Numerical study on closed-loop control of transonic buffet suppression by trailing edge flap

• A closed-loop control strategy is proposed to suppress transonic buffet.
• Effects of the balanced lift coefficient, gain, and delay time are investigated.
• Buffet can be completely suppressed when the phase lead is about 50 degrees.
• Unstable steady state can be obtained based on this control law.

Oscillating aerodynamic loads induced by buffet in transonic flight may cause structural fatigue and flight accidents, making transonic buffet control a research focus in aeronautical engineering. A study based on the Unsteady Reynolds-averaged Navier–Stokes equations and the Spalart–Allmaras turbulence model is proposed to decrease or suppress the buffet with a closed-loop control strategy. Buffet onsets and buffet loads of a stationary airfoil are verified by experimental data. A linear-delayed control law with a feedback signal of lift coefficient is designed. Its correlation parameters, namely the balanced lift coefficient, gain, and delay time, are investigated for a NACA0012 airfoil. It is found that the optimum balanced lift coefficient is the lift coefficient of the unstable steady state, which is different from the time-averaged flow field. Delay time significantly affects the active control. Buffet can be completely suppressed when the flap rotation is approximately 50° phase lead towards the lift response, where the flap rotation is in reversed phase towards the lift force making negative work to the buffet flow. An optimal parameter combination designed for a fixed Mach number and angle of attack also suits another buffet state. Meanwhile, the present closed-loop control does not lead to an undesirable increase of the drag force. Therefore, the present closed-loop control strategy is effective in transonic buffet suppression.