Unsteadiness is a common feature of shock-wave/boundary-layer interactions. When separation is present, the separated shear layer is unstable, leading to highly energetic flow features. In addition, broadband unsteadiness is commonly observed at an order of magnitude smaller frequency compared to the shear layer structures. In such situations, for example in ramp or shock impingement configurations, there is usually a closed separation bubble, which is observed to ‘breathe’ at this low frequency, increasing and decreasing in  size as the separation shock wave moves upstream and downstream. Multiple physical phenomena  are present in these flows and some aspects may be by-products of the unsteadiness, rather than the  fundamental cause. In this contribution, we will consider shock-related separation problems in a  number of different configurations and extract some of the common features, as well as noting where  there are significant differences between configurations. We consider upstream influences, in cases  with an upstream turbulent boundary layer, as well as downstream influences in cases with laminar turbulent transition cases and in shock trains. We conclude by pointing to some differences between  shock-impingement in supersonic flows and transonic buffet, where global modes appear to coexist  with local separation bubble modes.

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