[force_95]

Hi everyone,


I've got a problem with my UDF File. I want to simulate two bubbles moving against each other. Between the bubbles should be water as continous phase.
I wrote a UDF for one bubble. The other bubble cant move. The speed of the moving bubble should depend on the force that the fluid water acts on the bubble. When I'm trying to simulate, I immediatly receive a negative mesh error because the force and speed grow up very strongly. I think that the UDF uses the wrong pressure in order to calculate the force. I guess I need only the the viscous force .
Here s my UDF (0.3 m/s is the take-off speed and should decrease slowly)





#include "udf.h"

static real v_prev = 0.3;

DEFINE_CG_MOTION(bubble, 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 / 0.000055;
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;
}


Hope somebody can help me.
Thank you