When you load a model into the Meshing or Mechanical application, by default, connections are found between parts that have faces in proximity of each other. Depending on the application, you may want the boundaries common to two parts to be similar, so that contact definitions or non-conformal interface definitions may be more accurate. To get common boundaries between parts in an assembly, you should first imprint all the parts with each other in SpaceClaim or DesignModeler. Then, when you edit the model in the Meshing or Mechanical application, you should define specific contact conditions.
One of those conditions is tolerance, which controls the extent of contact between parts in an assembly. Tolerance is set as a percentage of the bounding box of the assembly. The bounding box is the smallest volume that the assembly will fit in. You can change the tolerance (between -100 and 100) in the Options dialog box under the Mechanical application's Connections category.
The higher the number, the tighter the tolerance. A loose tolerance generally increases the number of contact faces and areas of contact between parts, while a tight tolerance will decrease the number of contact faces.
Each face of a part is checked against the faces of other parts in the assembly. A connection is generated between any faces within the tolerance. You can use overlap tolerances to further limit which faces are in contact if you want only the faces that fully overlap to be found in contact.
When solving in the Mechanical solver, the elements for the two sets of faces that make up a contact pair are compared. Contact elements are generated for element pairs that are within the tolerance, but element pairs outside the tolerance are ignored.
Recommendations for Defining Contact for CFD Analyses
CFD users should be aware of the following recommendations:
The Auto Detect Contact On Attach option controls whether contact detection is computed upon geometry import. If you do not want contact detection to be computed, make sure that it is disabled by selecting from the Ansys Workbench main menu, and then selecting either the Mechanical or Meshing category as appropriate. The option is enabled by default in both applications.
If you are an Ansys Fluent user, you generally want to imprint all the parts with each other in SpaceClaim or DesignModeler as mentioned above. Failing to imprint parts may lead to connections that have cyclic redundancy and may fail to output to the solver.
For Ansys Fluent users, a boundary zone type of INTERFACE is assigned automatically to the contact source and contact target entities that compose contact regions at the time of mesh export. See Special Cases for details.