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No 3Dimensionality in Flow Past a Cylinder of Subcritical Re # Using RANS

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Old   December 9, 2015, 20:24
Default No 3Dimensionality in Flow Past a Cylinder of Subcritical Re # Using RANS
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Hi,

I am trying to simulate flow past a circular cylinder for Re = 4,000 . For now, I am using RANS models in Star-CCM+. I have noticed that any k-epsilon based models in Star-CCM+ (i.e. SKE, RKE, RSM) do not have any 3 dimensionality. Meaning, there is no amplitude modulations in forces when the phenomenon is model in a 3D configuration and the result of 3D and 2D simulations are almost identical.

In general, the results of k-epsilon models( drag, lift, and Strouhal number) are closer to the experiment compared to k-omega, although the model shows no 3D effects and the values of velocities and vorticity do not change in depth.

On the other hand, using 3D simulations with k-omega results in lower values of drag and lift and amplitude modulation is observed in results.

I do not think it is due to mesh refinement as I have tested different mesh settings and sizes. CFL number is kept less than 1.0 through out the simulation and y+ values are also less than unity.

Anyone has any idea why this happens? Any help is appreciated.

Thanks,
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Old   December 10, 2015, 17:32
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I am confused. When you say 3D, you have a finite length cylinder (not semi-infinite) that is giving the same results as 2D? If you have a semi-infinite model (meaning the cylinder spans the thickness of your domain and butts up against a wall or symmetry plane then you should see nearly identical results between a 2D mesh and a 3D mesh. However, if it is a finite cylinder (meaning it has free ends that the fluid can move around them) then you might still get similar results if the L/D is fairly high, although, you would have to be looking at z/L =0.5 (mid span of your cylinder).

Also, unless I miss understand you, the amplitude modulations of the forces are not a result of 2D vs 3D analysis. These are caused by von-karman vortex shedding and should be present for both models.
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Old   December 10, 2015, 18:37
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L/D is 3.0 for the 3D model and the top and bottom boundaries are modeled as periodic interfaces. The response looks like a sinusoidal signal which in reality is not. There should a periodic response with amplitude modulations (the maximum value of forces in a period changes through time). For this range of Re number vortex shedding is a 3D phenomenon and 3D-ness should help get a lower response and amplitude modulation. But I do not see this when I use k-epsilon.
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Old   December 10, 2015, 18:41
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k-e is not a good model for separation driven phenomena. Try using k-omega SST and see if that helps.

I am still confused by your choice of words though;

"The response looks like a sinusoidal signal which in reality is not. There should a periodic response with amplitude modulations..."

A sinusoidal response is periodic, isn't it? What am I missing in your explanation?
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Old   December 10, 2015, 18:55
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Thanks for your quick reply.

I have checked k-w sst and I see the 3D effects in the simulation. But the results are higher than the experiment.

The actual response should be a periodic signal with amplitude modulations while the k-epsilon result is a periodic response with a constant amplitude (no modulations)
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