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Solver settings for thermal expansion with steep temperature gradients?
Posted Feb 15, 2012, 6:33 a.m. EST Heat Transfer & Phase Change, Studies & Solvers, Structural Mechanics Version 3.5a, Version 4.2 9 Replies
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Hello everybody,
I'm still trying to convert a model from Comsol 3.5a to version 4.2, which simulates a selective laser melting process. In the first place, I had some problems for the distributed laser heat source but I was able to solve that with help from the community. Now I have a new problem:
I have two physic modules, one heat transfer module and one solid mechanics module with thermal expansion. When I try to solve the problem the solver just does not converge after a while. I have quite steep temperature gradients in time and in space, so I think, this might be a problem for the solver. The coupling between the temperature and the expansion is designed as a weak coupling with a seggregated solver. In the first step the temperature distribution is computed and the expansion is computed in the second step. Material properties are temperature dependent. For the time stepping generalized alpha is used, as it was used in the 3.5a version file. For the seggregated steps I use 25 iterations as break condition and a constant damping of 1. As the geometry model dimensions are about 1e-3 m, the displacement tolerances are set 1e-8 m.
If just the temperature distribution is computed, I get a solution which looks fine. As soon as the solid mechanics module is actived, the solver does not converge, so I think the problem might be somewhere in the solid mechanics solving process.
Does anybody have a hint for me, e. g. optimzied solver settings?
I'm still trying to convert a model from Comsol 3.5a to version 4.2, which simulates a selective laser melting process. In the first place, I had some problems for the distributed laser heat source but I was able to solve that with help from the community. Now I have a new problem:
I have two physic modules, one heat transfer module and one solid mechanics module with thermal expansion. When I try to solve the problem the solver just does not converge after a while. I have quite steep temperature gradients in time and in space, so I think, this might be a problem for the solver. The coupling between the temperature and the expansion is designed as a weak coupling with a seggregated solver. In the first step the temperature distribution is computed and the expansion is computed in the second step. Material properties are temperature dependent. For the time stepping generalized alpha is used, as it was used in the 3.5a version file. For the seggregated steps I use 25 iterations as break condition and a constant damping of 1. As the geometry model dimensions are about 1e-3 m, the displacement tolerances are set 1e-8 m.
If just the temperature distribution is computed, I get a solution which looks fine. As soon as the solid mechanics module is actived, the solver does not converge, so I think the problem might be somewhere in the solid mechanics solving process.
Does anybody have a hint for me, e. g. optimzied solver settings?
9 Replies Last Post Feb 15, 2012, 9:02 a.m. EST
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Posted:
1 decade ago
Hi
have you checked the dependent variables of HT and of TS ?
in fact you are doubling up the work.
HT solves for T but T is already included in the TS.
So from what I understand you only need TS without HT in addition
--
Good luck
Ivar
have you checked the dependent variables of HT and of TS ?
in fact you are doubling up the work.
HT solves for T but T is already included in the TS.
So from what I understand you only need TS without HT in addition
--
Good luck
Ivar
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Posted:
1 decade ago
I assume TS means thermal strain module? I've just used the solid mechanics module (solid) and added to the linear elastic material node the thermal expansion node. Because of numerical problems in the heat transfer module some modifications for the values where used, so that the heat transfer module calculates a logarithmic temperature. As input for the thermal expansion node the retransformed temperature is used. This one has the steeper gradients. Does the thermal expansion node also calculate the temperature again?
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Posted:
1 decade ago
Hi
Always check your dependent variables when you add physics.
COMSOl has now in v4.2 included many composed modules "pre-cooked".
But if you mix the complex one you end up duplicating dependent variables. I see a T in both TS and HT so for me you have 2xT, which is one too much. Just disable HT and copy all BC to the corresponding TS heat BC's, it should work ;)
--
Good luck
Ivar
Always check your dependent variables when you add physics.
COMSOl has now in v4.2 included many composed modules "pre-cooked".
But if you mix the complex one you end up duplicating dependent variables. I see a T in both TS and HT so for me you have 2xT, which is one too much. Just disable HT and copy all BC to the corresponding TS heat BC's, it should work ;)
--
Good luck
Ivar
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Posted:
1 decade ago
Thanks for your quick reply, but I'm not sure, if I understand what you mean. In the solid mechanic module (solid) I use, I cannot find any dependent temperature variable or I'm not able to identify it. I just have a temperature as model input. Does this make this temperature a new dependent variable?
Or are you proposing to forget my present model with solid mechanics and heat transfer module and build a new model with the thermal strain module?
Or are you proposing to forget my present model with solid mechanics and heat transfer module and build a new model with the thermal strain module?
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Posted:
1 decade ago
Hi
Under the main physics node TS you have at the bottom of the window a Dependent variable tab, open it up there you will find the dependent variables (they show also up when you select the physics for a new model). But you can also expand the "Equation view" tab and see what is therein
For me TS is enough: you define the "solid" structural BCs and the Thermal Heat BCs
--
Good luck
Ivar
Under the main physics node TS you have at the bottom of the window a Dependent variable tab, open it up there you will find the dependent variables (they show also up when you select the physics for a new model). But you can also expand the "Equation view" tab and see what is therein
For me TS is enough: you define the "solid" structural BCs and the Thermal Heat BCs
--
Good luck
Ivar
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Posted:
1 decade ago
OK, now I opened up a new file with a TS module. There I see also T as a dependent variable for that module, but if I open up the dependent variable tab in the solid module in the other file, there I just see u, v and w but no T. So I still do not understand, why you are still mentioning the TS module, but I will give it a try :-)
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Posted:
1 decade ago
Hi
solid is only u,v,w, while HT is only T but TS is both u,v,w & T
solid itself does not contain the alpha heat exansion in its equation, TS does hence the T
--
Good luck
Ivar
solid is only u,v,w, while HT is only T but TS is both u,v,w & T
solid itself does not contain the alpha heat exansion in its equation, TS does hence the T
--
Good luck
Ivar
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Posted:
1 decade ago
Thanks again. I tried to add all the boundary conditions to a thermal strain module but now, there is some new problem with the distributed heat source. I modeled my laser heat source with a weak form PDE and coupled it with a general extrusion operator to my process model. But now I get an error message, that no consistent initial values could be found and that no origin could be found for the extrusion coupling operator.
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Posted:
1 decade ago
Hi
check carefully all nodes and their links, if you make "selections" from your BC id's it's often easier to reconnect all boundaries to the corresponding BC nodes.
but it's difficult for us out here to understand exactly what is wrong, from a few lines explanation, there are so many fields ;)
--
Good luck
Ivar
check carefully all nodes and their links, if you make "selections" from your BC id's it's often easier to reconnect all boundaries to the corresponding BC nodes.
but it's difficult for us out here to understand exactly what is wrong, from a few lines explanation, there are so many fields ;)
--
Good luck
Ivar