A computational analysis of the aerodynamic interference between two birds flying in formation and in ground effect simultaneously

This work presents four independent stages of analyses leading to a description of the aerodynamics of two birds flying in formation and in ground effect simultaneously, using the vortex lattice computational method.; First, the aerodynamics and flowfield topography around the wing of a single bird flying out of a ground effect is quantified.; Second, the aerodynamic interference during flight formation is found to increase lift by 6.8% and decrease induced drag of both birds by 27% when flying side by side due to the existence of a flowfield topography that allows both birds to "slide down" an upwash velocity gradient caused by the wing tip vortex caused by the other bird. This was found to be analogous to dolphins "riding the bow wave" of a ship. The lagging bird increases the lead bird's induced drag by 3% when entering specific "forbidden" formation zones.; Third, the aerodynamics and flowfield topography around a wing of a single bird flying in ground effect is quantified. Transition towards ground effect flying while maximizing glide ratio is performed by keeping the bird's overall aerodynamic drag coefficient, the overall lift as well as the induced drag constant while the flight speed decreases and the wing effective aspect ratio, the lift coefficient and the angle of attack increase. While ground effect does flatten the downwash surrounding a wing, it is shown that upwash is increased 10% as compared with its upwash while out of ground effect. This explains why birds still benefit from flight formation while in ground effect.; The fourth case of two birds flying in formation and ground effect simultaneously proves that maximum lift savings for both birds while flying side by side are around 6.8% while induced drag is reduced by 27%. All results are applicable to flapping flight, provided the vertical wing tip speeds during flapping are negligible when compared to forward speed.