Time Averaged Shear Stress
 

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

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.

Hi,

Can Time average wall shear stress be calculated in Fluent?

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.

if do in excel/matlab, how to see the TAWSS contour plot on the image/geometry?

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?

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?

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

Lets say you defined a 3D function TWSS(x,y,z) then u define a new expresssin TWSS2= TWSS(x,y,z)

Good luck

Thank you so much.


Sorry, I have another question, where you defined the 3D function, is it in pre CFX?

Yes. In CFX-Pre, click on "User Function", then you can define 1D and 3D functions.

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?

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.

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

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.

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

I recommend you to use the method explained by Lance, it is straightforward.

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)

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

Hi Lance,

"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.
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. "

Based on the above, I have calculated the TAWSS in excel. Then, in the CFD post, I load results of my fluent at 1 time point. Then, in the CFD post, I go to the file/export and export wall shear at the wall part. I also check line and face connectivity.
Then, I replaced the value of wall shear stress into TAWSS and then import back the file as a user surface.
then the variable in the csv file show up in the variable list in post. When I checked the imported file, seems nothing happen.

Is there any mistake in my steps?

Thank you.

Regards,
Naimah

If the variable shows up in the list it should work. Could you give an example on the data you import to post?

Hi Lance,

I have imported the data in .csv file. there is X, Y, Z and wall shear stress value. There are 2 cases in .csv file. I changed the values of WSS to the TAWSS.
Actually in my CFD post there are 2 cases in which is cases 4.20 at 5 s and cases 4.20 at 4.20 s.
I only want to look at case 4.20 at 4.20 s.
However in the excel, this two cases are together.

When I imported into CFD post, the 2 cases separated.

Is there anything wrong?

Thank you.

Regards,
Naimah

I havent tried two cases at the same time, so I dont know if that works. Try to remove the case you dont need.
Also, I wouldnt use spaces in the variable names.

"Also, I wouldnt use spaces in the variable names"

Sorry, which spaces did you refered?
I don get it.

The variable name seems to be "wall part_1 in Case FFF 1 00420"

Hi Lance,

I m not successful plotting time average wall shear stress in the CFD post.
Herewith I attached the steps taken.
Can you help me to look into it?

Thank you.

Regards,
Naimah

Plot contours of Time Average Wall Shear Stress and Oscillatory Shear Index
 

I am simulating an unsteady blood flow and I need to plot contour of Time Averaged Wall Shear Stress (TAWSS) and Oscillatory Shear Index (OSI) at different time steps of the cardiac cycle. Is there any procedure to do that CFD-post. Please suggest me an apt solution for the same.

Have a look at the capabilities of CFD-Post, it can do quite powerful post processing.

If you define exactly what you want to see then we might help you. (Wall shear stress of what? A point on a wall? Averaged over a wall surface? And define Oscillatory Shear Index)

I want to calculate the values and plot the contours of TAWSS, OSI and RRT at different time steps of the entire transient cycle on the three-dimensional arterial wall. Suggest some way to plot these variables in either CFD post or in tecplot.
https://ibb.co/crBdKZd
https://ibb.co/64RHfFg
https://ibb.co/qxwmnhg

Please don't use jargon - you will need to define what Time Averaged Wall Shear Stress, Oscillatory Shear Index and RRT are. These terms are not general CFD terms so I don't know what their definitions are.

If you want to post an image here is the FAQ on it: https://www.cfd-online.com/Wiki/Ansy...n_the_forum.3F

But don't forget the basics. Have you done the CFD-Post tutorials? They are available on the ANSYS Customer website and ANSYS Academic sites. These tutorials teach you the basics.

How to define the new time-varying functions and further plot their contours in CFD post. Please suggest a suitable way.

Thanks.

TAWSS needs to be calculated as a transient statistic as the simulation runs. It cannot be done post-processing. So have a look at transient statistics in the output tab in CFX-Pre.

OSI is a similar equation to TAWSS and will require a transient statistic.

RRT is a simple function of TAWSS and OSI and can be done either as the simulation runs or in post-processing. This would be just a simple CEL expression - have a look in the CFX tutorials for examples of CEL expressions.

I am Fluent solver. Also, defining an expression for TAWSS and OSI in fluent after the simulation for post processing will be a correct method?

I cannot help you with calculating these equations in the Solver for Fluent. Try the Fluent forum for that.

Theoretically you can evaluate those equations in post processing providing you have saved transient results files at high enough time resolution. I am assuming you have not saved these files, which means you will need to do it in the solver.