Hello Rob, thank you for your help as always.

I am sure a real MPM volume fraction did not recognize in a domain!

To test my assumption, I started again with a simple small domain.

1) I created a cemented ball in a simple cylinder by using an MPM technique.

2) I patched the water over the cemented ball to notice when the water falls, whether it interacts with the ball.

3. I ran a simulation. The result is shown below.

The result indicated the VoF of water overlaps with the VoF of MPM. It means both MPM and water did not interact.

I've tried to fix it by adjusting many possible ways (following an Ansys Fluent User's guide), but the MPM still did not recognize in the domain (although the MPM volume fraction shows in a domain!).

Do you have any general suggestions I should try?