CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

Mass in and mass out BCs advice

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   September 20, 2020, 08:58
Default Mass in and mass out BCs advice
  #1
New Member
 
Mudit Agarwal
Join Date: Sep 2020
Posts: 2
Rep Power: 0
troublesome52 is on a distinguished road
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!
Attached Images
File Type: png Meshing.PNG (104.4 KB, 8 views)
File Type: png Residual.PNG (28.3 KB, 7 views)
troublesome52 is offline   Reply With Quote

Old   September 20, 2020, 17:28
Default
  #2
lei
New Member
 
Lei Chen
Join Date: Mar 2010
Posts: 21
Rep Power: 14
lei is on a distinguished road
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.
lei is offline   Reply With Quote

Old   September 21, 2020, 00:17
Default
  #3
New Member
 
Mudit Agarwal
Join Date: Sep 2020
Posts: 2
Rep Power: 0
troublesome52 is on a distinguished road
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!
Attached Images
File Type: png mass_out.PNG (55.6 KB, 3 views)
File Type: png Residual_2.PNG (22.8 KB, 1 views)
File Type: png Meshing_2.PNG (42.8 KB, 2 views)
troublesome52 is offline   Reply With Quote

Old   September 21, 2020, 09:42
Default
  #4
lei
New Member
 
Lei Chen
Join Date: Mar 2010
Posts: 21
Rep Power: 14
lei is on a distinguished road
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.
lei is offline   Reply With Quote

Reply

Tags
fluent 2020 r2, residuals fluent

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On



All times are GMT -4. The time now is 22:56.