Setting Up Morphing Motion and Boundary Conditions

The motion of the gerotor walls is realized using the morpher.

To avoid extreme deformation of the cells in the small gaps, you specify a Constraint morpher boundary for the Inner.Wall of the region PumpFluid so that it slides along the Wall surface of the Inner geometry part.

By giving vertices freedom to slide, less strong deformed cells appear than enforcing a rigid displacement. For morphing applications with small gaps this practice facilities morphing over several time steps without negative cells appearing.

In general, the morphing process must succeed for at least 2-3 time steps without negative cells—otherwise the Remeshing solver would activate constant remeshing without improving mesh quality.

You are advised to use the constraint approach for surfaces that have no sharp edges. Otherwise the surface would not be represented correctly during rotation.

The morphing concept is illustrated in the image below:



For more details of morpher boundary conditions, refer to Morpher Boundary Condition Reference.

  1. To create the morpher motion:
    1. Right-click the Tools > Motions node and select New > Morphing.
      Use the default Morphing Method, BSpline.
    2. Select the Morphing > BSpline Parameters node and activate Linear Fitter.
      For more details, see Linear Fitter.
  2. To assign the morpher motion to the region PumpFluid:
    1. Select the Regions > PumpFluid > Physics Value > Motion Specification node and set Motion to Morphing.
  3. You set the outer wall to a Displacement morpher boundary; this boundary becomes the guiding surface for the morphing process. To set this boundary condition:
    1. Select the Regions > PumpFluid > Boundaries > Outer.wall > Physics Conditions > Morpher Specification and set Specification to Displacement.
      The default Morpher Displacement Specification is Incremental with the Morpher Incremental Linear Displacement [0.0,0.0,0.0] m.
    2. To assign the rotation motion that specifies the displacement, select the Physics Values > Morpher Rigid Boundary Motion and set Rigid Motion to Rotation_outer:


  4. You set the inner wall to a Constraint morpher boundary (the sliding surface of the morphing process). To set this boundary condition:
    1. Select the Regions > PumpFluid > Boundaries > Inner.wall > Physics Conditions > Morpher Specification and set Specification to Constraint.
    2. Select the Physics Conditions > Morpher Constraint Specification node and set Constraint to Part Surface.
    3. To specify the part surface, select the Physics Values > Part Surface node and add Inner.wall to Part Surface.
    4. To make sure that the assigned Part Surface moves correctly, select the Physics Values > Morpher Rigid Boundary Motion node and set Rigid Motion to Rotation_inner.
    5. Since the boundary vertices can freely move on the Part Surface, select the Physics Conditions > Reference Frame Specification node and set Option to Local Reference Frame.
      This option allows the boundary to have its own rotating frame attributes.
    6. In the Physics Values > Boundary Reference Frame Specification node, set Reference Frame to ReferenceFrame for Rotation_inner.
  5. All the other boundaries of the region PumpFluid are also set to use the morpher Constraint boundary condition, but with the Boundary Plane constraint rather than Part Surface. The Boundary Plane constraint means that vertices can move only in the plane of the boundary. To set this boundary condition:
    1. Multi-select the boundaries Outer.bottom, Outer.top, Outer.top[Inlet/PumpFluid], Outer.top[Outlet/PumpFluid] , right-click and select Edit.
    2. In the Multiple Objects dialog, select the Physics Conditions > Morpher Specification node and set Specification to Constraint.
      The default Morpher Constraint Specification type is Boundary Plane.
    NoteWhen using the Boundary Plane constraint, the morpher computes a guide plane based on the vertices on the boundary surface using least-squared fitting. To make sure that the calculated guide plane intersects clearly with the part surfaces, you have previously extended the surfaces Inner.bottom and Inner.top in the direction normal to the boundary plane.
In the region PumpFluid, the boundaries Outer.bottom and Outer.top are not moving boundaries in the real case. Therefore, you set their Reference Frame Specification to follow the laboratory reference frame.
  1. Multi-select the Outer.bottom and Outer.top, righ-click on any one of them and select Edit.
  2. In the Multiple Objects dialog, select the Physics Conditions > Reference Frame Specification node and set Operation to Lab Frame.