full moment of inertia or half?

Henry Arrigo · February 27, 2015, 14:04

Hi all,
I want to model a rigid body motion. I am going to take the advantage of the symmetrical nature of the problem and just model the half of geometry. So my question is should I use the full moment of inertia and mass in my simulation or half moment of inertia and half of mass?

Thanks

ghorrocks · February 28, 2015, 06:42

If you are using symmetry you should halve it.

gautami · April 15, 2018, 12:22

Dear Mr. Horrocks,

To simulate a floating cylinder (using 6DOF rigid body motion solver) with symmetryPlane option, dividing the computational domain into half, does the mass and moment of inertia of cylinder defined in the dynamicMeshDict be divided to half. Taking full mass sinks the cylinder. Kindly please advise. Thank You.

Most&More,
Gautami Erukulla.

gautami · April 15, 2018, 12:34

Dear Mr. Horrocks,

To simulate a floating cylinder (using 6DOF rigid body motion solver) with 3 DOF motion in heave, surge and pitch, and be restrained with linear spring, should the spring stiffness be taken half value for half the domain using symmetry plane option. Kindly please advise. Thank You.

Most&More,
Gautami Erukulla.

ghorrocks · April 16, 2018, 06:23

You appear to be using OpenFOAM. Try the OpenFOAM forum.

gautami · April 16, 2018, 08:30

Dear Mr. Horrocks,

Thank you for your reply. Will do as suggested.

Most&More,
Gautami Erukulla.

vava10 · March 6, 2021, 14:18

Hey,

I am simulating a ship. Due to symmetry I am only working with half of the geometry. for Dynamic mesh I have following doubts

1. centre of mass - the centre of half or full geometry
2. moment of inertia -moment of inertia of half geometry or (moment of inertia of full geometry)/2

Thanks in advance
vava10

evcelica · March 15, 2021, 02:31

COM should be original location (on symmetry plane)
It should also be moment of inertia of full geometry / 2. Though this should be the same as half the geometry since the center of mass should be specified as the same.

vava10 · March 15, 2021, 05:21

Thank you for your reply.

I have few more question in dynnamicMeshDict

1. rhoInf : what exactly is this value? In some ship simulation cases it is one and in one I saw it was assigned the value of density of water.

2. In DTCHullMoving the setup uses the entire geometry for meshing even though one half of the geometry is analysed. Does this affect the values of mass entries in DynamicMesh

Thanks in advance

ghorrocks · March 15, 2021, 18:12

Please refer to post #5:

Quote:

You appear to be using OpenFOAM. Try the OpenFOAM forum.

February 27, 2015, 14:04	full moment of inertia or half?	#1
Henry Arrigo Senior Member Henry Arrigo Join Date: Jun 2010 Location: Italy Posts: 100 Rep Power: 15	Hi all, I want to model a rigid body motion. I am going to take the advantage of the symmetrical nature of the problem and just model the half of geometry. So my question is should I use the full moment of inertia and mass in my simulation or half moment of inertia and half of mass? Thanks

February 28, 2015, 06:42		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,692 Rep Power: 143	If you are using symmetry you should halve it. aero_head likes this.

March 6, 2021, 14:18	Center of mas annd moment of inertia	#7
vava10 Member Deutschland Join Date: Sep 2020 Posts: 69 Rep Power: 5	Hey, I am simulating a ship. Due to symmetry I am only working with half of the geometry. for Dynamic mesh I have following doubts 1. centre of mass - the centre of half or full geometry 2. moment of inertia -moment of inertia of half geometry or (moment of inertia of full geometry)/2 Thanks in advance vava10

March 15, 2021, 02:31		#8
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,167 Rep Power: 23	COM should be original location (on symmetry plane) It should also be moment of inertia of full geometry / 2. Though this should be the same as half the geometry since the center of mass should be specified as the same. vava10 likes this.

March 15, 2021, 05:21	half geometry	#9
vava10 Member Deutschland Join Date: Sep 2020 Posts: 69 Rep Power: 5	Thank you for your reply. I have few more question in dynnamicMeshDict 1. rhoInf : what exactly is this value? In some ship simulation cases it is one and in one I saw it was assigned the value of density of water. 2. In DTCHullMoving the setup uses the entire geometry for meshing even though one half of the geometry is analysed. Does this affect the values of mass entries in DynamicMesh Thanks in advance

April 15, 2018, 12:22		#3
gautami Member Gautami Erukulla Join Date: Mar 2009 Posts: 71 Rep Power: 17	Dear Mr. Horrocks, To simulate a floating cylinder (using 6DOF rigid body motion solver) with symmetryPlane option, dividing the computational domain into half, does the mass and moment of inertia of cylinder defined in the dynamicMeshDict be divided to half. Taking full mass sinks the cylinder. Kindly please advise. Thank You. Most&More, Gautami Erukulla.

April 15, 2018, 12:34		#4
gautami Member Gautami Erukulla Join Date: Mar 2009 Posts: 71 Rep Power: 17	Dear Mr. Horrocks, To simulate a floating cylinder (using 6DOF rigid body motion solver) with 3 DOF motion in heave, surge and pitch, and be restrained with linear spring, should the spring stiffness be taken half value for half the domain using symmetry plane option. Kindly please advise. Thank You. Most&More, Gautami Erukulla.

April 16, 2018, 06:23		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,692 Rep Power: 143	You appear to be using OpenFOAM. Try the OpenFOAM forum. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

April 16, 2018, 08:30		#6
gautami Member Gautami Erukulla Join Date: Mar 2009 Posts: 71 Rep Power: 17	Dear Mr. Horrocks, Thank you for your reply. Will do as suggested. Most&More, Gautami Erukulla.

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