Effect of Boundary Layers on Pressures
In addition to direct skin friction drag, the presence of the boundary
layer changes the effective shape of the body, leading to changes in the
pressure distribution and to the overall lift and drag.
The effective shape can be used to approximate the effect of the boundary
layer using inviscid analysis methods combined with the boundary layer equations.
Outside the boundary layer, the flow behaves much like an inviscid (and
usually irrotational) fluid. Thus, we can use the simpler analysis methods
outside the boundary layer, and if we could compute the streamlines just
outside of the boundary layer we could use these as boundary conditions.
(This is actually easier said than done as the coupled boundary layer --
inviscid solutions are poorly conditioned numerically.)
This leads to changes in the lift, drag, and moment compared with the inviscid
solution. Note that this change in pressure distribution leads to a non-zero
pressure drag in addition to the skin friction drag discussed previously.
The sum of the skin friction and pressure drag is often termed "profile
drag."
As the angle of attack changes, the boundary layer shape changes, with thicker
boundary layers developing toward the aft part of the airfoil at higher
angles of attack (because of the more severe adverse pressure gradients).
The effective shape of the airfoil thus changes with angle of attack. If
we look at the mean line of the effective shapes, it is clear that viscous
effects cause an effective decambering of the airfoil shape. This leads
to changes in the lift curve slope (up to a 10% reduction in Cl at Reynolds
numbers in the millions) and an aerodynamic center that is usually farther forward
than is predicted by inviscid theory.
The effect is of increasing importance as Reynolds number is reduced.