VIVs of a circular cylinder with a downstream strip-plate in cruciform arrangement

Earlier wind tunnel experiments by the present authors on the transverse vibration of a circular cylinder with a downstream counterpart in cruciform arrangement showed that either of the two longitudinal vortices – trailing or necklace vortices, depending on the gap-to-diameter ratio s/d – induces a resonant oscillation, like the Kármán-vortex-induced vibration (KVIV). In this study, the downstream cylinder is replaced by a strip plate with a width w comparable in magnitude with the cylinder diameter d, expecting that interference of a plate would be stronger than that of a circular cylinder. The characteristics of longitudinal vortices and VIV of the upstream cylinder are investigated through wind tunnel experiments over the range of Reynolds number 3 000−22 000, as well as a visualization experiment in a water tunnel at around Re=1 700. When the system is fixed, the two longitudinal vortices (trailing and necklace) shed periodically over respective ranges of s/d for plates with 0.5≤w/d≤0.62, while the trailing vortex sheds periodically but the necklace vortex is not observed for plates with 0.75≤w/d≤1.0. When the cylinder is elastically supported for transverse vibration, the plate effectively suppresses KVIV, at distances further downstream for larger w/d plates. An unexpected effect is that KVIV is enhanced by the plate with 0.5≤w/d≤1.0 over a certain range of s/d. The two longitudinal vortices induce resonant vibrations under conditions of w/d and s/d correlating with their characteristics for the fixed system. The velocity ranges of the trailing vortices are much wider than for the case of the two-circular-cylinder system. The effects of the downstream plate on VIV can be utilized for oscillation control, i.e., not only to suppress but also to induce oscillations for beneficial purposes.