This invention relates to a method of controlling vortex bursting on an aerodynamic surface (20) associated with separated flows and, in particular, relates to control of separated flows over aerodynamic or hydrodynamic surfaces (20) that may have highly swept leading edges (26). A method of controlling vortex bursting on an aerodynamic surface or a hydrodynamic surface (20) is provided, the surface (20) comprising a gas source (22) located on or in the surface (20) and the method comprising the step of repeatedly operating the gas source (22) thereby to eject a flow of gas into an airflow passing over the surface (20). Effective control of the frequency at which the gas source (22) is operated has been found to reduce pressures on the surface (20) caused by vortex bursting. The present invention also provides a synthetic jet actuator (22) and an aerodynamic or hydrodynamic surface (20) comprising a plurality of such discrete synthetic jet actuators (22).