Discussion Forum

Posted: 9 years ago Aug 24, 2015, 12:49 p.m. EDT

Hi, an important thing to specify is that my model is a 3D model.

Posted: 9 years ago Aug 24, 2015, 1:55 p.m. EDT

Hi I suspect that when you add a periodic condition, you still need to restrain one point, or some other boundary to fully constrain the solution, and that could be the issue, that your solution is modulo a constant and that sometimes the numeric make COMSOL to manage to +/- converge to one possible solution, but then by changing the mesh or something else it does not converge anymore. Always check that you have enough BC for your equations, acoustics are second order equations in time and space so you need at least two independent BC's, and reasonable initial conditions -- Good luck Ivar

Posted: 8 years ago Feb 21, 2017, 4:09 p.m. EST

[QUOTE] Hi I suspect that when you add a periodic condition, you still need to restrain one point, or some other boundary to fully constrain the solution, and that could be the issue, that your solution is modulo a constant and that sometimes the numeric make COMSOL to manage to +/- converge to one possible solution, but then by changing the mesh or something else it does not converge anymore. Always check that you have enough BC for your equations, acoustics are second order equations in time and space so you need at least two independent BC's, and reasonable initial conditions -- Good luck Ivar [/QUOTE] Dear Ivar Kjelberg, I also got the problem about the period boundary condition. I am dealing with a simple geometry and when I set the inflow and outflow boundary as the Period Flow Condition, it always tells me "Failed to find a solution. Maximum number of Newton iterations reached. There was an error message from the linear solver. The relative residual (0.041) is greater than the relative tolerance. Returned solution is not converged." But it actually is accuracy to meet the requirements, so where can I set it to avoid the error. Thank you! Jiaqing

Posted: 8 years ago Feb 22, 2017, 8:14 a.m. EST

Hello Jiaqing, It's hard to say for sure what is wrong with your set-up without seeing your mph file, but it's quite possible that Ivar's comment above applies directly to your model as well. Indeed, a Navier Stokes problem does not have a unique solution unless a reference pressure is defined somewhere. See e.g. https://www.comsol.com/community/forums/general/thread/117651 Best regards, Jeff

Posted: 8 years ago Feb 22, 2017, 3:23 p.m. EST

Hi Jeff, Thank you so much for the answering. I attach three pictures which you can have a closer look at the problem I am encountering. Picture 1 shows the model and periodic flow boundary, and Picture 2 shows the Solver Configuration where you can see, I just adopt the default settings and the relative tolerance is 0.001. Picture 3 shows the Converge plot, as you can see the Error is really tiny. But I do not know why the calculation is not stopped when the Error is below 0.001. I am trying to upload the mph file, so you can see it directly.

Attachments:

• Capture1.PNG
• Capture2.PNG
• Capture3.PNG

Posted: 8 years ago Feb 23, 2017, 8:47 p.m. EST

Dear Jeff, I solve it according to your comments, thank you so much. I have not got your meaning until I have a try. Thank you again, it really helps. Jiaqing