Transonic unsteady aerodynamics in the vicinity of shock-buffet instability

Abstract

A study of transonic unsteady aerodynamic responses in the vicinity of shock-buffet is presented. Navier–Stokes simulations of a NACA 0012 airfoil with a fitted 20% trailing edge flap are performed to compute the aerodynamic responses to prescribed pitch and flap motions, about mean flow conditions at shock-buffet onset, and while exhibiting shock buffet. The unsteady aerodynamic response is found to be fundamentally different from the response predicted by the linear aerodynamic theory. At mean angles of attack close to buffet onset noticeable damped resonance responses are observed at frequencies close to the buffet frequency. The responses grow as the mean angle of attack is increased towards buffet onset. Also, a phase lead is observed for the aerodynamic coefficients, for some range of frequencies. The large aerodynamic responses and phase lead appear in frequencies that are typical of structural elastic frequencies, suggesting that they may be responsible for transonic aeroelastic instabilities. At shock buffet conditions, prescribing sufficiently large pitch or flap harmonic motions results in synchronization of the buffet frequency with the excitation frequencies. At these conditions, the lift and pitching moment responses to prescribed pitch motion also result in resonance and phase lead, as in the pre-buffet case. Large prescribed flap motions eliminate the lift resonance response, and significantly reduce the lift coefficient amplitudes, indicating that the aerodynamic coefficients at these conditions can be controlled by prescribed structural motions.