Meeting Abstract

59.2  Saturday, Jan. 5  Unsteady aerodynamics of a flapping bird model HUBEL, Tatjana*; TROPEA, Cameron; Darmstadt University of Technology; Darmstadt University of Technology tatjana_hubel@brown.edu

This study investigates unsteady effects within the characteristic range of bird and large insect flight (Re 10⁴ to 10⁵). The influence of flapping frequency, angle of attack and amplitude was analyzed using PIV for qualitative and quantitative flow visualizations synchronized with direct force measurements of a physical model with an internal three component balance. The results of both measurement techniques were validated with one another in order to determine the utility of 2D flow visualization for such investigations. The study was performed in a low-speed wind tunnel using a flapping mechanical model based on the characteristics of a goose. Measurements parallel and perpendicular to the flow field were made to visualize the tip vortex and the transverse vortices. The circulation within the tip vortices and transverse vortices were calculated using Stokes�s theorem, then related to total lift production and changes in lift production over the wingbeat cycle, respectively. Both the direct force measurements and the flow visualization on the wing itself confirm the existence of the dynamic stall effect, which is indicated through significantly higher lift coefficients compared to static wings and the observation of a leading-edge vortex. This shows that even under bird flight conditions the flow on the mechanical model cannot be simplified as quasi-steady. However, the model operates in a range where the leading edge vortex is not stabilized over the wing beat cycle, which leads us to the prediction that, in bird flight, �adaptive wings� are necessary to fully control these unsteady flow features.