SPOD Analysis of the Transonic Buffet-Induced Unsteady Flow Field over a Supercritical Airfoil.*

Transonic buffet refers to the shock oscillations on an airfoil surface that coincide with strong variations in the flow field. This interaction causes fluctuations in lift and pressure on an airframe, which may reduce aircraft performance and can potentially lead to critical structural damage. A thorough understanding of this phenomenon is integral in ensuring the structural integrity, stability, and control of aircraft throughout the entire flight regime. To better understand the temporal and spectral behavior of transonic buffet, we carried out an experimental wind tunnel investigation of an OAT15A model at a Reynolds number of 2 x 106 and Mach numbers of 0.68 < M < 0.80. High-speed focusing schlieren images acquired across different buffet regimes, including buffet onset, developed buffet, and offset, are used to resolve the dynamic interaction in time. We use a spectral proper orthogonal decomposition (SPOD) approach to discern the spatially coherent structures attributed to the shock-induced separation. These results are compared to a power spectral density (PSD) analysis to describe both the dominant spatial structures of buffet and the associated spectral footprint across the captured flow domain and throughout the buffet regimes.