Periodic Condition
Use the Periodic Condition node to apply periodic boundary conditions along one of the x, y, or z directions. The Periodic Condition feature requires the selection of two sets of planar boundaries: source and destination.
Grains close to the source (destination) boundaries can interact with the periodic images of the grains near the destination (source) boundaries. Further, a grain crossing the source (destination) boundaries will automatically be placed at the corresponding locations on the destination (source) boundaries.
Boundary Selection
The selection of the boundaries is important and needs to satisfy the following criteria for the periodic boundary conditions to work correctly.
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The source and the destination boundaries need to be planar. Curved boundaries are not supported. Further, the normals of the selected boundaries must align with one of the x-, y-, or z-axes.
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If a Periodic Condition is applied along one axis, then all the boundaries in the geometry whose normals lie along that axis must be selected. Periodic boundary conditions cannot be selectively applied on a part of the geometry along a chosen direction.
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The source and destination boundaries must also be the exterior boundaries of the geometry along the chosen direction.
The software usually automatically identifies the boundaries as either source boundaries or destination boundaries, as indicated in the selection list. This works fine for cases like opposing parallel boundaries. In other cases, right-click Periodic Condition and select Manual Destination Selection to control the destination. By default it contains the selection that COMSOL Multiphysics identifies.
Destination Selection
This section is available for specifying the destination boundaries, if needed, when the Manual Destination Selection (right-click the parent node) option is selected in the context menu for the Periodic Condition node. You can only select destination boundaries from the union of all source and destination boundaries.