Oscillation of the shock train in an isolator with incident shocks

Abstract

The oscillation characteristics of the shock train in an isolator have been investigated in a direct-connect wind tunnel at Mach 2.7. High-speed schlieren imaging and high-resonance frequency pressure measurements were used to capture the flow features during the shock train movement. The oscillation features without the effects of incident shocks were analyzed first. As the shock train moved upstream, the low-frequency part of the oscillation was found to develop. The analysis was then extended to complex situations with incident shocks. It was revealed that the shock wave-boundary layer interactions considerably influence the shock train behavior. The interactions were classified into three patterns: (I) single interaction, (II) multi-interactions on the same side, and (III) multi-interactions on different sides. Experimental results indicated that the oscillation could be affected in temporal scale by pattern II and enhanced in spatial scale by pattern III. The data also showed that the pressure rise induced upstream propagates to the exit, causing phase offsets in the wall pressure histories and making the pressure distributions diverge from their stable state. This phenomenon suggested a possible physical mechanism for the oscillation during shock train movement, which was verified by additional tests with large backpressure rising rate. It was found that there exists a critical frequency which is related to the pressure ramping rate during the oscillation. If the dominant frequency of the backpressure varies beyond this critical frequency, the pressure distribution could be forced into a steady state before the oscillation was induced. Otherwise the oscillation could not be suppressed.