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Extracting effective modal masses with Comsol 4.1

Posted Jul 11, 2011, 3:57 a.m. EDT Studies & Solvers, Structural Mechanics Version 4.1 2 Replies

Lars Tiedemann
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I worked out how to extract effective modal masses and mass fractions with Comsol 4.1. Modal analysis support in version 4.x has been largely advertised, but I found it hard to find in the documentation how to actually do it. So here's a benchmark analysis of a bending beam with point masses in a 2D analysis.

The trick to get effective modal masses is to scale the solution to the mass matrix. This is not the default setting! In the "Solver", "Eigenvalue Solver" settings, select "Mass matrix" in the "Output", "Scaling of eigenvectors" field and compute the solution. Now you have new variables you can work with, namely the participation factors. In "Results", "Derived Values" add a new "Global Evaluation" and type "MPF_mod1.u^2*1[kg]" in the "Expression" field. This value is the effective mass in global x-direction. Repeat this for all other directions. If you want effective mass fractions, divide the effective mass by the total system mass. You can now plot the effective mass (fractions) over the frequency.
2 Replies Last Post Oct 21, 2011, 2:38 a.m. EDT
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Karen Grutter
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Posted: 1 decade ago Oct 21, 2011, 12:37 a.m. EDT
Thanks for posting this - very helpful. However, I'm having a little bit of trouble using this technique to extract the effective mass for radial contour modes in a ring. The effective mass values I get are several orders of magnitude smaller than they should be. (I expected ~10^-12 kg, but I got ~10^-37 kg)

I got reasonable values for effective mass for a big block on the end of a cantilever, but the same technique doesn't seem to work for the ring. Any ideas on how to tweak it so I get the right values?
Thanks for posting this - very helpful. However, I'm having a little bit of trouble using this technique to extract the effective mass for radial contour modes in a ring. The effective mass values I get are several orders of magnitude smaller than they should be. (I expected ~10^-12 kg, but I got ~10^-37 kg) I got reasonable values for effective mass for a big block on the end of a cantilever, but the same technique doesn't seem to work for the ring. Any ideas on how to tweak it so I get the right values?
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Oct 21, 2011, 2:38 a.m. EDT
Hi

you can also calculate the total mass, by integrating solid.rho over the model, and divide the square of the modal participation factor by the total mass, this gives you the relative participation factor. Based upon the fact that the total sum of all modal participation factor should sum up to the total mass.

If you do not get close enough to the total value, it might be that you are not considering enough modes

One thing is unfortunately still missing: the rotational Rx,Ry,Rz participation factors, summing to the inertia, this is for me the no + shortcoming of COMSOL in Structural all other FEM programmes have these normalisation included. And doing via matlab for a reasonable engineering model is useless, far too large matrices to handle. I hope it will come in a future version ;)

--
Good luck
Ivar
Hi you can also calculate the total mass, by integrating solid.rho over the model, and divide the square of the modal participation factor by the total mass, this gives you the relative participation factor. Based upon the fact that the total sum of all modal participation factor should sum up to the total mass. If you do not get close enough to the total value, it might be that you are not considering enough modes One thing is unfortunately still missing: the rotational Rx,Ry,Rz participation factors, summing to the inertia, this is for me the no + shortcoming of COMSOL in Structural all other FEM programmes have these normalisation included. And doing via matlab for a reasonable engineering model is useless, far too large matrices to handle. I hope it will come in a future version ;) -- Good luck Ivar

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