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Solution does not converge

Posted 25 mar 2010, 10:15 GMT-4 2 Replies

Sean Vogel
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Hi there

I'm using Comsol to simulate a oscillating Flow in a tube. At the beginning everything worked quite well. But as soon as I entered the material data of my fluid (in this case Water (rho=1000[kg/m^3] and mu=10^-3[Pa*s])), the solver has problems with the calculation. Most of the time steps are just skipped and marked with an 'out'. At some point Comsol stops solving and responds that there is no convergence.
I then tried, to at least be able to calculate a solution, to increase the tolerance. But it didn't help.

Does anybody has an idea why the solution is not converging?

Thank you
2 Replies Last Post 13 apr 2011, 12:26 GMT-4
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Hello Sean Vogel

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Mark Robison
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Posted: 1 decade ago 13 apr 2011, 10:58 GMT-4
I am having the same problem, if I set the density up to 100 kg/m^3 it works, but if I set it to 1000 kg/m^3 it won't converge. Also, I can't lower the viscosity or else it won't converge either.
I am having the same problem, if I set the density up to 100 kg/m^3 it works, but if I set it to 1000 kg/m^3 it won't converge. Also, I can't lower the viscosity or else it won't converge either.
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 13 apr 2011, 12:26 GMT-4
Hi

you can have many issues, I believe here as you describe it.

First of all "out" means that COMSOL has passed the desired value and is saving it to the results data set file (its not skipping. Depending on how you set the solver it will not necessarily calculate strictly for your points you defined (=strict time stepping) it might take large steps and interpolate. This behavior is OK for asymptotically converging solutions such as diffusion cases, but can be disastrous for periodic excitation functions, hence one need to tweak the time solver settings and change from "Auto" to "strict "or even "intermediate", see the doc

when you have such conversion issues, it can come from initial conditions too far from the final result, or difficulties to get the initial trend (I mostly add some initial conditions and start my stepping such that I have some small load/flux active for the first step. But it could as likely, or perhaps more likely ? come from a dependent variable scaling issue. Then one must estimate the result and give manual scaling values in the solver settings to normalize the result to around "1". This requires often to solve the case in a simple "one physics" variant and to use th preliminary results as scaling factor

--
Good luck
Ivar
Hi you can have many issues, I believe here as you describe it. First of all "out" means that COMSOL has passed the desired value and is saving it to the results data set file (its not skipping. Depending on how you set the solver it will not necessarily calculate strictly for your points you defined (=strict time stepping) it might take large steps and interpolate. This behavior is OK for asymptotically converging solutions such as diffusion cases, but can be disastrous for periodic excitation functions, hence one need to tweak the time solver settings and change from "Auto" to "strict "or even "intermediate", see the doc when you have such conversion issues, it can come from initial conditions too far from the final result, or difficulties to get the initial trend (I mostly add some initial conditions and start my stepping such that I have some small load/flux active for the first step. But it could as likely, or perhaps more likely ? come from a dependent variable scaling issue. Then one must estimate the result and give manual scaling values in the solver settings to normalize the result to around "1". This requires often to solve the case in a simple "one physics" variant and to use th preliminary results as scaling factor -- Good luck Ivar

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