Case Study: Starling, Sandpiper and Robin
Birds’ unique characteristics such as wing shape, flexibility, feathers, and flapping motion, result in high aerodynamic performance. Flapping flight mode is common in low Reynolds number flyers and the contribution of unsteady effects to lift and drag is not entirely understood. To gain insight about the unsteady contribution, a controlled study in an avian wind tunnel on the near wake flow behind freely flying birds was performed. Long duration, time resolved particle image velocimetry (PIV), combined with high speed imaging has been used to characterize the various flow features at the bird’s wake that are associated with flapping flight. Time series of the velocity and the vorticity fields have been expressed as composite wake plots, which reveal various characteristics of the wake during the upstroke (US) and downstroke (DS) phases of the flapping mode as well as the transition between US to DS and vice versa. Unsteady drag and lift components have been estimated over these wingbeat cycles, demonstrating how the bird manifests its aerodynamic performance through the kinematics of the wings. Comparison between the near wake fields of the three birds shows remarkable similarity in their wake structure. We have identified the presence of what appears to be an overlap of two distinct patterns during the transition from US to DS. Over these regions, the majority of net positive circulation is accumulated, which indicates that this overlap may be a key feature in producing lift, and thus contribute to the observed high aerodynamic performance.