The control of separation by periodic oscillations

It was recently demonstrated^9,15 tbat oscillatory blowing is an effective method for delaying separation from a lifting surface, much more so than the steady blowing traditionally used for this purpose. Experiments carried out on different airfoils revealed that this flow depends on many parameters such as; the location of the blowing slot, the steady and oscillatory momentum coefficients of the jet, the frequency of imposed oscillations and the shape and incidence of the airfoil. In airfoils equipped with slotted flaps, the flow is also dependent on the geometry of the slot and on the Reynolds number in addition to the flap deflection which is considered as a part of the airfoil shape. The incremental improvements in a single element airfoil characteristics are generally insensitive to a change in Reynolds number provided the latter is sufficiently large. It became clear that an effective control of separation must be further subdivided into two distinct tasks: 1 Preventing of a complete detachment of the flow from the surface. 2 Forcing a separated flow to change its direction and reattach to the surface. Each of the above flow conditions should be treated differently because the problem as a whole is bi-stable over a wide range of flow parameters which lead to hysteresis. The investigation is being expanded to novel designs of very thick airfoils which are only efficient when integrated with the active separation control systems, and to compressible flow. These aspects of the program are also discussed.