Morpher Motion Reference

You control the morpher mesh movement through various settings and properties that Simcenter STAR-CCM+ applies at boundaries, regions, and interfaces.

Morphing Motion Properties

Morphing Order
The order in which the morphing is carried out can affect the result. Choose from one of the following:
Regionwise

Each region associated with a motion is treated as an individual domain, which is morphed independently—one region at a time. Point sets are grouped into such one-region domains according to the morphing motion applied to the point sets.

Use this option, for example, in a fluid-solid case, where the solid region has a solid displacement motion and the fluid region has a morphing motion.

 Example:

Two regions without any point set.

  • Both regions are associated with the same morphing motion.
  • Two regions are associated with different morphing motions.
 Example:

Two regions with a point set.

  • The point set and both regions are associated with the same morphing motion.


    Internally, the point set is connected to two region domains with the same morphing motion. This association is not supported by the morpher solver. You get a warning as follows:



    Note In such situation you are advised to set the morphing order to Motionwise.
  • The point set and two regions are associated to different morphing motions.


    This association is supported by the morpher solver.
Motionwise

With this option all regions and point sets sharing the identical morphing motion are morphed as one unit using one set of vertices.

This approach is recommended for cases where the regions have the same motion applied and where internal interfaces are used between regions.

Two regions without any point set.

Two regions with a point set.
Morphing Method
There are two Morphing Methods that can be selected (Morphing Theory)—BSpline or RBF.

The BSpline morpher algorithm is based on cubic BSplines that perform the interpolation. The algorithm starts from a coarse grid and propagates down to progressively finer levels until no further correction is necessary. The BSpline algorithm is largely automated and requires less user intervention than the RBF morpher.

The RBF morpher collects the control points from the mesh boundaries and produces, by default, one control point per mesh vertex. The number of control points can be increased or decreased in order to optimise solution against calculation time. An automatic thinning algorithm uses an estimated deformation of the mesh to reduce (or 'thin-out') the number of control vertices. Manual control is also provided through Thin Factor values on morphed boundaries. A Thin Factor of greater than 1 increases the number of control points proportionately, whilst a factor less than 1 reduces the number of points.

You specify the error tolerance between the interpolated surface that the morpher creates and the grid vertices under the Solvers > Mesh Morpher > BSpline Settings node.
BSpline Parameters
  • Linear Fitter

    The linear fitter can be applied to both morphing methods: BSpline and RBF.

    The linear fitter uses the control point data to produce a least-squares fit to a linear transformation between new and old coordinates. Motion due to this linear fit is subtracted from the overall motion during interpolation and added back in afterwards to get the final motion (see Linear Fitter in Theory Guide).

    A linear transformation includes all the rigid body modes and some stretching modes:

    • 3 parameters for translation,
    • 3 parameters for rotation,
    • 6 parameters for stretching

    Therefore, the linear fitter solves for a total of 12 unknown degrees of freedom.

    Having deduced the linear transformation, the interpolation is used only for the portion of the overall motion that the linear transformation does not cover. The complete motion of the control points is the sum of the linear transformation of the points from their linear location, plus the deformation that is propagated from the interpolation.

    For example, if the control points undergo pure rigid body motion, the fit to the linear transformation is exact. In this case, the morpher would move the vertices exactly as if you had selected a rigid body motion model, and there would be no morphing due to deformation.

    As another example, consider a case where the control points are on a body that is slightly flexible but undergoes rigid-body motion. The linear fit for this case is not exact, but the difference between it and the full motion is small. These small variations are interpolated to the moving vertex and then the overall rigid body motion (the linear fit) is added back in for the total movement.

    This linear fit option is therefore suited for cases where the rigid body motion usually governs the fluid grid motion.

RBF Parameters
  • Linear Fitter : As for the BSpline morpher
  • Automatic Thin-Out

    Unlike the BSpline morpher, the RBF morpher does not incorporate a hierarchical refinement approach to optimising the number of control points used. However, some flexibility is provided by automatic and manual control of the vertex density using a function called Thin-out.

    When automatic thin-out is activated on the morpher motion, the morpher thins out input vertices automatically to reduce the morpher execution time. The automatic thinner attempts to thin out control vertices by estimating the deformation of the grid due to morphing and placing control vertices efficiently according to this information. This option is active by default.

    The Automatic Thin-out Cl control factor is provided to control the strength of the automatic thinning algorithm. Control vertices are thinned more aggressively as the factor increases. The suggested range of values is from 0.5 (dense) to 5.0 (sparse).

    Studies have shown that automatic thin-out works best when the Morph From Zero property is activated on the Morpher solver. Further improvements are possible when the Linear Fitter is also activated, which separates any rigid body motion of mesh vertices from their deformation motion.