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Gradient of a variable in CFX

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Old   February 9, 2011, 10:31
Default Gradient of a variable in CFX
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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.
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Old   February 9, 2011, 19:36
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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.
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Old   February 10, 2011, 00:12
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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: |
| DIVIDE-BY-ZERO |
| |
| 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 sub-expression: |
| 1/T.Gradient X
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Old   February 10, 2011, 02:56
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Your expression T.Gradient X is zero... due to uniform initial conditions maybe?
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Old   February 10, 2011, 05:15
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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?
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Old   February 10, 2011, 06:07
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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.
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Old   February 10, 2011, 06:59
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OK, no problem.
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Old   February 10, 2011, 09:29
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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?
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Old   February 10, 2011, 16:36
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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(.00001-t*1[s^-1])

for a quick and dirty solution. Make sure you change .00001 to something that suits your time step.
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Old   February 10, 2011, 18:00
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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?
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Old   February 10, 2011, 23:03
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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.
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Old   February 10, 2011, 23:22
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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 1e-10 [K m^-1] to T.Gradient X and it worked.
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