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Old   February 25, 2012, 05:22
Question Time Averaged Shear Stress
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Ashkan Javadzadegan
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Dear All,

I want to calculate time average wall shear stress(TAWSS) from following equation using CFX-Pre.

TAWSS=(1/T)*(Integral [(wall shear stress)*dt])

Please let me know how to do that.

Thank you,
Regards,
AshtonJ
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Old   February 25, 2012, 06:56
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I would export the history of wall shear stress as a text file and do the integration in another package (excel, matlab etc). But you may be able to do this in CFD-Post V14, I have not checked in detail.
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Old   April 23, 2012, 03:59
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Hi,

Can Time average wall shear stress be calculated in Fluent?
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Old   April 23, 2012, 04:02
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I agree with ghorrocks, the easiest way is to export the data as txt file and do the time average WSS in excel or matlab.
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Old   April 23, 2012, 04:09
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if do in excel/matlab, how to see the TAWSS contour plot on the image/geometry?
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Old   April 23, 2012, 04:15
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After finding the TAWSS, in CFX-pre, define a 3D function and import the time aweraged WSS, then using this function, define an expression and using the expression, define a new variable. Now, if you run your case just for one iteration, you can find your defined varibale in CFD Post and plot it.
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Old   April 23, 2012, 04:38
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Quote:
Originally Posted by ashtonJ View Post
After finding the TAWSS, in CFX-pre, define a 3D function and import the time aweraged WSS, then using this function, define an expression and using the expression, define a new variable. Now, if you run your case just for one iteration, you can find your defined varibale in CFD Post and plot it.
Or import your results as a user surface in cfd-post...

Also, wouldnt it be possible to use the transient statistics to get the time-averaged variable in post directly?
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Old   April 23, 2012, 05:04
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is that the things above mentioned is using CFX?
Can I applied the same method on FLUENT?
Is there any tutorials that can show me on how to do it?
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Old   April 27, 2012, 04:35
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Hi ashton j,

Based on this,
" After finding the TAWSS, in CFX-pre, define a 3D function and import the time aweraged WSS, then using this function, define an expression and using the expression, define a new variable. Now, if you run your case just for one iteration, you can find your defined varibale in CFD Post and plot it. "

What expression you have defined? Is that the TAWSS expression that you have mentioned above.

Hi Lance,

"Or import your results as a user surface in cfd-post...

Also, wouldnt it be possible to use the transient statistics to get the time-averaged variable in post directly? "

How to do this in CFD post. Would you mind to help me?

Thank you

Naima
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Old   April 27, 2012, 04:38
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Lets say you defined a 3D function TWSS(x,y,z) then u define a new expresssin TWSS2= TWSS(x,y,z)

Good luck
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Old   April 27, 2012, 04:48
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Thank you so much.


Sorry, I have another question, where you defined the 3D function, is it in pre CFX?
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Old   April 27, 2012, 04:50
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Yes. In CFX-Pre, click on "User Function", then you can define 1D and 3D functions.
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Old   April 27, 2012, 04:52
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I got another question,

You have calculated the time average wall shear stress.

Why do you need to find the 3D function?

The TAWSS cannot be directly imported into the CFD post?
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Old   April 27, 2012, 05:03
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Quote:
Originally Posted by wanna88 View Post
I got another question,

The TAWSS cannot be directly imported into the CFD post?
Yes you can, through a user surface. Say that you have calculated TAWSS in excel/matlab and know the value at each coordinate. Then, in CFD-post goto file/export and export any variable (say e.g. velocity) at the wall of interest. Check "line and face connectivity" and you'll get a file with coordinates and the variable + face connectivity.

Now, simply replace the variable you exported with your calculated values of TAWSS, and then import the file as a user surface.
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Old   April 27, 2012, 05:18
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Thank you so much.

After import the data, do I still need to define an expression and define a new variable in CFD Post?

naimah
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Old   April 27, 2012, 05:20
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Nope, whatever you name your variable to in the .csv-file will show up in the variables list in post.
No need for CFX-pre at all.
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Old   April 27, 2012, 05:20
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Before plotting your TAWSS in CFD Post you need to define a new variable for your computed TAWSS in CFX Pre.

In order to define a new variable, you need an expression and in order to import your computed TAWSS into CFX Pre, you need a new function.

TAWSS the you calculate is a 3D function, because at each node of your domain, you have a corresponding value for TAWSS and each node has three coordinates x, y, z
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Old   April 27, 2012, 05:22
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I recommend you to use the method explained by Lance, it is straightforward.
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Old   April 27, 2012, 05:31
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But if you only want the arithmetic average of WSS then I would use the transient statistics function instead. Then you would get the value directly in post.
(this needs to be setup in pre, before you run your simulation)
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Old   April 27, 2012, 05:43
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Dear all,

Thank you so much.

I will try my best first since i used fluent, but I can import my results into excel/matlab and calculate the TAWSS. I will try to bring this TAWSS results into CFD post.

Thank you so much for your help.



Naimah
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