Separation

When the flow near the surface reverses its direction and flows upstream, there must be a place, generally a bit farther upstream, where streamlines meet and then leave the surface. This is separation and it is caused by the presence of an adverse pressure gradient. When this occurs, the assumptions that the u component of velocity is larger than the v component and that certain derivatives in the x direction may be ignored, no longer are valid. Thus, coupling an inviscid analysis with a simple boundary layer calculation does not work. One must resort to experiment or Navier-Stokes solutions.

The changes in the flow pattern, and associated forces and moments are large. Drag usually increases substantially and airfoil lift usually drops. The effect is generally Reynolds number dependent.

The figure above shows how the flow pattern and pressure distribution is affected by separation. On the left, the pressures are modified slightly by the boundary layer; in the center image, separation near the trailing edge has reduced the Cp and lift; leading edge separation dramatically reduces the suction peaks and reduces lift.

The presence of an adverse pressure gradient (increasing pressure) causes a deceleration of the fluid. Just as when one coasts uphill, the fluid that starts up the (pressure) hill with little speed, starts rolling backward after a while.

This picture explains why flow does not separate as readily at higher Reynolds numbers. In that case, the velocity profile is "fuller" with the high external velocities extending down closer to the surface. Turbulent boundary layers also have greater velocity near the surface and are therefore better able to handle adverse pressure gradients.

Since the velocity near the surface in a laminar boundary layer has lower velocity than its turbulent counterpart, the laminar boundary layer is more likely to separate. When this occurs, the laminar boundary layer leaves the surface and usually undergoes transition to turbulent flow away from the surface. This process takes place over a certain distance that is inversely related to the Reynolds number, but if it happens quickly enough, the flow may reattach as a turbulent boundary layer and continue along the surface.

This phenomenon has significant effects on airfoil pressure distributions at low Reynolds numbers.

To compute when separation will occur, we can solve the Navier-Stokes equations or apply one of several separation criteria to solutions of the boundary layer equations.
Laminar Separation Criteria
Turbulent Separation Criteria