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Setting initial values for an iterative solver

Posted Dec 10, 2012, 7:40 p.m. EST Studies & Solvers, Structural Mechanics Version 4.3a 4 Replies

Chris Shor
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Comsol user interface shows some options for setting initial values at the stationary study step at 'Initial values of variables solved for'. These are either zero or a solution - but we want to set the initial values directly as an expression e.g. 'x'. Is there a way to do this via Matlab or Comsol user interface? If there is no way to do this: is it possible to set up a some solution object containing e.g. the values for 'x' and using that as initial value for our iterative solver?
4 Replies Last Post Dec 14, 2012, 12:15 p.m. EST
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Dec 11, 2012, 2:04 a.m. EST
Hi

you can set any function filed or variable in the equation for the initial conditions, just as for any BC in COMSOL, that is the strength of the tool.

Now, many people havent really understood the dfault notations, when you select an Entity (domain for physics, or a Boundary for BC's, you are in fact saying "for all x,y,z belonging to the selected entity" my value is my_expression. So you can write

5*sin(2*pi*freq_spatial*x)*exp(-y^2/sigmay^2)*exp(-kt*t)

if this makes any sens for you ;)

Juast as you can write a scalar that remains constant over all the surface respectively volume of your ENtity.

only some BC as the lumped ports do not accept spatially depending variables, only time variable, as these are defined as average values spread out over the full BC
--
Good luck
Ivar
Hi you can set any function filed or variable in the equation for the initial conditions, just as for any BC in COMSOL, that is the strength of the tool. Now, many people havent really understood the dfault notations, when you select an Entity (domain for physics, or a Boundary for BC's, you are in fact saying "for all x,y,z belonging to the selected entity" my value is my_expression. So you can write 5*sin(2*pi*freq_spatial*x)*exp(-y^2/sigmay^2)*exp(-kt*t) if this makes any sens for you ;) Juast as you can write a scalar that remains constant over all the surface respectively volume of your ENtity. only some BC as the lumped ports do not accept spatially depending variables, only time variable, as these are defined as average values spread out over the full BC -- Good luck Ivar
Chris Shor
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Posted: 1 decade ago Dec 12, 2012, 12:37 p.m. EST
Thank you for your help - unfortunately it doesn't work using the 'Initial values' node given by the Solid Mechanics physics. My displacement field variable 'u' starts at zero in each dimension. When I change one of the values e.g. to 'x' Comsol throws an error. Is this feature also valid for boundary value problems (I'm using periodic boundary conditions) or only for initial vaule problems?
Best regards
Thank you for your help - unfortunately it doesn't work using the 'Initial values' node given by the Solid Mechanics physics. My displacement field variable 'u' starts at zero in each dimension. When I change one of the values e.g. to 'x' Comsol throws an error. Is this feature also valid for boundary value problems (I'm using periodic boundary conditions) or only for initial vaule problems? Best regards
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Dec 12, 2012, 3:24 p.m. EST
Hi

first of all in the initial conditions for solid, you should define material frame hence use upper-case letters X,Y,Z,R

Then if you define only a displacement, the stress values etc are not set accordingly, for that you need to solve a stationary case before your time series (or whatever you plan to make as follow up simulation), and use this stationary case as initial conditions

Hope my simple model (v4.3a upd1) might help

--
Good luck
Ivar
Hi first of all in the initial conditions for solid, you should define material frame hence use upper-case letters X,Y,Z,R Then if you define only a displacement, the stress values etc are not set accordingly, for that you need to solve a stationary case before your time series (or whatever you plan to make as follow up simulation), and use this stationary case as initial conditions Hope my simple model (v4.3a upd1) might help -- Good luck Ivar

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Chris Shor
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Posted: 1 decade ago Dec 14, 2012, 12:15 p.m. EST
Hi Ivar,
thank you again for you superb help here...I finally found out that my problem was not the initial values...they work as you've described it. My model uses periodic boundary conditions and an integration constraint to make the solution distinct. Without the integration constraint everything works fine now...but when I enable it the convergence ot the solver 'jumps' and finally fails to find a solution. Unfortunately setting initial vaules doesn't help here. I've opened a Comsol support case for the issue.
Have a nice weekend and best regards,
Chris
Hi Ivar, thank you again for you superb help here...I finally found out that my problem was not the initial values...they work as you've described it. My model uses periodic boundary conditions and an integration constraint to make the solution distinct. Without the integration constraint everything works fine now...but when I enable it the convergence ot the solver 'jumps' and finally fails to find a solution. Unfortunately setting initial vaules doesn't help here. I've opened a Comsol support case for the issue. Have a nice weekend and best regards, Chris

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