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

• Published in: The International journal of heat and fluid flow Vol. 115; p. 109866
• Main Authors: Larose, Emil, Kerhervé, Franck, Fraigneau, Yann, Podvin, Bérengère, Morton, Chris, Martinuzzi, Robert J.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.09.2025
• Subjects: Boundary layer; Forward-facing step; Laminar; Recirculation regions; Separated flow; Turbulent; Unsteady dynamics; Turbulent; Forward-facing step; Recirculation regions; Separated flow; Laminar; Boundary layer; Unsteady dynamics
• ISSN: 0142-727X
• DOI: 10.1016/j.ijheatfluidflow.2025.109866

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∗=fH/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. [Display omitted] •For thin oncoming boundary layers (δ/H<1), the upstream recirculation region exhibits three states in response to the oncoming flow conditions: unsteady, transitional-unsteady and steady.•For oncoming turbulent boundary layer, the upstream dynamics are dominated by a low-frequency expansion and contraction, related to the intermittent ejection of separated flow from the upstream recirculation region (unsteady).•For oncoming laminar boundary layer, the upstream dynamics are dominated by a very low-frequency oscillation of the recirculation region about a steady state (steady). The presence of weak freestream disturbances excites an instability of the upstream separated shear layer around f∗=fH/U∞≈0.15. If the Reynolds number is sufficiently high, this instability is amplified and the flow enters an unsteady transitional state (transitional-unsteady).•The downstream recirculation region dynamics are strongly affected by the behaviour of the upstream recirculation region.