Necklace Vortex Excitation of Upstream Cylinder in Crisscross Circular Cylinder System : Influence of Cylinder Diameter, Gap and Damping Factor

The necklace vortex excitation, one of the longitudinal vortex excitations, appeared on the upstream cylinder in a crisscross circular cylinder system in uniform flow. The influence of the diameter ratio, gap-to-diameter ratio s/d and the logarithmic damping factor on the oscillation behavior caused by the necklace vortex excitation was investigated using a wind tunnel. The amplitude of the oscillation increases with increasing diameter ratio. The substantial fluctuating lift coefficient of the necklace vortex was introduced based on the spanwise dimension of the vortex, (C_<LR>)'_<rms>, and it was evaluated using the measurement of the oscillation amplitude. In the necklace vortex excitation region, the relationship between (C_<LR>)'_<rms> and the gap-to-diameter ratio collapses on a single curve irrespective of the diameter ratio of the two cylinders, and (C_<LR>)'_<rms> decreases with increasing gap-to-diameter ratio.