simple 2d open ended cylinder with moving piston: dynamic meshing query
 

Hi,
I am looking to do a 2D model of a moving piston in an open ended cylinder of constant diameter. I have seen relevant examples and they seem to use dynamic meshing, simulating the piston as a moving wall/ boundary condition. After reading through the Fluent manuals, I think that the layering scheme is sufficient for this simple problem. However, I am unsure of how to create a dynamic mesh zone to select which wall will be moving.
When I try to create a dynamic mesh zone it says there motion UDF/ profile is not specified. Do I have to write UDF code in order to create a dynamic mesh zone or is there a simpler way of doing this?


I have attached a screenshot of my issue.
Thanks in advance,
Lisa