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how to set up and write a cosine UDF

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Old   September 19, 2012, 10:16
Default how to set up and write a cosine UDF
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Chris K
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Hello everybody,

i ve got a problem with my inlet boundary conditions. i want to implement a cosine (or sine) function because, i got a pulsative flow at the inlet. my flow changes every second from 0 to 20 m/s.
one possible function would be 10*cos(x*pi)+10 with 0<x<2.

Now, i just dont know how to setup this function in fluent and how and where to wirte it. I know that i have to define this function somewhere and upload it after in fluent. and how can i tell fluent that one period is like 4 seconds?

Would be really great if someone can help me with this.

regards, chris
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Old   September 19, 2012, 12:47
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You should read the Fluent UDF manual here:
http://hpce.iitm.ac.in/website/Manua...f/main_pre.htm

Alternatively you can create a Profile file with discrete velocity vs. time data and use that. See here:
http://hpce.iitm.ac.in/website/Manua...ug/node301.htm
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Old   September 20, 2012, 08:02
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ok thanks i`ll try it!
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Old   September 20, 2012, 11:22
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Quote:
Originally Posted by chriskit View Post
Hello everybody,

i ve got a problem with my inlet boundary conditions. i want to implement a cosine (or sine) function because, i got a pulsative flow at the inlet. my flow changes every second from 0 to 20 m/s.
one possible function would be 10*cos(x*pi)+10 with 0<x<2.

Now, i just dont know how to setup this function in fluent and how and where to wirte it. I know that i have to define this function somewhere and upload it after in fluent. and how can i tell fluent that one period is like 4 seconds?

Would be really great if someone can help me with this.

regards, chris
Hi,
I don't understand very well: your function should be time dependent but there isn't time in your function..what is x?time?
From what I understand I think you want to change velocity at inlet with a time dependent cos function.
If so, try this udf.

Code:
#include "udf.h"
#include "math.h"
#include "mem.h"

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

  begin_f_loop(f, thread)
    {
          F_PROFILE(f, thread, position) = 10.*cos(t*3.14159)+10.;
    }
  end_f_loop(f, thread)
}
Am I right?
If so copy this udf in a textedit file and save it as somename.c (not txt extension but c extension) and put it in the folder of your cas and dat files.
Open fluent and go to define->user-defined->functions->interpreted.
Choose the c file and interpret it.
Then in your velocity boundary condition choose your interpreted udf ("my_velocity").
Choose an appropriate time step for your unsteady simulation and stop it after some periods (you know 1 period is 2 seconds).

Also, I don't understand this:
..and how can i tell fluent that one period is like 4 seconds?

your period is 2 seconds not 4.
If I don't understand post a sketch of your domain and explain better.

Daniele
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Last edited by ghost82; September 20, 2012 at 11:39.
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Old   September 21, 2012, 06:08
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Hey Daniele,

thanks a lot! You understood it absolute right. And really thanks for the code, my knowledge at C and C++ isnīt that big...

So now, i made it like your proposal but the new problem is that "my_velocity" doesn`t appear at the boundary conditions. I think interpreting worked well but i can`t choose this condition.

Do you know what kind of problem could be the reason for that?

Thank you and regards,

chris
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Old   September 21, 2012, 06:28
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Quote:
Originally Posted by chriskit View Post
Hey Daniele,

thanks a lot! You understood it absolute right. And really thanks for the code, my knowledge at C and C++ isnīt that big...

So now, i made it like your proposal but the new problem is that "my_velocity" doesn`t appear at the boundary conditions. I think interpreting worked well but i can`t choose this condition.

Do you know what kind of problem could be the reason for that?

Thank you and regards,

chris
Hi Chris,
after interpreting your udf, go to boundary condition panel, choose your inlet, then change from "constant" to udf my_velocity.

PS: if you start your simulation with your function velocity at time=0 s is 20 m/s; you should perform a steady state simulation first, with v=constant=20 m/s than switch to unsteady and apply your cosine udf.

Daniele
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Old   September 21, 2012, 09:38
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ok great, thank you, now it works!!!
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