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How to define the material to be completely incompressible

Posted Aug 9, 2012, 9:30 a.m. EDT Structural Mechanics Version 4.2a 3 Replies

Thongchai Asavaparjukul
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Hello,
I am using the Solid Mechanics module to simulate the hydraulic structure.
I would like the liquid inside my hydraulic structure to be completely incompressible (not only nearly-incompressible)
First, I have tried to define the Poisson's ratio equal to 0 but it still compressed.
Second, it is necessary to give a Young's modulus value (which normally it does not exist for liquid)
Please give me some suggestion how to define this liquid.
Thank you very much.

Thongchai
3 Replies Last Post Aug 9, 2012, 1:32 p.m. EDT
Steve Kranz
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Posted: 1 decade ago Aug 9, 2012, 11:02 a.m. EDT
I don't have an answer for you, but I may be having a similar problem. I'm modeling a microfluidic network with a single input an d multiple output. My issue is that my input flux is 2 to 6% higher than the sum of the output fluxes. I suspect the problem may be that the fluid is being treated as compressible when it should be incompressible. If you find an answer, please let me know.
I don't have an answer for you, but I may be having a similar problem. I'm modeling a microfluidic network with a single input an d multiple output. My issue is that my input flux is 2 to 6% higher than the sum of the output fluxes. I suspect the problem may be that the fluid is being treated as compressible when it should be incompressible. If you find an answer, please let me know.
Paolo Tricerri
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Posted: 1 decade ago Aug 9, 2012, 11:12 a.m. EDT
Good afternoon,

I also wanted to ask the same question. Using the Structural Mechanics module and modelling the NeoHookean material, I have seen that the constitutive law is in the compressible form.

Does it mean that we the simulation is performed, the material is compressible or the pressure (lagrange multiplyer for the incompressibility) is added implicitly at some point?

Many thanks,

Paolo
Good afternoon, I also wanted to ask the same question. Using the Structural Mechanics module and modelling the NeoHookean material, I have seen that the constitutive law is in the compressible form. Does it mean that we the simulation is performed, the material is compressible or the pressure (lagrange multiplyer for the incompressibility) is added implicitly at some point? Many thanks, Paolo
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Aug 9, 2012, 1:32 p.m. EDT
It seems that this issue is of concern to many.

Thongchai: Perfectly incompressible has Poisson’s ratio = 0.5, and that is harder for most solvers than say a ratio of 0.49999 (I can’t speak for COMSOL’s solvers so I’m not sure if this applies). However, both values of Poisson’s ratio should give the same solution for most models. There is a bigger problem however. You should not use Solid Mechanics to simulate hydraulics. Fluids are best modeled in an Eulerian framework and that’s what the fluid flow physics does and COMSOL interfaces it very accurately with any other structural parts that you may have.

Steve: Make sure you are checking the mass flux and not the volume flux. That could be different due to compressibility or thermal expansion. Also make sure you have a good mesh because the error can be in the post-processing integration of the flux.

Paolo: If the Neo-Hookean material has a low bulk modulus it is “compressible” and can be solved accurately using displacement degrees of freedom only and that’s what COMSOL does. If you select “Nearly incompressible material” the equations change and COMSOL switches to a formulation that uses Lagrange Multipliers to enforce the incompressibility.

Nagi Elabbasi
Veryst Engineering
It seems that this issue is of concern to many. Thongchai: Perfectly incompressible has Poisson’s ratio = 0.5, and that is harder for most solvers than say a ratio of 0.49999 (I can’t speak for COMSOL’s solvers so I’m not sure if this applies). However, both values of Poisson’s ratio should give the same solution for most models. There is a bigger problem however. You should not use Solid Mechanics to simulate hydraulics. Fluids are best modeled in an Eulerian framework and that’s what the fluid flow physics does and COMSOL interfaces it very accurately with any other structural parts that you may have. Steve: Make sure you are checking the mass flux and not the volume flux. That could be different due to compressibility or thermal expansion. Also make sure you have a good mesh because the error can be in the post-processing integration of the flux. Paolo: If the Neo-Hookean material has a low bulk modulus it is “compressible” and can be solved accurately using displacement degrees of freedom only and that’s what COMSOL does. If you select “Nearly incompressible material” the equations change and COMSOL switches to a formulation that uses Lagrange Multipliers to enforce the incompressibility. Nagi Elabbasi Veryst Engineering

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