This article presents a review of some recent work that deals with the phenomenon of unsteady separation on two- and three-dimensional lifting surfaces. The presently available experimental data are interpreted in light of the recent idea advanced by Ho (1986), in which unsteady separation is associated with a spatially developing local shear layer. No attempt is made to give a complete account of the vast number of papers written in the subject area. Instead, reference is made to a few excellent review articles available in the open literature. The unsteady motions considered include change of angle of attack, impulsive start from rest, and change of freestream velocity. The lifting surfaces studied are bodies of revolution and wings of different aspect ratios, planforms, and leading edge bluntness. Other, nonlifting surfaces are briefly considered. Velocity probe measurements in such complex flow fields are sparse. However, flow visualization results are ample and are extensively reviewed in this paper.

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