Global Structure of Buffeting Flow on Transonic Airfoils

The flow field associated with transonic airfoil buffet is investigated using a combination of global-stability theory and experimental data. The theory is based on perturbing a steady flow field obtained from the Reynolds-averaged Navier— Stokes equations. Linearized perturbations are described by an eigenvalue problem, with the frequency and growth rate given by the eigenvalue and global-flow structure provided by the eigenfunction. The experiments provide both steady and unsteady information on the airfoil surface and in the flow downstream of the shock. The theory and experiment show good agreement for the buffet onset conditions — including the critical angle of attack and the buffet-onset frequency. The post-buffet flow structure is also in good agreement, and shows a shock oscillation phase locked to an oscillating shear layer downstream of the shock.