In this paper theoretical analysis of transonic flow separation from a rigid body surface is presented. Two forms of separation are considered. The first one is observed when the boundary layer separates from a corner point of a rigid body contour, say, an aerofoil. The second takes place on a smooth part of the surface. For both cases the flow behaviour is studied based on the asymptotic analysis of the Navier-Stokes equations. In this analysis the Reynolds number is assumed large, and the Mach number of the inviscid flow at the separation point is close to one. We found that the flow separating from a corner is driven towards the separation by inviscid-inviscid interaction between the boundary layer and inviscid external flow. Meanwhile separation on a smooth surface is accompanied by a more traditional viscous-inviscid separation. However, unlike in subsonic or supersonic flow, the boundary layer immediately upstream of the interaction region has a preseparated form. This results in a hysteresis character of the flow behaviour in the interaction region.
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