Abstract
The authors describe the acceleration from rest of a buoyant sphere rising freely in water. They analyse in detail the first phase characterised by a constant acceleration due to the combined action of gravity, buoyancy and turbulent drag. The acceleration equation includes a contribution due to the flow of the fluid induced by the motion of the sphere (Kelvin drag term). In a first, accelerated, regime the authors measure this term while neglecting the viscosity as the boundary layer has not yet developed. The transition of this regime to a constant velocity one takes place, in an abrupt way, for a time which corresponds, for different spheres, to a displacement of 1.55 diameters of the sphere. They discuss qualitatively this transition and also a second in which the trajectory oscillates, in response to a periodic emission of non-asymmetric vortex rings, using a simple visual experiment. The existence of two different regimes: an accelerated, inviscid one, followed by a vortex regime, can be easily revealed by watching a buoyant sphere rise in water and 'pop off' across the upper free surface.