Velocity measurement of a settling sphere

We study experimentally the motion of a solid sphere settling under gravity in a fluid at rest. The particle velocity is measured with a new acoustic method. Variations of the sphere size and density allow measurements at Rey nolds numbers, based on limit velocity, between 40 and 7000. At ail Reynold s numbers, our observations are consistent with the presence of a memory-de pendent force acting on the particle. At short times it has a t(-1/2) behav iour as predicted by the unsteady Stokes equations and as observed in numer ical simulations. At long times, the decay of the memory (Basset) force is better fitted by an exponential behaviour. Comparison of the dynamics of sp heres of different densities for the same Reynolds number show that the den sity is an important control parameter. Light spheres show transitory oscil lations at Re similar to 400, but reach a constant limit speed.