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Velocity Fluctuations in Fluent

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Old   March 19, 2009, 02:30
Exclamation Velocity Fluctuations in Fluent
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Andrew
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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
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Old   March 20, 2009, 03:31
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User Defined Function (UDF)?
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Old   March 20, 2009, 05:05
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Hi Paka,

Yeah, I meant User Defined Function.

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Andrew
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Old   March 20, 2009, 05:35
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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,
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Old   March 20, 2009, 05:38
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Thanks Paka.

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

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Andrew
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Old   March 21, 2009, 14:40
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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
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Old   March 22, 2009, 17:17
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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
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Old   March 22, 2009, 22:41
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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.

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Old   August 10, 2015, 18:12
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Hello Andrew,
Did you find out how to calculate fluctuating velocities in fluent? Do you have any user defined function for this?
Thanks,
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Old   August 11, 2015, 12:07
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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.
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Old   August 11, 2015, 12:58
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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...
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Old   August 11, 2015, 16:19
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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.
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Old   August 18, 2015, 14:11
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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.
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Old   August 19, 2015, 10:08
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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.
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Old   August 24, 2015, 00:26
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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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