How to calculate shear stress within interior fluid?

malekan.cfd · March 7, 2018, 10:04

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

LuckyTran · March 7, 2018, 15:35

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.

malekan.cfd · March 13, 2018, 15:47

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.

arash.mp · February 26, 2019, 05:32

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

LuckyTran · February 26, 2019, 13:46

Quote:

Originally Posted by arash.mp

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.

arash.mp · February 26, 2019, 15:13

Quote:

Originally Posted by LuckyTran

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!

arash.mp · February 27, 2019, 14:06

Quote:

Originally Posted by arash.mp

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?

March 7, 2018, 10:04	How to calculate shear stress within interior fluid?	#1
malekan.cfd Member Join Date: Dec 2017 Posts: 34 Rep Power: 8	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

March 7, 2018, 15:35		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,674 Rep Power: 65	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. soheil_r7 and manuks007 like this.

March 13, 2018, 15:47	shear stress fomula	#3
malekan.cfd Member Join Date: Dec 2017 Posts: 34 Rep Power: 8	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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February 26, 2019, 05:32		#4
arash.mp New Member Join Date: Dec 2018 Posts: 17 Rep Power: 7	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