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 (β = 00, 20, 50 and 100). 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.

中文翻译：

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膨胀角上超音速湍流边界层的壁面剪切应力波动

通过对具有各种偏转角 (β = 00, 20, 50 和100）。发现WSS的二维场的特征是流向拉长的条纹结构在展向方向上排列，类似于流动缓冲区中的低速和高速条纹。由于再分层效应，这些 WSS 牛排在膨胀角后不久会突然减弱，但逐渐再生，其长度尺度甚至超过了远下游的平板案例。在最大偏转角的情况下，瞬时 WSS 矢量的强烈流向对齐是明显的，这表明重新分层过程中的间歇性明显减少。此外，WSS展向分量的特征时间尺度对膨胀效应是准不变的，而流向分量的峰值频率随着偏转角的增加而增加。