Laminar separation bubbles from a high Reynolds‐number asymptotic point of view

Classical boundary layer theory proved to lose its validity if flow separation is encountered. However, introduction of the triple deck concept – i.e. allowing for the displacement effect of the viscous boundary layer to interact locally with the induced pressure (disturbances) in the outer inviscid flow regime – led to a successful description of ‘short’ laminar separation bubbles in various fundamental flow problems. Here we want to focus on situations which share the following distinctive feature: two‐dimensional steady state solutions of the underlying interaction equations exist in a limited range of values of a parameter controlling the conditions leading to separation only. Furthermore, these solutions turn out to be non‐unique in the vicinity of the limiting critical value of this parameter. Typical examples include leading edge separation, smooth backward facing step flow, channel flow with suction (cases of so‐called marginal separation) and subsonic expansion ramp flow, asymmetric trailing edge separation (classical triple deck flows). More recent investigations which take into account (local) unsteady, three‐dimensional and flow control device effects and concentrate on near critical flows indicate that ( i ) exceeding the critical value of the controlling parameter is associated with self‐sustained vortex shedding (time periodic bubble ‘bursting’ even in the absence of external disturbances), ( ii ) coherent structure formation caused by bubble bursting events turns out to be essentially self‐similar, ( iii ) the high sensitivity of the separated flow region with respect to external disturbances allows for effective application of (optimized) flow control devices in order to force or delay the transition process to turbulent boundary layer flow. The aim of the presentation is to deliver insight into the power of modern asymptotic analysis. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)