[Benito89]

Hi everyone, I need some help with writing a UDF. I need write a code for simulating a car airbox for my final tesis, where you have an velocity inlet costant of 53m / s and 4 output tubes that have a mass flow outlet of 0.0045kg / s each. This outflows should have a pulsed output flow. Furthermore the exit from the pipes should take place in this order like a normal engine ignition configuration.(pipe 1;pipe 3;pipe 4;pipe 2).
Can you help me with this issue.
Thank you

[AlexanderZ]

you should describe functionality of your UDF in more details.
you should put here your code, so other members could fix it

if you wanna time dependent outflow mass-flow rate -> you can use DEFINE_PROFILE macro

you can find good example in Ansys Fluent Customization manual (use google)

best regards

[Benito89]

Thanks to your instructions, I found the udf code (2.3.20.10. Example 8 - Target Mass Flow Rate UDF as a Function of Physical Flow Time) and I adapted it to my needs.
#include "udf.h"
DEFINE_PROFILE(tm_pout2, t, nv)
{
face_t f ;
real flow_time = RP_Get_Real("flow-time");
if (flow_time < 0.2)
{
printf("Time = %f sec. \n",flow_time);
printf("Targeted mass-flow rate set at 0.0 kg/s \n");
begin_f_loop(f,t)
{
F_PROFILE(f,t,nv) = 0.0 ;
}
end_f_loop(f,t)
}
else
{
printf("Time = %f sec. \n",flow_time);
printf("Targeted mass-flow rate set at 0.045 kg/s \n") ;

begin_f_loop(f,t)
{
F_PROFILE(f,t,nv) = 0.045 ;
}
end_f_loop(f,t)
} }


Only it always gives me a solution as 0. How can I change it?
18 1.6981e-01 4.1807e-03 2.9109e-03 2.6440e-03 0:00:30 2
Time = 0.000000 sec.
Targeted mass-flow rate set at 0.0 kg/s
Time = 0.000000 sec.
Targeted mass-flow rate set at 0.0 kg/s
Time = 0.000000 sec.
Targeted mass-flow rate set at 0.0 kg/s
Time = 0.000000 sec.
Targeted mass-flow rate set at 0.0 kg/s
19 1.7504e-01 4.3283e-03 3.0188e-03 2.6914e-03 0:00:16 1
Time = 0.000000 sec.
Targeted mass-flow rate set at 0.0 kg/s
Time = 0.000000 sec.
Targeted mass-flow rate set at 0.0 kg/s
Time = 0.000000 sec.
Targeted mass-flow rate set at 0.0 kg/s
Time = 0.000000 sec.
Targeted mass-flow rate set at 0.0 kg/s
20 1.8807e-01 5.0708e-03 3.5272e-03 3.1685e-03 0:00:00 0

[AlexanderZ]

#include "udf.h"
DEFINE_PROFILE(tm_pout2, t, nv)
{
face_t f ;
real flow_time;
flow_time = RP_Get_Real("flow-time");
if (flow_time < 0.2)
{
printf("Time = %f sec. \n",flow_time);
printf("Targeted mass-flow rate set at 0.0 kg/s \n");
begin_f_loop(f,t)
{
F_PROFILE(f,t,nv) = 0.0 ;
}
end_f_loop(f,t)
}
else
{
printf("Time = %f sec. \n",flow_time);
printf("Targeted mass-flow rate set at 0.045 kg/s \n") ;

begin_f_loop(f,t)
{
F_PROFILE(f,t,nv) = 0.045 ;
}
end_f_loop(f,t)
} }

best regards

[Benito89]

Thank you very much AlexanderZ, it works perfectly.
But now I have another request, sorry if I always ask but it's the first time I use FLUENT. If instead of imposing a constant input speed, I set a pulsating speed written like this:
#include "udf.h"

DEFINE_PROFILE (my_velocity, thread, position)
{
face_t f;
real t = CURRENT_TIME;

begin_f_loop (f, thread)
{
F_PROFILE (f, thread, position) = - 26.76 * cos (t * 3.14159) + 26.76 ;
}
end_f_loop (f, thread)
}

I always associate this code with that of the always pulsating air outlet mass.
Can you do this? and above all the program how do you understand that the t of the two codes must be the same at the same time?For example if t = 2 should return 0 from both codes

[AlexanderZ]

I didn't understand, what you want

if you want to define 2 boundary conditions in your UDF you can use 2 DEFINE_PROFILE macros (same way as you did before):

#include "udf.h"
DEFINE_PROFILE(tm_pout2, t, nv)
{
...
}
DEFINE_PROFILE (my_velocity, thread, position)
{
...
}

your second UDF is correct also.

t (which is time probably) is the same for the whole your simulation as it is controlled by solver, but you only check its value using macro

flow_time = RP_Get_Real("flow-time");

I recommend you to use this macro
Dont do this:

real flow_time  = CURRENT_TIME;

but use

real flow_time ;
flow_time = CURRENT_TIME;
or
flow_time = RP_Get_Real("flow-time");

best regards

[Benito89]

Thanks AlexanderZ, you are of great help.
I tried to channel the air sequentially like this: tube1, tube3, tube4, tube2. I did it through this code:
#include "udf.h"
DEFINE_PROFILE(tm_pout, t, nv)
{
face_t f ;
real flow_time;
flow_time = RP_Get_Real("flow-time");
if (flow_time < 0.2)
{
printf("Time = %f sec. \n",flow_time);
printf("Targeted mass-flow rate set at 0.0 kg/s \n");
begin_f_loop(f,t)
{
F_PROFILE(f,t,nv) = 0.0 ;
}
end_f_loop(f,t)
}
else
{
printf("Time = %f sec. \n",flow_time);
printf("Targeted mass-flow rate set at 0.045 kg/s \n") ;

begin_f_loop(f,t)
{
F_PROFILE(f,t,nv) = 0.045 ;
}
end_f_loop(f,t)
} }
DEFINE_PROFILE(tm_pout3, t, nv)
{
face_t f ;
real flow_time;
flow_time = RP_Get_Real("flow-time");
if (flow_time < 0.3)
{
printf("Time = %f sec. \n",flow_time);
printf("Targeted mass-flow rate set at 0.0 kg/s \n");
begin_f_loop(f,t)
{
F_PROFILE(f,t,nv) = 0.0 ;
}
end_f_loop(f,t)
}
else
{
printf("Time = %f sec. \n",flow_time);
printf("Targeted mass-flow rate set at 0.045 kg/s \n") ;

begin_f_loop(f,t)
{
F_PROFILE(f,t,nv) = 0.045 ;
}
end_f_loop(f,t)
} }
DEFINE_PROFILE(tm_pout4, t, nv)
{
face_t f ;
real flow_time;
flow_time = RP_Get_Real("flow-time");
if (flow_time < 0.4)
{
printf("Time = %f sec. \n",flow_time);
printf("Targeted mass-flow rate set at 0.0 kg/s \n");
begin_f_loop(f,t)
{
F_PROFILE(f,t,nv) = 0.0 ;
}
end_f_loop(f,t)
}
else
{
printf("Time = %f sec. \n",flow_time);
printf("Targeted mass-flow rate set at 0.045 kg/s \n") ;

begin_f_loop(f,t)
{
F_PROFILE(f,t,nv) = 0.045 ;
}
end_f_loop(f,t)
} }
DEFINE_PROFILE(tm_pout2, t, nv)
{
face_t f ;
real flow_time;
flow_time = RP_Get_Real("flow-time");
if (flow_time < 0.5)
{
printf("Time = %f sec. \n",flow_time);
printf("Targeted mass-flow rate set at 0.0 kg/s \n");
begin_f_loop(f,t)
{
F_PROFILE(f,t,nv) = 0.0 ;
}
end_f_loop(f,t)
}
else
{
printf("Time = %f sec. \n",flow_time);
printf("Targeted mass-flow rate set at 0.045 kg/s \n") ;

begin_f_loop(f,t)
{
F_PROFILE(f,t,nv) = 0.045 ;
}
end_f_loop(f,t)
} }
DEFINE_PROFILE (my_velocity, t, position)
{
face_t f;
real flow_time;
flow_time = RP_Get_Real("flow-time");

begin_f_loop (f, t)
{
F_PROFILE (f, t, position) = - 26.76 * cos (flow_time * 3.14159) + 26.76 ;
}
end_f_loop (f, t)
}

but now I would like to impose a new condition:
the tube1 opens at 0.2s and closes at 0.3s;
the tube3 opens at 0.3s and closes at 0.4s;
the tube4 opens at 0.4s and closes at 0.5s;
the tube2 opens at 0.5s and closes at 0.6s.
When the tube2 closes, the tube1 must be opened again and then start again with the cycle, how can I do?

[AlexanderZ]

use sin with period 0.6s - 0.2 s = 0.4 s

best regards

[Benito89]

Excuse ignorance but where do I put this function?

[AlexanderZ]

in my vision, you should put it in if statement:

if (flow_time*sin(your_function) > 0)

best regards