Setting Up Extrusion and Free Surface Flow

Use the following steps for each extrudate continuum in the simulation. This workflow applies to steady-state extrusion simulations where the extrudate length is pre-defined.

To set up extrusion and free surface flow:

For most cases, it is best to create a multi-part volume mesh with an internal interface between the static and the free surface regions.

For the free surface, you must specify the extrusion direction and certain constraints subject to which the free surface deforms.

Edit the Models > Free Surface node and set the Free Surface Properties.

The outward boundary of the free surface must be a free stream boundary. To model symmetric extrudates, you can additionally apply a symmetry boundary condition on the corresponding boundary surface.

Edit the Regions > [free surface region] > Boundaries > [outer free surface boundary] and set Type to Free Stream.

The motion of the free surface is performed through the morpher. To control the motion, you specify constraints at the free stream boundaries. See Morpher Boundary Conditions Reference. You must apply different conditions to boundaries that are parallel to the extrusion direction from boundaries that are normal to the extrusion direction. Hence, before applying boundary conditions, split the free surface boundary into at least one boundary surface that is initially parallel to the extrusion direction and one outlet boundary surface that is normal to the direction.

Edit the Regions > [free surface region] > Boundaries > [outer free surface boundary parallel to the extrusion direction] > Physics Condition node and set the following:


Node	Property	Setting
`Morpher Specification`	Specification	Displacement
`Morpher Displacement Specification`	Specification	Internal

These settings cause a free surface to be calculated on this boundary.

To calculate the shape for a die to produce a desired cross section for an extrudate, perform the steps described in Setting Up Inverse Die Design Simulations.

Edit the Regions > [free surface region] > Boundaries > [outlet boundary normal to the extrusion direction] > Physics Condition node and set the following:

These settings put no kinematic condition at the boundary. The constraint condition leaves the boundary free to move along with activating free surface tracking.

For most free surface simulations:


Node	Property	Setting
`Morpher Specification`	Specification	Constraint
`Morpher Constraint Specification`	Specification	Boundary Plane
`Traction` Represents a pull exerted on the extrusion.	Method	Select one of: No Traction—does not apply any force at the Free Surface exit. Traction Velocity—applies a given pulling velocity at the end of the free surface. Aligned Exit—applies zero force in the plane normal to the extrusion direction and sets zero velocity in the tangential direction on the plane. Normal Velocity—applies a velocity normal to the outlet boundary and zero forces (traction) for the tangential directions in the outlet boundary plane. Open Boundary—applies traction in all three directions at the outlet, based on values upstream. Available only with viscoelastic models.

If you are modeling extrusion where there is a significant traction velocity at the end of the extrudate, that is, a very high draw ratio, such as in fiber spinning, select the Morpher Specification node and set Specification to Floating.

To model co-extrusion, perform the steps described in Setting Up Co-Extrusion.
To visualize the deformation of the free surface, you can set up a scalar scene and activate the Surface option on the free surface part.
When using field functions in a scene or report, make sure that you use Smooth Values, so that you visualize the nodal values.