How to calculate the RMS flow velocity in X,Y and Z directions

[aniruddhabose96]

Hi

I have run an unsteady flow in a pipe and in the Standard Data file quantities a lot of variables are present but when i am trying to visualise the result, I cannot find lots of variables. For eg I want to plot the RMS X velocity along a line but cannot see the variable in the post-processing window. How to get these variables?

[Rob]

Using https://innovationspace.ansys.com/forum/forums/topic/how-to-calculate-the-tke-of-particles-in-dpm/#post-459164 as a hint you may want to check if it's a cell value and linked to the averaging functions (time or iteration).

[aniruddhabose96]

So I need to calculate it seperately from the X, Y, Z flow velocities. Is it not directly available as a post processing variable?

[Rob]

Please rephrase as I have no idea what you mean. 

[aniruddhabose96]

Yes, so I want to plot RMS X, Y velocity of a flow. But it is not available as a variable in the Fluent Contour or data panel. Although it is showing in the 'Standard Data File quantities' under File option. So how can i plot this variable?

[Rob]

Have you switched on the Data Sampling and run sufficient iterations/time steps for it to be meaningful? 

[aniruddhabose96]

Yes, I switched on everything and ran for 4.8 s for time averaging. The variables i get in the contour panel are Mean X velocity, RMSE X velocity but do not get RMS X velocity.

[Rob]

But Mean is available. If I have some time with +2m/s x-velocity in a cell and an equal time at -2m/s  what should my mean value be? What should my RMS value be? 

[aniruddhabose96]

Yes so thats i was saying I have to calculate seperately by taking the data out in another software like python, etc. The RMS value is not directly available to plot. But how is RMSE calculated is it the function value - mean value until that time?

[Rob]

Read my question carefully. 

[aniruddhabose96]

Hi

for your above question, the mean is 0 but RMS is 2. But i did not get how are you trying to relate. If i want to plot the RMS value, it is not directly available, so I need to take the data out right? Or are you telling to calculate the RMS from the mean and RMSError values?

[Rob]

Decide what YOU want from the data. I was pointing out that you need to be more careful with what you're asking. If a specific value isn't available you'll need to calculate it, either within Fluent or externally.