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Old   May 7, 2010, 06:25
Default entropy generation with UDF
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mohammad
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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"
DEFINE_ADJUST(adjust_gradient, domain)
{
Thread *t;
cell_t c;
face_t f;
domain = Get_Domain(1);
/* Fill UDS with the variable. */
thread_loop_c (t,domain)
{
begin_c_loop (c,t)
{
C_UDSI(c,t,0) = C_T(c,t);
}
end_c_loop (c,t)
}
thread_loop_f (t,domain)
{
if (THREAD_STORAGE(t,SV_UDS_I(0))!=NULL)
begin_f_loop (f,t)
{
F_UDSI(f,t,0) = F_T(f,t);
}
end_f_loop (f,t)
}
}
DEFINE_ON_DEMAND(store_gradient)
{
Domain *domain;
cell_t c;
Thread *t;
domain=Get_Domain(1);
/* Fill the UDM with magnitude of gradient. */
thread_loop_c (t,domain)
{
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?
please help with this problem
thanks
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Old   May 7, 2010, 07:32
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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));
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Old   May 7, 2010, 12:43
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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"
DEFINE_ADJUST(adjust_gradient, domain)
{
Thread *t;
cell_t c;
face_t f;
domain = Get_Domain(1);
/* Fill UDS with the variable. */
thread_loop_c (t,domain)
{
begin_c_loop (c,t)
{
C_UDSI(c,t,0) = C_T(c,t);
}
end_c_loop (c,t)
}
thread_loop_f (t,domain)
{
if (THREAD_STORAGE(t,SV_UDS_I(0))!=NULL)
begin_f_loop (f,t)
{
F_UDSI(f,t,0) = F_T(f,t);
}
end_f_loop (f,t)
}
}
DEFINE_ON_DEMAND(store_gradient)
{
Domain *domain;
cell_t c;
Thread *t;
face_t f;
domain=Get_Domain(1);
/* Fill the UDM with magnitude of gradient. */
thread_loop_c (t,domain)
{
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)
}
thread_loop_f(t, domain)
{
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
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Old   May 8, 2010, 04:19
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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
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Old   May 9, 2010, 17:12
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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
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Old   March 12, 2013, 08:14
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Satish Gupta
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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)
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Old   March 12, 2013, 10:39
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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.
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Old   March 12, 2013, 13:41
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I want the gradient value on a wall i.e, surface.....will it be possible
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Old   March 14, 2013, 16:35
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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
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Old   February 13, 2014, 13:50
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hi,
could you please write the custom field function for entropy generation
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Old   February 24, 2014, 15:20
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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
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Old   August 30, 2014, 13:03
Default Regarding Entropy generation
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Ashok
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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
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Old   September 12, 2014, 13:18
Default how i enter attached equation in custom filed function?
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Abdul Sattar Dogonchi
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Quote:
Originally Posted by mohammadkm View Post
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
---------------------------------------------------------
Hi mohammad
how i enter equation that you attached in custom filed function?
please help me about this
thnx
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