Surfing birds: flight interactions with a structured vortex wake*

Understanding the coupling between avian flight behavior and unsteady flow structures is crucial for advancements in biological and engineering research.  We present results from a wind tunnel  study of the European Starlings' response to a mechanically generated wake.  We conduct synchronized measurements of the bird position  - measured using machine-vision tracking - and the wake velocity field - measured using planar Particle Image Velocimetry (PIV) aligned in the streamwise plane.   Measurements are conducted behind the birds flying both in a clean freestream, and in a vortex dominated flow, generated by a two-dimensional wing positioned upstream and pitching at the bird flapping frequency, (11 Hz).  The animal flight results are interpreted with the assistance of an engineering model system in which the aerodynamic forces and torques on a NACA 0012 airfoil are measured as it surfs in the identical mechanically-generated vortex wake. The experiments reveal a variation in lift induced by the wake, at a frequency locked into the wake shedding frequency.  The strength of this force variation varied linearly with the Strouhal number based on the flapping frequency and freestream velocity).