Abstract: Shock wave/Turbulent Boundary Layer Interaction(STBLI) is ubiquitous in internal and external of supersonic and hypersonic flow. The physical mechanism of low-frequency unsteadiness, which is found in shock-induced separation and appears as low-frequency shock motion accompanied with the expansion and contraction of the separation bubble, has been disputed. This research field has been widely concerned and studied. The driving mechanism of low-frequency unsteadiness could be generally divided into three categories. Some researchers believed that the source of the low-frequency unsteadiness originated in the upstream boundary layer. On the contrary some scholars held the opinion that the low-frequency dynamics was dominated by intrinsic nature of downstream separation flow. However, some recent researches trended to reconcile these two opposite views, believing that the upstream and downstream mechanisms co-existed with a weighting function depending on the state of the STBLI. The coupling between shock wave and boundary layer was assumed as a dynamics system, which could be represented as a first-order low-pass filter. This system responded selectively to the perturbations below certain frequency regardless of whether they came from upstream or downstream. The above three physical mechanisms are reviewed respectively. In addition, some research areas which need further attentions are present based on the existing results and the authors' knowledge.

Key words: shock wave, turbulent boundary layer, interaction, flow separation, low-frequency unsteadiness