Analysis of Dynamic Stall using Dynamic Mode Decomposition Technique

Dynamic mode decomposition (DMD) is applied to investigate the unsteady flow field around a pitching airfoil. The extracted flow structures are termed as DMD modes. Analyses are performed for both attached flow and dynamic stall cases. Initially flow field data generated from numerical simulations are investigated. The effect of exclusion of the flow field near the surface of the airfoil on the structure of the DMD modes is examined. This is of vital importance for the experimental measurements, as the flow field near the airfoil's surface is difficult to measure using particle image velocimetry. In the latter part of the paper, DMD modes extracted from the experimental data are analyzed. The structure of the DMD modes are compared with the modes obtained from the proper orthogonal decomposition (POD) technique. A lower order model is constructed from few DMD modes and compared with a model obtained using POD modes. Both techniques capture the overall dynamics of the flow field during dynamic stall. Finally, effects of phase averaging and insufficient measurement time on the modes are discussed.