Use the Symmetry node to define a plane of symmetry at selected boundaries in the geometry.

For particle tracing models, the physical interpretation of plane symmetry is as follows: for every particle that exits the domain through a boundary assigned a Symmetry node, a new particle enters the modeling domain at the same location at the same time, with an incoming velocity that mirrors the outgoing velocity of the exiting particle. Thus the model particle is specularly reflected as if it hit a Wall with the Bounce condition.