Defining aerodynamic modes of large-scale birds' covert feathers

The aerodynamic stability of large-scale birds, attributed to their covert feathers, has long fascinated biologists and bio-inspired design communities. However, existing theories cannot accurately describe the biological physics of coverts. Here, we propose a theory, based on their biological constraints, by defining covert behavior in terms of two modes: Mode Attach and Mode Deform. To validate the theory and to provide a greater understanding of the physics involved, we conduct a series of wind tunnel experiments using coverts on a man-made wing. Both the biological and physical aspects of this theory are well validated. The results show that in Mode Attach, the covert wake coincides with the wing-bound vortices, whereas in Mode Deform, it detaches from the wing wake vortices. The root constraint level affects the process of transitioning from Mode Attach to Mode Deform. Coverts positioned closer to the wing's leading edge result in smaller coherent turbulence structure scales exhibited in the wake region and higher shedding stability of the wake vortices. Furthermore, compared to those in the clean wing case, the nonlinear vortex packets attached to the wing's suction surface are modulated into self-sustaining attached vortices at multiple scales. Additional anechoic tunnel tests also demonstrated that this theory and definition provide a platform for studying both the aerodynamics and the aeroacoustics of coverts.