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accelerationDamping in DynamicMeshDict

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Old   October 3, 2018, 04:14
Question accelerationDamping in DynamicMeshDict
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Dear all,
I'm recently trying to deal with a body in waves, but the result shows that the linear and Angular velocity are particularly high, so I want to use accelerationDaming and accelerationrelaxation to solve this problem,but I am not sure the accuracy of the result if I reduce these parameters such as 0.001 or even smaller .
I would be appreciated if anyone could give me any advice.
Thanks in advance.
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Old   October 4, 2018, 09:01
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Search in the www.openfoamwiki.net for your keyword. There is exciting and helpful information.
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Old   October 4, 2018, 22:44
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Dear Tobias,
Thank you for your reply.
But I have already read DynamicMeshDict and Parameter Definitions, What I have learned from that is the following lines :
Be careful with this accelerationRelaxation. Too low of a value will mean that the Body does not respond to the fluid forces correctly.
What confused me was that I haven't seen a similar description of the accelerationDamping, so I want to change this value to solve my problem(I'm not sure about its accuracy, that's why I posted this thread).
I have just learned CFD for two months, my question may seem too simple, I'm sorry about that.
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Old   October 5, 2018, 00:58
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Hi,
itīs always a matter of knowledge. To know what the keyword and respectively the value does, you should check the source code and analyze it. I did some tests with 6DoF concerning flow-induced-rotations, but I donīt know if my set-up was correct (no validation was done). Thus, I cannot give you detailed feedback, sorry for that.
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Old   October 5, 2018, 02:16
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thank you any way
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Old   October 5, 2018, 03:08
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Checked it and the accelerationDamping and accelerationRelaxation factors are one if not specified. However, it will influence the system as follow:

https://cpp.openfoam.org/v6/sixDoFRi...8C_source.html
Code:
 void Foam::sixDoFRigidBodyMotion::updateAcceleration
  270 (
  271     const vector& fGlobal,
  272     const vector& tauGlobal
  273 )
  274 {
  275     static bool first = true;
  276 
  277     // Save the previous iteration accelerations for relaxation
  278     vector aPrevIter = a();
  279     vector tauPrevIter = tau();
  280 
  281     // Calculate new accelerations
  282     a() = fGlobal/mass_;
  283     tau() = (Q().T() & tauGlobal);
  284     applyRestraints();
  285 
  286     // Relax accelerations on all but first iteration
  287     if (!first)
  288     {
  289         a() = aRelax_*a() + (1 - aRelax_)*aPrevIter;
  290         tau() = aRelax_*tau() + (1 - aRelax_)*tauPrevIter;
  291     }
  292     else
  293     {
  294         first = false;
  295     }
  296 }
As one can see, it is a common relaxation method (as we do it for field relaxation).

One remark. I do not get the point of the if/else loop. Actually, I would expect that the last iteration in a PIMPLE loop should be without relaxation (so vice versa).
Maybe there is a bug, or I donīt get the point correctly. However, doing a correct numerical analysis should give you the same velocity (relaxation is for stability).
Maybe you are searching for a damping rather than relaxation or !!! your solution is not stable and thus, you get higher values.

Please see point 5.2 here https://openfoamwiki.net/index.php/P...onSolverFvMesh

PS: A useful discussion might be here (a bit old but I guess it is still interesting to read through): https://bugs.openfoam.org/view.php?id=1675
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