Velocity measurement of a settling sphere
École Normale Supérieure de Lyon, Laboratoire de
Physique, 46 Allée d'Italie, 69007 Lyon,
Revised: 13 July 2000
Published online: 15 November 2000
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 Reynolds numbers, based on limit velocity, between 40 and 7 000. At all Reynolds numbers, our observations are consistent with the presence of a memory-dependent force acting on the particle. At short times it has a behaviour as predicted by the unsteady Stokes equations and as observed in numerical simulations. At long times, the decay of the memory (Basset) force is better fitted by an exponential behaviour. Comparison of the dynamics of spheres of different densities for the same Reynolds number show that the density is an important control parameter. Light spheres show transitory oscillations at , but reach a constant limit speed.
PACS: 06.30.Gv – Velocity, acceleration, and rotation / 43.60.+d – Acoustic signal processing / 47.27.Vf – Wakes
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 2000