Near-wall dynamics of compressible boundary layers

The coherent structures populating the inner-region of a compressible boundary layer with free-stream Mach number equal to 2.5 are analyzed by means of direct numerical simulations of the Navier-Stokes equations. This study shows similarity with the incompressible case in the sense that turbulence in the near-wall region can be sustained without fluctuations in the outer region, proving the existence of a local cycle within the near-wall region. The dynamics are further simplified by making use of the coherence of the inner region. The wall-normal velocity component in this region is split into two: one coherent part representing vortices spanning all the inner-region, and one incoherent part representing the background turbulence. By damping the latter part, the statistical features of the flow are only slightly influenced, showing that the coherent part is essential in determining the flow characteristics. Flow dynamics and turbulence structures within this coherent part are examined. It is shown that the near-wall region is populated by crescent-shaped vortical structures, which are associated with regions with strong positive Reynolds shear stress production. The structure of the flow associated with these regions shares several features with the so-called internal shear layers. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3600659]