Characterization of Unsteady Flow Scales on a Pitching NACA 0012 Across a Range of Reynolds Numbers.

Dynamic stall is a well-known phenomenon in rotary wing flows, leading initially to an overshoot in lift performance followed by a rapid loss in such performance. During the dynamic stall process, the flow over a lifting surface becomes unsteady and evolves into a flow dominated by large-scale vortex structures. The emergence and evolution of these vortex flow structures contribute to large variations in loading, thus affecting both the aerodynamic performance and the structural integrity of aircraft.

Previous studies on airfoils have identified some of the existing flow features and have revealed dominant frequencies linked to the formation of the leading-edge vortex. However, the lack of precise spatial localization and temporal evolution of dominant frequencies limits the implementation of effective flow control methods.

The present work focuses on the dynamic stall flow features leading up to the inception of the dynamic stall vortex at chord-based Reynolds numbers, Re_c, between 10,000 and 100,000 using time-resolved PIV data acquired for a dynamically pitching NACA 0012 airfoil at a reduced frequency of k = 0.05. A combination of log-Gabor filter and Riesz-transform was applied to the 2-D PIV images to provide a better localization and a multiscale, multi-orientation quantification of the wavenumber amplitudes associated with the flow features. In general, the amplitudes associated with these wavenumbers were observed to reach an amplified state during the initial ejection of flow structures.