[saadyw]

I have a 2D dynamic mesh case for a piston-cylinder arrangement as shown in the attached image. I can create a udf to move the piston under certain velocity and the simulation proceeds with no problems. this udf is:
#include "udf.h"

DEFINE_CG_MOTION(fluid_move,dt,vel,omega,time,dtim e)

{

vel[1] =-10.0;

}

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

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

Can you help me on this udf ?