The flow around a circular cylinder is controlled by attaching O-rings to reduce drag force acting on the cylinder. Wind tunnel experiments on the flow around a circular cylinder with and without ring type surface protrusions are carried out to investigate the flow characteristics of the controlled wake. Four experimental models are tested in this study; one smooth cylinder of diameter D (60 mm) and three cylinders fitted with longitudinal O-rings of diameters d = 0.0167D, 0.05D and 0.067D with various pitches. The drag force, mean velocity and turbulence intensity profiles in the near wake behind the cylinders are measured for Reynolds numbers based on the cylinder diameter in the range of Re-D = 7.8 x 10(3) similar to 1.2 x 10(5). Flow field around the cylinders is visualized using a smoke-wire technique to see the flow structure qualitatively. The results are compared with those for a smooth cylinder having the same diameter. At ReD = 1.2 x 10(5), the cylinder fitted with O-rings of d = 0.0167D in a pitch interval of 0.165D shows the maximum drag reduction of about 9%, compared with the smooth cylinder. The drag reduction effect of O-rings of d = 0.067D is not so high and it has nearly the same value as that of the smooth cylinder. For the O-ring circular, as the Reynolds number increases, the location of peak turbulence intensity shifts downstream and the peak magnitude is decreased. In addition, the vortex shedding frequency has nearly same value as that of the smooth cylinder up to a Reynolds number of 3.2 x 10(4). Thereafter, the shedding frequency increases and finally disappears as the Reynolds number increases. The visualized flow for the smooth cylinder does not show distinct spanwise variation of flow pattern. However, the size of vortices and vortex formation region formed behind the O-ring cylinder are smaller, compared with the smooth cylinder. In addition, the instantaneous topological flow image shows spanwise variation of V-shaped flow pattern. Consequently, the simple attachment of O-rings effectively controls the vortex shedding from the cylinder and elongates the vortex formation region, reducing drag force acting on the cylinder at high Reynolds numbers. (C) 2004 Published by The Japan Society of Fluid Mechanics and Elsevier B.V. All rights reserved.
