First of all you need to define a C function which would take time as an argument and return the temperature, something like

real mytemp(real mytime)

{

/* some kind of interpolation that takes your 27 points {ti, Ti}, i=1..27 */

return result;

}

This function should be at least continuous.

This function you will use in your DEFINE_PROFILE in the line

Assuming that the temperature will have the same time-dependent values everywhere on the boundary, your DEFINE_PROFILE simplifies to this:

DEFINE_PROFILE(transient_temp,th,i)

{

face_t f;

real flow_time = CURRENT_TIME;

begin_f_loop(f,th)

{

F_PROFILE(f,th,i) = mytemp(flow_time);

end_f_loop(f,th);

}

}

Now the most interesting question is how to implement the interpolation and which interpolation scheme to choose. Depending on your problem an the character of the temperature time-curve you choose your interpolation scheme. You can always start with the piecewise linear interpolation. You can also use approximation here, it all depends on your temperature-time data points.

There are two ways to implement this. Either you can program it yourself in C (google "pieceweise interpolation" for formulae) or use a library such as GNU Scientific Library (https://www.gnu.org/software/gsl/doc/html/interp.html?highlight=interpolation) or https://people.sc.fsu.edu/~jburkardt/c_src/pwl_interp_1d/pwl_interp_1d.html

I am sure there are other libraries available. This is a very often occuring task.

It depends on what is easier for you: to program your own code, or to get the UDF to call a library function and to compile with the external library.

I had good experiences calling GSL library subroutines from UDF. It is quite easy to do under Linux.

I hope this helps. Good luck!

@ Anton Ishmurzin

Hi Anton, thank you for answering my question. I'm an absolute beginner in coding with C++, so I'm looking for the easiest way to write my UDF. The UFD will be interpreted in ANSYS Fluent software. I did some search and I found a schematic diagram of the UDF for wall temperature for a case similar to mine, please see attached file. Now my question is do I still need to use the library that you mentioned in your answer? is there a simple way to code the function shown in the schematic diagram?

Thank you

The absolute minimum of programming would be to try to approximate your temperature-time curve using Excel or similar software and get an analytic formula that would give you the values "close" to your 27 points. Then you take the formula and program it into your UDF, it will be only one line of code. See the example in the attached screenshot. First you need to plot an XY-Plot with your data, then do "insert trend line", there will be an option (checkbox) to show the approximation formula.

@ Anton Ishmurzin YOU ARE MY HERO. This is really good information and good start to code my UDF. I have found so many examples on coding the transient temperature using formulas, so this should not be so hard especially after your explanation on how to obtain the formula out of my data.

I really can't find enough words to thank you for your effort on helping me with my question. Regards

I'm happy I could help and got you started on UDF programming :)

Hi people,

I need your suggestion on a similar problem.

I have UDF program with time and velocity

(shown below)

#include "udf.h"

DEFINE_PROFILE(velocity_magnitude, t, i)

{

real velocity;

real the_current_time;

face_t f;

the_current_time = CURRENT_TIME;

if ((the_current_time>=0) && (the_current_time=100) && (the_current_time=3900) && (the_current_time=0 to time=3900 to time

I need to run entire UDF for 30 degree first then for 40 degree moving on till 70 degree. How can i hook it to this UDF. Present UDF gives the inlet velocity for an assigned time say 0.03 m/s for 0-99 sec,0.06 m/s  for 100-199 sec till 1.21 m/s for 3899-3999 sec so total 40 inlet velocities at 0.03 individual increment. I need this inlet velocity for given inlet temperature i.e. for 0 to 3999 sec i need to vary my inlet velocity as well as inlet temperature.

Please find the attached image file.

Regards,

Vighnesh

Anton Ishmurzin Oula Fatla

Please share your thoughts.

Vignesh Shenoy

Sorry for the delay, I do not log in here daily.

if I understand you correctly, you want a time-dependent temperature profile in addition to your existing time-dependent velocity profile.

You can do this by implementing this time-dependent temperature profile in a separate DEFINE_PROFILE subroutine.

Then it should be possible to hook the UDF under "Thermal" tab under Boundary conditions for the boundary.

Thanks for the reply..

U mean similar to velocity UDF, i need to write one more UDF?

Can u please elaborate..I am not much in to UDF.

As a work around, i am giving the temp input manually under Thermal tab. It would be better if i get all in just one UDF.

Waiting for your early response.

Regards,

yes, you need to write another, second DEFINE_PROFILE UDF just like the one you use for the velocity, but with a different name and it should calculate temperature depending on time, similarly to the UDF code you provided above, like

if (t_0

Thanks mate!!

" but with a different name and it should calculate temperature depending on time, " (Previous answer)

Actually temp is my input value similar to velocity. I don't need to calculate it.

Issue:

Once im done taking temp_value=X degree Centigrade for time 0 to 4000 sec, i need to give input as temp_value=Y degree Centigrade again for same time 0 to 4000 sec.

I would not mind taking manually temp values if 2-3 different temp_values but i need to do it for more than 40 different input temp_values.

Regards

> I would not mind taking manually temp values if 2-3 different temp_values but i >need to do it for more than 40 different input temp_values.

that's what the UDFs are for :)

Hi,

Can u please suggest me how can i hook my temp inlet with the existing velocity UDF.

I have in mind a logic of giving temp input as X for whole velocity UDF once then changing temp to Y and again whole velocity UDF to run and so on. How can i write a UDF for this?

Vignesh Shenoy you need to code two separate DEFINE_PROFILE user-defined functions, one for time-dependent velocity profile and the other one for time-dependent temperature profile.

The UDF for the velocity profile you pick in the "Momentum" tab of the BC settings window of FLUENT and the UDF for the temperature profile you pick in the "Thermal" tab of the same BC settings window.