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November 24, 2012, 12:47 |
Flow-split boundary and reverse flow
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#1 |
New Member
Nicoḷ Demicheli
Join Date: Nov 2012
Location: Italy
Posts: 4
Rep Power: 13 |
Hi,
I have some troubles with a simulation with the flow-split boundary condition. My geometry is similar to a T junction between two pipes, with an inlet and two outlets. The fluid is water, the Re number (and Mach) is quite low so the flow should be laminar, and I am using the segregated solver. The inlet is a velocity-inlet, while the two outlets are set to flow-split. I tried two RANS simulations, one steady and one unsteady. In the steady simulation the residuals go down very well but, after a lot of iterations they start to increase and the solution diverges, because there is a reverse flow on the two outlets. Then I have tried with an unsteady simulation (because there is a vortex shedding near the junction of the two pipes), but I still get the reverse flow. I know that the flow-split doesn't support the reverse flow . In both cases it starts from a cell near the wall of the pipes, where there are very thin prism cells (I need to solve a thin boundary layer for the transport of a passive scalar), and then it expands to the other cells of the outlet. The mesh is of good quality (4 million of cells, polyhedral + prism layer cells), and I have tried to use a low under-relaxation factor, but the reverse flow still occur. The outlets are far away from the junction (more than 20 diameters) so the flow should be straight near them; nothing changes if I use two longer pipes, or a coarser mesh near the outlets. I don't know the pressure at the outlets, so I cannot use two pressure-outlets, and I need to prescribe the flow split between the two pipes. Now I am using one pressure-outlet and two velocity-inlet as boundary conditions, but I don't know if it is correct. Can I use the velocity-inlet for an outlet? Otherwise, what kind of boundary conditions could be suitable for this simulation? Last edited by Nick88; November 25, 2012 at 12:52. |
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November 25, 2012, 22:25 |
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#2 | |
Senior Member
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,740
Rep Power: 66 |
Quote:
You can trick Star into using a velocity inlet or pressure inlet as an outlet, but this is not recommended. Flow split outlet seems to be the correct choice. Remember that the sum of flow-split values should equal 1, did you double-check those? |
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November 26, 2012, 06:01 |
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#3 | |
New Member
Nicoḷ Demicheli
Join Date: Nov 2012
Location: Italy
Posts: 4
Rep Power: 13 |
Quote:
The sum of flow-split values is 1 (0.2 and 0.8). I have tried to use the velocity-inlet as an outlet boundary condition, it seems to work. If I can use it, I will do so. |
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Tags |
boundary condition, flow split, reverse flow |
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