In transonic aircraft, vortex generators are usually mounted farther aft on an airfoil to minimize drag by energizing which layer?

• A. The boundary layer.
• B. The core flow.
• C. The wake.
• D. The free stream.

Answer: (A)

Vortex generators energize the boundary layer. In transonic flight, shocks can create strong adverse pressure gradients that make the near-wall fluid separate from the surface, dramatically increasing drag. The small vortices produced by the generators mix higher-momentum air from outside the boundary layer into the near-wall region, boosting its momentum and helping it resist detachment. Placing them farther aft targets the region where separation is most likely to occur on the airfoil, so the energized boundary layer stays attached longer, reducing drag. The other regions—core flow, wake, or free stream—aren’t the layers being energized in this mechanism.