Viscous Flow
Viscous Flow is a finite element approach for use with viscoelastic materials and other highly viscous non-Newtonian fluids, such as liquid plastics and rubber, dough and similar foodstuffs, molten glass, and mud. Viscoelastic materials resemble elastic materials, but also exhibit viscous effects, rebounding slowly from deformations.
Use the Viscous Flow models to calculate:
- Heating and normal stresses inside the viscous flows
- Dissipation energies
- Non-linear behavior of viscoelastic fluids
- Residence time distributions
- Species distributions
- Exerted fluid forces on walls to perform solid stress analysis
Different viscoelastic models can be coupled with each other, each model using the material properties under all models, to improve convergence. See Viscoelastic Model.
Viscous flow simulations are designed to run in double precision. You can deactivate the Solve in double precision property of the Viscous Flow solver, but do so only when memory is severely restricted. See Viscous Flow Solver.
Several rheological models/constitutive equations are available to model non-Newtonian fluids:
| Use Cases | Model | Rheology Law |
|---|---|---|
| High-viscosity fluids in which stress, fluid velocity, and viscosity are interrelated. | Generalized Newtonian | Generalized Newtonian Fluid |
| High-viscosity fluids in which viscosity reduces over time as shear increases, followed by a gradual recovery when the stress is removed. | Thixotropic | Thixotropic Flow |
| High-viscosity fluids with elastic properties | Viscoelastic | Viscoelastic Fluids |
 Uncured molten rubber and similar materials
|
Viscoelastic mode
|
Giesekus Leonov |
| Academic cases with multiple normal stresses and constant shear viscosity
|
Viscoelastic mode
|
Oldroyd-B |
|
Molten polyethylenes and similar materials
|
Viscoelastic mode
|
Linear Phan Thien-Tanner / JS(Johnson-Segalman) |
| Exponential Phan Thien-Tanner / JS(Johnson-Segalman) | ||
| Inhomogeneous flows of branched polymers
|
Viscoelastic mode
|
eXtended Pom-Pom |
|
Inhomogeneous flows of entangled linear polymers
|
Viscoelastic mode
|
Rolie-Poly |
Scope
- The Viscous Flow models are best suited to low Reynolds numbers, less than 10 and often less than 1.
- Viscous Flow is not suitable for simulating convection, though it is compatible with diffusion.
- The fluid flows treated by Viscous Flow are laminar; turbulence models are not compatible with Viscous Flow.
- Use Viscous Flow models only with single-phase flows.
- Viscous Flow models are not compatible with overset meshes or conjugate heat transfer.
- Viscous Flow models do not work with radiation modeling.
Use Case: Rubber Seals
A manufacturing process runs liquid rubber through a mixer and extrudes it in a strip. The strip is then cooled in water and sliced into thin sheets to be used as seals. Viscous Flow models can simulate the behavior of the rubber in the mixer, at the point of extrusion, and as it is cooled. This way, the manufacturer can evaluate the heat and forces in the mixer, the normal stresses affecting extrusion, and the rate of cooling in the water. All these factors affect the maximum speed for the process. See the tutorials Generalized Newtonian Fluid Flow in a Static Mixer and Viscoelastic Flow: Basic Extrusion.