Hi, in unsteady boundary layer flows there seems to be some confusion about determining the point of laminar separation followed by turbulent reattachment. As for separation, the criterion of vanishing wall shear stress no longer holds, and surprisingly little is found in the literature about the topic. Who can help? Regards, Jens.

Hi, Jens,

Try to find a book by Sychev V.V., Ruban A.I., Sychev Vik.V, and Korolev G.L., ed. Sychev V.V., title approximately - Asymptotic theory of separated flows. But it is heavy with mathematics, the reader is assumed to be quite familiar with the method of matched asymptotic expansions. Further references can be found there.

For unsteady flows, instead of zero skin friction, the Moore-Rotta-Sirs (not at all sure about the spelling, usually abbreviated as MRS) criterion is used: laminar boundary layer separates at the point where both u and du/dn are zero. Such a point appears, however, in the bulk of the boundary layer and not on the wall, when the separation point moves upstream. Little is known in case it moves downstream.

Rgds, Sergei

Small correction, or additional explanation. For MRS, u=0 implies u measured in the reference system based on the separation point itself. In other words, the system of reference is such that the separation point is at rest but the body, and, hence, the wall is moving.