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Problem with rigidBodyMeshMotion library

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Old   March 7, 2024, 19:46
Default Problem with rigidBodyMeshMotion library
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Adwaith Nath
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Hi,

I am trying to run a simulation for bare hull resistance for KCS using interFoam. I first ran a steady state simulation with LTS till the forces on the hull converged and then using this as the initial solution, I ran anunsteady simulation where the hull was allowed to move along the z direction and rotate about the y axis.

I was using rigidBodyMeshMotion library but I noticed that the trim was around 0.4 degrees which is incorrect.

When I use the sixDoFRigidBodyMesh library the trim is correct which is around 0.16 degrees.

Please note that all the other parameters are the same for both the cases . I have attached the both the dynamicMeshDict files along with this for reference.

When I say correct, it means I have experimental data, and simulations from both starccm and finemarine and all these show the same results which is 0.16 degree trim.

Is this a bug in the rigidBodyMeshMotion libarary or am I doing something incorrect. I checked the tutorial files DTCHullMoving and my setup is similar.

Thanks in advance.

dynamicMeshDicts.zip

sixDof_trim.png

rigidbody_trim.png

Last edited by adwaith.nath; March 7, 2024 at 21:09.
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Old   March 8, 2024, 10:47
Default Differences in code when compared with reference
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Adwaith Nath
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I checked the book mentioned in the post regarding the implementation of the rigidBodyMeshMotion library:
Question on the differences of rigidBodyMotion and sixDoFRigidBodyMotion.

On comparing the pseudo code with the implementation, I noticed some differences. Could this be the source of the error? The code used transpose of the spatial transform matrix Xlambda_ instead of the inverse in a certain part of the code. (I apologize, it was not Hodge Dual but inverse. I misread the book)

I have attached the portions of the code where I noticed the discrepancy.

I wanted to add that I need to use rigidBodyMeshMotion library for the next step of project which is to simulate self-propulsion with a meshed propeller. I have already simulated self-propulsion (using AMI) integrating a PI controller to guess the rps based on the hull resistance and interestingly the simulation did not crash but the results including the trim and the rps were not correct. So, I am sure that the problem with library, if any, is not related to the transformation of the joints as if this was the case then the simulation would have crashed due to the ami surfaces not being correctly aligned. So, my best guess would be that the problem should be related to the forwardDynamics function.

Did any one else have a problem with this library?

Thanks in advance

reference.png

forwardDynamics.C.png

Last edited by adwaith.nath; March 8, 2024 at 22:38.
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