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errror: mass matrix is zero

Posted 21 mag 2016, 05:22 GMT-4 Version 5.0, Version 5.1, Version 5.2 3 Replies

August Mikkelsen
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When I try to run the attached script which solves an eigenvalue equation with different parameters, I get the error: mass matrix is zero.
Does anyone know why this happens?

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3 Replies Last Post 22 lug 2016, 12:11 GMT-4
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 8 years ago 22 mag 2016, 10:36 GMT-4
Hi

But the egenfrequency solves for lambda that is a multiplier of "da" and "ea" (see the equations of the Coefficient PDE node, these are first and second time derivative of your PDE), but you have set both to "0" so the "mass" matrix is null.

What are you trying to analyze exactly ?

--
Good luck
Ivar
Hi But the egenfrequency solves for lambda that is a multiplier of "da" and "ea" (see the equations of the Coefficient PDE node, these are first and second time derivative of your PDE), but you have set both to "0" so the "mass" matrix is null. What are you trying to analyze exactly ? -- Good luck Ivar
August Mikkelsen
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Posted: 8 years ago 24 mag 2016, 07:19 GMT-4
Okay thanks I had not realized that. I have now set d_a=0, but the solution I get makes no sense. Do you know what goes wrong numerically since I get a solution that looks like that?
What I am trying to solve is the eigenvalue equation for a more difficult geometry but I get solutions that do not make sense so I have made this test geometry to try and analyze what goes wrong with the solution.
Okay thanks I had not realized that. I have now set d_a=0, but the solution I get makes no sense. Do you know what goes wrong numerically since I get a solution that looks like that? What I am trying to solve is the eigenvalue equation for a more difficult geometry but I get solutions that do not make sense so I have made this test geometry to try and analyze what goes wrong with the solution.

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Shahin Mohammadnejad
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Posted: 8 years ago 22 lug 2016, 12:11 GMT-4
Hello,

I have built a model using Pressure acoustic, frequency domain module. I am trying to extract the state space matrices using LiveLink for MATLAB. The problem is the mass matrix is evaluated as a Null Matrix.
My system is a simple cylindrical tube which I have especified pressure boundary condition at its inlet and sound soft boundary at the outlet. The walls are also considered sound hard boundry conditions.

I will be really thankful if you give me so hints for solving my problem.

Kind regards,

Shahin Mohammadnejad
Hello, I have built a model using Pressure acoustic, frequency domain module. I am trying to extract the state space matrices using LiveLink for MATLAB. The problem is the mass matrix is evaluated as a Null Matrix. My system is a simple cylindrical tube which I have especified pressure boundary condition at its inlet and sound soft boundary at the outlet. The walls are also considered sound hard boundry conditions. I will be really thankful if you give me so hints for solving my problem. Kind regards, Shahin Mohammadnejad

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