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 = 20000. 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, 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.
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Lee et al. (2004) studied this question.
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