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specified shear at wall - temperature gradient - UDF - access violation error

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Old   July 29, 2010, 12:04
Default specified shear at wall - temperature gradient - UDF - access violation error
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Deb
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Hi
A simple rectangular geometry. Have temp gradient in domain as walls at different temperature. Need to specify shear at wall. The specified shear depends on temperature gradient. Wrote udf to specify it.
the udf uses the x- and y- components of temperature gradient to calculate shear stress. Getting 'ACCESS VIOLATION ERROR'

NOTE:
[1] Tried to even run a few steps and then hook the udf, still error.
[2] runs ok if I use C_T i.e. it can access the temp but cannot access the gradient!!!

ANY HELP WOULD BE VERY MUCH APPRECIATED
Doing in Fluent

The UDF is as follows:

#include "udf.h"
#define SurfTenGrad -0.43e-3 /*temperature coefficient of surface tension in N/(m-K)*/

/*Specifying x-component of shear on top surface*/
DEFINE_PROFILE(xShearStress,t,j)
{
cell_t c;

begin_c_loop(c,t)
{
/*C_T_G(c,t)[0] = returns x-component of cell temp gradient vector*/

C_PROFILE(c,t,j) = SurfTenGrad*C_T_G(c,t)[0];

}
end_c_loop(c,t)
}

/*Specifying y-component of shear on top surface*/
DEFINE_PROFILE(yShearStress,t,l)
{
cell_t c;

begin_c_loop(c,t)
{
/*C_T_G(c,t)[1] = returns y-component of cell temp gradient vector*/

C_PROFILE(c,t,l) = SurfTenGrad*C_T_G(c,t)[0];

}
end_c_loop(c,t)
}
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Old   January 18, 2011, 21:41
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m facin similar problem .. if u got thru thn plz help me out
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Old   January 21, 2011, 13:27
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Nikolopoulos Aristeidis
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Sorry but i don't understand why are you using
C_PROFILE instead of F_PROFILE.

I think that this udf is for boundary faces and not for boundary cells

Nikolopoulos Aristeidis
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Old   February 2, 2011, 05:24
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i am getting access violation if i try to use C_T_G(c,t) ..if i dont use it then everything works fine
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Old   February 2, 2011, 06:04
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I think your problem and solution is discribed in UDF manual:

"Table 3.2.9 shows a list of cell gradient vector macros. Note that gradient variables are available only when the equation for that variable is being solved. For example, if you are defining a source term for energy, your UDF can access the cell temperature gradient (using C_T_G), but it cannot get access to the x-velocity gradient (using C_U_G). The reason for this is that the solver continually removes data from memory that it doesn't need. In order to retain the gradient data (when you want to set up user-defined scalar transport equations, for example), you can prevent the solver from freeing up memory by issuing the text command solve/set/expert and then answering yes to the question Keep temporary solver memory from being freed?. Note that when you do this, all of the gradient data is retained, but the calculation requires more memory to run. "
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Old   February 2, 2011, 23:19
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hi Nikolopoulos,

thnx a lot for ur prompt responses

i later found that on other threads and things r wrkin fine now... but m facing another problem - m gettin floating point error
wen i am trying to display any profile..the udf compiles well widout givin ne error..it iterates too givin a correct indicative range of soln but wen i try to get a "display" its gives an FPE widout givin ne display ..is it a problem with my graphics card.

cheers
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Old   February 3, 2011, 02:39
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I don't think it is a problem of graphics.
Can you post your new code that is compiled?
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Old   May 12, 2012, 18:23
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i have the same problem ,also with using solve/set/expert and chose yes in the question
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Old   September 8, 2014, 04:24
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Hi calvin,

I know this is a old thread, but it would be very nice if you would share your solution or post the link where you found the solution.

Thank you
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Old   September 18, 2014, 07:29
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becouse u didnt include its header file
mem.h
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