Investigation for the drag reduction by introducing a passage through a circular cylinder

A series of experimental studies have been conducted in a subsonic wind tunnel on circular cylinders with passages through them to investigate influence of the passages on drag coefficient. The passages have been placed through the cylinder at various longitudinal positions and at various angles. The aim of the study is to find the effect of a small narrow passage placed at different positions and angles through the cylindrical body, on the aerodynamic drag. Experimental results show that, the placement of the passage through the cylinder does not contribute on reducing drag coefficient at low Reynolds number region, except for the passage through the center of the smallest cylinder. However, the drag coefficients are much reduced as compared with smooth cylinder when the Reynolds numbers are reached in the critical zone. The experiments are further extended for double passages with converging and diverging orientations and show that, the drag reduction occurs for higher Reynolds numbers. The experimental results suggest that, placement of passage through the cylinder with any orientation can reduce the drag up to 31% at high Reynolds number. On the contrary, for lower Reynolds number region, the drag reduction is much influenced by the placement and orientation of the passage through the cylinder. And, it is found, while the passage is placed at the center along the longitudinal direction for the smallest tested cylinder, drag can be reduced up to 30%. The experimental results are validated with available literature and have obtained good agreement.