How do I implement cyclic boundaries where all variable should be transfered excpet from the pressure in order to generate a mass flow where the outlet velocities (and all other variables) equal to those at the inlet? I would like to do so, in order to model a section of a (infinte long) piping system. I use CFX5-5.

Michael Bo

You need to use translational periodicity + a momentum source to set the pressure gradient. You may have to iterate (manually) a little to get the mass flow rate you want.

Neale.

Does anyone have any experience with complicated 2D crossections and obtaining a truly periodic flow rate? For example say a square duct with four large fins sticking 1/3 the edge length towards the center. Is the momentum source simple ie constant everywhere, or is it a complex 2D function in order to create the periodic flow / pressure field? I'm just forseeing a complicated manual iteration process in front of me that I think the code should do for me, but maybe it's not as bad as I'm thinking.

One other option is to use Fluent because it has a periodic / fully developed flow boundary condition available directly to the user without messing around with source terms and flow rates: you can just set the pressure gradient OR the flow rate.

>you can just set the pressure gradient OR the flow rate. That's because the pressure drop is a function of the flow rate and vice versa; e.g. in laminar flow cf=64/Re.

I've been talking with CFX and they will probably include a peridoc boundary with specification of mass flow: "I have requested that the boundary condition your require be in a future release."