Use the Outlet node to determine what should happen to particles when exiting the modeling domain. The Accumulator (Boundary) subnode is available from the context menu (right-click the parent node) or from the Physics toolbar, Attributes menu.

The Outlet is essentially a simplified Wall with that can only absorb particles, not reflect them.

Select a Wall condition: Freeze (the default) or Disappear. When Freeze is selected, the particle position and velocity remains frozen once the particle has made contact with the outlet boundary. This allows information about the particle velocity, energy and so forth to be recovered at the instant the particles left the modeling domain. For Disappear it means that the particles are not visualized once they strike the outlet boundary.