Journal of Fluid Mechanics



On the formation and suppression of vortex ‘shedding’ at low Reynolds numbers


P. J.  Strykowski a1 and K. R.  Sreenivasan a2
a1 Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455, USA
a2 Mason Laboratory, Yale University, New Haven, CT 06520, USA

Article author query
strykowski pj   [Google Scholar] 
sreenivasan kr   [Google Scholar] 
 

Abstract

Vortex ‘shedding’ behind circular cylinders can be altered and suppressed altogether (or ‘controlled’) over a limited range of Reynolds numbers, by a proper placement of a second, much smaller, cylinder in the near wake of the main cylinder. This new and dramatic suppression of vortex ‘shedding’ is the subject of this paper. Details of the phenomenon are documented through parallel experimental and numerical investigations, including flow visualization. Temporal growth rate measurements of the velocity fluctuations reveal that the presence of the smaller cylinder reduces the growth rate of the disturbances leading to vortex ‘shedding’, and that its suppression, accompanied by the disappearance of sharp spectral peaks, coincides with negative temporal growth rates. It is argued that the presence of the secondary cylinder has the effect of altering the local stability of the flow by smearing and diffusing concentrated vorticity in the shear layers behind the body; a related effect is that the secondary cylinder diverts a small amount of fluid into the wake of the main cylinder. A unified explanation of the formation and suppression of the vortex street is attempted, and it is suggested that the vortex ‘shedding’ is associated with temporally unstable eigenmodes which are heavily weighted by the near field. It is also shown that absolute instability is relevant, up to a point, in explaining vortex shedding, whose suppression can similarly be associated with altering the instability in the near wake region from absolute to convective.

(Published Online April 26 2006)
(Received January 9 1989)
(Revised December 6 1989)



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