Numerical investigation of transonic buffet on a prescribed-pitching OAT15A airfoil

The transonic buffet over a prescribed-pitching OAT15A airfoil was numerically investigated using a well-validated delayed detached eddy simulation approach. The present approach successfully reproduced self-sustained shock wave oscillations. The computational results show that the amplitude of the lift fluctuations in the pitching case is approximately six times that of the fixed case. Compared to the fixed case, the presence of a larger shock motion region, two shock wave interactions, and severe boundary separation give rise to higher pressure and velocity fluctuation levels on the whole upper surface in the pitching case. In particular, the highly disordered flow replaces the periodic vortex shedding in the wake, resulting in the disappearance of the secondary fluctuation in unsteady lift coefficients, which is observed at 2274 Hz in the fixed case. A proper orthogonal decomposition analysis shows that the buffet flow in the fixed case exhibits the feature of regular motion, which is closely related to the coherent flow structure of shock oscillations and Karman vortices. Strong interactions between the Karman vortex structure and the shock oscillations are also observed. Compared to those in the fixed case, the fluctuation amplitudes of the coefficients of the first mode pair and the second to fourth mode pairs in the pitching case are 2.5 times and 5 times larger, respectively, and the energy ratio of the nonlinear component in the total mode energy is significantly increased. Furthermore, the main frequencies of modes 1–10 in the pitching case are all located at the shock buffet frequency or its higher-order harmonics.