Abstract: A system for controlling unwanted flow separation. One or more microjets are placed to feed auxiliary fluid into a region of suspected flow separation on a surface. If the separation is intermittent, sensors can be employed to detect its onset. Once separation is developing, the microjets are activated to inject a stream of high-velocity gas in a direction that is transverse to the prevailing flow. This injected stream forms counter-rotating vortices in the prevailing flow and thereby transfers momentum from the prevailing flow into the boundary layer proximate the surface.