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Studies in offshore engineering have shown that vortex-induced vibration (VIV) of a slender offshore riser can be fundamentally different from that of a cylindrical spar structure. Under similar environments, the primary difference between the two structures affecting vortex shedding relates to the aspect ratio (length to diameter ratio, L/D). In the present investigation, careful experiments were carried out on the VIV of an elastically mounted cylinder with a varying aspect ratio. Experiments were conducted by forcing the vertical cylinders in still water. Of particular interest was the measurement of the response amplitude, hydrodynamic forces, Strouhal number (St) and the lock-in region while the aspect ratio of the cylinder varied from L/D = 0.5 to L/D = 13. The experiments were conducted in the subcritical flow region (7.4 × 10 3 < Re < 2 × 10 5 ), corresponding to a range of reduced velocity Ur from 2 to 14. Comparisons with experimental results obtained from the literature showed meaningful trends. It was found that the response amplitude of the structure decreased as the aspect ratio was reduced. The decrease in response amplitude was accompanied by a decrease in the correlation length of the vortex shedding and the hydrodynamic forces experienced by the structure. The Strouhal number and lock-in region, too, showed a significant reduction with a lower aspect ratio. It is apparent that the aspect ratio influenced the occurrence of the VIV, which affected the hydrodynamics of the fluidstructure interaction between the fluid and the bluff body.

EXPERIMENTS ON VORTEX-INDUCED VIBRATION OF A VERTICAL CYLINDRICAL STRUCTURE: EFFECT OF LOW ASPECT RATIO