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Resolved Reynolds Stresses in LES...??!

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Old   November 28, 2014, 11:30
Post Resolved Reynolds Stresses in LES...??!
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Teagyoung Kim
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Running 3D LES on FLUENT.

Clicked Data Sampling for Time Statistics options.
Now I see Resolved UV,UW,VW Reynolds Stresses on export quantities.

Q1. Why suddenly showing up after Time Statistics options??

Q2. Can I use this as the input for Acoustic Analogies?! Is it mean valued throughout the simulation?!

Q3. Do I just square the resolved x velocity to get Resolved UU Reynolds Stress? Is this correct?

Anyone experienced in dealing with Lighthill's Stress Tensor??

Last edited by ktgfeel; November 28, 2014 at 11:32. Reason: confusing title
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Old   November 29, 2014, 02:46
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Quote:
Originally Posted by ktgfeel View Post
Q1. Why suddenly showing up after Time Statistics options??
The Reynolds stresses are time-averaged quantities. Once you activate time statistics, many time-averaged mean and rms quantities suddenly become available (such as mean velocity, rms velocities).
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Originally Posted by ktgfeel View Post
Q2. Can I use this as the input for Acoustic Analogies?! Is it mean valued throughout the simulation?!
It is mean valued based on the sampling interval that you set (default is every time-step). And yes you can use it for a lot of stuff.
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Originally Posted by ktgfeel View Post
Q3. Do I just square the resolved x velocity to get Resolved UU Reynolds Stress? Is this correct?
If you do it to the rms velocities. Yes.

Sorry I am not familiar with Lighthill's Stress Tensor.

I am not sure why in the GUI the reynolds stresses are labeled with "Resolved". In the field variables they are not named that way but are simnply named UU UV and UW Reynolds stresses. Information on the meaning of "resolved" is somewhat obscure.
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Old   December 22, 2014, 00:12
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Thank you very much for your reply. I know understand more. but still haven't reached my goal.

Fluent doesn't have any option on Time Staticstics to have set the length of time to calculate. It only has option to set sampling intervals.
However, what I am trying to get is somewhat like Instantaneous Reynolds Stress.

I have moved on to LES calculating method because having Reynolds Stress Model converge was almost impossible. ( Reynolds Stress Model,i.e. RSM, does calculate Reynolds stresses right away).

Q1. If I just multiply LES calculated(not rms) velocities, what would it mean?? Would it be something like instantaneous reynolds stresses??

Thank you again for your reply and careful explanation.
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Old   December 22, 2014, 00:13
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Quote:
Originally Posted by LuckyTran View Post
The Reynolds stresses are time-averaged quantities. Once you activate time statistics, many time-averaged mean and rms quantities suddenly become available (such as mean velocity, rms velocities).

If you do it to the rms velocities. Yes.
Thank you very much for your reply. I know understand more. but still haven't reached my goal.

Fluent doesn't have any option on Time Staticstics to have set the length of time to calculate. It only has option to set sampling intervals.
However, what I am trying to get is somewhat like Instantaneous Reynolds Stress.

I have moved on to LES calculating method because having Reynolds Stress Model converge was almost impossible. ( Reynolds Stress Model,i.e. RSM, does calculate Reynolds stresses right away).

Q1. If I just multiply LES calculated(not rms) velocities, what would it mean?? Would it be something like instantaneous reynolds stresses??

Thank you again for your reply and careful explanation.
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Old   March 1, 2016, 07:58
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Hi ktgfeel,

I know this is quite an old thread, I was just wondering if you or anybody else reading eventually got how the Resolved Reyonlds Stresses are calculated in Fluent when doing LES and collecting statistics.

My idea is that, for instance, the resolved uv stress should be defined as:

Resolved uv Reynolds stress = {u''v''}
where {} indicates ensemble average over time and

u''=[u]-{[u]}
v''=[v]-{[v]}
where [u] and [v] are the filtered (i.e. resolved) instantaneous velocity components, and {} again indicates time averaging.

Cheers
Andrea
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Old   March 1, 2016, 10:46
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I have created custom field functions for the variables defined in my previous post and I am matching the Resolved Reynolds Stressed reported by Fluent under Unsteady Statistics.

I guess this answers LuckyTran's question about the label "resolved", since the Reynolds stressed are calculated based on the resolved velocity field and the contribution coming from the subgrid scales is neglected.
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Old   March 2, 2016, 18:18
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Quote:
Originally Posted by Andrea1984 View Post
I have created custom field functions for the variables defined in my previous post and I am matching the Resolved Reynolds Stressed reported by Fluent under Unsteady Statistics.

I guess this answers LuckyTran's question about the label "resolved", since the Reynolds stressed are calculated based on the resolved velocity field and the contribution coming from the subgrid scales is neglected.
Thanks I've always wondered but never got around to testing it myself. So it seems like Fluent needs to do a better job labeling internal variables and work on their documentation.

Quote:
Originally Posted by Andrea1984 View Post
Hi ktgfeel,

I know this is quite an old thread, I was just wondering if you or anybody else reading eventually got how the Resolved Reyonlds Stresses are calculated in Fluent when doing LES and collecting statistics.

My idea is that, for instance, the resolved uv stress should be defined as:

Resolved uv Reynolds stress = {u''v''}
where {} indicates ensemble average over time and

u''=[u]-{[u]}
v''=[v]-{[v]}
where [u] and [v] are the filtered (i.e. resolved) instantaneous velocity components, and {} again indicates time averaging.

Cheers
Andrea
In most circles, this is the meaning of resolved Reynolds stress, calculated based on resolved/filtered velocity and does not include the contribution from the SGS model. It is good to know that the Fluent definition is the same. I appreciate the work you did to verify this.
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