Fluid mechanics of flapping wings

Based on the results of two-dimensional flapping wing studies, the selection of Strouhal number for steady cruising is explored. The hypothesis that Strouhal number selection in natural flapping wing propulsion is a limit cycle process is proposed and the implications of this hypothesis for the design, control and computational/experimental modeling of freely flying systems are explored. It is suggested that the steady-state propulsive efficiency of man-made systems that use flapping propulsion must be designed-in, rather than actively controlled and that sensory feedback may be more important for control during maneuvering/acceleration and play only a minor role in the regulation of a flying/swimming system's steady forward motion. A brief literature review of the structure of the flow around three-dimensional translating and revolving wings is also presented. In the case of translating wings the flow structure consists of a chain of interconnected vortex loops and has a strong Strouhal number dependence. For revolving wings the leading edge vortex is especially important for the development of lift and thrust. The stability of this leading edge vortex is strongly dependent on the development of a spanwise flow that creates a balance between the creation of vorticity at the leading edge of the wing and the transport of vorticity into the wake.