Warning: this probably sounds like a dumb question

I have tried to reproduce the classic problem of flow through a tube in fluent. It works with pressure boundary conditions, but I want to prescribe a velocity boundary condition instead (and solve for the pressure gradient needed to produce laminar flow). How do I get the theoretical solution using this boundary condition? When I run the solution this way, the velocity profile never reaches a truly parabolic shape - it looks like plug or turbulent flow. I have already accounted for the transition to fully developed flow - the fully developed solution is not parabolic.

The reason I want to know how to do this is that I have some problems for branched tubes I want to solve using a velocity boundary condition. I am trying to reassure myself that I understand the physics of the way Fluent is solving the problem of straight tube flow before I solve a problem that has no analytical solution. Thanks.

Dear Rosy, it's a good idea to start using fluent with some basic flow. When I started with fluent three years ago, I have tried to solve the same problem as you. And I think that the mistakes was the reynolds number; I had a turbulent flow with a number of reynolds over 5000. I think that this is your problem... (for the pressure don't change any thing, just put your right velocity in the inlet and no thing in the outlet...) goog luck lagha