Aerodynamic lift and drag fluctuations of a sphere

An experimental and theoretical investigation is made of the unsteady lift and drag exerted on a sphere in a nominally steady, high Reynolds number, i ncompressible flow. The net force on the sphere has previously been ascribe d to fluctuations in the bound vorticity in the meridian plane normal to th e force, produced by large-scale coherent structures shed into the wake. A simplified model of vortex shedding is proposed that involves coherent eddi es in the form of a succession of randomly orientated vortex rings, interco nnected by pairs of oppositely rotating line vortices, and shed at quasi-re gular intervals with a Strouhal number similar to 0.19. The rings are rapid ly dissipated by turbulence diffusion, but it is shown that only the nascen t vortex ring makes a significant contribution to the surface force, and th at the force spectrum at Strouhal numbers exceeding unity is effectively in dependent of the shape of the fully formed vortex. Predictions of the lift and drag spectra at these frequencies are found to be in good accord with n ew towing tank measurements presented in this paper.