[esha]

I want to study flow over a 2D plate. I set inlet and outlet as periodic. How may I set boundary condition as the pressure difference for the flow of fluid? Thank you!

[RaiderDoctor]

I don't think what you're looking for will work for your problem. First, how to set up a periodic boundary condition. Try this link (http://www.afs.enea.it/project/neptu...ug/node252.htm) for a description of what it does, and how to use it. As for actually setting up the periodic boundary condition, it appears as though you have to manually do this through the TUI. Try this out if you have an older version of fluent (https://www.youtube.com/watch?v=e-aDm7ihcr0). If you're running a newer version, you can try this command:
mesh/modify-zones/make-periodic
And then just follow the on-screen prompts. Once that's completed, you can click on the newly created boundary condition in the tree and specify a constant pressure gradient across the surfaces (which is what I think you are after).
However, I do not think this will work for your project. The main reason is due to the formation of the boundary layer. The equation for the boundary layer is:
,
where , x = particular distance in question, = density of material, V = free stream velocity, L = characteristic length, and = viscosity of the material. As you increase your length, the boundary layer will increase as well, meaning that this is not something that can be modeled using periodic conditions. I could be wrong, however, but that's just what I'm thinking. Try checking out this as well (http://web.iitd.ac.in/~pmvs/courses/...ndarylayer.pdf).