Mass moment of inertia

Cluain · July 17, 2012, 14:01

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, 20:21

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, 07:50

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, 08:52

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

Cluain · July 18, 2012, 10:36

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

July 17, 2012, 14:01	Mass moment of inertia	#1
Cluain Member Join Date: Oct 2011 Posts: 32 Rep Power: 15	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?

July 18, 2012, 10:36		#5
Cluain Member Join Date: Oct 2011 Posts: 32 Rep Power: 15	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 Last edited by Cluain; July 18, 2012 at 14:24.

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July 17, 2012, 20:21		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	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.

July 18, 2012, 07:50		#3
Cluain Member Join Date: Oct 2011 Posts: 32 Rep Power: 15	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

July 18, 2012, 08:52		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Can you post an image, including the axis directions and the symmetry planes?