hi,can you give me some information about periodicity especially in traslation.what 's the purpose?while peiodic does it stop iterating once we achieve the number of periodic iterations we prescribe or it can stop once we reach stable sollution? i hope you understand what i need unless i 'll try to describe the pb in details. thanks

That is to model a channel flow, for example, to simulate the flow characteristic at a point infinitely downstream. Hope this is the answer you want.

if i'm working on a long channel can i use a short one for example a half-length one and make it twice periodic?

imagine this, outlet of the the channel section is fed back to the inlet of the same section, what do you think? Isnt that when the solution converged it will represent a point at infinitely downstream?

Utlizing periodic boundary condition is an excellent way of reducing the necessary number of computational cells. One may in fact simulate an infinte long channel using a "2D" domain.