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The low-frequency unsteady motions behind a backward-facing step (BFS) in aturbulent flow at $Ma=1.7$ and $Re_\infty=1.3718\times 10^5$ is investigatedusing a well-resolved large-eddy simulation (LES). The instantaneous flow fieldillustrates the unsteady phenomena of the shock wave/boundary layer interaction(SWBLI) system, including vortex shedding in the shear layer, the flappingmotions of the shock and breathing of the separation bubble, streamwise streaksnear the wall and arc-shaped vortices in the turbulent boundary layerdownstream of the separation bubble. A spectral analysis reveals that thelow-frequency behaviour of the system is related to the interaction betweenshock wave and separated shear layer, while the medium-frequency motions areassociated with the shedding of shear layer vortices. Using a three-dimensionaldynamic mode decomposition (DMD), we analyse the individual contributions ofselected modes to the unsteadiness of the shock and streamwise-elongatedvortices around the reattachment region. G\"ortler-like vortices, which areinduced by the centrifugal forces originating from the strong curvature of thestreamlines in the reattachment region, are strongly correlated with thelow-frequency unsteadiness in the current BFS case. Our DMD analysis and thecomparison with an identical but laminar case provide evidence that theseunsteady G\"ortler-like vortices are affected by fluctuations in the incomingboundary layer. Compared to SWBLI in flat plate and ramp configurations, weobserve a slightly higher non-dimensional frequency (based on the separationlength) of the low-frequency mode.

Low-frequency unsteadiness mechanisms in shock wave/turbulent boundary layer interactions over a backward-facing step