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-   -   Velocity Fluctuations in Fluent (http://www.cfd-online.com/Forums/fluent/62759-velocity-fluctuations-fluent.html)

Andrew March 19, 2009 02:30

Velocity Fluctuations in Fluent
 
Hi Everyone.

I'm running simulations using Fluent 6.2.16 for flow over an elastically mounted circular cylinder. I want to monitor and write to file the velocity fluctuations u', v' and w', but cannot seems to find these options anywhere within Fluent. I see that you can monitor the absolute velocity and relative velocities in all directions, but no fluctuations.

Does anybody know how I can monitor and record these parameters during a simulation? Are they available in the GUI within Fluent, or do I have to create an UDF to obtain the values?

Thanks.

Andrew

paka March 20, 2009 03:31

User Defined Function (UDF)?

Andrew March 20, 2009 05:05

Hi Paka,

Yeah, I meant User Defined Function.

Regards

Andrew

paka March 20, 2009 05:35

That's what I'm saying, read tutorials about UDF, finally the idea should show up, unless someone else on this forum can provide you ready to go solution.

Best,
K

Andrew March 20, 2009 05:38

Thanks Paka.

I'll try to read through the UDF tutorials and manual to see if there is anything relating to fluctuating velocities.

Regards

Andrew

O.D.Y. March 21, 2009 14:40

what if you just monitor the absolut velocities and then calculate the deviations yourself by substracting the time-mean-value or simple doing a DFT on it? Is that what you want?

regards

paka March 22, 2009 17:17

That's what I was thinking. As soon as you get familiar with UDF (which might look scary at the beginning), it should be relatively easy to write your own routine. It is easy to access velocity data, so this shouldn't be a big deal.

If you do not work on UNIX system, it means you use Windows, which means you need Microsoft C compiler, which means you need to spend money and get it - I absolutely do not support that idea :). In this case, I would just dump velocity data into text file and then do post-processing on your own writing for example simple Python script.

Best

Andrew March 22, 2009 22:41

Thanks Guys,

I appreciate the feedback. I will try to monitor the velocities into text files and post-process the data for fluctuating velocities using DFT or another method. This seems like the easiest option that Fluent will allow.

Regards

Andrew

maphd August 10, 2015 18:12

Hello Andrew,
Did you find out how to calculate fluctuating velocities in fluent? Do you have any user defined function for this?
Thanks,

LuckyTran August 11, 2015 12:07

Quote:

Originally Posted by maphd (Post 559014)
Hello Andrew,
Did you find out how to calculate fluctuating velocities in fluent? Do you have any user defined function for this?
Thanks,

If you know the average velocities (which you must in order to even define the decomposition in the first place) it is straightforward to define a custom field function through the GUI.

For example:
uprime = x-velocity - (average x velocity)

where you must enter the average x velocity

Do that for all three velocities and then you can export those plot/export those velocities. Because it's a simple scale shift, it's easily doable outside Fluent (say tecplot, matlab, excel, etc).

It's a little more cumbersome to define a custom field function through TUI, so I recommend to do it first through the GUI if you have access to the GUI.

maphd August 11, 2015 12:58

LuckyTran,

Thank you for your reply.

The problem is that fluent does not give average velocities directly. Do you know any way to calculate the average velocities?

Thank you very much...

LuckyTran August 11, 2015 16:19

Quote:

Originally Posted by maphd (Post 559156)
LuckyTran,

Thank you for your reply.

The problem is that fluent does not give average velocities directly. Do you know any way to calculate the average velocities?

Thank you very much...

Fluent does give mean velocities if you enable data sampling for time statistics. There will be new variables for the mean quantities. You need to incorporate those new mean variables as part of your analysis.

If you still want fluctuating velocities in Fluent then you need to be a little creative. You need to export the mean values from the last time-step and onto the data at previous time-steps and then use that in the custom field function. Basically you need to run your transient simulation and save the data at all time-steps of interest, and then you can come back and calculate the fluctuating velocity after-the-fact.

maphd August 18, 2015 14:11

LuckyTran,

Thank you for your reply.
I enabled the data sampling for time statictics. However, I could not see the new variables for the mean quantities?

is it like other variables? should I look under velocity ?

Thanks, I appreciate your help.

LuckyTran August 19, 2015 10:08

In the GUI, they are listed under unsteady statistics or unsteady wall statistics. If you're using the TUI, they will have mean as a prefix, e.g. mean-x-velocity, mean-y-velocity, etc. The mean fluctuating components have the prefix RMSE. If you are using older versions of Fluent they may just be RMS instead of RMSE.

maphd August 24, 2015 00:26

LuckyTran,

I appreciate all your help. I am kind of new to this unsteady calculations in fluent. I finally get the unsteady statistics in fluent.
I have a question regarding the fluctuating velocities. I run my program for total of 1000 time steps. I also saved fluent files in every 50 time steps and keep running the fluent until 1000 time steps. I also saved my latest file which is in 1000 time steps.

So when I am writing custom define function; Should I define it in my last file which is in 1000 time steps?
To calculate fluctuating velocity; I will take the difference between instantaneous velocity which is x velocity in the GUI and mean x velocity. My question is will I choose these variables in my last file which is 1000 time steps? Will I be using my previous files at all which I saved in different time steps?

Thank you...


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