What I mean is, during the deformation of Dynamic mesh, I need to define the expanding wall, Piston and compressing wall as Rigid Body Motion. A rigid body motion need a UDF. How do we specify the UDF when we do not know at what velocity the Piston is moving ? 

How can we make Fluent to automatically move the piston, expanding wall and compression wall as per given input pressure ? (The velocity might vary with different pressure)