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eigenfrequency after calculation of the equilibrium state

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Hello,

I try to model a water bubble between two stones with smooth texture. I describe the problem theoreticaly with the Navier-Stokes equation and create a 2D-model in Comsol with the use of PDE's to calculate in the first step the equilibrium state. My starting geometry is a square and I can calculate the equilibrium state with the use of time dependent studies.
My problem is the calculation of the eigenfrequencies of this new structure.

When I just add a secound step to evaluate the eigenfrequency the results are trivial and make no sense.
I tried to get the mass- and stiffnessmatrix of the FE-mesh with the matlab link to calculate the eigenvalues with Matlab but all values in the massmatrix are zero.


Does anyone find a mistake in my approach or has another idea to calculate the eigenvalues or frequencies?

Maybe some more words to my model:
the only loads are the surrounding pressure and the surface tension.

Thank you very much!
Greetings,
Christopher


1 Reply Last Post 2011年11月7日 GMT-5 05:27

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Posted: 1 decade ago 2011年11月7日 GMT-5 05:27
Maybe I should add that I only have a student license. Thats why I cannot create a laminar flow model.
The navier stokes equation describes the velocity of the flow. The massmatrix is connected with the secound derivative of the displacements of the FE nodes. Maybe I described the problem with the wrong terms and comsol cant connect the velocity with the displacement.
Does anyone ever calculated the eigenvalues of a static fluid problem which is described with the navier stokes equation?
Maybe I should add that I only have a student license. Thats why I cannot create a laminar flow model. The navier stokes equation describes the velocity of the flow. The massmatrix is connected with the secound derivative of the displacements of the FE nodes. Maybe I described the problem with the wrong terms and comsol cant connect the velocity with the displacement. Does anyone ever calculated the eigenvalues of a static fluid problem which is described with the navier stokes equation?

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