Effect of compressibility on the small-scale structures in hypersonic turbulent boundary layer

Abstract

The effects of the wall temperature and local compressibility on the small-scale structures, local flow topology, and enstrophy production in the hypersonic turbulent boundary layer are numerically investigated. The colder wall temperature significantly enhances the unstable topologies and non-focal topologies near the wall, mainly due to stronger compressibility and more sheet-like structures. The eigenvalue decomposition of the strain rate tensor is introduced to investigate the mechanism of the enstrophy production and small-scale structures. It is shown that in the near wall region, the enstrophy production is mainly governed by the intermediate eigenvalue of strain-rate tensor through flow topologies unstable node/saddle/saddle (UN/S/S), stable focus/stretching (SFS), unstable focus/stretching, and unstable node/unstable node/unstable node, while the enstrophy destruction is primarily contributed by the intermediate eigenvalue through flow topologies stable node/saddle/saddle, stable focus/compressing (SFC), and stable node/stable node/stable node. Moreover, in the far wall region, the third eigenvalue in flow topology SFS and the intermediate eigenvalue in UN/S/S make major contributions to the enstrophy production, while the first eigenvalue in flow topologies unstable focus/compressing and SFC, and the intermediate eigenvalue in SFC mainly lead to the enstrophy destruction.