The Periodic Condition can be used to remove particles that make contact with a source boundary and re-insert them at some other boundary, the destination. The boundaries must be planar and cannot be curved. At least two boundaries must be selected.

Select an option from the Type of periodicity list: Continuity or User defined.

If Continuity is selected, the particle velocity at the destination boundary is equal to the velocity at the source boundary.

If the source and destination boundaries are parallel, the components of the particle velocity vector are unchanged. If the source and destination boundaries are not parallel, then the two boundaries are interpreted at the extents of a unit cell exhibiting sector symmetry; thus the velocity vector will be rotated based on the orientation of the destination with respect to the source. This is illustrated in Figure 3-1; the angle between the incident particle velocity v1 and the reinitialized particle velocity v2 is equal to the angle between the source and destination boundaries (shown in red).

If User defined is selected, enter values or expressions for the components of the Reinitialized particle velocity vr (SI unit: m/s). The default is zero. Expressions for the reinitialized particle velocity are evaluated at the source boundary, not the destination; for example, expressions in terms of tangential and normal vector components will use the directions tangent or normal to the source boundary. If the velocity is such that the particle will immediately leave the modeling domain through the destination boundary, the particle will instead disappear, to prevent the particle from being subjected to an arbitrarily large number of successive boundary interactions.

In other words, if a particle were to get stuck in an infinite loop between the source and destination boundaries in a Periodic Condition, the particle would instead go through a large but finite number of wall interactions, then disappear if the interaction is still not resolved.

This works in the same way as for the Wall feature. The expressions are evaluated at the source boundary. See New Value of Auxiliary Dependent Variables.