Swirling flow of viscoelastic fluids. Part 2. Elastic effects

A torsionally driven cavity has been used to examine the influence of elast icity on the swirling how of constant-viscosity elastic liquids (Boger flui ds). A wealth of phenomena is observed as the degree of inertia, elasticity and viscous forces are varied by using a range of low- to high-viscosity f lexible polyacrylamide Boger fluids and a semi-rigid xanthan gum Boger flui d. As the inertia is decreased and elasticity increased by using polyacryla mide Boger fluids, the circulation rates for a 'Newtonian-like' secondary f low decreases until flow reversal occurs owing to the increasing magnitude of the primary normal stress difference. For each polyacrylamide fluid, the flow becomes highly unstable at a critical combination of Reynolds number and Weissenberg number resulting in a new time-dependent elastic instabilit y. Each fluid is characterized by a dimensionless elasticity number and a c orrelation with Reynolds number is found for the occurrence of the instabil ity. In the elasticity dominated flow of the polyacrylamide Boger fluids, t he instability disrupts the flow dramatically and causes an increase in the peak axial velocity along the central axis by as much as 400%. In this cas e, the core vortex spirals with the primary motion of fluid and is observed in some cases at Reynolds numbers much less than unity. Elastic 'reverse' how is observed for the xanthan gum Boger fluid at high Weissenberg number. As the Weissenberg number decreases, and Reynolds number increases, counte r-rotating vortices flowing in the inertial direction form on the rotating lid. The peak axial velocity decreases for the xanthan gum Boger fluid with decreasing Weissenberg number. In addition, several constitutive models ar e used to describe accurately the rheological properties of the fluids used in this work in shear and extensional flow. This experimental investigatio n of a complex three-dimensional flow using well-characterized fluids provi des the information necessary for the validation of non-Newtonian constitut ive models through numerical analysis of the torsionally driven cavity flow .