pressure eq. "converges" after few time steps
1 Attachment(s)
Hi everybody,
weird simpleFoam convergence over here, need your help! I have a complex pipes geometry, similar to what sketched in the geom.png file. The two main pipes are connected by fans to the outside, which is represented by a spherical domain. Reynolds is around 21000 on the smallest pipe, thus a launderSharmaKE model is applied, using wallfunction to keep low the cell number. In any case, the mesh is not really fine since I first want to evaluate my setup. BC are standard:
Code:
grad faceMDLimited Gauss linear 0.5; Code:
this is the first one: Code:
p Code:
Time = 1 This does not change when:  reducing the nNonOrthogonalCorrectors;  using the second fvSchemes files;  switching off the turbulence. Any explanation on that? Really no ideas... thank you! maddalena 
Hi,
try lowering the tolerances of the linear solvers. Something in the order of 1e12 might be appropriate. Remember, your simulation is not converged if the residuals of just one equation fall below a certain tolerance. The pressure field depends on the velocity field and vice versa  since your velocity field clearly hasn't converged after the second iteration your whole problem hasn't (would be very surprising if it had) and the pressure equation might and probably will be resolved at some point. Greetings, Felix. 
Dear Maddalena,
I would try also changing the initial conditions: I am not aware of your problem but maybe you can try to force the raising of pressure gradients by assigning non zero values for the velocity in the pipes and zero outside. Regards, Franco 
Thanks Felix and Francesco for your suggestions.
Some observations after some more try: Quote:
Quote:
Quote:
What I have got up to now is a converging but unstable solution: I have bounding epsilon and k warnings, but a convergent solution till time step 295. After that, the max epsilon and k raise suddently and the solution blows away: Code:
Time = 295 Ideas? maddalena 
You could look at the individual cell residuals? That might help you figure out what's causing the instability.

Quote:

I need to do it myself and am just about to look into it :D
You can jump ahead here: http://www.cfdonline.com/Forums/ope...residuals.html 
Quote:
Stay in touch. mad 
Quote:
mad 
Same. Popped it up on github:
http://github.com/lrm29/OpenFOAM.loc...dualCalculator OpenFOAM1.7.x has some errorEstimation libraries which would be nice to use. The residual that simpleFoamResidual calculates needs to be normalised (by the initial state possibly?) so that the changes from step to step can be seen more clearly. 
Quote:
Another thing that may be worth checking is that the mass flow into and out of your domain matches. 
Why my solutiion is not converging?
Hello,
still here with the same problem on a different case: pressure & velocity fields look good and their residual's trend is "converging"; however local value of residuals (simpleFoamResidual) does not looks good and I have weird oscillations on the initial residual of every time step. My mesh (tet mesh with no boundary layer) is nice. I have tried tons of different BCfvSchemesfvSolution settings. No way. May this be due to the high number of cyclic patches I am using? Two fans BC + cyclic sides. I do not what to think now. Suggestions? mad 
Reversed problem!
Ok, maybe this is not the right place where to post, but I hope to get some answer on a problem that is similar to what posted above...
I started this thread speaking of a pressure equation that converges too soon... And I am writing now for a pressure equation that is never solved within the 1000 iterations of a time step! Geometry and bc are similar to what described above, only the pipe geometry is a little bit more complex than what sketched. Check mesh does not complain about that: Code:
Overall domain bounding box (37.4532 6.70564 3.99289e17) (42.605 6.70578 27.2094) fvSchemes and fvSolution are: Code:
grad faceMDLimited Gauss linear 0.5; Code:
p Code:
DILUPBiCG: Solving for Ux, Initial residual = 0.0028965, Final residual = 2.16561e11, No Iterations 7 What I should do? What to change? I really need a help from you! mad 
maddalena
Are you doing some internal loops? 
Hello,
fastest answer ever! Quote:
mad 
:cool:
could U please post more of your log? 
1 Attachment(s)
Yes, sure. Here it is.
As you can see, something strange happens around time step 17 on Uz. On the contrary, pressure does what explained above.:confused: mad 
... and your fvScheme and fvSolution in the studio please)))

2 Attachment(s)
Quote:
mad 

All times are GMT 4. The time now is 19:08. 