vortex formation length
Hi, everyone, I am puzzled with the defination of vortex formation length behind an circular cylinder. Some paper use the distance between the center of the cylinder and the u=0 position on the symmetry line of the wake. Some use the postion of the maximum fluctuating value of u on the symmetry line of the wake. Do they coincide with each other? Can anybody give me some explanation? Thanks!

Re: vortex formation length
I think these two are different things. In case of steady flow past circular cylinder, the wake length is defined as the distance between the center of cylinder (or rear stagnation point of the cylinder) and the point on the symmetry line at which U=V=0. I have not come up with the other definition but I guess it might be used to define the wake length in case of unsteady flow. You can get more detail on this subject in the following books:
1. Zdravkovich, M. M. (1997). Flow around circular cylinder Volume 1, Fundamentals. Oxford University Press, New York, USA. 2. Zdravkovich, M. M. (2003). Flow around circular cylinder Volume 2, Applications. Oxford University Press, New York, USA. 
Re: vortex formation length
Thanks very much! I have got three methods to define the vortex formation length from the papers. These are: (1) the mean U=0 position on the symmetry line of the wake; (2) the maximum fluctuating u position and (3) the maximum vortex position. Can you give more explanation for the defination in terms of its usage? In fact it may be difficulty for me to get your recommanded books.Thanks again!

Re: vortex formation length
You can get the detailed information of vortex formation and length in the frist 34 pages of the following paper: C. H. K. Williamson, Vortex dynamics in the cylinder wake, Annual Reviews of Fluid Mechanics, Vol. 28, pp. 477539, 1996. If you can not find this paper, please let me know. I'll write about these things in detail or I can forward you a copy of it.

Re: vortex formation length
You are great! I am sorry to say that I can not find this paper. I would like to get an ecopy of it or you can write more information about this question. Best regards!

Re: vortex formation length
[1] In the steady flow regime, the wake behind the cylinder is symmetric and closed. In this case, if the solution is steady, the 'recirculation or wake length' is defined as distance between the rear stagnation point or center of the cylinder and the point on the symmetry line having U=V=0. In case of the unsteady solution in this flow regime, its defined with the point on the symmetry line having mean U=V=0 and it is called as 'mean recirculation length'.
[2] In the steady periodic flow regime, the vortex is neither closed nor symmetric. In this case the maximum fluctuating u position is used to defined the vortex length. [3] I am not sure about the third definition but I guess it might be defining the vortex position in unsteady flows. If you still need to read more about it, I'll send you scaned copy of first 34 pages of that review paper by Williamson, let me know about it with ur email id but it might take bit time. 
Re: vortex formation length
Thanks. Please send me the first 34 pages copy of that review. By the way, I have the ecopy review paper(2004) by Williamson. If you need it, please tell me.
My email adress: mefpz@sina.com Thanks again! Pingzhi Fang, Tongji University, Shanghai, P. R. of China. 
Re: vortex formation length
P.S. Please divided the copy several parts, for the attachment of my email is limited to 10M

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thanks in advance 
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