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Scaling of motional mass
Posted Feb 10, 2013, 5:06 a.m. EST Structural Mechanics 1 Reply
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Hello everyone,
Thanks to this forum I have found out how to calculate the motional mass for different vibrational structures. The calculation of (MPF_mod1.u^2)/(TotalMass) and for v and w gives a nice distribution of motional masses, and taking enough eigenmodes sums up to the total mass. However I think there must be a further normalization I do not understand because for the clamped membrane case I find a participation factor of a half the total mass instead of the 1/4 it is in reality. The same is the case for a clamped wire/beam where the motional mass should be half the total but instead is about 0.7 of the total mass. I attach this case.
Am I just confusing the normalization?
Thanks to this forum I have found out how to calculate the motional mass for different vibrational structures. The calculation of (MPF_mod1.u^2)/(TotalMass) and for v and w gives a nice distribution of motional masses, and taking enough eigenmodes sums up to the total mass. However I think there must be a further normalization I do not understand because for the clamped membrane case I find a participation factor of a half the total mass instead of the 1/4 it is in reality. The same is the case for a clamped wire/beam where the motional mass should be half the total but instead is about 0.7 of the total mass. I attach this case.
Am I just confusing the normalization?
1 Reply Last Post Feb 11, 2013, 7:17 a.m. EST
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Posted:
1 decade ago
Hi
I'm not sure I follow you, why should only a part of the mass participate, I agree if you clampe once, or twice the total amount of "mass" participating in each mode changes, but if you add up all (to INF) you should get to 1 or total mass.
Anyhow it's not fully true to call it mass, its something like an equivalent mass, as relative mass " mode shape " requency give an energy ratio relation.
And do not forget that COMSOl only defines u,v,w mass, not Rx Ry Rz inertias, these *body rotations" are missing, unfortunately
--
Good luck
Ivar
I'm not sure I follow you, why should only a part of the mass participate, I agree if you clampe once, or twice the total amount of "mass" participating in each mode changes, but if you add up all (to INF) you should get to 1 or total mass.
Anyhow it's not fully true to call it mass, its something like an equivalent mass, as relative mass " mode shape " requency give an energy ratio relation.
And do not forget that COMSOl only defines u,v,w mass, not Rx Ry Rz inertias, these *body rotations" are missing, unfortunately
--
Good luck
Ivar