Journal of Fluid Mechanics

On vortex formation from a cylinder. Part 1. The initial instability

M. F.  Unal a1 and D.  Rockwell a1
a1 Department of Mechanical Engineering and Mechanics, Lehigh University, Bethlehem, PA 18015, USA

Article author query
unal mf   [Google Scholar] 
rockwell d   [Google Scholar] 


Vortex shedding from a circular cylinder is examined over a tenfold range of Reynolds number, 440 [less-than-or-equal] Re [less-than-or-equal] 5040. The shear layer separating from the cylinder shows, to varying degrees, an exponential variation of fluctuating kinetic energy with distance downstream of the cylinder. The characteristics of this unsteady shear layer are interpreted within the context of an absolute instability of the near wake. At the trailing-end of the cylinder, the fluctuation amplitude of the instability correlates well with previously measured values of mean base pressure. Moreover, this amplitude follows the visualized vortex formation length as Reynolds number varies. There is a drastic decrease in this near-wake fluctuation amplitude in the lower range of Reynolds number and a rapid increase at higher Reynolds number. These trends are addressed relative to the present, as well as previous, observations.

(Published Online April 21 2006)
(Received October 24 1985)
(Revised November 6 1987)