# User guide for dynamicMeshDict

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 July 11, 2019, 04:22 User guide for dynamicMeshDict #1 Senior Member   Sita Drost Join Date: Mar 2009 Location: Arnhem, The Netherlands Posts: 227 Rep Power: 17 Hello all, Is anybody aware of the existence of an up-to-date user guide/tutorial on the use of dynamic mesh in OpenFOAM 6? I'm trying to use the DTCHullMoving tutorial as a basis for my own simulations, but I'm not quite sure about the meaning of some parameters in dynamicMeshDict: - I'm guessing "inertia" represents the inertia tensor, with only 6 moment of inertia values because of symmetry, in the order Ixx, Ixy/yx, Ixz/zx, Iyy, Iyz/zy, Izz. Is that correct? - What exactly does "transform" mean/do? It looks like it moves centreOfMass to the position of the centre of mass/rotation, but why not define centreOfMass at the correct position directly then? - Regarding "innerDistance" and "outerDistance": how are these distances measured, i.e. relative to what point? Any help would be appreciated! Sita Last edited by sita; July 11, 2019 at 07:17.

July 12, 2019, 06:37
#2
Senior Member

Tom Fahner
Join Date: Mar 2009
Location: Breda, Netherlands
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Hello Sita,

Quote:
 I'm guessing "inertia" represents the inertia tensor, with only 6 moment of inertia values because of symmetry, in the order Ixx, Ixy/yx, Ixz/zx, Iyy, Iyz/zy, Izz. Is that correct?
Yes that is correct.

Quote:
 - What exactly does "transform" mean/do? It looks like it moves centreOfMass to the position of the centre of mass/rotation, but why not define centreOfMass at the correct position directly then?
I am not sure about this one. It may be related to having the inertia tensor defined in the (0,0,0) point and the tutorial just shows the possibilities that if your body happens to start in a location/orientation that is not aligned with the Cartesian axes. Is is maybe not exactly the most convenient.

Quote:
 - Regarding "innerDistance" and "outerDistance": how are these distances measured, i.e. relative to what point?
If I understand correctly, every cell with their center point within <innerDistance> of the solidBody will move rigidly with the solid body (these cells are not morphed). Everything outside of the <outerDistance> will not move at all and inbetween there will be an interpolation based on slerp (according to the source code). So it is always with respect to the distance to the body that is moving.

Hope this helps.
Regards,
Tom

 July 12, 2019, 08:05 #3 Senior Member   Sita Drost Join Date: Mar 2009 Location: Arnhem, The Netherlands Posts: 227 Rep Power: 17 Hello Tom, Thanks a lot for your replies, things are getting much clearer for me now! Cheers, Sita

October 12, 2020, 13:57
#4
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Sachin
Join Date: Aug 2014
Location: India
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Quote:
 Originally Posted by sita Hello Tom, Thanks a lot for your replies, things are getting much clearer for me now! Cheers, Sita
What is the unit of inertia term?

Is it area moment of inertia or mass moment of inertia?

 October 13, 2020, 03:27 #5 Senior Member   Sita Drost Join Date: Mar 2009 Location: Arnhem, The Netherlands Posts: 227 Rep Power: 17 Hi Sachin, I never actually checked the equations I'm afraid, but I'd say it's the mass moment of inertia; the area moment of inertia wouldn't make much sense for a 3D object. Also, in the Wiki the momentOfInertia is listed under Mass Properties: https://openfoamwiki.net/index.php/P...onSolverFvMesh. And finally, if you go through this forum, you'll notice that you're not the first one to wonder: moment of inertia in dynamicMeshDict Hope this helps, Sita

December 20, 2020, 16:39
#6
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Chandan Bose
Join Date: Feb 2015
Location: Chennai
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Quote:
 Originally Posted by Sachin m What is the unit of inertia term? Is it area moment of inertia or mass moment of inertia?
Mass moment of inertia, of course.

Best wishes,
Chandan

 March 13, 2021, 13:20 Mass property #7 Member   Deutschland Join Date: Sep 2020 Posts: 69 Rep Power: 5 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 20, 2021, 07:32
#8
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Sachin
Join Date: Aug 2014
Location: India
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Quote:
 Originally Posted by sita Hi Sachin, I never actually checked the equations I'm afraid, but I'd say it's the mass moment of inertia; the area moment of inertia wouldn't make much sense for a 3D object. Also, in the Wiki the momentOfInertia is listed under Mass Properties: https://openfoamwiki.net/index.php/P...onSolverFvMesh. And finally, if you go through this forum, you'll notice that you're not the first one to wonder: moment of inertia in dynamicMeshDict Hope this helps, Sita
Hi Sita,

I have tried using the surfaceInertia tool in openfoam to calculate the value for the dtchull form in openfoam tutorial. But the values obtained using the tool does not match with the values in dynamicmeshdict inthe tutorial.

Did anybody try this?

With Regards,

Sachin M

 May 18, 2021, 22:01 #9 New Member   RICHARD LEONEL VASQUEZ MEZA Join Date: Jun 2018 Location: PERU Posts: 4 Rep Power: 7 hello.. maybe those videos can you help in order to found the properties of the body. also the center of pressure. https://www.youtube.com/watch?v=0gmVWp38KLo https://www.youtube.com/watch?v=-MXho20qm1k&t=2s

December 26, 2021, 13:37
#10
Member

Join Date: Jan 2017
Posts: 70
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Quote:
 Originally Posted by Sachin m Hi Sita, Thank you for the reply. I have tried using the surfaceInertia tool in openfoam to calculate the value for the dtchull form in openfoam tutorial. But the values obtained using the tool does not match with the values in dynamicmeshdict inthe tutorial. Did anybody try this? With Regards, Sachin M
Yes, I faced the same issue. Did you find out what is missing in the calculation?