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entropy generation with UDF

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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

 dmoroian May 7, 2010 07:32

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:
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));`

Dear dragos
How can i directly use gradient temperature provided with fluent? there was no gradient temperature(dT/dx) in custom field function panel to use!!
I did that(solve/set/expert) but my result was not changed.
I tried this code to compute gradient face 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;
face_t f;
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)
}
{
begin_f_loop(f,t)
{
F_UDMI(f,t,0) =(pow(NV_MAG(F_UDSI_G(f,t,0)),2));
}
end_f_loop(f, t)
}
}
/////////////////////////
but i got this error:
..\..\src\test.c(48) : error C2109: subscript requires array or pointer type
..\..\src\test.c(48) : error C2109: subscript requires array or pointer type
..\..\src\test.c(48) : error C2109: subscript requires array or pointer type
..\..\src\test.c(48) : error C2109: subscript requires array or pointer type
///////////////////////////////////////////////
please help me i don't know how to solve my problem. i am reletively new to UDF.
thanks

 dmoroian May 8, 2010 04:19

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

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

 Rahul123 March 12, 2013 08:14

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)

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.

 Rahul123 March 12, 2013 13:41

I want the gradient value on a wall i.e, surface.....will it be possible

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

 Rahul123 February 13, 2014 13:50

hi,
could you please write the custom field function for entropy generation

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

 ashok_iit@hotmail.com August 30, 2014 13:03

Regarding Entropy generation

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

 setty September 12, 2014 13:18

how i enter attached equation in custom filed function?

Quote:
 Originally Posted by mohammadkm (Post 476546) 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

 Ras0ul July 11, 2015 12:29

entropy generation custom field function

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!!!
thanks

 pakk July 14, 2015 10:05

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.

 Ras0ul July 14, 2015 10:58

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 !

 pakk July 14, 2015 11:16

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.

 A. Kinhar February 26, 2016 06:52

fluid friction of volumetric entropy generation rate

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

 dmoroian February 26, 2016 10:14

I'll posed problem

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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