On the effect of rapid area change in perching-like maneuvers

A perching bird is able to rapidly decelerate at a high angle of attack while maintaining lift and control. However, the underlying aerodynamic mechanism is poorly understood. We perform a study on a simultaneously decelerating and pitching airfoil section as a simple perching model. First, we use analytic arguments to establish the inertial non-circulatory force on a wing section, and its dependence on the shape change number, τ*, the ratio between the rate of change of frontal dimension and the initial translational velocity. Next, we report that forces measured on a towed and pitched airfoil at Re = 22000, and forces from simulations at Re = 2000, are found to be well above non-circulatory predictions, exhibiting high lift and drag. Flow-field visualizations, both from Particle Image Velocimetry and simulations, reveal strong coherent vortical structures in the wake and near the body. At a higher shape change number, vortices in the wake convect more quickly than at a lower shape change number, generating higher drag. Additionally, separation of the LEV and positive vorticity near the body is reduced at a higher shape change number, increasing lift. Thus wake manipulation through rapid area change provides a means through which a perching bird can maintain high lift and drag simultaneously while slowing to a controlled stop.