How do vortex generators work and when are they used?

Vortex generators (VGs) are small vanes that create streamwise vortices to energize the boundary layer by mixing high-momentum outer flow with low-momentum near-wall flow. This delays separation, allowing surfaces to operate at higher angles or in stronger adverse pressure gradients. VGs are used on wings (especially near control surfaces), engine nacelles, and vertical tails to prevent or delay separation. While they add parasitic drag, the reduction in separation drag and improved control effectiveness often provides a net benefit. Typical VG height is boundary layer thickness.