On the transition zone in shock wave/boundary layer interactions

According to the state of incoming boundary layer, shock wave/boundary layer interaction can be classified into three groups: shock wave/laminar boundary layer interaction (SLBLI), shock wave/transitional boundary layer interaction (STrBLI) and shock wave/turbulent boundary layer interaction (STBLI). Although STBLI has been comprehensively studied, the difference between SLBLI and STrBLI is still not fully understood, especially in the framework of laminar–turbulent transition. In this work, a compression-ramp flow at Mach 6.5 is studied using experiment, direct numerical simulations (DNS) and stability analysis. Two distinct transition scenarios are revealed. On the one hand, without introducing external disturbances in the DNS, transition to turbulence is observed downstream of reattachment. This transition process is excited by the global instability in the separation bubble. On the other hand, the experiment captures a much smaller separation bubble under the same flow conditions. When realistic disturbances are superimposed on the freestream of DNS, a good agreement between the DNS and the experiment is achieved. The first mode is shown to be responsible for the transition. It is found that in the first transition scenario, the transition zone is located downstream of the reattachment region, and as a result, the size of the separation bubble is nearly not affected by the transition. This identifies an SLBLI. In the second transition scenario, however, the reattachment region is fully covered by the transition zone, accompanied by the shrinking of the separation bubble. This features an STrBLI. Therefore, the relative position between the reattachment region and the transition zone can be used to identify whether a flow belongs to SLBLI or STrBLI.