Message via Patreon from Steve Bussolari

[Paragraph breaks add by Jack.]

In your comments on laminar vs turbulent flow, you mentioned several times the notion of "separation" or "attached" flow over a wing.

It is important to distinguish between flow separation (which is a large scale phenomenon that can occur in either laminar or turbulent flow) and the transition from laminar to turbulent flow which is fundamentally a flow stability phenomenon.

Turbulent flow is characterized by a churning motion consisting of many very small eddies. The classic (if somewhat dated) example of the transition from laminar to turbulent flow can be seen in the smoke plume from a cigarette in a still room. the initial plume is smooth (laminar) but becomes unstable and breaks up into many smaller eddies.

A boundary layer can transition from laminar to turbulent without causing separation. In fact, transition to turbulence is often used to keep a flow attached. The classic example of this is the dimples on a golf ball.

A smooth golf ball will not fly nearly as far as a dimpled one because the turbulent boundary layer caused by the dimples stays attached further back on the ball surface and the ball has less drag.

Fowler flaps and vortex generators are also examples of injecting high energy flow into the boundary layer to keep it attached. The boundary layer suction wing that you mentioned just pulls the low energy boundary layer flow away and brings the higher energy flow closer to the wing surface to keep the flow attached.