Formation of two-layer structures in relaminarized turbulent boundary layers at supersonic expansion corner

Direct numerical simulations of supersonic turbulent boundary layers passing through an expansion corner with varying Reynolds number and Mach number conditions are conducted to investigate the relaminarization phenomena of turbulent boundary layers and the related formation of two-layer structures, in which the turbulence characteristics differ significantly between the inner and outer layers of the boundary layers. The inner and outer layer turbulence in the post-corner region are found to exhibit distinct characteristics in the magnitudes and length scales of the dominant velocity fluctuations.

In the near-corner region, phenomena indicative of relaminarization were observed, including the extension of the near-wall linear velocity profile and a decrease in the Reynolds stress components. Spectral analysis of the Reynolds stresses revealed that the outer layer turbulence downstream of the corner exhibits large velocity fluctuations in the low spanwise wavenumbers. Further analyses suggest that these fluctuations are related to the undulations of the expansion fan in the spanwise direction originating from the outer-layer streamwise-elongated large-scale structures. In contrast, the effects of the spanwise fluctuations of the expansion fan are observed to remain limited in the inner layer. The near-wall high-wavenumber velocity fluctuations rapidly regenerate shortly after the corner, leading to the differences in the turbulence characteristics between the inner and outer layers.