Initial conditions - 3D piston/cylinder
 

I am very new to Fluent and have used other CFD programs in the past. The simulation I am attempting to run uses a 3D dynamic mesh to recreate a piston/cylinder bore during the expansion stroke.

I would like to set the initial pressure/temperature inside the cylinder, expand the mesh and calculate the new pressure/temperature throughout the volume.

I've got the dynamic mesh component working well. However, and I imagine this is an amateur's oversight on my part, I cannot seem to figure out how to set the initial condition. I can initialize the solution but as I understand it that simply provides a best-guess to help the solver. Where in Fluent do I actually state that there is a fluid in my volume with a given temp/pressure to start with?

Thanks in advance!