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May 21, 2010, 06:21 
How to determine if vortex shedding will occur?.2

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Matteo Viery
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How to determine if vortex shedding will occur?.
This question is more from the theory point of view rather form a CFD simulation point of view. With given object dimensions and speed moving inside a fluid. How do I decide if vortex shedding will occur knowing a strouhal number?. Does it mean that every thing immerse into a fluid will shedding ( for any reynolds number obviusly with fluid separetion)?. what happen in hight reynolds numbers where inertial forces will dominate the fluid?..will you spect sheadding?. 

May 24, 2010, 02:38 

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May 25, 2010, 06:13 

#3  
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Matteo Viery
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Quote:
Rugabug, the wikipedia just gives you a quick answ, as example I have seen charts for blunt objects where strouhal number goes from zero to hundreds, but still it does not tell you if sheadding wil appear...that is my quetion. 

May 25, 2010, 14:03 

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Sorry I didn't really read the wiki article I just assumed it would have a discussion on Reynolds number. Anyways back in undergrad I seem to remember a teacher saying vortex shedding occurs at between Re=10005000 and that it is not comepletly dependent on Re.
This link on page 4 says 3000 for the needed Re. http://www.scribd.com/doc/9674101/Fl...ontrolJuly20 

May 31, 2010, 06:27 
vortex shedding

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unver
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Vortex shedding will occur indefinitely once the critical Reynolds number is reached. As an example take the case of Tacoma Bridges where Reynolds number is very high and the flow is definitely turbulent. Coherent vorticies will be turbulent but nevertheless they will be there.


June 1, 2010, 09:31 

#6 
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Matteo Viery
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OK, OK..
all these numeber (5005000 reynolds) are for a cylinder, but what happen when reynolds is 100000 or even 1E6?... you will se graphs for cylinders showing you strouhal number less of 0.15..that does it mean that you will see vortex sheadding?...again: what happen in hight reynolds numbers where inertial forces will dominate the fluid?..will you spect sheadding?. will you spect to have a stady state without sheadding due to inertial forces for hight RE numbers.? 

June 1, 2010, 10:15 

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Vortex shedding from a cylinder has been found for 10^2 < Re < 10^7. The average Strouhal number of such shedding is 0.21. I don't know if anyone has investigated beyond that Re range.


June 1, 2010, 16:18 

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Ertan Karaismail
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To my understanding, (orderly) vortex shedding is a phase or (route) to reach chaos (turbulence) at higher velocities. Since fluid flow is a nonlinear phenomena, at a critical point (Re) bifurcation starts in the flow, leading to orderly vortex shedding. For this range of Re where velocity is low, boundary layer is laminar and flow separates upstream of (or around) the symmetry axis of the immersed object. Viscous stress is high there, but can be beaten by the effect of adverse pressure gradient. It's worth noting that any roughness, sharp edges etc..on the surface of the object will ease triggering separation and hence will cause vortex shedding at low velocities. At higher Re, the wake tends to become chaotic and the boundary layer will become turbulent which can resist more to separation (turbulent viscosity, and hence viscous stress is higher). Therefore, separation point will shift downstream. This late separation will limit (narrow) the size of the vortices in the wake. Furthermore, since the turbulence in the wake is diffusive it will dampen out the gradients in the flow to some extent and the vortex shedding weaken or die away as well. However, the flow will always be unsteady with a frequency larger than the frequency of turbulent fluctuations. These flow structures are coherent structures. All in all, I think whenever you simulate a flow past a blunt body, you should expect coherent structures rather than vortex shedding, which, in my mind, covers orderly vortex shedding as well. I believe they can vanish at very high Re where flow gets extremely diffusive. Fluid flow is a matter if relative importance of forces which is mostly hard to predict beforehand without a dimensional analysis.


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