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Convergence problem

Hello,
in my problem, there are an inlet and 3 outlets, I know mass flow inlet and the mass fraction of each outlet. The flow is incompressible and laminar and I use Ansys Fluent 14.0 to solve it. I have decided to use velocity inlet and pressure outlet as boundary condition. However, I do not know the amount of pressure outlet. Therefore, I have done over a number of trials to achieve suitable mass fraction in each of my outlets. I have used transient mode for this problem. The amount of residual is good (10-4 for continuity and 10-6 for velocity X & Y), also mass balance is very good; it is less than 10-5%. My problem is that after reaching to suitable mass fraction in each of outlets it stayed constant beyond 700-1000 iterations but then it again started to change and the changes is not periodically. Can I consider it as converged solution during that 500 or 700 iterations? What is your suggestion? I should mention that, mesh study has been done and it did not affect solution. It is kind of instability problems in its physics. How can I get completely steady mass flow outlet.

 flotus1 July 26, 2013 08:36

Since you only have 1 inlet and know the mass flow rate at each of the 3 outlets, you could use velocity outlets and a pressure inlet.
Because the fluid is incompressible, the pressure level at the inlet wont change the solution.
There are other possible combinations of boundary conditions for such a setup, but i am not not familiar with all of them.

As long as the solution changes significantly with additional iterations, you can not consider it as converged somewhere in between.