[Old Wang]

Hello.
I am trying to simulate heave and pitch motion of a block in 5th order wave. The size of the block is 2.5m*0.273m*0.175m. It's easy to determine the center of mass, and I set half of the block's volume to be submerged, so it is easy to decide the buoyancy, then the weight of the block. And it's also easy to decide the moment of inertia for a cuboid.
I set the 5th order wave length to be L=2.5m, wave height H=0.06m. The computational domain is set to be 10m*6m*6.5m, with water depth 4m. The free surface mesh size of x direction is L/80, and z direction is a quarter of x mesh. Cells slowly get larger as they go far away from the static water level. The time step is selected to satisfy the Courant number smaller than 1. I set it to be 0.004s.
The size of overset mesh is set approximately equal to the background mesh. The red cells in the mesh sence are overset mesh cells.
After about 3s or 4s of physical time, the block flies away or sinks. I don't quite understand why this happens. Do you have any idea about this? Thank you VERY MUCH for your time.
The free surface sence, mesh sence are as follows.

[Old Wang]

Solved. OS mesh didn't match the background mesh. Thanks for your attention.