Sliding pipes

Clara

Hello to everyone! I've got 2 pipes (a shock absorber). They've got the same diameter, but one of these, has striction on the top, so it's got the same external diameter, but a smaller internal one. One pipe moves by define -> boundary condition -> wall -> moving. I imposed "grid interface" with the little hole of the static pipe as "interface zone 1" and all the section of moving pipe as "interface zone 2". In this way, running fluent, only a small part of fluid pass through the hole: i've got a big reflux along axis of moving pipe. Anyone knows what's error I'done?

Thank you,

Clara.