Boundary condition problem

when i generate the mesh with a simple hexahedron, i always meet the follow problem. I adjust the domain extent, it’s not worked. I hope someone can help me with this and explain why this happen.
5_2021-04-18T220247.145559.zip (42.2 KB)

If your geometry is a simple rectangular box, you should not use cut cells and you do not need an stl file. The MFiX domain is already set to use a box by default.

thank you for your reply, i will try your method.