An analysis of the structure of laminar separation bubbles in swept infinite geometries

The influence of sweep on the general structure of short separation bubbles in strictly laminar flow fields on swept infinite geometries is investigated by theoretical analysis and direct numerical simulations (DNS). In this situation the ‘independence principle’ of the Navier–Stokes equations for incompressible flow enforces a unique topology for the mean flow, which includes the much better understood unswept separation bubbles as a special case: swept laminar separation bubbles form leading edge parallel streamtubes with a spanwise outflow and a helical motion inside directed parallel to the separation line. If chordwise inflow conditions are kept constant, their cross-sections stay independent of a rising sweep angle, as the spanwise velocity field is then merely superimposed over the unchanged flow of the corresponding unswept case. Their mean flow field follows strict scaling rules that may be derived analytically from the generic 45°-solution, as confirmed by DNS-results for a series of pressure-induced separation bubbles subjected to a systematic variation of the sweep angle between 0° and 60°.