MOTION OF A SPHERE FALLING UNDER GRAVITY IN A CONSTANT-VISCOSITY ELASTIC LIQUID

We describe experiments on the fall of a single sphere through two dif ferent types of Boger fluids. Particular attention is given to the dep endence of the equilibrium drag on the sphere on the Weissenberg numbe r and the ratio beta between the radius of the sphere and that of the containing cylinder. Some attention is also given to the transient bui ld up to steady state flow, where it is shown that a significant overs hoot in velocity is sometimes observed over an inordinately long times cale. In general terms, the experiments show that the normalized drag can be very high for the limiting cases beta --> 0 and beta --> 1, wit h much smaller values at some intermediate values of beta. Numerous qu estions remain and there is at present a significant gulf between avai lable experimental data and reliable numerical simulations for simple viscoelastic models.