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"centerOfRotation" handling in sixDoFRigidBodyMotion 

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January 29, 2015, 13:34 
"centerOfRotation" handling in sixDoFRigidBodyMotion

#1 
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Stefano
Join Date: May 2014
Location: Genova, Italy
Posts: 2
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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); ... Looking at the source code at src/sixDoFRigidBodyMotion/sixDoFRigidBodyMotion/sixDoFRigidBodyMotion.C (lines 103111), I found strange the following: Code:
... initialCentreOfMass_ ( dict.lookupOrDefault ( "initialCentreOfMass", vector(dict.lookup("centreOfMass")) ) ), initialCentreOfRotation_(initialCentreOfMass_), ... 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 03:40. 

August 4, 2016, 09:49 

#2 
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anonymous
Join Date: Jul 2015
Posts: 21
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Hi,
if you check dynamicMeshDict from pimpleDyMFoam's wingMotion case, you will notice that centreOfRotation is set in the constraints. If you built your case on a specific example, it would have been nice to know which one. iya. 

March 13, 2021, 12:27 
Mass property

#3 
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Deutschland
Join Date: Sep 2020
Posts: 69
Rep Power: 2 
Hey,
I am trying to simulate a ship flow. I have few questions regarding dynamic mesh. Hope someone can help. Its urgent 1. in DTCHull case entire ship is considered in the snappyHexDict even though one half is simulated. Does this not affect the parameters in the entire case setup? So is it fine if I use my entire geometry rather than cutting the geometry into half and then using it? If I use the entire geometry wall all think should I pay attention to? 2. In another thread I asked if I am using the half geometry should the mass properties be of the half geometry and I was told yes. Then if so doesn't it affect the centre of mass and moment of inertia? 3. why is the rhoInf in the dynamic mesh 1. should it not be the density of the ship (mass/volume). I have the weight for my geometry (77 kg) and I got the volume using the Solidwork. 4. The moment of inerta. there are 3 moment of inertia a. Principal axes of inertia and principal moments of inertia: ( kilograms * square meters ) Taken at the center of mass. b.Moments of inertia: ( kilograms * square meters )Taken at the center of mass and aligned with the output coordinate system c. Moments of inertia: ( kilograms * square meters ) Taken at the output coordinate system. My guess for the moment of inertia was about the center of mass since that is what asked in the dynamic mesh. Bot I try to clarify it using Solidwork, I am not getting the same moment of inertia nor mass using the rhoInf as 1 so I tried to find the density using the mass in the dynamicMeah and the volume I got from the software. But I still don't have answers Please please help. I am lost and out of ideas Kind regards vava10 

April 25, 2021, 12:49 

#4 
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Ran
Join Date: Aug 2016
Posts: 68
Rep Power: 6 
When the model is defined to move along the yaxis only, the user
needs to explicitly specify it. It means the model is not allowed to move along the xaxis or zaxis. In terms of the rotation, the user needs to specify how the rotation is defined. For example, the following code only allows the object to rotate along the zaxis, but it is unclear how the rotation exactly is. `centreOfRotation' bridges this gap. Both conditions meet the requirement of rotating along the zaxis, but a rotating centre at Pt1 results in a different rigid body motion compared to the centre at Pt2. ,  constraints  {  yLine  {  sixDoFRigidBodyMotionConstraint line;  centreOfRotation (0.25 0.007 0.125);  direction (0 1 0);  }   zAxis  {  sixDoFRigidBodyMotionConstraint axis;  axis (0 0 1);  }  } ` screen_20210425_122839.png
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Yours in CFD, Ran 

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centerofrotation, pimpledyfoam, sixdofrigidbodymotion 
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