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 for infinitely long domains into smaller ones.
Key points to remember before applying the periodic boundary condition:
The two faces need 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 to be congruent
Speaking math, the flux across the two faces is same in magnitude with opposite signs.
Such a mesh cannot be ensured in SimScale currently and hence needs to be locally created and uploaded to the SimScale platform.
Some common examples where the periodic condition can be used are heat exchangers and fans or impellers.
In case of fans, a quarter of a geometry can be modelled by correctly utilizing the cyclic nature of the geometry and the flow, saving approximately a fourth of the computational resources.
Figure 1 shows a perfect example of a periodic boundary condition applied on the faces (marked with red arrows) of a quarter model of a rotating fan saving plenty of time on mesh and other expenses.
Last updated: October 12th, 2020
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