Hello, I know there are analytical solutions to these flows but I am trying to show my class a short tutorial on fluent, and this is the topic that was most recently covered.

I am trying to model laminar fully developed flow in arbitrarily shaped pipes. The first one I am trying to do is flow inside of a square pipe. I started by just making a square, adding a grid to it and then setting the four sides to walls under the boundary conditions.

From there I'm not sure what to do. I can't use periodic boundary conditions on the face because it is just a 2-D representation...... any ideas?

Again, it is just a square pipe where the flow is into or out of the screen. I'm not sure how to get the periodic BC's to work and also not sure how I can set a pressure gradient (into or out of the screen).

To get what you want you have to move to a fully 3D model of the squared duct and, once put the grid in fluent, give the periodic boundary condition on inflow and outflow (which you can temporarily define as just velocity inlet in gambit) through the TUI...if i remember well you have to type:

grid/modify/mp

then you will be asked about the ID numbers of the two faces you want to make periodic (this information is available under the boudary condition panel of the GUI) and if you want a translational (your case) or rotational periodic boundary.

paola,I want to know hao defind the periodic boundary condiction and velocity inlet boundary condiction at the sametime?

You can't, the inlet velocity profile will emerge as part of the solution.

If you are primarily looking for the steady laminar velocity profile in a section of the duct, you can use a very short duct, set the periodic condition on the inflow-outflow boundaries, initialize the field with 0 and solve with the segregated pressure-based solver.

When you'll get your final solution, you will see that the inlet velocity profile is just the same as any other profile in any section of the duct. Then just get the contour in one of this sections and the work is done.