Lift on Stationary and Rotating Spheres Under Varying Flow and Surface Conditions

We examined the coefficient of lift for rotating and nonrotating spheres of varying surface texture under a variety of flight conditions. At Reynolds number less than 500,000 the direction of lift on a nonrotating polished sphere in a uniform flowfield is nonzero and in a direction that varies randomly with Reynolds number. However, once established, the direction and magnitude of lift appears to be stable over time at a particular Reynolds number. This phenomenon has not been reported previously. Additionally, the texture of a rotating sphere in a uniform flow can modulate direction and magnitude of lift generated by the spin. Although the expected direction of the net force on a spinning sphere is in a direction from the advancing to retreating surface ("Magnus effect"), in certain flight regimes the net force is in the opposite direction ("negative Magnus effect"). In this paper we present the first systematic wind-tunnel characterization of the flow parameters generating a negative Magnus effect on a sphere.