Experimental study of the unsteady aerodynamics during flapping flight of birds; test case: Starling, Sandpiper and Robin

Birds’ unique characteristics such as wing shape, flexibility, feathers, flapping motion, etc., result in high aerodynamic performance. Using various flight modes such as gliding, bounding, and flapping, birds can use a single propulsion system for multiple functions. In low Reynolds number flyers using flapping flight mechanisms, the contribution of unsteady effects on lift and drag is not entirely understood. To gain insight about the unsteady contributions, a controlled study on the near wake flow behind freely flying birds is performed. Long duration, time resolved particle image velocimetry (PIV), combined with high speed imaging is used to characterize the various flow features in the wake that are associated with flapping flight. The specially designed PIV system can sample the flow field for twenty minutes yielding a continuous measurement, sampling several wingbeat cycles consecutively. Time series of the vorticity fields are expressed as composite wake plots, which reveal various characteristics of the wake during the upstroke (US) and downstroke (DS) phase of the flapping as well as the transition between US to DS and vice versa. Comparison between the near wake fields behind the three birds show remarkable similarity in their wake structure. We have identified over multiple wing beat cycles the presence of what appears to be an overlap of two distinct wakes during the transition from US to DS. Over this region, the majority of net positive circulation is accumulated. Indicating this may be a key feature in producing lift, and thus contribute to the observed high aerodynamic performance.