Unsteady Aerodynamics of Pitching and Perching Wings
Webb, Joshua Eric
Birds are very graceful creatures that fly effortless through the air. Many of their maneuvers and capabilities are unmatched by current man-made airplane technology. To investigate the fundamental unsteady fluid dynamics and force production of the flapping wings of birds, two types of idealized wing motions with combined linear translation and pitching were considered in a two-dimensional context, one for characteristic pitching and the other for perch landing. Two validation cases were first carried out to verify the model and assess accuracy of the COMSOL Multiphysics software package. After successful verification, the pitch rate in each type of wing motion was systematically varied. The force production, pressure distribution, velocity field, and vorticity field were examined in detail in each case. For the pitching-only motion, as the pitch rate goes from the slowest (K=1/64) to the fastest (K=1/2), the peak lift increases by about four times; while for the perching maneuver, the peak lift increases about ten times. Associated with the force enhancement are significantly improved stability of the boundary layer on the dorsal side of the wing and delay of flow separation. In actual bird flight, such great force enhancement could be critical for the animals to perform highly controlled maneuvers. The present study thus confirms the significant role of pitch rotation in the unsteady aerodynamics of flapping wings.