# "centerOfRotation" handling in sixDoFRigidBodyMotion

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 January 29, 2015, 14:34 "centerOfRotation" handling in sixDoFRigidBodyMotion #1 New Member   Stefano Join Date: May 2014 Location: Genova, Italy Posts: 2 Rep Power: 0 Hi everyone, I am a bit confused having an issue with the sixDoFRigidBodyMotion solver. First of all, what is exactly, from a physical point of view, the object called "centreOfRotation"? I presume it represents the point for which the equation that rules the rotation (i.e. Newton's second law for angular momentum) is written. In fact, all the moment arms are calculated as it was. If I'm right, the theory says that one can choose any arbitrary point to be the center of rotation, having the same result (the moments and the moment of inertia will vary accordingly). Of course, this point moves with the body, i.e. the distance between centreOfRotation and centreOfMass is a constant vector in the body local reference, while in the fixed reference only the magnitude of the vector is constant. In the problem that I am studying, I need to choose "smartly" this point, so that the contribution of certain forces is zero, and the point does not coincide with the center of mass. Now, I'm not being able to set the case successfully. I tried with setting in the constant/dynamicMeshDict an additional centreOfRotation entry, for example: Code: ```... centreOfRotation (-5 0 0); centreOfMass (0 0 0); ...``` that I think is somehow read by the code, but I see in the output that "centreOfMass" is overwritten by "centreOfRotation". Looking at the source code at src/sixDoFRigidBodyMotion/sixDoFRigidBodyMotion/sixDoFRigidBodyMotion.C (lines 103-111), I found strange the following: Code: ```... initialCentreOfMass_ ( dict.lookupOrDefault ( "initialCentreOfMass", vector(dict.lookup("centreOfMass")) ) ), initialCentreOfRotation_(initialCentreOfMass_), ...``` Why is the centreOfRotation assumed to be coincident with the centreOfMass? Did anyone have a similar problem before? Any help is appreciated! Stefano P.S.: the solver application I use is pimpleDyMFoam. Last edited by ste_ge; January 30, 2015 at 04:40.

 Tags centerofrotation, pimpledyfoam, sixdofrigidbodymotion

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