During the localization procedure in BC Mesh, some percentage of points could not be located. What might be the possible reason?

During the localization procedure in BC_MESH, if some percentage of points cannot be located, several reasons are possible based on the information in the sources:

• Tolerance parameters might be too restrictive. The localization process in BC_MESH relies on projecting nodes from the external mesh (defined in BC_FILE) onto the coupling surface. This projection is controlled by tolerance parameters defined in the TOLERANCE_LIST and TOLERANCE data blocks, specifically GAP_TOL and PLANE_TOL.

  • If the GAP_TOL (an absolute distance value) is too small, nodes that lie at a distance from the coupling surface greater than this value will not be projected. The default GAP_TOL is 0.01.
  • If the PLANE_TOL (a relative value based on the size of the closest element) is too small, nodes whose projections fall just outside the face plane of the coupling surface by more than this relative amount will not be captured. The default PLANE_TOL is 0.01.
  • You can specify a list of tolerance values, and the projection process will iteratively try with each value. If all tolerance values are exhausted and points remain unprojected, they are not considered.

• Geometrical incompatibility between the BC_MESH and the coupling surface. The nodes of the external mesh might not have a corresponding location on the coupling surface within the defined tolerances. This can occur due to:

  • Different mesh resolutions: The mesh in the BC_FILE might be coarser or have a different discretization than the coupling surface mesh.
  • Geometrical mismatches: The shapes or extents of the two surfaces might not perfectly align, causing some nodes to fall outside the projection range.
  • Nodes located too far from the coupling surface: As mentioned earlier, if the distance between a node in the BC_MESH and the coupling surface exceeds the GAP_TOL, it will not be localized.

• Mesh quality issues. Problems with the mesh of either the BC_FILE or the coupling surface (e.g., distorted elements, gaps, overlaps) could hinder the projection process and lead to localization failures.

• Incorrect definition of the coupling surface. If the coupling surface is not defined correctly to cover the region where the boundary conditions from the BC_MESH are relevant, some points might not find a suitable projection target.

• Unit system discrepancies. Although less likely to cause partial localization, if the unit system of the external mesh file specified in BC_MESH is significantly different from the unit system of the Actran analysis, it could lead to geometrical mismatches that prevent proper localization. The user is expected to ensure the local UNIT_SYSTEM specified in the BC_MESH block matches the one in which the external files are defined.

To address the issue of unlocalized points, you might need to:

• Adjust the GAP_TOL and PLANE_TOL values in the TOLERANCE_LIST or a specific TOLERANCE block referenced by the PROJECTION_TOL keyword in the BC_MESH block. You could try increasing these tolerances to capture points that were initially missed.
• Examine the geometry of the BC_MESH and the coupling surface to identify areas of mismatch or large distances. You might need to refine one or both meshes or redefine the coupling surface.
• Ensure the mesh quality is adequate for both the source and target surfaces.
• If you are using the SAMPLED_POINTS method for BC_MESH, note that tolerance values are not used, and points are mapped to the closest identified nodes within the coupling surface. In this case, the density and distribution of nodes on the coupling surface are crucial for successful mapping.

ActranVI provides tools like the Projection Manager that can be used to check the consistency of projection parameters and the quality of the projection, including the ratio of localized nodes. You can use this tool to test different tolerance parameters and visualize the mapping quality.