How to report force in les model

aref_ehteshami · May 31, 2021, 05:34

Hi guys
I'm working on a numerical simulation of wind turbine in a turbulent flow.
as far as I understood although my problem is turbulent and unsteady But when I take time averaging from flow parameters like velocity field the contours become statistically steady.
for example after enough flow time let's say 40 s if I take time average from velocity contours from 40 s until 42 s it is identical to when I take time average from 42 until 44.
The problem is My whole Turbine is inside a Body and I want calculate The force that are exerting on my body by flow. But when I get time averaging in the the same periods that I mentioned they are not equal.
How I should report this forces?
Is it necessary to take time-averaging from them or just report them as a function of time for example by a diagram?
Thank you in advance

flotus1 · June 2, 2021, 03:29

From what you describe, it appears that the largest time scales for the two quantities might be different.
Whatever the case may be, 2s seems to be too small of an averaging interval for the body forces you are trying to extract. Don't forget that the averaged quantities taken from two finite-length intervals will always be different though, that's just statistics. The question is how much of a difference you accept to still call it statistically steady.
Reporting "transient average" results from LES which are averages over some shorter time interval isn't something that is often done. You would need a rather solid reason to do that. One of them might be a boundary condition changing at a much larger time scale than any of the time scales inherent to the flow itself.
Since you are working with a wind turbine, i.e. what I assume is a rotating body: make sure your averaging intervals are always over a whole number of rotations. Otherwise you introduce another error to your averages, causing a larger difference between intervals even if the averaging window was "long enough". Alternatively, if the rotation has "equivalent configurations", that can be the smallest unit of averaging. E.g. for a wind turbine with 3 blades and no other obstructions, the averaging interval can be a whole multiple of 1/3 of a rotation. Then again, you might want to know the maximum/minimum forces exerted on the structure in such a case. Which means that you can't really work with averaged quantities.

May 31, 2021, 05:34	How to report force in les model	#1
aref_ehteshami New Member aref Join Date: Oct 2019 Posts: 25 Rep Power: 7	Hi guys I'm working on a numerical simulation of wind turbine in a turbulent flow. as far as I understood although my problem is turbulent and unsteady But when I take time averaging from flow parameters like velocity field the contours become statistically steady. for example after enough flow time let's say 40 s if I take time average from velocity contours from 40 s until 42 s it is identical to when I take time average from 42 until 44. The problem is My whole Turbine is inside a Body and I want calculate The force that are exerting on my body by flow. But when I get time averaging in the the same periods that I mentioned they are not equal. How I should report this forces? Is it necessary to take time-averaging from them or just report them as a function of time for example by a diagram? Thank you in advance

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June 2, 2021, 03:29		#2
flotus1 Super Moderator Alex Join Date: Jun 2012 Location: Germany Posts: 3,428 Rep Power: 49	From what you describe, it appears that the largest time scales for the two quantities might be different. Whatever the case may be, 2s seems to be too small of an averaging interval for the body forces you are trying to extract. Don't forget that the averaged quantities taken from two finite-length intervals will always be different though, that's just statistics. The question is how much of a difference you accept to still call it statistically steady. Reporting "transient average" results from LES which are averages over some shorter time interval isn't something that is often done. You would need a rather solid reason to do that. One of them might be a boundary condition changing at a much larger time scale than any of the time scales inherent to the flow itself. Since you are working with a wind turbine, i.e. what I assume is a rotating body: make sure your averaging intervals are always over a whole number of rotations. Otherwise you introduce another error to your averages, causing a larger difference between intervals even if the averaging window was "long enough". Alternatively, if the rotation has "equivalent configurations", that can be the smallest unit of averaging. E.g. for a wind turbine with 3 blades and no other obstructions, the averaging interval can be a whole multiple of 1/3 of a rotation. Then again, you might want to know the maximum/minimum forces exerted on the structure in such a case. Which means that you can't really work with averaged quantities.