Thank you vert much, sir.  I 'll try unstruct mesh if it doesn't work.

By the way I really want to know if I can use C-topology or H-topology for overset method. As far as I know, the component zone and background zone have to get similar-size mesh at the overset boundary. So if I don't use O-topology, the very tiny boundary mesh will be exposed, mesh size can't match well.

thank you again, I appreciate it.