Quasi-steady shock refraction phenomenon at a diffusive fast–slow interface

The quasi-steady shock refraction at a diffusive air–SF $_6$ interface (fast–slow type) is investigated numerically and theoretically. A new refraction pattern where both shock and expansion waves are simultaneously present in the reflected waves (named RRR-E) is first observed at the diffusive interface. The new refraction pattern is a regular pattern that is not expected to occur in classical shock refraction at a sharp fast–slow interface. Through the shock polar method, continuous refraction processes occur within the diffusion layer to satisfy the kinematic relationship between the reflected wave and the transmitted shock, which results in the RRR-E formation. Subsequently, the conditions for the RRR-E occurrence are obtained theoretically and verified numerically. In the phase diagram of the refraction patterns, the presence of RRR-E results in the transition boundaries of different refraction patterns at the sharp fast–slow interface no longer being valid. Specifically, the appearance of RRR-E delays the Mach reflection refraction (MRR) process, which is of great significance for the design of scramjet engines.