Abstract: An acoustic jet disposed within an aerodynamic surface, such as a wing or a blade, has a resilient wall supporting a mass. Vibrations of the blade cause oscillatory pressure waves within the acoustic jet, the nozzle of which directs fluid particles having high momentum flux essentially tangentially into the boundary layer of the suction surface of the blade, the resonant cavity of the synthetic jet being replenished with particles having low momentum flux drawn from the flow in a direction normal to the surface, thereby to provide a net time averaged flow of fluid particles of increasing momentum flux into the boundary layer to defer or prevent the onset of boundary layer separation. Single and double chambers drive nozzles separated streamwise or spanwise on airfoils (blades, wings) and fuselages. Applications include helicopters, airplanes, air moving machines and wind energy electric power generators.