Model of a small displacements piston

Smagmon

Hi there, I am trying to simulate a diaphragm in a complex pressure vessel in CFX. I mean, I was wondering, how can we model a piston with very small displacements. I tried in these to ways:

1- I used a opening to impose the pressure inside the volume and to one of walls I gave a prescribed displacement, one described simply as the displacement of a spring with known stiffness. (displacement = Pressure in wall * Area of wall / Stiffness);

2- An other tried possibility was, to use also a wall with prescribed displacement as input, instead of the opening.

The idea is to simulate a experiment in which the input varies with frequencies and to understand the state of pressure of the compressible fluid in different geometric configurations of the vessel.

Thanks in advance