Effects of oncoming flow conditions on the flow upstream of a forward-facing step subject to a thin boundary layer
Résumé
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 ∫ * = ∫ H/U∞ ≈ 0.03 and the recirculation region exhibits a natural instability around ∫ * ≈ 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.
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