Investigations on Unsteadiness and Instability of Shock/Boundary Layer Interactions of the Ramp Flow by DNS

Abstract

Shock/boundary layer interaction is an important issue in the high-speed flow. The interaction may cause the boundary layer separation, the reduction of total pressure, and the flow unsteadiness. These phenomena will result in performance reduction, structural fatigue, and other unfavorable effects. With regard to unsteadiness, it is known from literatures that there are oscillations with specific low frequencies, which are found to be much lower than that of the incoming turbulent boundary layer. The phenomena are believed to relate to shock/boundary layer interaction (SBLI) and the separation caused by it. Although series of investigations have been made, the physical origin of the low-frequency oscillation is still open for discussion. Till now, at least three explanations have been reported. The first one still thought the unsteadiness of low frequency came from the incoming flow boundary layer, especially from the superstructures with long streamwise length scale [1, 2]. The second one thought that the unsteadiness came from the feedback mechanism of the separation bubble [3, 4]. The last one was that the unsteadiness resulted from the coherent instability of the separated flow [5]. In addition, some spanwise instabilities were also observed, which were characterized by irregular distributions in spanwise direction. Some studies [4] reported the owl eye-like pattern from the surface oil flow, and recent delicate experiments have shown the irregularity of the shock wave with certain length scale.