Flow around a circular cylinder with axially or helically arranged holes

In this study, we investigate the characteristics of flow around a circular cylinder with axially or helically arranged holes. The experiment is performed in a wind tunnel at Re = 80,000 while varying the angle of attack (α; the angle between the reference hole center axis and the streamwise direction) from 0° to 90°. We directly measure the drag forces on a circular cylinder with axially or helically penetrated holes and compare them with that on a circular cylinder with no hole. Axially arranged holes reduce the drag at low α but rather increase the drag at high α, as compared to that with no hole. On the other hand, the drag is reduced by helically arranged holes, regardless of the angle of attack. The velocity fields around a circular cylinder with and without holes are measured by particle image velocimetry, and the flow mechanisms responsible for the drag reduction are to be suggested in the presentation.