Troubleshooting Negative Volume Cells

Poor quality or negative volume cells may occur when setting up a morpher analysis. These can be identified by examining the grid at various stages using visualisation tools: such as reports, plots and scalar field functions.

If negative volume cells are reported during the morphing process, the first step is to create a threshold derived part in order to identify the location of these cells. This can be done using the Volume field function for a maximum value of 0 to identify the location of these rogue cells:
Identifying Negative Volume Cells
  1. Right-click the Derived Parts node and choose New Part > Threshold...
  2. In the Create Threshold edit panel, do the following:
    • Ensure that the morphed region is selected in the Input Parts menu.
    • Choose Volume from the Scalar menu.
    • Set the Extraction Mode to All Below.
    • Leave the Threshold Value at its default setting of 0.
    An example of a panel that is set with these properties is shown below. Click Create to generate the threshold part.

    Add the part to a scalar scene to identify the location of any negative volume cells.

    Suggestions for Correcting the Model:
  3. Reduce the morpher solver tolerance.

    Reduce the Morpher Tolerance to 10E-7 to 10E-9. This setting can avoid negative cell volumes occurring during morphing, especially If the mesh contains extremely long and thin prism layers cells.

  4. If the motion includes both deformation and linear displacement, activate the Linear Fitter.
  5. If the RBF morpher method is selected:
    1. If the negative volume cells are close to a moving body, increase the Vertex Thin Factor for the boundary on the body. If possible, split the moving body boundary and apply a larger Vertex Thin Factor just for the area near the negative volume cells. This arrangement allows you to use a lower value (0.25 or below) for the remainder of the boundary.
    2. If the negative volume cells are close to a fixed boundary, follow the same procedure as in (a).
    3. If the negative volume cells are close to a fixed plane boundary, check to see if compression normal to the boundary is causing the negative volumes. If so, increase the damping factor so that a greater number of cell layers are accounted for in the damping zone. If the negative volume cells are being stretched parallel to the boundary, change the boundary to the in-plane type instead.
    4. If the negative volume cells are close to an in-plane boundary, increase the damping factor.
    NoteThe above approach is not relevant for the BSpline morpher.