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

Abstract

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.

KEYWORDS:

• Wall-shear stress
• expansion corner
• supersonic turbulent boundary layer
• direct numerical simulation

Acknowledgements

We are deeply grateful to Dr. Chong Pan from Beihang University for his fruitful discussion and help in improving the quality of this paper.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the National Key Research and Development Program of China Natural Science Foundation of China Key Research and Development Program of China (2019YFA0405300, 2016YFA0401200), NSFC Projects (11972356, 91852203), National Numerical Windtunnel project, Science Challenge Project (TZ2016001), and Strategic Priority Research Program of Chinese Academy of Sciences (XDC01000000).