The effect of a plane boundary and the velocity gradient on force and vortex shedding characteristics of a smooth circular cylinder is investigated experimentally at Reynolds numbers ranging from 1.0×104 to 2.2×104. It is observed that the drag coefficient and the Strouhal number are affected by both the gap ratio and the velocity gradient, while the lift coefficient is dominated by the gap ratio only. A new method for identifying the vortex shedding suppression point is proposed.
Flow around a circular cylinder near a plane boundary has been the subject of much research due to its engineering relevance. Although a great deal of work has been published on this subject, there remain some confusing, or even controversial aspects of that research. When fluid flows around a circular cylinder above a critical Reynolds number, vortex shedding occurs. Therefore, the cylinder is subjected to a fluctuating drag and lift in addition to the mean drag and lift. The drag, lift and vortex shedding frequency are normally expressed in non-dimensional forms as the drag coefficient CD, lift coefficient CL, and Strouhal number St. These three non-dimensional parameters depend on the nature of both the approach flow (such as Reynolds number, flow pattern and freestream turbulence etc.) and the cylinder (such as aspect ratio and surface roughness etc.). When the cylinder is located near a plane boundary, the bed proximity and the velocity profile in the boundary layer also have some influence on these parameters. The effect of a plane boundary on force and vortex shedding characteristics has been investigated by many researchers. The experiments conducted by Bearman and Zdravkovich (1978) in a wind tunnel revealed that for gap ratios greater than 0.3 the Strouhal number was remarkably constant.