Free Surface Model Reference

The Free Surface model allows for the simulation of movement and change of shape in the surface of a viscous liquid. This model is supported for both single and multiphase flows.


Provided by	[physics continuum] > Models > Optional Models
Example Node Paths	Continua > Physics 1 > Models > Free Surface
Requires	Space: Three Dimensional Flow: Viscous Flow
Properties	See Free Surface Properties.
Activates	Boundary Inputs	See Free Surface Boundary Settings.
	Region Inputs	Physics Conditions > Alignment Option . See Free Surface Region Settings.
	Solvers	Mesh Morpher
	Field Functions	Boundary Displacement Grid Flux Grid Displacement Morpher Displacement SavedCoord
	Motions	Morphing

Free Surface Properties

Extrusion Direction: Specifies the axis along which the viscous flow moves, X, Y, or Z. The default is Z.
Kinematic Condition: Locates the boundary according to the velocity direction. Line constrains the cell edges and the extrusion direction to be aligned with the velocity vector on the boundary. The looser Surface setting only constrains the cell faces to be parallel to the velocity vector. The default is Line. See Kinematic Conditions.
Restrain X
Restrain Y
Restrain Z: When one of these properties is On, it sets the integral of the velocity component in the given direction to zero. Use these properties when there is no plane of symmetry in a given direction. The default is Off.
Restrain Torque: In some cases, the free surface has rotational freedom around the axis of extrusion—for example, when periodic boundary conditions apply, but without a symmetry boundary condition. This rotational degree of freedom can cause arbitrarily large rotations of the free surface, leading to negative volume errors. When this property is On, this rotation is constrained. The default is Off.
Relax Free Surfaces: When this property is On, Simcenter STAR-CCM+ introduces the kinematic condition progressively. Use it if the fixed domain model converges but the free surface one does not, to locate free surface topological problems. By default, the property is Off.
Extrusion Moments: When any of the Restrain properties is On, this property specifies the number of moments in the extrusion direction that are set to zero. The higher the number, the stiffer the extrudate becomes. It is seldom useful to go higher than 5. The default value is 3.
Cross-FS Stabilization: A numerical stabilization term to eliminate irregularities that result from the Line Kinematic Condition. The default value is 0.01.

Free Surface Boundary Settings

All boundary types but Symmetry Plane

Morpher Specification

Controls how the morphing for a boundary is performed. For more information, see Morpher Boundary Condition Reference. Activated when the Free Surface model is selected. For some boundaries, only a subset of the methods that are listed below is available.

Method	Corresponding Physics Value Nodes
Fixed	Morpher Thinout Factor
Constraint Adds the following nodes: Morpher Constraint Specification Morpher Plane Constraint Specification Traction	Morpher Plane Options
Displacement Adds the Morpher Displacement Specification node.	Morpher Thinout Factor
Floating	None

Traction

For extrusion simulations, controls the traction velocity at the boundary normal to the extrusion direction. Becomes available when [boundary] > Physics Conditions > Morpher Specification is set to Constraint. See Traction.

Method	Corresponding Physics Value Nodes
No Traction No force is applied at the free surface exit.	None.
Traction Velocity Applies a pulling velocity at the end of the free surface.	Traction Velocity Sets the pulling velocity using the chosen Method.
Aligned Exit Impose a zero Dirichlet condition for tangential components of velocity vector on the outlet boundary but, in the normal direction, sets a zero force (or traction) condition.	None.
Normal Velocity Applies a pulling velocity at the end of the free surface and zero traction in the tangential directions of the outlet.	Traction Velocity Sets the pulling velocity using the chosen Method.
Open Boundary Applies traction in all three directions at the outlet, based on values upstream.	None.

Symmetry Plane

Morpher Plane Options: This node gets added to Physics Values. For more information, see Morpher Plane Options.

Free Stream

Represents the outer surface of an extrudate in a free surface simulation. The position of that boundary is computed as part of the solution by an Arbitrary-Lagrange-Eulerian (ALE) approach. The vertices of the boundary are moved by the morpher according to the selected morpher settings. See Free Stream.

Free Surface Option

Activated when the Free Surface model and the Implicit Unsteady time model are selected in the physics continuum. Determines which method is used to constrain the motion of the extrudate.

Option can be set to the following:

ALE—constrains the mesh motion in the extrusion direction. Suitable for steady-state extrusion simulations.
Lagrangian—allows the mesh to move in the extrusion direction. Suitable for transient extrusion simulations.

Free Surface Region Settings

Applies to fluid regions.

Alignment Option

Determines in which direction the mesh cells of the extrudate are allowed to move.

Aligned—allows mesh cells to move in the direction of the extrusion flow. Suitable for steady-state extrusion only.
Unaligned—allows mesh cells to move in all directions. Suitable for steady extrusion only.
Unaligned with stretching—allows mesh cells to move in extrusion direction. Suitable for unsteady extrusion flows only.