Abstract
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 + 0.083 d. 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.
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