The evolution of unsteady flow structures in the wake of flapping bird: Experimetnal and numerical analysis

Understanding the physics of flapping wings at moderate Reynolds number flows takes on greater importance in the context of avian aerodynamics. Analyzing the characteristics of wake vortices generated downstream of flapping wings can help to explain the unsteady contribution to the aerodynamics loads. In this study, a combined effort of flow measurements behind freely flying birds in an avian wind tunnel and numerical simulations of flow over a bio-inspired pseudo-2D flapping wing model were conducted to characterize the evolution of unsteady flow structures in the downstream wake of flapping wing. The near wake flow field behind a Western sandpiper and a European starling were measured using long-duration time-resolved Particle Image Velocimetry (PIV) system. The measurements were performed in a hypobaric avian wind tunnel where the bird was freely flying over long period of time at its comfort speed. For the simulations, the wing was modeled based on the starling's wing and the wingbeat kinematics were incorporated to simulate a free-forward flight. The starling's wingbeat kinematics were extracted from the experiments conducted in the avian wind tunnel using high-speed imaging which yielded a series of kinematic images sampled. The simulations were carried out at a Reynolds number of 54,000 and Strouhal number of 0.16. Large eddy simulation was performed using a second order, finite difference code ParLES. Characteristics of wake vortex structures during the different phases of the wing strokes were examined. The flow patterns identified at the near wake region, suggesting that the unique flapping mode of small migratory birds may play a role in their ability to fly in an efficient manner through unsteady flow mechanisms.