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ypchen January 2, 2012 09:13

calculate integral in UDF
Hi, everybody.
My case has a heat flux cross the wall, and it's not a constant value.
The value is calculated by integrating the domain from z1 to z2, the variable in the integral is temperature (function of z), others are just some constants.
I wanna calculate the integral in UDF.
I have a idea by using "TRAPEZOIDAL RULE", but I don't now how to do it. Does anyone knows how to calculate integral in UDF or how to apply TRAPEZOIDAL RULE ??
thank you very much !

Sixkillers January 2, 2012 10:22

Hi! You have two options how to solve this problem:

1) Write your own integration method (as you proposed)
2) Link your UDF against some numerical library (e.g. GSL) and use it inside your code.

If you are fine with number 1) and you don't need such robust integration routine. Here is my implementation of Gauss–Kronrod quadrature formula, which I use in my UDFs. It requires 15 function evaluation and its accuracy is comparable with trapezoidal rule when integration interval is divided into 100 parts.

PHP Code:

static const real nodes[7] =

static const 
real weights[8] =

real gk15(real (*fce)(real), real areal b)
real trans1 = (b-a)/2.0;
real trans2 = (a+b)/2.0;

real integral 0.0;

int i;
integral += weights[i]*((*fce)(trans1*nodes[i] + trans2) + (*fce)(trans1*(-nodes[i]) + trans2));

integral += weights[7]*(*fce)(trans2);


So you just have to pass pointer to your function, lower and upper integration limit (e.g. gk15(&myFun,0,10)).

ypchen January 3, 2012 10:14

Thank for your reply, sixlillers.
Actually, I wanna calculate the heat flux at a cylinder wall, I need to get the temperature data near the wall so that I can calculate the temperature difference, so I have one more question : how do I get temperature data near the wall and how to use it in the UDF that you gave me to calculate the integral.
thank you ! ypchen

Sixkillers January 3, 2012 17:25

Well I thought you need compute an integral of a scalar function (one variable). Fluent can compute surface integral by demand after simulation (Results -> Reports -> Surface Integrals) or you can setup a surface monitor, which will compute your desired integral and write its value to a file after each time step and then you will have to read it from your UDF.

saksham.pachar January 4, 2012 08:53

Hi Six KIllers
I have a Problem in defining udf For Spanwise Average Nussel Number Along Flow

I have the .cas, .dat, and tecplot layout in my mail.
kindly give me your , so that I can mail them the entire problem.

Is it Possible to Integrate in Fluent

I can Plot h*X/k along z at a line along the flow on the bottom wall.
But there can be a lot of lines along the breadth.
I dont want to use excel.

Secondly i want the contours of Nussel no on the bottom plate, which I am able to show in Fluent, But I also want to show, the number along the contour, So that i can identify which contour is for which number

, how to calculate Nussel Number, and average Nussel Number In Tecplot, By Specifying Equations is also welcome

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