# vortex length scale

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 March 16, 2006, 12:49 vortex length scale #1 ZubenUbi Guest   Posts: n/a hello, Does anybody knows how to calculate an average length scale of a vortex flow. I mean, if you look at a flow behind a cylinder at low Reynolds number, you find an alley of vortices. Using a ruler, it is really easy to measure their radius. But how should I do if I can not or don't want to use a plan cutter ? Of course, if the center of the vortex is known then the radius can be computed from velocity spatial cross correlation. But how should I do if I don't know where the center is ? Thanks in advance for your answer, if any, ZubenUbi

 January 19, 2014, 08:28 #2 New Member   LouisK Join Date: Aug 2013 Location: Shanghai,China Posts: 9 Rep Power: 6 I have come across the same question, based on my calculation results, the velocity in the end of vortex is the lowest, so the distance between the two lowest velocity is vortex length, but do you have some reference papers for this method?

 January 19, 2014, 08:45 #3 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 3,819 Rep Power: 42 but the first issue is: how do you detect a vortex? Many methods can be used and can produce different results...

January 19, 2014, 08:51
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LouisK
Join Date: Aug 2013
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Quote:
 Originally Posted by FMDenaro but the first issue is: how do you detect a vortex? Many methods can be used and can produce different results...
I have gained the friction velocities around the gable roof. According to the streamline and the results of velocities, i think the vortex end velocity is the lowest, so the length between the two lowest velocities is the length of a vortex, but is there some reference for this method?

 January 19, 2014, 14:22 #5 Senior Member   Joachim Join Date: Mar 2012 Location: Providence, RI Posts: 144 Rep Power: 8 Hey! You could use the vortex detection scheme of Graftieaux I guess. Graftieaux, L., Michard, M., and Grosjean, N., \Combining PIV, POD and Vortex Identi cation Algorithms for the Study of Unsteady Turbulent Swirling Flows," Measurement Science and Technology, Vol. 12, No. 9, August 2001, pp. 1422-1429. Good luck! Joachim

January 19, 2014, 22:59
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LouisK
Join Date: Aug 2013
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Quote:
 Originally Posted by Joachim Hey! You could use the vortex detection scheme of Graftieaux I guess. Graftieaux, L., Michard, M., and Grosjean, N., \Combining PIV, POD and Vortex Identi cation Algorithms for the Study of Unsteady Turbulent Swirling Flows," Measurement Science and Technology, Vol. 12, No. 9, August 2001, pp. 1422-1429. Good luck! Joachim

Thank you! I have read this paper in the morning, it really helps about the concept of vortex length and center.
However, it does not solve my problem. I have gained the RANS results of a flow field by CFD, then I want to estimate the time averaged mean eddy length in an easy way without fluctuating velocities. The main purpose for me is to determine the influence length of a separation bubble on the wall.

 January 20, 2014, 02:23 #7 New Member   LouisK Join Date: Aug 2013 Location: Shanghai,China Posts: 9 Rep Power: 6 Thank you! I have read this paper in the morning, it really helps about the concept of vortex length and center. However, it does not solve my problem. I have gained the RANS results of a flow field by CFD, then I want to estimate the time averaged mean eddy length in an easy way without fluctuating velocities. The main purpose for me is to determine the influence length of a separation bubble on the roof. Kang Luyang

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