On the use of lagrangian variables in descriptions of unsteady boundary-layer separation

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Abstract
The lagrangian description of unsteady boundary-layer separation is reviewed from both analytical and numerical perspectives. We explain in simple terms how particle distortion gives rise to unsteady separation, and why a theory centred on lagrangian coordinates provides the clearest description of this phenomenon. Included in the review are some of the more recent results for unsteady three-dimensional compressible separation. The different forms of separation that can arise from symmetries are emphasized. Current work includes a possible description of separation when the detaching vorticity layer exits the classical boundary-layer region, but still remains much closer to the surface than a typical body length-scale.