Mechanics

The flow separation that is responsible for lack of pressure recovery is illustrated opposite. This boundary layer separation is occurring on the rear surface of a streamlined shape where the flow is expanding due to the curvature of the solid surface. Due to this expansion, the flow velocity is decreasing and the pressure is increasing. This process sets up an adverse pressure gradient such that each fluid element experiences a force opposing its motion in the direction of the flow. Fluid elements close to the solid surface have a low momentum and can be brought to rest or moved in a counterflow direction (red zone on the diagram) by the inverse pressure gradient.

From: Wegener,
"What Makes Airplanes Fly?"
Springer-Verlag (1991)

Flow separation starts at the point that the shear force (wall friction) on the near surface fluid goes to zero (i.e. the velocity gradient goes to zero). Past this point, near surface flow may be in a direction opposite to the free stream flow and the streamlines shown illustrate this reverse flow. Because the initial velocity gradient is lower in a laminar boundary layer than in an equivalent turbulent boundary layer, the latter can remain attached further downstream. As long as the boundary layer is attached pressure recovery continues. The turbulent boundary layer, therefore, permits more pressure recovery and hence reduces pressure drag. This pressure drag reduction more than compensates for the increased skin friction of the turbulent boundary layer.