Originally Posted by Cubdriver

The wiki article is pretty good for a short summary but this is always a painfully abstract subject for those who do not worry about aerodynamics and prefer to leave it to the engineers. Boundary layers affect drag in a variety of ways. To bring home how significant this can be, think of a golf ball without dimples. It will not go half as far as one with, simply because the dimples force the air on the ball to remain turbulent which in turn energizes the flow field around the ball enough to affect its Reynolds number. Reynold's number is the ratio of viscous (syrupy) properties to inertial (mass times speed) in the flow. The higher the Re, the better the ability of the field to avoid separation from the body, thus minimizing drag.

It is sort of like "greasing" the air flow. Grease as we know is literally a viscous fluid, it sticks to things, but it acts like grease when placed in a wheel bearing which has the opposite effect. Same thing with vortex generators- they make the boundary layer turbulent on a tiny scale so the large scale flow is eased around the aerodynamic body. This makes the larger flow able to stay close to the aerodynamic body and do more useful work there.

An example. I was riding on a jungle jet yesterday near the wing in the passenger zone, and I saw it has VGs located upstream of the ailerons. The swept wing tends to experience lateral flow in addition to the normal (= 90 degrees) flow, and this lateral path tends to negate the usefulness of the ailerons. In turn, it is important to get the most out of the normal flow component and retain more aileron authority.

VGs are used a a tool for boundary layer transition management, but there are others. When I was in school I designed a heavy-lift STOL airplane that used boundary layer air pumps to retain flow at high angles of attack. It was a novel idea and not mine by the way, but one which can do amazing things for STOL aircraft.