Ansys Polyflow offers a wide variety of constitutive models for both non-Newtonian inelastic and viscoelastic fluids. It is, however, essential to be aware that none of these models leads to realistic predictions in all types of deformation of any particular non-Newtonian fluid.
In many cases, the use of non-Newtonian inelastic models will capture the physics of the flow with sufficient accuracy. The only modeling issue then is to identify a suitable nonlinear viscosity function that will fit available viscometric data. The problem is more complex, however, when viscoelastic effects must be taken into account.
Important: If the modeling procedure is to be realistic, you should select a viscoelastic model that at least gives satisfactory predictions in the standard rheometrical tests that appear most relevant for the flow under consideration. To help determine an appropriate viscoelastic model, you can use the viscoelastic fitting tool in the Ansys Polymat preprocessor. See Curve Fitting for Material Properties and the Ansys Polymat User’s Guide for details.
Current formulations of viscoelastic flows lead to highly nonlinear problems whose mathematical nature combines ellipticity and hyperbolicity in a rather subtle way. In addition, most viscoelastic flows of practical interest involve internal and boundary layers in the stress and velocity fields, as well as strong singularities.
Differential Viscoelastic Models describes the differential approach to computing viscoelastic flow, and Integral Viscoelastic Models describes an integral approach. The differential approach is appropriate for most practical applications, while the integral approach is provided mainly as a tool for rheology research. The numerical methods are more robust for the differential approach; for the integral approach, the available numerical methods converge slowly. Finally, Simplified Viscoelastic Model describes a simplified viscoelastic model, which, via appropriate simplifications from the point of view of rheology and mathematics, enables the qualitative prediction of viscoelastic effects at a reduced computational cost.
This chapter assumes a basic familiarity with the phenomena associated with viscoelastic flow, and will not serve as an initial introduction to the topic. For background information about viscoelastic flow, see [2] or any similar handbook or textbook on rheology.