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Old   February 26, 2021, 03:19
Question Fluent UDF - polynomial profile description
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Mihnea Costin
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Good morning everyone. Young and inexperienced Fluent user here.
I am trying to describe a temperature profile for my boundary condition. Please see Image1. Image1.PNG
My domain is formed of a nozzle which injects fuel in a coflow domain. Both jet and coflow have their velocities, temperatures, compositions. I want to mention the domain is 3D as I aim to use LES in a later phase.
I want to describe a profile for the coflow based on a polynomial. For this I have taken some available experimental data points and have used Matlab to fit a 4th degree polynomial. The function reads:
yt=p1t*r_exp_T.^4 + p2t*r_exp_T.^3 + p3t*r_exp_T.^2 + p4t*r_exp_T + p5t;
Matlab implementation to check the polynomial behaviour on the Fluent extracted coordinates
I have tried implementing the same function in a UDF for Fluent. Image2 shows the UDF function for the coflow. Image2.jpg
I have plotted the temperature vs radial coordinate. The profile obtained is flat. I then extracted my radial coordinates and put them in Matlab. The polynomial works fine and gives a good profile for my coordinates.
My UDF for describing the profile reads:

DEFINE_PROFILE(coflow_T_profile, thread, position)

real xt[ND_ND];
real yt;
real zt;
real rt;
face_t ft;
#define p1t -4.941e+09
#define p2t -4.007e+08
#define p3t -1.272e+07
#define p4t -1.884e+05
#define p5t 398.6

begin_f_loop(ft, thread)
yt = xt[1];
zt= xt[2];
F_PROFILE(ft, thread, position)=p1t*pow(rt,4.)+p2t*pow(rt,3.)+p3t*pow(rt ,2.)+p4t*pow(rt,1.)+p5t;
end_f_loop(ft, thread)

I have tried using pow(rt,4.), pow(rt,4), rt*rt*rt*rt. The polynomial seems unresponsive. I came with the idea to check if it is working at all. So I used an if statement to set a fixed value along the coordinate. It seems to work. but the polynomial is still flat.
Would you be so kind to indicate what I might be doing wrong in my profile definition. Thank you in advance!
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profile boundary cond., udf code

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