  The low-frequency unsteady motions behind a backward-facing step (BFS) in a
turbulent flow at $Ma=1.7$ and $Re_\infty=1.3718\times 10^5$ is investigated
using a well-resolved large-eddy simulation (LES). The instantaneous flow field
illustrates the unsteady phenomena of the shock wave/boundary layer interaction
(SWBLI) system, including vortex shedding in the shear layer, the flapping
motions of the shock and breathing of the separation bubble, streamwise streaks
near the wall and arc-shaped vortices in the turbulent boundary layer
downstream of the separation bubble. A spectral analysis reveals that the
low-frequency behaviour of the system is related to the interaction between
shock wave and separated shear layer, while the medium-frequency motions are
associated with the shedding of shear layer vortices. Using a three-dimensional
dynamic mode decomposition (DMD), we analyse the individual contributions of
selected modes to the unsteadiness of the shock and streamwise-elongated
vortices around the reattachment region. G\"ortler-like vortices, which are
induced by the centrifugal forces originating from the strong curvature of the
streamlines in the reattachment region, are strongly correlated with the
low-frequency unsteadiness in the current BFS case. Our DMD analysis and the
comparison with an identical but laminar case provide evidence that these
unsteady G\"ortler-like vortices are affected by fluctuations in the incoming
boundary layer. Compared to SWBLI in flat plate and ramp configurations, we
observe a slightly higher non-dimensional frequency (based on the separation
length) of the low-frequency mode.
