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Old   December 15, 2016, 11:03
Default About flow around a circular cylinder using Fluent
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Dear all. I am performing flow around a circular cylinder in Fluent. The density is 1000kg/m3, the viscosity is 5 pa*s. The cylinder diameter is 0.5m and the width of flow path is 1m, the length of it is 4m. The inlet velocity is 0.5m/S.
So Reynold number Re=1000*1*0.5/5=100
After calculation I get a pair of stable vortices in the wake.
We all know there is laminar vortex when 40<Re<150.
And the cell Reynolds number contour shows that the reynolds above the cylinder is about 13. Why? I got 100 from my calculation.
I use laminar of viscous model and donít have a boundary mesh.
Thank you very much.
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Old   December 15, 2016, 11:14
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as you say, it's the cell reynolds number. It's based on the fluid properties in that grid cell, the velocity in that grid cell, and the lengthscale of that grid cell. It has nothing to do with the cylinder reynolds number that you calculated.
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Old   December 15, 2016, 21:44
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Thank you.
I know cell Reynolds number is not the same with that i calculated.
But the flow field seemed to be not consistent with the reynolds I calculated.
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Old   December 15, 2016, 21:45
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Quote:
Originally Posted by CeesH View Post
as you say, it's the cell reynolds number. It's based on the fluid properties in that grid cell, the velocity in that grid cell, and the lengthscale of that grid cell. It has nothing to do with the cylinder reynolds number that you calculated.
Thank you.
I know cell Reynolds number is not the same with that i calculated.
But the flow field seemed to be not consistent with the reynolds I calculated.
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Old   December 15, 2016, 21:52
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A few comments:
  • What are your boundary conditions? There appears to be a no-slip condition imposed on the top and bottom sides. This condition is similar to modelling flow around a cylinder within a box and the effective Re is higher due to the blockage effect.
  • As you've discovered, the cell Re is different to the cylinder Re -- these are two different metrics.
  • Is the drag on the cylinder matching experiments?
  • Consider applying inflation layers around the cylinder to capture the velocity gradient in the laminar boundary layer. Furthermore, is your solution grid independent?
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Old   December 15, 2016, 22:05
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Quote:
Originally Posted by `e` View Post
A few comments:
  • What are your boundary conditions? There appears to be a no-slip condition imposed on the top and bottom sides. This condition is similar to modelling flow around a cylinder within a box and the effective Re is higher due to the blockage effect.
  • As you've discovered, the cell Re is different to the cylinder Re -- these are two different metrics.
  • Is the drag on the cylinder matching experiments?
  • Consider applying inflation layers around the cylinder to capture the velocity gradient in the laminar boundary layer. Furthermore, is your solution grid independent?
Thank you.
I use no-slip condition on the top and bottom sides. The effective Re is higher. But the flow field seems to be the same with ever.
I will try to consider the drag.
After applying inflation layers I get the same results as before. After increasing the mesh size it seems the same.
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