Disappearence of a body
 

I'm looking for help with a particular problem in ansys fluent. It comes to studying the disappearance of a body inside a viscous flow: at t = 0 the body (a cylinder) is invested by a flow at V = V1, while at t = t +1 such body must disappear leaving only the flow in the duct. The interior of the cylinder is fluid.
The problem is that in Fluent I can not change the wall of the cylinder into interior.
Why can not I?
Thank you! ;)