Flow control of circular cylinders with longitudinal grooved surfaces

Flow around circular cylinders with different grooved surfaces (U and V shaped) was investigated experimentally. The drag force, mean velocity, and turbulence intensity profiles of the wake behind the cylinders were measured for Reynolds numbers based on the cylinder diameter (D = 60 mm) in the range Re-D = 8 x 10(3) similar to 1.4 x 10(5). Flow around the cylinders was visualized using a particle tracer technique to see the flow structure qualitatively. The results were compared with those for a smooth cylinder of the same diameter D. At ReD = 1.4 X 10(5), the U-type groove surface reduced the drag coefficient by 18.6% compared with the smooth cylinder, whereas the drag reduction of V-type grooves was only 2.5%. For the circular cylinder with a U-grooved surface, as the Reynolds number increases, the location of peak turbulence intensity moves downstream, and the turbulence intensity is decreased. The two longitudinal-grooved surfaces tested in this study show different surface pressure distributions at high Reynolds numbers due to their different flow structures near the cylinder surface. Flow visualization results show that the U-grooved cylinder elongates the vortex formation region more than 50% compared with the smooth cylinder. This is consistent with the results of turbulence intensity distribution measured along the wake centerline. The large-scale longitudinal vortices formed behind the grooved cylinders seem to be broken into smaller ones due to the existence of longitudinal grooves. On average, the scale of longitudinal vortices behind the U-grooved cylinder is much smaller in size compared with the other two cases.