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density change due to deformation

Posted Aug 23, 2021, 6:41 a.m. EDT Parameters, Variables, & Functions, Structural Mechanics, Equation-Based Modeling Version 5.6 2 Replies

Boris Sandor
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I have multiple custom PDE's coupled that are modeling wood pyrolysis. They are all solving for the densities of the constituents of the pyrolyzing material. The equations them selves are basically just reaction equations. I now have also coupled a structural mechanics node with a LinElMat with thermal expansion. I would like the densities to change with the deformation but i cant find a way to do so during the simulation. In post thats not a problem since you can just multiply the results with the deformation gradient (everything is calculated in the spatial frame). But since the reaction rates depend on the densities that seems unsatisfying to me. Any help is greatly appreciated, thanks.

2 Replies Last Post Aug 25, 2021, 3:27 a.m. EDT
Henrik Sönnerlind COMSOL Employee

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Posted: 3 years ago Aug 25, 2021, 2:53 a.m. EDT

You can use the multiplication with determinant of the deformation gradient (solid.J) also in your own equations.

This said, having changing densities is always tricky. Is mass conservation to be valid in your calculation domain, or are you losing mass to the surrondings? Note also that the thermal coefficent of expansion in general is not imdependent of density, since it describes the change in volume of a piece of material containg a certain number of molecules.

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Henrik Sönnerlind
COMSOL
You can use the multiplication with determinant of the deformation gradient (solid.J) also in your own equations. This said, having changing densities is always tricky. Is mass conservation to be valid in your calculation domain, or are you losing mass to the surrondings? Note also that the thermal coefficent of expansion in general is not imdependent of density, since it describes the change in volume of a piece of material containg a certain number of molecules.
Boris Sandor
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Posted: 3 years ago Aug 25, 2021, 3:27 a.m. EDT
Updated: 3 years ago Aug 25, 2021, 3:28 a.m. EDT

Thanks for the fast reply!

I assume you mean the derivative of solid.J? Because thats what i came up with as well (as an absorption coefficient) and it seems to be working.

As I have set it up right now I shouldnt loose any mass but when I integrate all densities over the domain the total mass does change and I dont know why.

The density dependence of the thermal expansion shouldnt be too much of a concern in the model since the pointwise transitions happen relatively fast.

Thanks for the fast reply! I assume you mean the derivative of solid.J? Because thats what i came up with as well (as an absorption coefficient) and it seems to be working. As I have set it up right now I shouldnt loose any mass but when I integrate all densities over the domain the total mass does change and I dont know why. The density dependence of the thermal expansion shouldnt be too much of a concern in the model since the pointwise transitions happen relatively fast.

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