Federico Alzamora Previtali

What you are describing sounds like Overset meshing, where a component mesh will overlap and move through a background mesh. In that case, you will mesh the background and component mesh independently (with overlap), and append each to your Fluent case. Ideally, you would create both domains at the CAD level using Discovery or SpaceClaim.

Regarding Negative volume error, this error results from excessive cell skewness due to excessive boundary motion. Make sure that your time step is small enough to avoid any boundary zone movement to go beyond 1 cell at each time step. If you're using Remeshing and/or Smoothing, make sure your settings are set correctly for your mesh distribution.