# entropy generation with UDF

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May 7, 2010, 06:25
entropy generation with UDF
#1
Member

Join Date: Apr 2010
Posts: 42
Rep Power: 8
Dear all
I want to compute entropy generation in tube with constant heat flux.
I wrote a udf to compute gradient temperature:
# include "udf.h"
# include "math.h"
{
cell_t c;
face_t f;
domain = Get_Domain(1);
/* Fill UDS with the variable. */
{
begin_c_loop (c,t)
{
C_UDSI(c,t,0) = C_T(c,t);
}
end_c_loop (c,t)
}
{
begin_f_loop (f,t)
{
F_UDSI(f,t,0) = F_T(f,t);
}
end_f_loop (f,t)
}
}
{
Domain *domain;
cell_t c;
domain=Get_Domain(1);
/* Fill the UDM with magnitude of gradient. */
{
begin_c_loop (c,t)
{
C_UDMI(c,t,0) =(pow(NV_MAG(C_UDSI_G(c,t,0)),2));
}
end_c_loop (c,t)
}
}
////////////////////////////////
after the iteration in custom field function, i defined:
dissip=(2 * ((daxial-velocity-dx) ^ 2)) + ((daxial-velocity-dy) ^ 2)
S(generation)=((thermal-conductivity-lam * udm-0) / temperature ^ 2) + ((viscosity-lam * dissip) / temperature)
with this way i compute entropy generation in fluent.I can compile my code and get answer without any problem but my result is wrong(not validated with paper). I think my code for compute gradient temperature is incomplete and i need to compute gradient temperature in face too, but i dont know how can i do that?
thanks
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 May 7, 2010, 07:32 #2 Senior Member     Dragos Join Date: Mar 2009 Posts: 648 Rep Power: 12 I think you can easyly skip all the troubles by directly using the temperature gradient provided by fluent? However, to be able to access gradients you need to tell fluent to store them (check the udf manual section 3.2.3 - solve/set/expert). A suggestion: instead of Code: `C_UDMI(c,t,0) =(pow(NV_MAG(C_UDSI_G(c,t,0)),2));` you could use Code: `C_UDMI(c,t,0) =NV_MAG2(C_UDSI_G(c,t,0));`

 May 8, 2010, 04:19 #4 Senior Member     Dragos Join Date: Mar 2009 Posts: 648 Rep Power: 12 It seems either I missunderstood again your problem, or you mixed up concepts. To clarify: if you need temperature gradient only as a field function, then you don't need udf's. To get the temperature gradient available in field function you have to tell fluent to store it after each iteration (presuming that you have enabled the energy equation first). If you had red the manual page I suggested, you would have found, that storing the gradients is straight forward: Code: ```/solve/set/expert ... Keep temporary solver memory from being freed? [no] yes ...``` After at least 1 iteration you will see three new fields in the derivatives of the field function: dT-dX, dT-dY, and dT-dZ

 May 9, 2010, 17:12 #5 Member   mohammad Join Date: Apr 2010 Posts: 42 Rep Power: 8 Dear dragos you solved my problem, thank for your help. As you said there is no need to write udf, i just used "/solve/set/expert". After that i changed criteria of energy from 1e-6 to 1e-12 and my solution was validated. thanks again

 March 12, 2013, 08:14 #6 Member   Satish Gupta Join Date: Jun 2012 Posts: 30 Rep Power: 6 Hi, I am solving a 2D problem. I want to write the value of temperature gradient on a surface (I need the values). Can anyone tell me how to do it......I think a udf is to be written but I dont understand which macro to use. I have the end point coordinates of the surface(line)

 March 12, 2013, 10:39 #7 Member   mohammad Join Date: Apr 2010 Posts: 42 Rep Power: 8 you don't need to write udf for calculating the gradient. you should just keed the data by typing a command in the console. I can't remember the command but you can find it in the UDF PDF file.

 March 12, 2013, 13:41 #8 Member   Satish Gupta Join Date: Jun 2012 Posts: 30 Rep Power: 6 I want the gradient value on a wall i.e, surface.....will it be possible

 March 14, 2013, 16:35 #9 Member   mohammad Join Date: Apr 2010 Posts: 42 Rep Power: 8 I think it is possible. And the command to keep the memory is : /solve/set/expert ... Keep temporary solver memory from being freed? [no] yes good luck

 February 13, 2014, 13:50 #10 Member   Satish Gupta Join Date: Jun 2012 Posts: 30 Rep Power: 6 hi, could you please write the custom field function for entropy generation

 February 24, 2014, 15:20 #11 Member   mohammad Join Date: Apr 2010 Posts: 42 Rep Power: 8 Dear Rahul, Custom field function can be written based on the formula that I have attached in the first post. For adding gradient temperature you need to type the following statement in the cosole: /solve/set/expert...Keep temporary solver memory from being freed? [no] yes...Good luck

 August 30, 2014, 13:03 Regarding Entropy generation #12 New Member   Ashok Join Date: Aug 2014 Posts: 1 Rep Power: 0 I tried to calculate the entropy generation using the command: solve/set/expert Now, I got the reconstructed dT-dx, dT-dy and dT-dz. Can these values directly be used in my custom Field function to compute entropy. Can any one help me out mail me: ashok_iit@hotmail.com

September 12, 2014, 13:18
how i enter attached equation in custom filed function?
#13
New Member

Abdul Sattar Dogonchi
Join Date: Sep 2014
Posts: 1
Rep Power: 0
Quote:
 Originally Posted by mohammadkm Dear Rahul, Custom field function can be written based on the formula that I have attached in the first post. For adding gradient temperature you need to type the following statement in the cosole: /solve/set/expert...Keep temporary solver memory from being freed? [no] yes...Good luck
---------------------------------------------------------
how i enter equation that you attached in custom filed function?
thnx

 July 11, 2015, 12:29 entropy generation custom field function #14 New Member   Rasoul Join Date: Jun 2015 Posts: 3 Rep Power: 3 Dear all I entered the entropy generation equation in custom field function.but my result is wrong(not validated with paper). I examined my results and found that Fluent calculate the temperature gradient, but it can not properly calculate the square of the temperature gradient!!! please help with this problem thanks

 July 14, 2015, 10:05 #15 Senior Member   Join Date: Nov 2013 Posts: 919 Rep Power: 12 There are two options: - Fluent has a bug and calculates the square of the gradient of temperature wrong. - You did something wrong when you tried to let Fluent calculate the square of the gradient of the temperature. You seem to think that the second option is very unlikely. I think the first option is very unlikely.

 July 14, 2015, 10:58 #16 New Member   Rasoul Join Date: Jun 2015 Posts: 3 Rep Power: 3 Dear pakk of course, i think the first option is very unlikely. But I really do not know what's the problem. For a specific position within the computational domain, when i account the entropy based on the components separately, there is difference between the answer with custom field printing result. Interestingly, this difference can be seen when i want to calculate the square of the temperature gradient !

 July 14, 2015, 11:16 #17 Senior Member   Join Date: Nov 2013 Posts: 919 Rep Power: 12 So you agree that it is possible that you did something wrong when you tried to calculate the square of the temperature gradient, but you don't know what it is you could have done wrong. And what now? I think it is impossible to get help on which of your steps could be wrong, if you don't say which steps you took. But that is just my opinion.

 February 26, 2016, 06:52 fluid friction of volumetric entropy generation rate #18 New Member   Altayyeb kinhar Join Date: Feb 2016 Location: Bucharest Posts: 1 Rep Power: 0 Good day All: i have problem with the fluid friction of volumetric entropy generation rate, i already used the steps of Solve/set/expert and i get the heat transfer of entropy generation with the accepted results but i have problem with the friction of entropy generation The results of the friction are so low its around 0.0000029 when the Reynolds number is 100 and the heat flux is 100 000 w I used this formula in the Custom field function to get the fluid friction ( Viscosity * strain rate^2/static temperature) will you help me to know why the results are so low .... thanks and kind regards

 February 26, 2016, 10:14 I'll posed problem #19 Senior Member     Dragos Join Date: Mar 2009 Posts: 648 Rep Power: 12 Hello Kinhar, I cannot answer your question because your problem is not very well described, and I am not knowledgeable enough to guess it.

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