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How is drag oscillations past a circular cylinder characterized?

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Old   February 20, 2020, 05:59
Default How is drag oscillations past a circular cylinder characterized?
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filippo zacchei
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Hi guys,

I simulated the flow past a circular isolated cylinder using U-RANS simulation with PHOENICS program (Re=10^4). My object was studying the oscillation of the drag force and Drag coefficient on the cylinder. I appreciated the fact that the oscillations are characterized by a frequency 2 times bigger than the lift oscillations one. However I do not understand why these oscillations, as in the figure, are charcterized by another intrinsic frequency which gives me the altenations of two average minima. I expect that the amplitude differs a little bit, but I don't understand why this pattern is so repetitive also for much longer simulations.
Thank you very much

Filippo

https://ibb.co/jv9Fwbh
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Old   February 20, 2020, 06:50
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Why are you so surprised by the fact that you do not see a singular frequency? Only for a laminar vortex shedding you get an oscillation at a single wavenumber
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Old   February 20, 2020, 07:15
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Quote:
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Why are you so surprised by the fact that you do not see a singular frequency? Only for a laminar vortex shedding you get an oscillation at a single wavenumber
Yes that's true. However can you tell me a reason for this seond dominant frequency? It looks like the vortex from one of the side of the cylinder is Always bigger than the other.
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Old   February 20, 2020, 07:35
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Quote:
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Yes that's true. However can you tell me a reason for this seond dominant frequency? It looks like the vortex from one of the side of the cylinder is Always bigger than the other.
Actually, you have a very wide range of frequencies, they are filtered out by URANS formulation in the large part, some are solved and appear.
Are you solving the 3d problem?
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Old   February 20, 2020, 07:47
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Actually, you have a very wide range of frequencies, they are filtered out by URANS formulation in the large part, some are solved and appear.
Are you solving the 3d problem?
No, Itís the bidimensional approximation.
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Old   February 20, 2020, 08:10
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No, Itís the bidimensional approximation.
There is no real sense in a 2D URANS
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Old   February 20, 2020, 08:58
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There is no real sense in a 2D URANS
Actually the only task of my work was to see the shedding vortex behind the cylinder, and comparing it with lift and drag oscillations. However looking at the drag coefficient in the picture we can see the presence of two frequencies. I didn't know if there was a possible explanation to the alternations of the two averages minima. I agree with you that this is not a real and physical problem, since it's 2D. But maybe as using this approximation we can see the oscillation in the drag due to the vortex shedding behind the cylinder, also the presence of different peaks can be related to a reason I can't find at the moment.
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Old   February 20, 2020, 09:58
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Have a look to the first part of this paper
https://www.researchgate.net/publica...LIKE_BEHAVIOUR
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Old   February 20, 2020, 15:01
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Have a look to the first part of this paper
https://www.researchgate.net/publica...LIKE_BEHAVIOUR
Thank you ver much
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