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Non-periodic wake, is it possible? (flow past a I-beam)

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Old   July 26, 2017, 08:13
Default Non-periodic wake, is it possible? (flow past a I-beam)
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Hello,
I'm simulating a flow past a I-beam and, according to my solution, it has a periodic pattern in the beginning (periodic vortex shedding). But after sometime (aprox. 50 s) it seems to loose its periodicity. Please, see the attached figures.
I would like to know if it's possible to have non-periodic wakes or it's just a simulation error?

P.S.: I'm using a RANS turbulence model (k-omega SST) and I am using a unsteady solver.

Thanks in advance.
1.jpg
2.jpg
Cd.png
Cd-closed.png
residuo-inicial.png
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Old   July 26, 2017, 12:21
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Well, you are actually trying a URANS formulation. From a physical point of view, when the flow becomes transitional towards the turbulence, there is no more reason to have a periodic vortex shedding (In DNS or LES you will see that). Now, the question is if the URANS can justify the presence of more oscillating frequency...
This is somehow more tricky, the unsteady solution could contain some low frequencies but their physical meaning is not so clear.
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Old   July 27, 2017, 07:25
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Yep, that's exactly what I was wondering... My solution tells that the mean velocity does not have a periodic pattern. What would you say about it? Probably a simulation error or is it possible?
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Old   July 27, 2017, 08:44
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Quote:
Originally Posted by LThomes View Post
Yep, that's exactly what I was wondering... My solution tells that the mean velocity does not have a periodic pattern. What would you say about it? Probably a simulation error or is it possible?
The key is in understanding what is the "mean" field in URANS. In my opinion, this is not clear theoretically for such a problem. In principle, URANS has a more clear framework when the unstedy component in the mean field is due to an external force. A typical well suited case is the in-cylinder flow with a moving piston. Using URANS in a vortex shedding flow problem has some lack, at least in my opinion. If you would consider that, instead of an esemble averaging, the time-averaging is used, it would be more similar to a time-filtering in LES. In this case the range of resolved frequencies can give reason of a non exactly periodic solution.
I cannot say more, I never explored URANS for such problems...
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