The generalized Newtonian flow models discussed in Generalized Newtonian Flow are unable to describe viscoelastic phenomena related to normal stresses and stress relaxation, for example. Typically, vortex generation, extrudate swelling of some melts, and drag enhancement are due to normal-stress differences and high extensional viscosity that are characteristic of viscoelastic fluids. Information about differential modeling for viscoelastic flow is presented in this section. See Integral Viscoelastic Models for information about the integral modeling approach.

The differential approach to modeling viscoelastic flow is appropriate for most practical applications. Many of the most common rheological models for viscoelastic flow are provided in Ansys Polyflow, including Maxwell, Oldroyd, Phan-Thien-Tanner, Giesekus, FENE-P, POM-POM, and Leonov. Appropriate choices for the viscoelastic model and related parameters can yield qualitatively and quantitatively accurate representations of viscoelastic behavior.

You can sometimes improve accuracy if you use multiple relaxation times to better fit the viscoelastic behavior at different shear rates. If required, you can even use different viscoelastic models for the different relaxation times, although this has a very limited physical basis.

Information about the models available in Ansys Polyflow and the equations solved for each is presented in Theory and Equations, and instructions for using these models are provided in Problem Setup for Differential Viscoelastic Flows – Computing Differential Viscoelastic Flow.