Effects of oncoming flow conditions on the flow upstream of a forward-facing step subject to a thin boundary layer

The influence of the oncoming flow on intrinsic flow dynamics of a forward-facing step (FFS) is investigated using planar, two-component particle image velocimetry (PIV) and direct numerical simulations (DNS) for thin ( δ / H < 1 ) laminar and turbulent boundary layers (LBL & TBL). Transition of the upstream separation region between steady and unsteady behaviour is shown to be sensitive to weak freestream disturbances above a critical Reynolds number (Re). Unsteady behaviour of the upstream separation, for oncoming TBLs, is characterised by the intermittent large-scale ejection of fluid over the step. Steady behaviour is observed for oncoming LBLs below a critical Re or disturbance-free. It is characterised by a low-frequency oscillation of the separating shear layer below f ∗ = f H / U ∞ ≈ 0 . 03 and the recirculation region exhibits a natural instability around f ∗ ≈ 0 . 15 . Above a critical Re, this instability can be amplified by very small freestream disturbances and the behaviour of the upstream recirculation region becomes transitional-unsteady. The dynamics in this state – between steady and unsteady – exhibit intermittent ejections along with enhanced convective dynamics due to the formation of larger-scale motions at the unsteady laminar separation point. Unlike immersed steps ( δ / H > 2 ), scales from the oncoming boundary layer are not imprinted on the separation region, requiring a more complex description relating the oncoming flow to the recirculation dynamics about the step.