Meshing for 6DOF Model - Dropping motion of rigid structure in fluid

  • Last Post 19 November 2018
eufrat posted this 20 March 2018

Hi guys,

I am currently trying to learn to simulate structures dropping into fluid with the 6DOF model. There are two tutorials on this topic out there: One from ANSYS and one from CFD-Ninja on youtube. These are basically the same tutorials, the only difference is that in the first tutorial, the structure is a quadratic box and a sphere in the second. 

Both tutorials are about the dropping of a rigid structure through air into water in 2D. 

Both of these models have in common, that there is a mesh zone around the structure, which gets assigned passive 6DOF properties in the dynamic mesh settings in Fluent setup. This seems to be quite important, as all of my efforts to get a dynamic mesh to work properly without such a mesh zone have lead to negative volume cells immediately . 

Therefore, I really would like to know how to create such mesh zones around a rigid wall object. I fooled around a little bit with inflation in meshing and I got results which were optically quite similar, but I need the mesh zone as a choosable sub-part of the mesh, so that I can assign 6DOF properties to it in the Fluent setup. 

I attached a picture of the mesh which CFD-Ninja provided along with his tutorial. The mesh around the rigid sphere wall is listed as interior-interior2 in the mesh list.

Also I would appreciate some insights of why this particular mesh zone is needed for these type of simulations. All i can see is, that it seems to be important for the process of remeshing during the rigid bodys motion. In the tutorials, the background of this is not explained.

Thanks in advance!




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eufrat posted this 20 March 2018

Raef.Kobeissi posted this 20 March 2018

I have 2 tutorials for you one in 3D and one in 2 d which could help you:





Let me know if this helps

Raef Kobeissi

eufrat posted this 21 March 2018

Hi Raef,

I already watched your tutorials on youtube with great interest, but they don´t help me regarding my question on how to create the mesh zone around the rigid structure. As I wrote before, I get negative cell volumes immediately when I try to set up a dynamic mesh without this zone.

Thanks for your reply!



Kevinsandoval posted this 16 November 2018

Hola tengo el mismo problema , no entiendo bien el fondo de este mallado , pudiste aclarar la duda? estaria agradecido si puedieras compartirla conmigo , saludos y gracias! 


Please post in English

Hello, I have the same problem, I do not understand the background of this mesh, could you clarify the doubt? I would be grateful if you could share it with me, greetings and thanks!

rwoolhou posted this 19 November 2018

If you've getting negative volumes check the time step size as it's usually because the step is too big. You need to make sure you don't pass through a cell in a single time step.