Investigation of transonic buffet using high speed PIV

Transonic buffet of a supercritical airfoil (OAT15A) is investigated by means of schlieren visualization and high speed PIV. The buffet conditions were observed to be strongest at M = 0.7 and a = 3.5o at a frequency of 160 Hz. By means of conditional averaging according to the shock location and movement, the typical buffet cycle is illustrated, confirming observations from literature. In addition, proper orthogonal decomposition is used to further characterize the unsteadiness and it was found that the first 3 modes contain the bulk (> 80%) of the fluctuating energy. Furthermore, through spectral analysis it was found that the typical frequency connected to these modes is equal to the buffet frequency. For mode 4 and higher no peak in the power spectrum is found at the buffet frequency.