Effect of a plane boundary on oscillatory flow around a circular cylinder

This study deals with the flow around a circular cylinder placed near a plane wall and exposed to an oscillatory flow. The study comprises instantaneous pressure distribution measurements around the cylinder at high Reynolds numbers (mostly atRe∼ 105) and a flow visualization study of vortex motions at relatively smaller Reynolds numbers (Re∼ 103–104). The range of the gap-to-diameter ratio is from 0 to 2 for the pressure measurements and from 0 to 25 for the flow visualization experiments. The range of the Keulegan–Carpenter numberKCis from 4 to 65 for the pressure measurements and from 0 to 60 for the flow visualization tests. The details of vortex motions around the cylinder are identified for specific values of the gap-to-diameter ratio and for theKCregimes known from research on wall-free cylinders. The findings of the flow visualization study are used to interpret the variations in pressure with time around the pipe. The results indicate that the flow pattern and the pressure distribution change significantly because of the close proximity of the boundary where the symmetry in the formation of vortices breaks down, and also the characteristic transverse vortex street observed for wall-free cylinders for 7 <KC< 13 disappears. The results further indicate that the vortex shedding persists for smaller and smaller values of the gap-to-diameter ratio, asKCis decreased. The Strouhal frequency increases with decreasing gap-to-diameter ratio. The increase in the Strouhal frequency with respect to its wall-free-cylinder value can be as much as 50% when the cylinder is placed very close to the wall with a gap-to-diameter ratio ofO(0.1).