I am modeling a structure in open air. Fluent works fine when I use a pressure inlet/pressure outlet boundary condition but the code breaks down when I have a velocity inlet/pressure outlet boundary condition. I was wondering if the latter boundary conditions were inappropriate for modeling open air conditions. I will be glad to have a response. Thanks.

Some boundary condition sets allow for more stable convergence, but are not necessarily the best choice of b.c.'s. If you are unsatisfied with your results or choice of b.c.'s, examine other parts of your set up, like the mesh, ur factors, etc.

can you give us details on your problem? What is your explanation for using pout and pin? and then why vel inlet and p out?