Unsteady Separation Processes at High Reynolds Number and Their Control

Physical situations where a viscous boundary layer breaks down and interacts strongly with an effectively inviscid external flow are common place. For large Reynolds numbers, viscous effects are normally confined to thin boundary layers on all solid surfaces for the majority of any observation time. In most practical situations, exposure of such layers to an adverse pressure gradient is inevitable and in this circumstance, a sequence of events commences near the wall that culminates in an eruption and a strong viscous-inviscid interaction with the external flow. The events leading up to eruption are known as the Van Dommelen–Shen process and the eruption itself is referred to as boundary-layer separation; here the term ‘separation’ denotes the first process of interaction between a hitherto thin boundary layer and the external flow. The event is sufficiently complicated that extraordinary measures are needed to compute its evolution. In most situations, the onset of separation is subtle and hard to detect and thus development of rational control procedures is a challenging task. Here recent calculations of unsteady separation events are discussed for two- and three-dimensional flows. The phenomena involved are generic but leading-edge separation on airfoils and rotorcraft blades is emphasized. Recent studies on various control mechanisms are described, which are found to have the effect of slowing down and/or weakening the separation process. For some control processes, it has proved possible to eliminate separation entirely.