Abstract
The flow around two-dimensional circular cylinders at high Reynolds numbers has been a subject of intense attention for a long time both from academic and practical points of view; therefore a great deal of information has been accumulated. After the Roshko's classification of the flow ranges in 1961 based on the boundary-layer behavior, many researchers have modified it. The terminology and the definitions themselves vary among authors, which sometimes leads us to confusion and misunderstanding. In this paper, the classifications of the flow ranges defined or used in the literature are reviewed togeter with the definition of the critical Reynolds number. The fundamental nature of the flow is also presented. This paper is restricted to low-turbulence flows around smooth and rough-walled cylinders at Reynolds numbers larger than about 104.
