Surfing Birds: Bird flight and engineered wing responses to an unsteady vortex wake

Birds must accommodate unsteady flow structures encountered during flight, yet the coupling mechanism between flapping flight and flow patterns are not well studied. We report on a series of wind tunnel flight experiments in which European Starlings carrying accelerometer backpacks are exposed to an unsteady vortex wake generated by an upstream mounted airfoil, pitching close to the birds' flapping frequency. PIV measurements show that the wake can be generated dominated by positive vortices (upwash), negative vortices (downwash) or one with alternating signs of vorticity (von Karman street). When the birds fly in the unsteady wake, power spectra of the birds' vertical accelerations suggest that the wing flapping motion interacts with the dominant frequency of the pitching wing. We complement the live animal experiments with those using a model system comprised of a stationary NACA 0012 wing "surfing" in the wake of the upstream flapping foil. The wing is mounted on a force/torque transducer to capture the transient aerodynamic forces. Comparisons between the spectra measured in the model system and the accelerations experienced by the bird flights will be discussed.