# 4.10.6. Periodic

A periodic boundary condition is tied to an opposing sideset such that flow out of one side of the domain enters the other side of the domain. The periodic boundary should list another sideset to tie itself to, and the usual rule about all sidesets having an applied boundary condition is adjusted slightly in this case, and the tied surface does not need a separate boundary condition block.

    begin periodic boundary condition on surface surface_4
        paired with surface_1
        Search tolerance = 1e-6
    end

Note

Periodic boundary conditions tie nodal values together, so the two sidesets must have identical meshes. The sidesets will be translated based on the vector connecting the sideset centroids and each node much match locations with a translated node to within the specified search tolerance.

The complete list of available commands for the periodic boundary condition and the syntax for the different options can be found in Periodic Boundary Condition On Surface.