Water entry of spinning spheres

The complex hydrodynamics of water entry by a spinning sphere are investigated experimentally For low Froude numbers. Standard billiard balls are shot down at the free surface with controlled spin around one horizontal axis. High-speed digital video sequences reveal unique hydrodynamic phenomena which vary with spin rate and impact velocity. As anticipated, the spinning motion induces a lift force oil the sphere and thus causes significant curvature in the trajectory of the object along its descent, Similar to a curveball pitch in baseball. However, the splash and cavity dynamics are highly altered for the spinning case compared to impact of a sphere without spin. As spin rate increases, the Splash curtain and cavity form and collapse asymmetrically with a persistent wedge of fluid emerging across the centre of the cavity. The wedge is formed as the sphere drags Mild along the surface, due to the no-slip condition; the wedge crosses the cavity in the same time it takes the sphere to rotate one-half a revolution. The spin rate relaxation time plateaus to a constant for tangential velocities above half the translational velocity of the sphere. Non-dimensional time to pinch off scales with Froude number as does the depth of pinch-off: however, a clear mass ratio dependence is noted in the depth to pinch off data. A force model IS Used to evaluate the lift and drag forces oil the sphere after impact, resulting forces follow similar trends to those found For Spinning spheres ill Oncoming flow, but are altered LIS a result of the subsurface air cavity. Images of the cavity and splash evolution, as well as force data, are presented for a range of spin rates and impact speeds; the influence of sphere density and diameter are also considered.