# Why does vortex shedding occur?

 Register Blogs Members List Search Today's Posts Mark Forums Read

 February 24, 2011, 11:03 Why does vortex shedding occur? #1 Senior Member   TWB Join Date: Mar 2009 Posts: 119 Rep Power: 9 I have read some articles and papers on vortex shedding. Most simply describe the phenomenon and where and when it occurs, but does not give a physical explanation as to why it occurs. For example, a simple flow past a cylinder. Why does vortex shedding occurs after a while? Can someone give a clear physical explanation? Thanks!

 February 28, 2011, 19:55 #2 Senior Member   Join Date: Nov 2009 Location: Michigan Posts: 135 Rep Power: 9 you must already be aware that it happens because of adverse pressure gradient etc i had the same question and i believe it is the inherent non uniformity in the inlet flow that triggers the vortex shedding (this might answer why shedding happens first from top or bottom of cylinder) but what about numerical simulation? what if the inlet flow i specify is uniform, without turbulence and the grid generated is also symmetric about, say, horizontal line passing through center of cylinder? which side of cylinder (top or bottom) will first trigger seperation? do round off errors play a role here? I was digging into this at some point but then left halfway.... thanks for reminding.... i will again work on it.... let me know if you find something useful

March 1, 2011, 06:33
#3
Senior Member

Join Date: Jan 2011
Posts: 247
Blog Entries: 5
Rep Power: 9
Quote:
 Originally Posted by quarkz . Why does vortex shedding occurs after a while? Can someone give a clear physical explanation? Thanks!
The simplest explanation in my view reminiscent to that of Kelvin-Helmholtz instabilities when strong velocity shear is present (in BL with velocity inflection point or in recirculation regions): A non-zero curvature perturbation of "dividing" streamline will lead to a slight centrifugal force which in turn leads to a change in pressure thereby amplifying the ripple and stronger and stronger amplification and shedding the vortices.

March 1, 2011, 10:28
#4
Member

private
Join Date: Mar 2009
Posts: 74
Rep Power: 9
Quote:
 Originally Posted by doubtsincfd but what about numerical simulation? what if the inlet flow i specify is uniform, without turbulence and the grid generated is also symmetric about, say, horizontal line passing through center of cylinder? which side of cylinder (top or bottom) will first trigger seperation? do round off errors play a role here? I was digging into this at some point but then left halfway.... thanks for reminding.... i will again work on it.... let me know if you find something useful
I had the opportunity to work in the T3 group at LANL for a while in the 70's. This was shortly after J. Fromm and Frank Harlow published a (I think the first) simulation of vortex shedding.

I asked Dr. R. A. Gentry of that group how the shedding was initiated numerically. As best I recall, he answered "I'm sure we perturbed the inlet flow. We couldn't wait around for some random fluctuation to trigger it."

Hope this is useful.

OTD

 March 1, 2011, 20:37 #5 Senior Member   Hua Zen Join Date: Mar 2009 Posts: 116 Rep Power: 9 I have the same question. In reality it could be the perturbation of the incoming flow that trigger the vortex shedding. But what cause it in numerical simulation with homogeneous incoming flow and symmetrical configuration? Now the only explanation I could find is round off error when using the floating point calculation.

March 1, 2011, 20:59
#6
Member

private
Join Date: Mar 2009
Posts: 74
Rep Power: 9
Quote:
 I have the same question. In reality it could be the perturbation of the incoming flow that trigger the vortex shedding. But what cause it in numerical simulation with homogeneous incoming flow and symmetrical configuration? Now the only explanation I could find is round off error when using the floating point calculation.
Read the post just above yours. It refers specifically to numerical simulations. The Fromm and Harlow study was done by deliberately perturbing the inflow initially to break the symmetry. At least that's what one of their colleagues told me.

I suppose round-off or using difference algorithms that break the symmetry imposed by the boundary conditions could trigger shedding in a homogeneous inflow as well.

 March 1, 2011, 23:11 #7 Member   Derwin Parkin Join Date: Feb 2010 Posts: 35 Rep Power: 8 Yes Round-off can trigger it. I have run laminar simulations past cylinders and noticed shedding occur, with 100% uniformity in the flow. The only explanation I and my colleagues agreed on was Round-off error. But also, if you were to use a turbulence model, applying a level of turbulence at the inlet will speed up the process. The turbulence model you select will also play a role in the shedding of vortices - for example, L.E.S. is much better than just running a transient SST simulation.

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Julien Pralong FLOW-3D 1 October 23, 2010 15:24 aay023 Main CFD Forum 0 September 15, 2010 21:49 ROOZBEH FLUENT 0 July 12, 2008 10:28 ROOZBEH Main CFD Forum 0 July 12, 2008 10:27 lcw Main CFD Forum 3 September 1, 2005 13:40

All times are GMT -4. The time now is 02:28.