periodic boundary conditions
 

what is the actual significance of periodic boundary conditions(especially in problems pertaining to flow over a cylinder)? how do we create them?

The periodic boundary conditions are used to simulate repeated patterns of geometry. There are two types of periodic boundaries.

a) Allowing pressure drop: In this case there is a definite pressure drop across the periodic boundary, typically in streamwise direction and hence it is also called as streamwise periodic.

b) Not allowing pressure drop: In this case there is no pressure drop across the boundary and also called as translational periodic. The same is called as rotational periodic if you are simulating a cylindrical wedge and specifying the cut sections of wedge as periodic boundaries.

This is very well covered in the documentation, have you had a look at it?

OJ