it is tiny andrnapproximately invariable for hypersonic if the altitude exceeds 40 Km.