Multiscale vortex characteristics of dynamic stall from empirical mode decomposition

The multiscale development of dynamic stall was studied using a scale-based modal analysis technique. Time-resolved velocity field data around an airfoil during dynamic stall were used for this analysis, with experimental conditions corresponding to Re_c 9.2 × 10⁵ and k 0.05. Intrinsic mode functions were determined by a multivariate, multidimensional empirical mode decomposition method, which ensures mode alignment between velocity components and modal dependency on the full spatiotemporal flowfield. Individual intrinsic mode functions were linked to physically significant phenomena in the flowfield, including the formation of vortical structures due to the Kelvin–Helmholtz instability, interactions between shear-layer vortices leading to vortex pairing, and ultimately shear-layer roll-up and the formation of the dynamic stall vortex. In addition to qualitatively describing these flow structures and interactions, their characteristic length and time scales within the intrinsic mode functions were quantified. These quantitative measures were found to be consistent with time and length scales reported in the literature on experimental investigations of canonical fluid dynamic processes.