[force_95]

Hey all,


I have some problems with my UDF-Code. The code should calculate the decrease of the speed of a body depending on the viscous force that the fluid (water) bears on that body.


this is my code:


#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 Wall_Shear_Force;
real viscous;
real 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 */
viscous = 0.0;


begin_f_loop (f, t)
{
F_AREA (A, f, t);

Wall_Shear_Force = -F_STORAGE_R_N3V(f,t ,SV_WALL_SHEAR)[0];

viscous += Wall_Shear_Force ;


}
end_f_loop (f, t)

/* compute change in velocity, i.e., dv = F * dt / mass
velocity update using explicit Euler formula */

dv = dtime * viscous / 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;
}



I'm really hoping that somebody can help me.

[AlexanderZ]

what kind of problem do you have? describe in details!

for you code if you are using version v19.0 or earlier:

#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 Wall_Shear_Force;
real viscous,force;
real 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;
viscous = 0.0;


begin_f_loop (f, t)
{
F_AREA (A, f, t);
Wall_Shear_Force = -F_STORAGE_R_N3V(f,t ,SV_WALL_SHEAR)[0];
viscous += Wall_Shear_Force ;
}
end_f_loop (f, t)

# if RP_NODE
{
/* Reduce in parallel */
PRF_GRSUM1 (viscous);
}
# endif

/* compute change in velocity, i.e., dv = F * dt / mass
velocity update using explicit Euler formula */

dv = dtime * viscous / 0.000055;

v_prev += dv;

Message0("time = %f, x_vel = %f, force = %f\n", time, v_prev, 
force);

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

compile this code

best regards

[force_95]

thank you for your answer!

The problem with my UDF code is that I get error messages all the time when I try to compile the code in Fluent.
I guess its because I'm not familiar with programming in C.
The goal of the code should be that the body for which I use the code should slow down depending on the viscous force that a fluid exerts.

Kind regards

[AlexanderZ]

code, I've provided is compiled with no errors,
show what error did you get

best regards

[force_95]

I just tried to compile it. It seems to be working. Thank you !


Do you also know what I have to change in the code when I want to extract a text file which shows the viscous force over the flow time? I want to plot the viscous force.


Best regards

[AlexanderZ]

you may use UDMs to store variable you want.
modify this part

begin_f_loop (f, t)
{
F_AREA (A, f, t);
Wall_Shear_Force = -F_STORAGE_R_N3V(f,t ,SV_WALL_SHEAR)[0];
viscous += Wall_Shear_Force ;
F_UDMI(f,t,0) = Wall_Shear_Force ;
}
end_f_loop (f, t)

allocate memory for UDM in fluent GUI:
user-defined -> memory -> UDM locations change to 1

to monitor your UDM_0 make surface report area-weight average of udm_0

best regards

[force_95]

thank you.


Meanwhile I've got another problem. When I insert particles (with a Matlab generated file), I expect the bubble to slow down very faster than without particles. But there is no difference between simulating without and with particles. I dont understand why , because I enabled "Interaction with continous phase". Thats really weird.


best regards

[force_95]

Is it possible that the viscous force I'm calculating is ignoring the particles ? Do I have to enable the energy equation ?