Is it possible to have a separated boundary layer without having reversal flow?

Question

To fix the ideas, let's consider an airfoil instead of a generic shape.

As far as I understand, the separation of the boundary layer takes place after there is a region of reversal flow on the suction side of the airfoil.

In the other hand, I can find that the separation point is different from the point of reversal flow.

For me, this raises a question if it is possible to have a separation point on the suction side of the airfoil without having any reversal flow point? in other words, is it possible for the boundary layer to separate be it laminar or turbulent without having first a region of reversal flow? or maybe is the flow reversal a necessary condition for the boundary layer to separate?

Could you please provide me a pointer?

Thank you

Answer 1

I think I have a partial answer that I have found in a paper [1]:

The following is a quote from the introduction:

It is too narrow a view to use vanishing surface shearing stress or flow reversal as the criterion for separation. Only in steady two-dimensional flow do these conditions usually accompany separation. In unsteady two-dimensional flow the surface shear stress can change sign with flow reversal without the occurrence of breakaway. Conversely, the breakdown of the boundary-layer concept can occur before any flow reversal is encountered.

[1]: Simpson, R. L. (1989). Turbulent boundary-layer separation. Annual Review of Fluid Mechanics, 21(1), 205-232.