Tackling High pressure flow simulation in Fluent
Experience divergence when solving a high pressure choked flow
The procedure for tackling this type of high pressure flow is simple but not always intuitive. Here are the strategies that be used to obtain a solution:
1) First assess your domain and find out which parts of your flow domain have high and low pressure. Based on that patch pressure accordingly: patch high pressure zone with high pressure and low pressure zone with low pressure. You may also patch with realistic velocity values but this may not be essential.
2) Now run your case in steady state. If it fails (experience divergence) then follow step 3.
3) (a) If you experience divergence with steady state simulation , turn on the unsteady solver and run the calculations for several hundred or more time steps depending on how quickly the flow field reaches closer to the steady flow. Remember that even with unsteady state solver you may need
to lower the time step size should you experience divergence. Once the solution reaches closer to steady state (you may monitor some points), save the case and data file and then turn on the steady solver and continue running. If you still experience divergence follow the next step (3(b))
(b) If you still experience divergence with the steady solver, you may need to read the unsteady case & data again and run the unsteady case further and repeat 3(a) again.
Caution!! for very high pressure flows such as 2000 psi or more, you may not get a steady solution, especially for 3d choked flow leading to shock waves and the complex phenomenon.