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PDE
Posted Jul 22, 2013, 10:12 p.m. EDT MEMS & Nanotechnology, Plasma Physics, MEMS & Piezoelectric Devices, Computational Fluid Dynamics (CFD), Battery Design, Geometry, Materials, Modeling Tools & Definitions, Parameters, Variables, & Functions, Results & Visualization, Studies & Solvers, Structural Mechanics Version 4.3a 6 Replies
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Can anybody tell me how the variables for stress are defined in COMSOL?
Since I want to define a Coefficient PDE, which its diffusion coefficient (c) is coupled with Mechanical -Stress( such as Von Mise Stress) through decay exponential law:
C= exp(-0.2*s),
where s represent the yield stress such as Von Mises Stress.
Thanks so much in advance.
please answer my question I really need it.
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Go to solid mechanics node, linear elasticity and click on equation view. Here you see all variables used. This is the same for all physics nodes.
For instance, solid.mises is the von mises stress variable, assuming that your interface identifier is called "solid".
You must have the equation view switched on in order to see the variables. This is possible if you go to options, preferences, show and select all.
Suresh
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Thank you very much for your consideration.
I put the variable name of Von Mises stress( solid.mises) in my equation in PDE , but I get this error -1)"attempt to evaluate power of zero" -2) faild to evaluate temporary symbolic derivative variable"-3) failed to evaluate expression.
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Sorry I do not know your equation system, so I cannot comment. There are several topics on this in this forum.
Note that von Mises stress is a positive quantity.
Suresh
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thank you so much for your response.
I am using the coefficient form of pde equation for modeling the tumor growth, and I want to apply the mechanical stress on the diffusion coefficient at this equation . I use one- eightth of circle in 2D Geomery, and use linear elastic material for calculating the stress and displacement, when I put just for example the (soild.pw) in the exponential equation of diffusion coefficient ( c= 0.001*exp(-solid.pw) it works but when I enter (solid.mises) I recently get this error:
Attempt to evaluate negative power of zero.
- Function: ^
Failed to evaluate temporary symbolic derivative variable.
- Variable: mod1.solid.mises@VDN$mod1.vY
- Defined as: ((((3*mod1.solid.II2s)^-0.5)*0.5)*((((((((((mod1.solid.D12)))-(0.33333333333333331483*(((((mod1.solid.D23)))+((((mod1.solid.D22)))+(((mod1.solid.D12))))))))*(2*(mod1.solid.sx-(mod1.solid.I1s/3))))+(((((mod1.solid.D22)))-(0.33333333333333331483*(((((mod1.solid.D23)))+((((mod1.solid.D22)))+(((mod1.solid.D12))))))))*(2*(mod1.solid.sy-(mod1.solid.I1s/3)))))+(((((mod1.solid.D23)))-(0.33333333333333331483*(((((mod1.solid.D23)))+((((mod1.solid.D22)))+(((mod1.solid.D12))))))))*(2*(mod1.solid.sz-(mod1.solid.I1s/3)))))*0.5))*3))
Failed to evaluate expression.
- Expression: d(d((-(mod1.u2t-0.05*(1-mod1.u)*mod1.u)*test(mod1.u2)-0.0010*exp(mod1.solid.mises)*mod1.u2x*test(mod1.u2x)-0.0010*exp(mod1.solid.mises)*mod1.u2y*test(mod1.u2y))*(dvol_spatial),{test@8}),mod1.vY)
I searched a lot to find "coupling mechanical stress on PDE equation " but I could not find any thing.
Thank you so much
Sahar
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Ok, may be I see the problem now, though not sure.
If you specifically look at the expression you have printed above ((((3*mod1.solid.II2s)^-0.5)*0.5), when the term mod1.solid.II2s=0, you will end up with 0^(-0.5) which is probably what COMSOL error refers to. This term I think is a result of COMSOL taking a differential of solid.mises which is introduced in your 'c' definition.
So just before it crashes, check what is the values of solid.II2s, it is possible that all your deviatoric components are zero. So if you replaced with solid.pw (did you mean solid.pm ?) it may have worked because it is non-zero. This also you can check.
Once again I must admit I am not sure.
What you can try to do is create a new variable, let us say, 'von_mises' under 'variables' and then provide the following expression:
von_mises = solid.mises+eps
And then redefine your c = 0.001*exp(-von_mises)
where eps is a small number that COMSOL knows by default or you can specify your own small number.
Suresh
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I am now implying two linear solid mechanic (linear elastic material) in two rectangular(( Rectangular1)and rectangular2 ) ), there is a common inner boundary( number 2) between these two rectangular, i imposed the displacement of 1cm at this boundary 2 in the fallowing figure to see the behavior of the mechanical stress:
My problem is that when I use the identity pair for the boundary 2 and then use continuity for it , I got this error :
"Illegal domain selection".
- Pair: p1
I---------------------------------I(1)
I--------------------------------I (2)
I---------------------------------I(3)
thank you so much ,
No body answer me in linear elastic material , could you please answer me here?