turbulent vortex street behind a cylinder

Flashidashi · June 13, 2015, 09:19

Hello guys,

there are many studies and experiments about flows around cylinders. I have a basic question about ANSYS CFX operations to simulate the vortex behind the cylinder in turbulent flows.

My Question is: How does CFX simulate which of the two vertical vortex systems behind a cylinder collapse and detach first in a symmetric simulationroom? I read in books that in reality, the totality of little processes (e.g dust) leads to the detachment of a particular vortex, but you cant say, which one it will be.

Does anyone knows the procedure in CFX to simulate this phenomenon in transient simulations?

Thanks in advance, Flashi

ghorrocks · June 14, 2015, 07:49

As the simulation starts two vorticies will form behind the cylinder. They will be almost the same size and shape, but due to small variations one will be slightly dominant over the other. This variation will be tiny - but enough such that it gets amplified and then the dominant vortex pushes the other vortex out of the way. And then the vortex street starts up behind this.

The variation will be caused by a combination of meshing, numerical errors, convergence and the geometry not being completely straight.

Flashidashi · June 14, 2015, 08:27

Thanks Ghorrocks, thats what i wanted to know!

June 13, 2015, 09:19	turbulent vortex street behind a cylinder	#1
Flashidashi New Member Andreas Join Date: Dec 2014 Posts: 16 Rep Power: 12	Hello guys, there are many studies and experiments about flows around cylinders. I have a basic question about ANSYS CFX operations to simulate the vortex behind the cylinder in turbulent flows. My Question is: How does CFX simulate which of the two vertical vortex systems behind a cylinder collapse and detach first in a symmetric simulationroom? I read in books that in reality, the totality of little processes (e.g dust) leads to the detachment of a particular vortex, but you cant say, which one it will be. Does anyone knows the procedure in CFX to simulate this phenomenon in transient simulations? Thanks in advance, Flashi

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June 14, 2015, 07:49		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	As the simulation starts two vorticies will form behind the cylinder. They will be almost the same size and shape, but due to small variations one will be slightly dominant over the other. This variation will be tiny - but enough such that it gets amplified and then the dominant vortex pushes the other vortex out of the way. And then the vortex street starts up behind this. The variation will be caused by a combination of meshing, numerical errors, convergence and the geometry not being completely straight.

June 14, 2015, 08:27		#3
Flashidashi New Member Andreas Join Date: Dec 2014 Posts: 16 Rep Power: 12	Thanks Ghorrocks, thats what i wanted to know!