The differential approach to modeling viscoelastic flow is appropriate for most practical applications. Many of the most common numerical models for viscoelastic flow are provided in Ansys Polymat, 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.
Improved accuracy is possible 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.
Note: While differential viscoelastic models are compatible with 2D and 3D models, they are not compatible with the shell model.
For a differential viscoelastic flow, the constitutive equation for the extra-stress tensor is
 | (6–26) |
(the viscoelastic component) is computed differently for each
type of viscoelastic model. 
(the purely viscous component) is an optional component, which
is always computed from
 | (6–27) |
where 
is the rate-of-deformation tensor and 
is the viscosity factor for the Newtonian (that is, purely
viscous) component of the extra-stress tensor. The viscosity ratio
is defined as 
. The relationship of 
and 
to 
is expressed by
 | (6–28) |
and
 | (6–29) |
When a multi-mode viscoelastic model is used, the purely viscous component of the extra-stress tensor is defined through the first mode only.
To specify the viscosity model for a differential viscoelastic flow, you will click the Differential viscoelastic models menu item in the Material Data menu
Differential viscoelastic models
and then choose 1-st viscoelastic model.
1-st viscoelastic model
If you want to specify different parameters for different relaxation times, click Addition of a viscoelastic model.
To specify the temperature dependence of viscosity for a differential viscoelastic flow, you will click the Temperature dependence of viscosity menu item.
Temperature dependence of viscosity
See Non-Automatic Fitting and Automatic Fitting for information about where and how the material data specification occurs in the non-automatic and automatic fitting procedures, respectively.
See Differential Viscoelastic Models and Temperature Dependence of Viscosity for details about the parameters and characteristics of each fluid model.