[Jacob_Bing]

Hello,

I am working on creating a simulation to model a laser beam incident on the surface of a material. Because the beam of the laser has an intensity distribution similar to a normal distribution curve in 2D, I have been trying to write a UDF to apply that profile to the flux in my model.

The problem I am having is, when I run the simulation, the flux distribution appears to only be calculated at four points. It starts and ends at the correct values of approximately zero flux. However, it creates a horizontal line for the majority of the surface's width at the value that the flux should obtain at the center of the curve. The end points connect to this line with a straight line at each end.

My mesh in this area contains 14 cells along a 7um wide surface the flux is applied to and my UDF is shown below. If anyone could help me fix the UDF or tell me what is going wrong that would be great. Thank you.


#include "udf.h"
#define sig 0.0000035/3.

DEFINE_PROFILE(gauss_flux_small, thread, position)
{
real y[ND_ND];
real x;
face_t f;

begin_f_loop(f, thread)
{
F_CENTROID(y,f,thread);
x = y[2];
F_PROFILE(f, thread, position)=728000000.0*exp(-x*x/(2.0*sig*sig))/(sig*sqrt(2.0*3.14159265));
}
end_f_loop(f, thread)
}

[obscureed]

Hi Jabob_Bing,

At first sight, your UDF looks OK.

(Well, one warning: "#define sig 0.0000035/3." is very dangerous. If you later have "xyz = abc * sig;", then this is expanded to "xyz = abs * 0.0000035/3.", which is what you wanted. If you have "xyz = abc / sig;", then this is expanded to "xyz = abs / 0.0000035/3.", which is NOT what you wanted! -- wrong by a factor of 9. The preprocessor does a copy-and-paste, with no thought about numerical meaning. Therefore, you should definitely put brackets in "#define sig (0.0000035/3.)". As it happens, you have escaped this danger.)

The equation you have typed for the intensity is a function only of the z-coordinate, y[2], which you have called "x" (which is possibly a strange name). So if the face with this profile is perpendicular to the x-axis, for example, then the source will be a line, centred on the y-axis. These names are already confusing me, so I'd advise you to check this is what you intended. If you wanted a circular spot, you presumably should have put something like "exp(-(y[1]*y[1]+y[2]*y[2])/...".

With so few faces, it won't hurt to print out every value, as in something like this:

#include "udf.h"
#define sig (0.0000035/3.)
/* Change the following to NO_DEBUG when not required: */
#define DEBUG 1

DEFINE_PROFILE(gauss_flux_small, thread, position)
{
  real fcent[ND_ND];
  real z, flux;
  face_t f;
#ifdef DEBUG
  real farea[ND_ND], totalrate=0.;
#endif

  begin_f_loop(f, thread)
  {
    F_CENTROID(fcent,f,thread);
    z = fcent[2];
    flux=728000000.0*exp(-z*z/(2.0*sig*sig))/(sig*sqrt(2.0*M_PI));
    F_PROFILE(f, thread, position)=flux;
#ifdef DEBUG
    Message("face %d, coordinate %16.7g, flux %16.7g\n",f,z,flux);
    F_AREA(farea, f, thread);
    totalrate += flux * NV_MAG(farea);
#endif
  }
  end_f_loop(f, thread)
#ifdef DEBUG
  Message("Total rate = %16.6g\n",totalrate);
#endif
}

It also wouldn't hurt to multiply flux by area (from F_AREA, which gives you an array, then NV_MAG) and report a total applied power. With so few faces, the total might be some way off the theoretical value.

I hope this helps.
Ed

[Jacob_Bing]

Hello,

I think I figured out what the problem was. I have a face that runs in the x direction and I am trying to get the flux to vary along that face. When I changed the code to;

f_centroid(y,f,thread);
x=y[0];

It seems to work correctly. Because I was used to Matlab, I thought that the array positions started at 1 instead of 0. Is there a different way that I could write the code to make it less confusing? Thank you for the help.

[gouravjee]

can anyone tell me how to view flux udf profile in fluent ?

[pakk]

Maybe if you explain what you mean with "flux udf profile"... Do you mean the heat flux as defined by a UDF? In that case, it is the same way as you would show any flux, it does not matter that it was generated by a UDF...

[gouravjee]

Originally Posted by pakk

Maybe if you explain what you mean with "flux udf profile"... Do you mean the heat flux as defined by a UDF? In that case, it is the same way as you would show any flux, it does not matter that it was generated by a UDF...

Thanks pakk for your reply,
I have got the solution for my problem.
But i am stuck in an another problem. I have to write a UDS for an energy equation.
Can you suggest some source or thread or post from where i can read
about it ??

[Nourhan Bahgat]

please , how write code udf for this graph to insert as a bc in fluent

[pakk]

Take the code above and change the equation.

[Nourhan Bahgat]

Originally Posted by pakk

Take the code above and change the equation.

Thanks for your response, I will try .