Journal of Fluid Mechanics



Three-dimensional vortex structures in a cylinder wake


J.  Wu a1, J.  Sheridan a2, M. C.  Welsh a1 and K.  Hourigan a2
a1 CSIRO Division of Building, Construction and Engineering, Highett, Victoria 3190, Australia
a2 Department of Mechanical Engineering, Monash University, Clayton, Victoria 3168, Australia

Article author query
wu j   [Google Scholar] 
sheridan j   [Google Scholar] 
welsh mc   [Google Scholar] 
hourigan k   [Google Scholar] 
 

Abstract

The three-dimensionality of the velocity field in the wake of a circular cylinder has excited considerable interest and debate over the past decade. Presented here are experimental results that characterize the underlying vorticity field of such wakes. Using particle image velocimetry (PIV), instantaneous velocity fields were measured and from these the vorticity of the longitudinal vortices lying in the region between Kármán vortices was found. Near the saddle point, induced by the stretching of the Kármán vortices, the vorticity of the longitudinal vortices was found to be greater than the Kármán vortices themselves. Their circulation was of the order of 10% of the Kármán vortices. The high levels of vorticity result from the stretching of the longitudinal vortices, as evident in the topology of the vortices. It is shown that the longitudinal vortices are locked in phase to the KármánK vortices, effectively riding on their backs in the braid region. While only one mode of longitudinal vortex formation was observed, evidence was found of a step change in the vorticity levels at a Reynolds number of approximately 200. This is consistent with the transition point between the two modes of vortex shedding shown to exist by Williamson (1988). It had previously been proposed that the observed vortex patterns were consistent with the evolution of the longitudinal vortices from perturbations of vortex lines in the separating shear layer which experience self-induction and stretching from the Kármán vortices. Evidence is presented that supports this model.

(Published Online April 26 2006)
(Received May 5 1995)
(Revised November 17 1995)



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