Refer to figure.
The unfavorable characteristics associated with compressibility are due to boundary layer separation behind the shock wave (shock stall).
Flow separation occurs because the boundary layer loses kinetic energy as it flows against the adverse pressure gradient. Shock wave formation increases the adverse pressure gradient so the loss of kinetic energy in the boundary layer will be greater.
Increasing the kinetic energy of the boundary layer will reduce flow separation. Simple devices called vortex generators are used to re-energize the boundary layer.
Vortex generators are small plates, vanes, blades, or wedges mounted in spanwise rows along the wing surface.
Each vortex generator produces a vortex at its tip which will induce high energy air from the free stream flow to mix with the boundary layer, thus increasing its kinetic energy and helping it flow through the shock wave with much less separation.
Vortex generators are usually located on the upper wing surface, particularly ahead of control surfaces, but may be used anywhere where separation is causing high drag or reduced control effectiveness.
It should be noted that vortex generators may also be used on subsonic aircraft to prevent separation caused by high adverse pressure gradients due to the contours of the surface.
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