I have a 2D dynamic mesh case of a piston-cylinder arrangement as attached. I can create a udf to move the piston under certain velocity and the simulation proceeds with no problems. 

What I need is to set the dynamic mesh zones according to a udf expressing the equation of motion of the piston which is :

a = (F + p1*A1 - p2*A2) / m  or if represented with velocity  dv = (F + p1*A1 - p2*A2) * dt / m

where: a .. acceleration, F .. External force, p1,p2 .. pressures on upper and lower piston boundaries respectively, A1,A2 .. areas at same locations and m .. mass of the piston.

I have got through a similar udf in the udf manual for equation of motion in x-dir, but I do not understand how the pressure forces are computed. So, I want to know how the pressure on both sides is represented in the udf in my case.


This udf link is: https://www.cfd-online.com/Forums/fluent/87021-code-dynamic-meshing-udf.html 

Can you help me on this udf ?