The negative wake behind a sphere sedimenting through a viscoelastic fluid

The phenomenon of the negative wake behind a sphere sedimenting through a viscoelastic fluid is investigated. It is shown that there are two competing viscoelastic forces at work in this flow. The relaxation downstream of shear stresses generated near the side of the sphere drives a flow directed away from the sphere, giving rise to a negative wake. This force is opposed by the extensional stresses generated in the extensional flow at the rear of the sphere, which drives a flow towards the sphere producing an extended wake. The parameter controlling the balance between these forces is the extensibility of the polymer, which limits the extensional viscosity, with high extensibility producing an extended wake and smaller values giving negative wakes. (C) 2002 Elsevier Science B.V. All rights reserved.