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 Cluain July 17, 2012 13:01

Mass moment of inertia

For a rigid body simulation that uses symmetry, in which order should you compute the mass moment of inertia.

I am modelling a section that is 1/14 of the entire width of the complete floating structure.

So should I,

Calculate the mass moment of inertia for the entire 14/14 and divide by 14?

Or,

Calculate the mass moment of inertia for 1/14 of the model?

 ghorrocks July 17, 2012 19:21

Obviously for the degrees of freedom which remain you have 1/14 of the full amount. But for the constrained DOF you can put anything you like in there as it is constrained to not move in those directions anyway. But something close to the unconstrained DOF will probably converge better.

 Cluain July 18, 2012 06:50

I take it that means the first option Calculate mass moment of inertia for the whole structure and then use 1/14 of it.

I wouldn't say it is that obvious. My fellow students and supervisor are unsure. As the difference is quite significant I just wanted clarity.

Thanks

 ghorrocks July 18, 2012 07:52

Can you post an image, including the axis directions and the symmetry planes?

 Cluain July 18, 2012 09:36

We looked into the problem in more detail and your first post makes perfect sense now.

I have only one degree of rotational freedom remaining so I only need one value and in that case the two methods provide the same answer.

I’ll still put in the two others like you suggested.

Thanks Ghorrocks

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