Re-examination of the effect of a plane boundary on force and vortex shedding of a circular cylinder

The hydrodynamic forces and vortex shedding of a smooth circular cylinder immersed in different boundary layers were experimentally investigated at Reynolds numbers from 1.30×10⁴ to 1.45×10⁴. The effects of the bed proximity, the thickness of the boundary layer, and the velocity gradient in the boundary layer on the pressure distribution, the hydrodynamic forces and the vortex shedding behavior were examined. The experimental results show that both the drag and lift coefficients strongly depend on the gap ratio, and are affected by the boundary layer. A downward lift is observed at certain gap ratios in rod-generated boundary layers, and an explanation of this downward lift is given. Two different criteria for calculating the Strouhal number in the literature are discussed in this paper. It is found that the variation of the root-mean-square (RMS) lift coefficient reveals the onset or suppression of the vortex shedding. A quantitative method for identifying the vortex shedding suppression point is proposed. The observations show that the vortex shedding is suppressed at a gap ratio of about 0.2–0.3, depending on the thickness of the boundary layer. This critical gap ratio decreases as the thickness of the boundary layer increases.