Structure of longitudinal vortices shedding periodically from two cylinders in a cruciform arrangement

It was reported in earlier papers by the present authors that longitudinal vortices shed periodically from two cylinders in a cruciform arrangement with a gap s between them, placed in otherwise uniform flow. The longitudinal vortices can cause a strong vibration of the upstream cylinder. When the gap-to-diameter ratio s/d≤0.25, trailing vortices are shed periodically, and when 0.25≤s/d≤0.5, necklace vortices are shed periodically.In this work, the structure and the shedding process of the two types of the longitudinal vortices were investigated through flow visualization in a water tunnel. The vortex shedding frequency fv was obtained by a hot-film anemometer. Results concerning St-Re (St :Strouhal number, Re :Reynolds number) relationship for the longitudinal vortices obtained by the water tunnel measurements were found to correlate well with those by wind tunnel measurements.