How to create derivative terms to couple air Pipe Flow with Pysical Piston and due air Pressure piston will come in motion

AS i want to couple Air Pipe with Physical(body) Piton and due to that air force Piston will come under motion. As i got information that i have to create derivative terms of Air Pipe Flow and then have to attach with Piston is it correct? Please guide me for same like how to or any example or tutorial.