udf problem
 

hi, I am trying to solve a problem in Fluent, and I define some "interior" surfaces zone in computational domain for the convenience of post processing. After calculation, I would like to take the contour plots and get the surface integrals at those surfaces. So, I have written a udf to get the pressure value at the certain surface with mass flow average. I found that the UDF can work well at boundary inlet or outlet surface while at those surfaces defined as interior type the UDF can not work. I am really confused.

I would greatly appreciate for your help.

#include ¡°udf.h¡±

DEFINE_ON_DEMAND(calculate_pre) {

Domain *d;

Thread *t;

face_t f;

int ID = 4;

/* ID is the zone number,which can be set as the number of the boundary inlet surface or that of the interior surface*/

real p_s = 0; /*static pressure*/

real p_s_temp = 0;

real massflow = 0;/*total mass flow rate at the surface*/

d = Get_Domain(1);

t = Lookup_Thread(ID);

begin_f_loop(f, t) {

p_s_temp += F_P(f, t) * F_FLUX(f, t);

massflow += F_FLUX(f, t);

}

end_f_loop(f, t)

p_s = p_s_temp / massflow;

printf("p_s = %f", p_s);

}

Re: udf problem
 

Hi,

We are not able to get any values at interior(defalut interior type) via UDF. As your remarks, boundaries are O.K. We can get it.

FJ

Re: udf problem
 

Thanks for your response. As you know,we can get those values at the certain surfaces ,including interior type surfaces, with "Report/Surface Integrals..." command in FLUENT. Could you please tell me how FLUENT calculate that value?

In the UDF file,Can I get the value obtained from "Report" command.

I really need your hlep. Thanks in advance.

Re: udf problem
 

Hi,

The value of "Report/Surface Integrals..." is calculated by interpolated values and this interpolation scheme is linear interpolation. So this value is not exact value .

"Report-Fluxes" at boundaries, the exact values are calculated . This function calculates really exact flux value from Fluent solver's values.

If you want to get exact value at specified face, you have to put a certain face at that place with GAMBIT or other preprocessor.

In Fluent, this operation can be done. With "Grid-Seprate-Cells" ,you can separate of your grid(default interior) .Fluent will makes a certain face in your grid and it's named as a new interior ,automatically. (Your grid is tet mesh , the separated interior will be a serrated face).

Then ,you select the new interior face in the panel of "Report-Fluxes" .The exact value is calulated.

This is the only way to get the values. Via UDF, any interior values can not be got.

Thank you

FJ

Re: udf problem
 

so, you mean,in the udf file, I can not get the exact values that calculated from "Report-Fluxes".

you konw, those exact values will be shown in the console window. and I have to copy the values from the console window for postprocessing. That is so inconvenient for me since there are so many cases.

Whether there has a method that can exported those values to a txt file?

Thank you in advance, FJ

shancfd

Re: udf problem
 

Hi,

Yes. If an interior face, we can not get exact values.

I had the same problem before as well. To avoid it, I put the interior boundary(iterior face) when making mesh and made a udf to write external file.

The UDF's algorithm is calculating average values between C0, C1(adjucent cell at the interior). This calculated value is not so bad compared with "Report-Fluxes" in case of using hex mesh. In case of tet mesh , the error becomes a little large. I think this reason is the algorithm to calculating average values. If face vector components are used in average calculation, this error will be small, probably

Thank you

FJ

Re: udf problem
 

FJ, I greatly apperacite your help. Your expericnce is very precious to me. I will try it again with your method.

Thanks and regards,

shancfd