Viscoelastic fluid mechanics: Interaction between prediction and experiment

The extent to which elastic effects in the flow of viscoelastic fluids can be predicted is reviewed. For this purpose, constant-viscosity elastic liquids (Boger fluids) are used for observation where shear-rate-dependent viscosity effects and inertial effects can be eliminated. By comparing observations with predictions in squeeze film flows, jet flows, isothermal spinning, and for instabilities observed in various torsion flows, it is tempting to conclude that elastic effects in complex flows of viscoelastic fluids can be predicted with a relatively simple constitutive equation like the Oldroyd B model. However, this conclusion is not valid for flows for which the elongational rates are high or for regions of high stress concentration like tubular entry hows and creeping flow around a sphere. The importance of the elongational viscosity and its variation with strain rate in such, strong flows is clearly illustrated. Elongational viscosity is often a foreign concept in classical fluid mechanics, and hence its importance in viscoelastic fluid mechanics is stressed.