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Nearly incompressible material

Posted 2012/06/06 9:55 GMT-4 Structural Mechanics Version 3.5a 2 Replies

Giacomo Cerretti
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Hi,

I have a question about the nearly incompressible materials: in the hyperelastic materials model there is an option for nearly incompressible material. If it is active I have to insert the value of the bulk modulus, so the question is: activating this option the material is considered as incompressible or not? If the material is incompressible I do not understand why it asks me to enter a value of the bulk modulus.

Thanks so much,

Giacomo Cerretti
2 Replies Last Post 2012/06/13 9:01 GMT-4
Jean-Christophe Lavocat
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Posted: 1 decade ago 2012/06/13 6:27 GMT-4
Indeed, I had exactly the same question. If you find out, I would be very interested in knowing the answer!
Indeed, I had exactly the same question. If you find out, I would be very interested in knowing the answer!
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago 2012/06/13 9:01 GMT-4
Interesting question. When you select nearly incompressible COMSOL applies a mixed formulation meaning that it adds pressure as an independent degree of freedom. This mixed formulation is good because it does not “lock” for high values of bulk modulus. It also works in theory for the case of perfectly incompressible materials (infinite bulk modulus). However, the infinite bulk modulus is more demanding when it comes to solvers since it results in zeros on the diagonal terms of the pressure equations. A very high value of bulk modulus (using the mixed formulation) would be equally accurate and easier to solve. This is my reasoning based on FE theory and I guess it applies to COMSOL as well.

Nagi Elabbasi
Veryst Engineering
Interesting question. When you select nearly incompressible COMSOL applies a mixed formulation meaning that it adds pressure as an independent degree of freedom. This mixed formulation is good because it does not “lock” for high values of bulk modulus. It also works in theory for the case of perfectly incompressible materials (infinite bulk modulus). However, the infinite bulk modulus is more demanding when it comes to solvers since it results in zeros on the diagonal terms of the pressure equations. A very high value of bulk modulus (using the mixed formulation) would be equally accurate and easier to solve. This is my reasoning based on FE theory and I guess it applies to COMSOL as well. Nagi Elabbasi Veryst Engineering

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