Moving Boundary Problem
 

Hi, I am dealing with a simple problem like this.
http://www.youtube.com/watch?v=nWX6On8H3Mg

Will the geometry simply be drawn in usual manner by subtracting the smaller rectangle from larger one and then just change the boundary conditions for the smaller one in fluent accordingly (translational in this case). This doesn't work actually, so can any one help me through this.
What if smaller one is actually a vertically oscillating circle. Do I need to use dynamic mesh or sliding mesh in such problems(if necessary in this case).

Thanks.

Any suggestions?/