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February 9, 2011, 10:31 
Gradient of a variable in CFX

#1 
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Shahid Parvez
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Hi
Is it possible in CFX to calculate the Gradient of "My defined Variable" with respect to temperature? i,e My defined Variable.Gradient T It is not working for me saying "T" is not recognized. When I replace "T" with "X" then it is OK. 

February 9, 2011, 19:36 

#2 
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Glenn Horrocks
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You cannot do it directly. You can only do gradients like that WRT space. But dV/dT=dV/dX * 1/(dT/dX) so you should be able to calculate it from the spatial gradients of your variable and temperature.


February 10, 2011, 00:12 

#3 
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Shahid Parvez
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Thanks Glenn. Your response always helped me.
I tried your suggestion and got the following error message. Any comments Please...  PROBLEM ENCOUNTERED WHEN EXECUTING CFX EXPRESSION LANGUAGE     The CFX expression language was evaluating:   Additional Variable Value     The problem was:   DIVIDEBYZERO     FURTHER INFORMATION     The problem was encountered in executing the expression for:   GradSurfTension   The complete expression is:   Surface Tension.Gradient X*(1/T.Gradient X)   The error occurs on subexpression:   1/T.Gradient X 

February 10, 2011, 02:56 

#4 
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Lance
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Your expression T.Gradient X is zero... due to uniform initial conditions maybe?


February 10, 2011, 05:15 

#5 
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Glenn Horrocks
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Um, why are you doing this? If you have a surface tension set as a function of temperature you already know the gradient of the surface tension WRT to temperature, so why get CFX to calculate it?


February 10, 2011, 06:07 

#6 
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Shahid Parvez
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Hi Glenn,
If you look at the expression, Surface Tension is NOT set as a gradient of Temperature rather set as "X" and I want to calculate the Surface Tension as a Gradient of Temperature in a way as you mentioned in your previous reply. 

February 10, 2011, 06:59 

#7 
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Glenn Horrocks
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OK, no problem.


February 10, 2011, 09:29 

#8 
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Shahid Parvez
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Just noticed in Cfx Post that "T.Gradient X" is zero at t=0, and then nonzero. Any solution then? How to skip t=0 in transient analysis?


February 10, 2011, 16:36 

#9 
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Edmund Singer P.E.
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Perhaps add a non zero value to the T.Gradient.X term and a step function. Like
(T.Gradient X+0.0001 [K m^1]*step(.00001t*1[s^1]) for a quick and dirty solution. Make sure you change .00001 to something that suits your time step. 

February 10, 2011, 18:00 

#10 
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Glenn Horrocks
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Back to my previous post.... I am still puzzled by what you are trying to do. Surface tension is usually constant (so dS/dT=0 where S is surface tension and T is temperature) so if it is not constant you must have set it to be a function of something. So why not simply work out the derivative of that function WRT temperature?
Can you explain what function you have set surface tension to? 

February 10, 2011, 23:03 

#11 
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Shahid Parvez
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I want to calculate the Marangoni Force such that MF=dS/dT*dT/dX [N m^2] (where S is in N/m) and apply it as a BC.
So in my case S is not constant but a function of temperature and temperature is a function of "X". In CFX, as you mentioned earlier, dS/dT can not be calculated directly and that is why all this stuff come. 

February 10, 2011, 23:22 

#12 
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Shahid Parvez
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Problem solved. Thanks to all who replied to this post.
I followed Singer's instructions by adding a small value of 1e10 [K m^1] to T.Gradient X and it worked. 

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