Vortex Shedding from Tapered Cylinders at high Reynolds Numbers

: Helical strakes are known to reduce or suppress vortex-induced vibrations (VIV) of circular structures. The design of the strakes is generally recommended to be a triple-start helical strake system, with a strake height corresponding to approximately 10 percent of the structures diameter and 15 percent for strakes of circular cross section. The present paper argues that this height can be reduced for structures where the critical wind velocity for vortex shedding is in the Supercritical Reynolds number regime. The present investigations are aimed for suppressing VIV on offshore wind turbine towers during transportation. Preliminary wind tunnel experiments have been conducted with a tapered cylinder representing these towers. Static tests have been performed up to Re = 7.2x10^5. Dynamic tests at which the system oscillated due to VIV at Re = 1.48x10^5 (Subcritical) and Re = 4.96x10^5 (Supercritical). Results indicate that circular strakes with a diameter corresponding to 3 percent of the structures mean diameter can be used to efficiently reduce VIV in the Supercritical Reynolds number regime.