[siamak1438]

Hey guys!
I am trying to use the command "Velocity u.Gradient X" and other similar commands to extract the velocity gradients. It shows that the unit of those variables is [1] or non-dimension, but the unit of velocity gradient should be [s^-1]. So, I am confused . I just was wondering the "Velocity u.Gradient X" or similar commands return the velocity such as du/dx or they are other variables!!??
Thank you for your help.
Cheers,
Siamak

[Lance]

Then you are doing something wrong. Shown an example of your CEL code that give the wrong unit on Velocity u.Gradient X ?

[siamak1438]

Originally Posted by Lance

Then you are doing something wrong. Shown an example of your CEL code that give the wrong unit on Velocity u.Gradient X ?

for example when I use maxVal(Velocity u.Gradient X)@geom that should return the maximum of du/dx, it returns a number without unit. or when I am using volumeInt(Velocity u.Gradient X)@geom it returns m^3 but it should be m^3/s.

[Opaque]

What version of ANSYS CFX are you using ?

I tried with ANSYS CFX R16.0, and the expressions you suggested return the correct units..

[siamak1438]

Originally Posted by Opaque

What version of ANSYS CFX are you using ?

I tried with ANSYS CFX R16.0, and the expressions you suggested return the correct units..

I am using ANSYS 15.0. Please check the derivatives in CFD-Post and FLUENT, they are different! I contact with Ansys support center and they told me derivatives in CFD-Post and FLUENT are different!
“There may be substantial differences between gradients calculated in the Fluent solver and gradients calculated in CFD-Post. The Fluent solver uses options such as boundary treatments and limiters to calculate gradients; CFD-Post calculates gradients independently of the Fluent solver, and does not have access to all of the same data.”

[Opaque]

Ok. You are post-processing Fluent results using CFD-Post R15.0

The comment from Ansys support does not apply here. The gradient computed in CFD-Post may be different in value, but the units must remain the same.

[siamak1438]

Originally Posted by Opaque

Ok. You are post-processing Fluent results using CFD-Post R15.0

The comment from Ansys support does not apply here. The gradient computed in CFD-Post may be different in value, but the units must remain the same.

Yes. I have problem with both! It is not returning unit and the value is different. However, I decided to go back fluent and use custom field function to define& extract my desired parameters. Thank you for your replies.

[hamide]

hi all
i'm using fluent 15;
i wanna compute "x-velocity gradient in y direction" and "y-velocity gradient in x direction" to calculate liftSaffman force. how can do this?
should i export "velocity u.Gradient Y" and "velocity v.Gradient x" from results or calculate them by my self from u and v exported from results?

[siamak1438]

Originally Posted by hamide

hi all
i'm using fluent 15;
i wanna compute "x-velocity gradient in y direction" and "y-velocity gradient in x direction" to calculate liftSaffman force. how can do this?
should i export "velocity u.Gradient Y" and "velocity v.Gradient x" from results or calculate them by my self from u and v exported from results?

Hi Hamide,
Velocity gradients will be calculated automatically by CFD-post, which they are termed “Velocity u.Gradient x” and similar expressions for others (you can access them in Variables/derived). They are a little bit different with those calculated by Fluent (check the following link in my reasearhcgate profile to more info https://www.researchgate.net/post/Ho...nt_in_CFD-Post).
If you wish to export the velocity gradients calculated by Fluent to use in the CFD-Post, you should select velocity gradients (dX-Velocity/dX and similar terms) in the “calculation activity” before simulation. In this case, they (DX velocity DX ,…. ) will appear in the CFD post (you can access them in Variables/Solution).
Regards,
Siamak

[hamide]

thank you siamak
i have an other question, when i export data from all domain of channel to excel format it begins from the end of channel (as i attached)!!! i don't know how to fix this problem!!!

[siamak1438]

Originally Posted by hamide

thank you siamak
i have an other question, when i export data from all domain of channel to excel format it begins from the end of channel (as i attached)!!! i don't know how to fix this problem!!!

I don't understand your question completely. Do you export data from fluent? Because your data is not too much, you can manually reverse in the excel file by sorting from smallest to largest or vice versa (make sure to select "expand selection" to sort other columns accordingly)

[hamide]

Originally Posted by siamak1438

I don't understand your question completely. Do you export data from fluent? Because your data is not too much, you can manually reverse in the excel file by sorting from smallest to largest or vice versa (make sure to select "expand selection" to sort other columns accordingly)

yes, i export data from fluent.
thank you very much siamak for you help :-)