Buffet onset prediction and flow field around a buffeting airfoil at transonic speeds

To find a simple method with good accuracy for buffet onset prediction as well as to study the flow field around a buffeting airfoil in transonic flow, four different predicting methods are employed and the results are compared based on steady numerical simulations, then the unsteady flow characteristics are investigated at buffet condition. The steady numerical results indicate that, the methods of lift curve and pitching moment curve curvature variations are difficult to predict buffet onset because the obtained curves are fairly smooth, and they can not give satisfying results; the method of pressure center variation gives the most accurate result comparing to the other three methods, and it is applicable for predicting buffet onset in both wind tunnel tests and numerical simulations. The method of separation bubble occurrence can also give satisfying result, but it is only applicable for numerical results. The unsteady simulation results of transonic flow for an airfoil at buffet condition show that, both shock wave and separation bubble evidently oscillate on the upper surface, inducing the forward and rearward movements of pressure center. The peak value of unsteady lift resides itself at low frequency range.