[pipin]

Hi,
I have to model a problem of a piston in a cylinder and i want to make an UDF in order to move the piston 2 mm before then 6 mm behind.
have you any idea about the UDF to do this? and any advices for this kind of problem?
Thanks

[-mAx-]

you may use the macro DEFINE_CG_MOTION.
Please refer to chapter 2.6.1 from help
*************
Example:

/************************************************** **********
* 1-degree of freedom equation of motion (x-direction)
* compiled UDF
************************************************** **********/

/#include "udf.h"
static real v_prev = 0.0;

DEFINE_CG_MOTION(piston,dt,vel,omega,time,dtime)
{
Thread *t;
face_t f;
real NV_VEC(A);
real force, dv;

/* reset velocities */
NV_S(vel, =, 0.0);
NV_S(omega, =, 0.0);

if (!Data_Valid_P())
return;

/* get the thread pointer for which this motion is defined */
t = DT_THREAD(dt);
}

/* compute pressure force on body by looping through all faces */
force = 0.0;
begin_f_loop(f,t)
{
F_AREA(A,f,t);
force += F_P(f,t) * NV_MAG(A);
}
end_f_loop(f,t)

/* compute change in velocity, i.e., dv = F * dt /mass velocity update using explicit Euler formula */
dv = dtime * force / 50.0;
v_prev += dv;
Message ("time = %f, x_vel = %f, force = %f\n", time, v_prev, force);

/* set x-component of velocity */
vel[0] = v_prev;
}

[zdeth]

Can anyone explain to me the use of resetting the velocity? "NV_S (vel, =, 0.0);"

I am also trying to model a simple piston in a tube but am having problems with the motion/results