Buffet and buffeting control in transonic flow

Abstract

In transonic flow conditions, the shock wave/turbulent boundary layer interaction and the flow separations on the upper wing surfaces of a civil aircraft induce flow instabilities, “buffet” and then structural vibrations, “buffeting”. Buffeting can greatly affect aerodynamic behavior. The buffeting phenomenon appears when the aircraft's Mach number or angle of attack increases. This phenomenon limits the aircraft's flight envelope. The objectives of this study are to cancel out or decrease the aerodynamic instabilities (unsteady separation, movement of the shock position) due to this type of flow by using control systems. The following actuators can be used, “Vortex Generators” situated upstream of the shock location and a new moving part designed by ONERA, situated at the trailing edge of the wing, the “Trailing Edge Deflector” or TED. It looks like an adjustable “Divergent Trailing Edge”. It is an active actuator and can take different deflections or be driven by dynamic movements up to 250 Hz. Tests were performed in transonic 2D and 3D flow with models well equipped with unsteady pressure transducers. For high lift coefficients, selected deflections of the “Trailing Edge Deflector” increase the wing's aerodynamic performances and delays the onset of “buffet”. Furthermore, in 2D flow “buffet” condition, the “Trailing Edge Deflector”, driven by a closed-loop active control using the measurements of the unsteady wall static pressures, can greatly reduce “buffet”. In 3D flow “buffeting” conditions, the 2D flow control principle is available but some differences must be considered. Vortex generators have a great impact on the separated flows. The separated flow instabilities are greatly reduced and the buffet is totally controlled even for strong instabilities. The aerodynamic performances of the airfoil are also greatly improved.