HI I want to simulate suction phenomena in an internal combustion engine. I put a standard pressure boundary condition for the inlet. But how can I simulate the moving wall of the piston? Thanks for any help.

its very simple. mesh the geometry when piston is at TDC.In Domain tab in cfxpre , in Mesh defoemation click on Region of motion specifed. In the insert tab click on Expressions and enter the expression for piston movement.While giving the boundary condition to piston give mesh motion as specified displacement and write the name that you have given for piston motion.