Moment calculation for 6DoFs motion object

Ya_Squall2010 · August 18, 2023, 04:56

Hi Foamers,

We know that there're two different libs in OF to calculate the 6DoFs motions of a floating object. The ways they calculate the moment of the object using the functionObject::force lib are however different:

in rigidBodyMeshMotion::solve() we have:
forcesDict.add("CofR", vector::zero);

but in sixDoFRigidBodyMotionSolver::solve() we have:
forcesDict.add("CofR", motion_.centreOfRotation());

the latter make more sence to me. Just wonder how does the former lib corrently calculate the moment without passing the latest CofR to force lib? Or is this a bug?

Thanks and please correct me if I'm wrong.

Ya_Squall2010 · August 20, 2023, 23:25

It turns out that in the rigidBodyDynamic lib, the Articulated-Body Algorithm is implemented, and wherein, the Zero'th body in the linkage tree is fixed no matter how the rest of the "linked" bodies move. This implies that the CoR of entire linked system is always at the ZERO location.

August 18, 2023, 04:56	Moment calculation for 6DoFs motion object	#1
Ya_Squall2010 Member YS Join Date: Jan 2010 Posts: 96 Rep Power: 16	Hi Foamers, We know that there're two different libs in OF to calculate the 6DoFs motions of a floating object. The ways they calculate the moment of the object using the functionObject::force lib are however different: in rigidBodyMeshMotion::solve() we have: forcesDict.add("CofR", vector::zero); but in sixDoFRigidBodyMotionSolver::solve() we have: forcesDict.add("CofR", motion_.centreOfRotation()); the latter make more sence to me. Just wonder how does the former lib corrently calculate the moment without passing the latest CofR to force lib? Or is this a bug? Thanks and please correct me if I'm wrong.

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August 20, 2023, 23:25		#2
Ya_Squall2010 Member YS Join Date: Jan 2010 Posts: 96 Rep Power: 16	It turns out that in the rigidBodyDynamic lib, the Articulated-Body Algorithm is implemented, and wherein, the Zero'th body in the linkage tree is fixed no matter how the rest of the "linked" bodies move. This implies that the CoR of entire linked system is always at the ZERO location.