Wall-shear stress fluctuations in a supersonic turbulent boundary layer over an expansion corner

The effect of wall expansion on the structural and statistical characteristics of wall-shear stress (WSS) fluctuations was investigated by direct numerical simulations of a supersonic turbulent boundary layer over a sharp expansion corner with various deflection angles (beta = 0(0), 2(0), 5(0)and 10(0)). It is found that the two-dimensional fields of WSS are characterised as streamwise-elongated streaky structures being aligned in the spanwise direction, resembling low- and high-speed streaks in the buffer region of the flow. Due to the relaminarization effect, these WSS steaks experience a sudden weakening shortly after the expansion corner, but present gradual regrowth with their length scales even exceeding those of the flat-plate case in the far downstream. A strong streamwise-alignment of the instantaneous WSS vector is evident in the case of the largest deflection angle, suggesting a distinct reduction of the intermittency in the relaminarization process. Furthermore, the characteristic time scale of the spanwise component of WSS is quasi-invariant to the expansion effect, while the peak frequency of the streamwise component increases with the increase of the deflection angle.