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Periodic Boundary Condition

The Periodic or cyclic boundary condition (BC), is used in computational fluid dynamics simulations. It defines a cyclic/repeating situation of the flow across the boundary surface. For this condition, it is mandatory to select two boundary faces that will be treated as if they are physically connected. The flow exiting/entering from one face then enters/exits the other face.

Generally, this boundary condition is used to approximate infinitely long domains into smaller ones.

Symmetry Boundary Condition

Please note that Periodic boundary condition is different from the Symmetry for fluid dynamics and also the Symmetry plane boundary condition for finite element analysis. You can read more about them here.

Key points to remember before applying the periodic boundary condition:

  • The two faces need to be of same size and shape
  • The two faces need not be in the same orientation
  • The face elements of the mesh on the two faces need not be congruent. This means both structured and unstructured meshes can be used.

Speaking math, the flux across the two faces is same in magnitude with opposite signs.

Such a mesh can be ensured in SimScale currently using the standard mesher. To know more about the algorithm click here.

Some common examples where the periodic condition can be used are heat exchangers and on pipes.


SimScale supports only the translational periodic boundaries. This means turbines or fans cannot be used with this boundary condition.

simscale supports translational periodic boundary condition
Figure 1: Two types of periodic boundary condition, (a) Rotational (b) Translational. Only the translational type is supported in SimScale. Source OpenFOAM.
Some common examples where such translational periodic condition can be used are heat exchangers and long pipes.

Last updated: August 12th, 2022