Study on the drag reduction and vortex-induced vibration of an 18-sided polygon cylinder

Tall buildings and transmission wires with polygon cross-sections have been shown to have lower drag forces than circular cross-sections. In this study, flow around an 18-sided polygonal cylinder in both stationary and elastic states is numerically investigated. For a stationary polygon cylinder, the effect of Reynolds number (Re) on the drag coefficient (CD) is studied in the range Re = 2.3 x 104-10.6 x 104 and the effect of surface roughness (K/D) on CD is studied at Re = 7.1 x 104. The vortex-induced vibrations (VIV) of an elastically mounted polygon cylinder with m*C = 0.185 is studied in the range Re = 1.8 x 103-3.7 x 103 and Ur = 3.0-6.0. The results show that the CD of the stationary polygon cylinder is reduced by up to 47 % compared to a circular cylinder and that K/D affects the boundary layer separation and thus CD. The VIV of the polygon cylinder exhibits many features similar to a circular cylinder in the response, phase difference, and wake characteristics though the maximum vibration amplitude is reduced by 11.4 %. Based on the lift coefficient, displacement, and instantaneous frequency, the vibration mechanism of the VIV of the cylinder is discussed.