Effect of the number of grooves on flow characteristics around a circular cylinder with triangular grooves

In the flow around a circular cylinder, a sudden decrease in the mean drag coefficient occurs at a high Reynolds number, but the same phenomenon occurs at a lower Reynolds number in the case where there exist grooves or roughness on the cylinder surface. In this paper, in order to make clear the flow characteristics around a cylinder with 20, 26 and 32 triangular grooves, the mean drag coefficient, pressure distribution, velocity distribution and turbulence intensity distribution were measured. Moreover, the flow around the cylinder was analyzed by applying the RNGk − ɛ turbulent model, and the surface flow pattern was investigated using the oil-film technique. From these results, it was found that a sudden decrease in the mean drag coefficient of a cylinder with 32 triangular grooves occurs at a lower Reynolds number compared with 20 and 26 triangular grooves.