Mass in and mass out BCs advice

troublesome52 · September 20, 2020, 08:58

Hello, I'm a fairly new user of CFD and Fluent and was wondering if I could get some advice on why I might be getting these residual plots. I have a simple bifurcation with a mass inlet and a mass outlet condition, however the the residual plot is a bit all over the place. I have played around a bit with the meshing, and the simulation runs well if I use a pressure outlet condition, but fails with a mass outlet one. The mesh here is a made of quadratic, tetrahedral element I believe and the total element count is 123461 if that helps.

I have attached the meshing and the plot I'm getting. Any advice would be appreciated. Let me know if there is anything I can clarify. It is my first post so please let me know if I have broken any rules or anything. Thank you!

lei · September 20, 2020, 17:28

Hi Mudit, there is no "mass-out" boundary condition in FLUENT, do you mean "outflow" boundary condition?

If it is, then you need to assign the flow out portion of fluid in each outlets (if you have more than 1) and FLUENT will calculate back pressures for those outlets for you. Numercially, it is a bit difficult to converge than the pressure outlet b.c. in which the back pressure is assigned as input. That might be the reason you have high residuals using this b.c.

I can see that your mesh quality need to be improved as well, for instance tet mesh with large cells produce high residuals. You can consider refine your mesh with b.l. inflations, and extend your outlets for several pipe diameters long, in order to get better convergence.

troublesome52 · September 21, 2020, 00:17

Hi Lei, thank you for the response, it was really helpful. I was actually using the mass_flow_outlet condition as shown in the attached image. I don't know what this is for as it clearly seems to fail in my scenario. Upon switching to the outflow condition, I got a much better residual plot, I have also attached this. Everything seems to be good except for the continuity which I've always found difficult to converge in this model. I also refined my mesh, I've shown it in the attached image. Is this what you meant by inflation from the b.l? Thank you again, you were very helpful!

lei · September 21, 2020, 09:42

Yes the new mesh/solution looks great now, thanks for your feedbacks.

The Mass-flow-outlet boundary condition is similar to mass-flow-inlet BC but just to define a negative mass flow rate leaving domain at your outlet. When you use both "mass-flow-inlet" and "mass-flow-outlet", you actually have redundant B.C. (over-constraint to your problem), unless you have identical mass flow rates at inlet and outlet, even though, probably your pressure is not anchored anywhere, so the problem is still unstable. It is recommended to use mass-flow-inlet or velocity inlet at the inlet, combining the pressure outlet or outflow bc at outlet for a well-defined flow problem.

Please let me know if I made some mistakes in understanding it. Hope this helps.

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September 20, 2020, 17:28		#2
lei New Member Lei Chen Join Date: Mar 2010 Posts: 21 Rep Power: 16	Hi Mudit, there is no "mass-out" boundary condition in FLUENT, do you mean "outflow" boundary condition? If it is, then you need to assign the flow out portion of fluid in each outlets (if you have more than 1) and FLUENT will calculate back pressures for those outlets for you. Numercially, it is a bit difficult to converge than the pressure outlet b.c. in which the back pressure is assigned as input. That might be the reason you have high residuals using this b.c. I can see that your mesh quality need to be improved as well, for instance tet mesh with large cells produce high residuals. You can consider refine your mesh with b.l. inflations, and extend your outlets for several pipe diameters long, in order to get better convergence.

September 21, 2020, 09:42		#4
lei New Member Lei Chen Join Date: Mar 2010 Posts: 21 Rep Power: 16	Yes the new mesh/solution looks great now, thanks for your feedbacks. The Mass-flow-outlet boundary condition is similar to mass-flow-inlet BC but just to define a negative mass flow rate leaving domain at your outlet. When you use both "mass-flow-inlet" and "mass-flow-outlet", you actually have redundant B.C. (over-constraint to your problem), unless you have identical mass flow rates at inlet and outlet, even though, probably your pressure is not anchored anywhere, so the problem is still unstable. It is recommended to use mass-flow-inlet or velocity inlet at the inlet, combining the pressure outlet or outflow bc at outlet for a well-defined flow problem. Please let me know if I made some mistakes in understanding it. Hope this helps.