Hu, W., Hickel, S., van Oudheusden, B.W. (2022)
Journal of Fluid Mechanics 949: A2. doi: 10.1017/jfm.2022.737

The flow over a forward-facing step (FFS) at MaÔł×=1.7 and Re­ŁŤ┐ = 13 718┬áis investigated by well-resolved large-eddy simulation. To investigate effects of upstream flow structures and turbulence on the low-frequency dynamics of the shock wave/boundary layer interaction (SWBLI), two cases are considered: one with a laminar inflow and one with a turbulent inflow.

The laminar inflow case shows signs of a rapid transition to turbulence upstream of the step, as inferred from the streamwise variation of Cf and the evolution of the coherent vortical structures. Nevertheless, the separation length is more than twice as large for the laminar inflow case, and the coalescence of compression waves into a separation shock is observed only for the fully turbulent inflow case.

The dynamics at low and medium frequencies is characterized by a spectral analysis, where the lower frequency range is related to the unsteady separation region, and the intermediate one is associated with the shedding of shear layer vortices. For the turbulent inflow case, we furthermore use a three-dimensional dynamic mode decomposition to analyse the individual contributions of selected modes to the unsteadiness of the SWBLI. The separation shock and G├Ârtler-like vortices, which are induced by the centrifugal forces in the separation region, are strongly correlated with the low-frequency unsteadiness in the current FFS case.

Based on the above observations and analysis, we consider that the physical mechanism of the low-frequency unsteadiness in the FFS is very similar to the one for other canonical SWBLI cases. Similarly as observed previously for the backward-facing steps, we observe a slightly higher non-dimensional frequency (based on the separation length) of the low-frequency mode than for SWBLI in flat plate and ramp configurations.

Instantaneous vortical structures visualized by isosurfaces of  the lambda2 criterion. A numerical schlieren is included in the background. A snapshot of the case with laminar inflow is shown (a) upstream of the separation bubble, (b) close to the bubble, and (c) shows a snapshot of the fully turbulent case.