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How to calculate shear stress within interior fluid?

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Old   March 7, 2018, 10:04
Question How to calculate shear stress within interior fluid?
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Dear All,

I have a fluid flowing inside of a tube. I need to calculate shear stress within interior fluid, and not only at walls. Is there any way to do that? I know UDF can handle that, but I have no idea how to do that! Can anyone help me please?

Thanks
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Old   March 7, 2018, 15:35
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You can do it with custom field functions. If you know the mathematical formula, it's quite straightforward to define the custom field in the GUI.

If you are completely lost you can checkout an old example I uploaded for calculating Reynolds stresses. You just need to replace turbulent viscosity by regular viscosity and remove the turbulent kinetic energy term to make it molecular viscous stresses.

Btw do you want only the molecular stresses, Reynolds stresses, or the combined effective stress? Make sure you actually calculate what you want.
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Old   March 13, 2018, 15:47
Default shear stress fomula
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Dear LuckyTran,

Thanks again for your help. I have read your example and prepared following code:

(custom-field-function/define
'(((name shear-stress-ux) (display "viscosity * (dx-velocity-dx)") (syntax-tree ("*" "viscosity" ("dx-velocity-dx"))) (code (field-* (field-load "viscosity") (field-load "dx-velocity-dx"))))
((name shear-stress-vy) (display "viscosity * (dy-velocity-dy)") (syntax-tree ("*" "viscosity" ("dy-velocity-dy"))) (code (field-* (field-load "viscosity") (field-load "dy-velocity-dy"))))
((name shear-stress-magnitude) (display "viscosity * (dx-velocity-dx + dy-velocity-dy)") (syntax-tree ("*" "viscosity" ("+" "dx-velocity-dx" "dy-velocity-dy"))) (code (field-* (field-load "viscosity") (field-+ (field-load "dx-velocity-dx") (field-load "dy-velocity-dy")))))
))

Since, shear stress for fluid is calculated as:

tau = mu*du/dy

So, I used this formula for x and y direction. what do you think?
I have one issue, for shear magnitude, which I decided to calculate using tau_x and tau_y, I need to use sqrt function, but don't know how it works in tui command.
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Old   February 26, 2019, 05:32
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Dear all,
i have a serious problem in calculating Shear stress in an internal wall ( the interface between a free flow and a porous media) ! i search all the web for answer and i just got confused!
can anyone help me with what LuckyTran said in this post? i read the code above but i cant get it!
thanks
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Old   February 26, 2019, 13:46
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Quote:
Originally Posted by arash.mp View Post
Dear all,
i have a serious problem in calculating Shear stress in an internal wall ( the interface between a free flow and a porous media) ! i search all the web for answer and i just got confused!
can anyone help me with what LuckyTran said in this post? i read the code above but i cant get it!
thanks

On surfaces you use the wall shear stress which is a built-in variable. You don't need any of this complicated formula.
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Old   February 26, 2019, 15:13
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Quote:
Originally Posted by LuckyTran View Post
On surfaces you use the wall shear stress which is a built-in variable. You don't need any of this complicated formula.
thanks for your prompt answer!
unfortunately the surface between free flow and porous domain is "INTERIOR"! so I CAN NOT use Wall shear stress in fluent! and also im sure that (mu*Shear_rate) in fluent doesn't give shear stress! So i have to use udf or something else!
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Old   February 27, 2019, 14:06
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Quote:
Originally Posted by arash.mp View Post
thanks for your prompt answer!
unfortunately the surface between free flow and porous domain is "INTERIOR"! so I CAN NOT use Wall shear stress in fluent! and also im sure that (mu*Shear_rate) in fluent doesn't give shear stress! So i have to use udf or something else!
can anyone help me?
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