Experimental investigation on the influence of herringbone riblets on the incident shock/turbulent boundary layer interactions

Abstract

Shock/turbulent boundary layer interactions (STBLIs) seriously affect the structural safety and aerodynamic performance of supersonic/hypersonic engine inlets. Herringbone riblet (HR) is a promising passive flow control device, yet its influence on the incident STBLI remains unclear. In this paper, the influences are experimentally investigated in a Mach 3.4 wind tunnel. The results indicate that under the incident shock-induced adverse pressure gradient, HRs disrupt the smooth separation structure, yielding spanwise-varying separation zones. At the convergence line, the separation zone enlarges and shifts upstream, accompanied by an intensified near-wall recirculation within it and enhanced downward flow above its rear half. At the divergence line, sunken HRs reduce the size of the separation zone, whereas the protruding HRs slightly expand it. These topological changes within and downstream of the separation zone impart spanwise variations to the local compression waves, which in turn yields the separation shock with both spanwise variations and heightened unsteadiness. Compared with the sunken configuration, protruding HRs induce stronger spanwise variations in the boundary layer and shock-induced separation zone, along with greater dispersion of the separation shock. Furthermore, spectral proper orthogonal decomposition results suggest that without shock incidence, both HR configurations shift the pulsating energy of the turbulent boundary layer from low to high frequencies. With shock incidence, protruding HRs sustain low-frequency pulsations, whereas sunken HRs lead to a more uniform energy distribution. In both cases, the HRs extract energy from the mean flow and channel it into specific modal ranges depending on the background flow conditions.