Abstract
In order to clarify the formation of Kármán vortex street, the present paper examines experimentally the disturbance growth in the formation region behind a circular cylinder at a Reynolds number, 3.2×103. At this Reynolds number, it is found that the disturbance growth is governed initially by the instability of the shear layers close to the separation points and subsequently by that of the wake profile downstream. Although these two kinds of instabilities really contribute to the disturbance growth leading to the formation of Kármán vortex street, the wake-type instability is found to be more dominant in terms of high growth rate and sharp frequency-selectivity. These observations are not in harmony with the recent work by Unal & Rockwell, who emphasized the importance of the initial disturbance behaviour in the region mainly governed by the instability of the separated shear layer. To obtain a better understanding of the formation of Kármán vortex street, it is highly desired to clarify more details of disturbance structure and behaviour in the process.
