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Old   February 25, 2012, 16:28
Default Thermal conductivity UDF
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jack
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Hi friends...
i want to define thermal conductivity of fluid that varies with temperature and position. in other words i want to use this formula:
k= 3*T^2+6*y
that T is temperature and y is position in y-direction.
I wrote the following UDF and run the case but when i compared this result with constant thermal conductivity it has no effect in my results.

#include "udf.h"

DEFINE_PROPERTY(knew,c,t)
{
real ktc;
real y;
real temp = C_T(c,t);
ktc = (3. * pow (temp,2.)) + (6. *y);
return ktc;
}

can any body help me???
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Old   February 26, 2012, 04:41
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Well in your code variable "y" isn't initialized, so I guess that is the problem. Here is an quick fix:

PHP Code:
DEFINE_PROPERTY(knew,c,t)
{
  
real ktc;
  
real pos[ND_ND];
  
real y;
  
  
C_CENTROID(pos,c,t);
  
pos[1];
  
real temp C_T(c,t);
  
ktc = (3. pow (temp,2.)) + (6. y);
  return 
ktc;

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Old   February 26, 2012, 14:58
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Quote:
Originally Posted by Sixkillers View Post
Well in your code variable "y" isn't initialized, so I guess that is the problem. Here is an quick fix:

PHP Code:
DEFINE_PROPERTY(knew,c,t)
{
  
real ktc;
  
real pos[ND_ND];
  
real y;
 
  
C_CENTROID(pos,c,t);
  
pos[1];
  
real temp C_T(c,t);
  
ktc = (3. pow (temp,2.)) + (6. y);
  return 
ktc;

Thanks a lot Sixkillers for your answer...
i use this UDF and my results changed!!! But my result is not satisfactory.
i have some other question:
i want to use this UDF in 2d modeling of pipe, and use this UDF to change thermal conductivity of fluid in each section of pipe from inlet to outlet with temperature and position of each point in every section.
can i use this UDF in axisemmetric model with y or i should replace y with r???
if yes, can i replace y with r in the previous UDF???

And if i want to use this formula :
k=3*T+ \partial(U)/\partial r
how can i change previous UDF ???

Last edited by jack.s; February 27, 2012 at 07:02.
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Old   February 28, 2012, 17:08
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I think that you can use this code without any modification for an axisymmetric problem, if a symmetry axis is parallel with x axis. If it is parallel with y axis than you will have to change
PHP Code:
pos[1]; 
to
PHP Code:
pos[0]; 
To obtain a derivation of velocity according to spatial variable, you can use methods: C_U_G, C_V_G (take a look to this chapter).

Once again it depends, which spatial variable represents radius. Finally I would like to say, that I haven't test it and UDF manual isn't much speaking about axisymmetric problems in general. However, this is the way how I expect it should work
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Old   February 29, 2012, 12:57
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Quote:
Originally Posted by Sixkillers View Post
I think that you can use this code without any modification for an axisymmetric problem, if a symmetry axis is parallel with x axis. If it is parallel with y axis than you will have to change
PHP Code:
pos[1]; 
to
PHP Code:
pos[0]; 
To obtain a derivation of velocity according to spatial variable, you can use methods: C_U_G, C_V_G (take a look to this chapter).

Once again it depends, which spatial variable represents radius. Finally I would like to say, that I haven't test it and UDF manual isn't much speaking about axisymmetric problems in general. However, this is the way how I expect it should work
Thank you very much...
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