Vortex interaction with a moving sphere

This paper details the experimental results of the axisymmetric collision of a vortex ring with a sphere. The experiments were conducted in water and a neutrally buoyant sphere was free to move in response to the impulse delivered by the vortex ring during the interaction. Good agreement has been achieved between kinematic data for the sphere speed and acceleration and the behaviour of the moment and rate of change of the moment of vorticity, measured using particle image velocimetry (PIV). The interaction of the vortex ring with the sphere creates secondary vorticity on the sphere surface. This initially results in a reduction of the fluid impulse and an acceleration of the sphere. However, within the measurement window of the interaction, the rate of increase of the positive moment of vorticity is slightly larger than the rate of increase of the negative moment of vorticity and the sphere gradually slows. A movie is available with the online version of the paper.