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!