Dynamic stall: stability and evolution at transitional Reynolds numbers

Detailed herein is a campaign of experiments aimed at building an understanding of unsteady flow phenomena arising due to the rapid pitching motion of an airfoil. The physical problem investigated is simplified significantly from the extensive parameter space occupied by natural flyers. A pitch ramp–hold–return of a flat plate is performed in a water tunnel facility, and the vortex structures separating from both the leading and trailing edges are studied using multi–component, multi–dimensional laser–based particle image velocimetry (PIV) techniques. The experiments are conducted in the Reynolds number range 10³ ≤ Re ≤ 10⁴, within which there is some evidence that laminar–turbulent transition occurs. For this reason, the analysis presented here focusses on the smaller scale features within the flow, including the initiation and development of three–dimensional structure in this nominally two–dimensional flow. Vortex shedding frequencies are identified, the centrifugal instability initiating three– dimensional structure in the leading edge dynamic stall vortex is quantified, and a scale analysis using the discrete wavelet transform is performed to reveal its structure. The correlation between the leading and trailing edge flow structures is investigated, and correlative timescales calculated. This correlation analysis is extended also to the three–dimensional features associated with the centrifugal instability