Learning Forum

[helena_mel]

I get different results in fluent and in cfd Post when i plot the contour of Wall shear stress (fluent) or the wall shear(cfd-post).

[peteroznewman]

Please describe this in more detail.

[helena_mel]

I want to calculate the wall shear stress in fluent and plot the results in cfd post. In fluent postprocessing  I get the wall shear stress under wall flux and then I select wall shear stress and the surface and the contur plot is generated.

When I opened the results in cfd post, there is wall shear too but when i plot the wall shear at the same surface i get different results. Is the wall shear stress in fluent the same as wall shear in cfd post?

 

[Raef.Kobeissi]

Yes it is maybe the contour was different (contour map) and not the values. Can you send 2 photos for comparison please.

cheers

[shaheen wahab]

 Hello Helena,

How did you solve that problem of Wall shear stress? Does it come very small values as well? (e.g. order of -3)

Thanks 

[KR]

Hello Shaheen, 

Have to tried to use the same scales?

Also, another important difference between Fluent and CFD post is the fact that CFD post displays node based results.

Can you try and plot your contours in Fluent based on the node values? Please make sure you have this option checked.

Thank you.

Best Regards,

Karthik

[shaheen wahab]

Thank you so much, Karthik

[Jamasp]

Hello Peter,

I have the same issue with WSS. I did the following simulation with V17.0 and noticed that the WSS between Fluent and CFD post is different. Could you please explain the reason? The result is different for 10-6!! 

I did new simulations with V19.2 and did not have this issue.

Thanks,

Jimmy

[Jamasp]

Hello Kremella,

Could you please address my question regarding the difference in WSS between Fluent and CFD post?

 

Thanks,

Jimmy

[Jamasp]

Hello Raef,

Could you please address my question regarding the difference in WSS between Fluent and CFD post?

 

Thanks,

Jimmy

[Keyur Kanade]

Can you please insert some zoomed in image. 

As you mentioned, 19.2 is working, please use 19.2.

17 is very old build. 19 might have some fixes. 

Regards,

Keyur

 

If this helps, please mark this post as 'Is Solution' to help others.

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[Jamasp]

Hello Keyur,

Thank you so much for your reply. I would appreciate it if you or ANSYS team can provide a clear answer to this issue. Is it a bug in CFD post? Please check out the high quality images in the following. The simulations here are FSI and I'm just showing the results of the fluid domain. Notice that the simulations shown in v17 and v19 are two different problems with different boundary conditions. Do not compare the values of WSS. My concern is why WSS in CFD post is 10-6 smaller than Fluent??!! The WSS in CFD post is divided by 10e+6 for some reason. I'm opening the same case and data files in Fluent and CFD post but I'm getting two different values for WSS!!!

Simulations in Ansys V17.0 showing at time step 2.36s.

Wall shear stress in Fluent:

Wall shear stress in CFD post, which is 10e-6 smaller than Fluent!! Why??!

 

New simulations in v19 showing at time step 2.36 s.

Wall shear stress in Fluent:

Wall shear stress in CFD post, which matches the results in Fluent. 

 

 

Best,

Jimmy

[Keyur Kanade]

This is strange. 

Can you please quickly check scale of the problem using scale provided at the bottom as shown in image below. Please check it in fluent and cfd-post. 

[Jamasp]

Hello Keyur,

I also added the ruler. It shows the same scale in Fluent and CFD-Post!! This is very strange!!! I'm opening the same cases in Fluent and CFD-post. Is it a bug in CFD-post? Or is it kind of non-dimensionalized values of WSS showing in CFD-post? As I mentioned, the WSS values in CFD-post are divided by 10e6. Could you please ask Ansys team to explain the reason for this difference between Fluent and CFD-post? Knowing the reason is very important for us.

Results in Fluent

CFD-post results

Thanks,

Jimmy

[Jamasp]

Hello Keyur,

Thank you for following up regarding this issue in CFD post. I would appreciate it if you find out the issue for us.

 

Hello Peter,

Could you also provide some information regarding this problem, please? It's very critical for us to know why this is happening.

 

Thanks,

Jimmy

[Keyur Kanade]

Can you please check in latest 2020R1. 

Soon 2020R2 will be released. 

[Jamasp]

I did the simulation that shows different results between Fluent and CFD-post in V17.0 but I opened the results in V19.2. I also opened the results in 2019R3 and got the same thing. We don't have 2020R1 on campus, unfortunately. Is the issue related to the version? It seems that CFD-post shows the WSS values in MPa but the unit in the legend shows Pa, instead. Any ideas?