Shock receptivity: characteristics of shock oscillation modes and induced boundary-layer disturbances

The shock layer represents a region of complex wave interaction phenomena, characterised by the presence of different types of disturbances, including both waves coming from the freestream as well as waves produced by molecular oscillations of the shock. The induced post-shock waves interact with the boundary layer and may cause generation and growth of additional instability modes. The waves associated with the inner shock molecular collisions have been recently demonstrated to be linked to a bimodal behaviour of the particle-energy probability distribution function (PDF) inside the shock, and to cover a low-frequency range. In the present contribution, direct numerical simulation (DNS) of the compressible flow governing equations15, 21 are performed to simulate oblique shock oscillations, within a range of Mach numbers (M = 2 − 6), and at frequencies predicted by direct simulation Monte Carlo (DSMC) method,17 as well as to study the characteristics of the resulting waves propagating in the shock layer and in the boundary layer. It is found that postshock waves are radiated by the oscillating shock at the fundamental frequency, and that the interaction of these waves with the boundary layer generate modes with an amplified amplitude, including a progressive generation of higher harmonics of the fundamental wave as moving downstream. These, in turn, can synchronize with instability modes and promote their growth.