Reducing the drag on a circular cylinder by upstream installation of a small control rod

The effects of installing a small control rod upstream of a circular cylinder are investigated experimentally, with a focus on the drag characteristics and the wake structure behind the cylinder. The Reynolds number based on the main cylinder diameter (D=30 mm) is about Re=20 000. Control rods with diameters d ranging from d/D=0.133 to 0.267 are tested. Wake velocity and surface-pressure distributions are measured with varying the pitch distance L (i.e., center-to-center distance) between the main circular cylinder and the control rod. The form drag coefficient is obtained by integrating the surface pressure on the cylinder surface. The critical pitch distance L_c at which vortices start to shed from the control cylinder is found to obey the relation L_c/D=1.5+0.083d. Characteristic flow quantities such as pressure and drag force change substantially at the critical pitch distance. The drag coefficient of the main circular cylinder decreases about 29% when a control rod of diameter d=7 mm (d/D=0.233) is installed at a pitch ratio close to the critical value of L_c/D=2.081. However, the maximum reduction of the total system drag including that of the control rod (about 25%) is observed at a pitch ratio of L/D=1.833 with a control rod diameter of d/D=0.233.