Investigation of models to estimate flight performance of gliding birds from wakes

Mathematical models based on inviscid flow theory are effective at predicting the aerodynamic forces on large-scale aircraft. Avian flight, however, is characterized by smaller sizes, slower speeds, and increased influence of viscous effects associated with lower Reynolds numbers. Therefore, inviscid mathematical models of avian flight should be used with caution. The assumptions used in such models, such as thin wings and streamlined bodies, may be violated by birds, potentially introducing additional error. To investigate the applicability of the existing models to calculate the aerodynamic performance of bird flight, we compared predictions using simulated wakes with those calculated directly from forces on the bird surface, both derived from computational fluid dynamics of a high-fidelity barn owl geometry in free gliding flight. Two lift models and two drag models are assessed. We show that the generalized Kutta-Joukowski model, corrected by the streamwise velocity, can predict not only the lift but also span loading well. Drag was predicted best by a drag model based on the conservation of fluid momentum in a control volume. Finally, we estimated force production for three raptor species across nine gliding flights by applying the best lift model to wake flow fields measured with particle tracking velocimetry.