Experiments on Vortex-Induced Vibration of a Vertical Cylindrical Structure: Effect of Low Aspect Ratio

Abstract

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 U r 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.

Large eddy simulation of wind flow through an urban environment in its full-scale wind tunnel models

“…Insufficient ventilating mechanism could cause temperature rise and pollutant concentration which in turn result in health problems and increased power consumption by air conditioning systems usage [1,2]. In addition, the wind flow induces alternating lift and drag forces and hence causing vibrations on submerged buildings [3] that could potentially lead to failure of civil structures. Another factor motivating the research in atmospheric flow is the design of more efficient wind turbines [4].…”

Analytical modelling prediction by using wake oscillator model for vortex-induced vibrations

“…That is because a large safety factor is applied during the design stage due to lack of knowledge and understanding on VIV prediction. Extensive study about VIV was done for a few decades, such as [2] [3][4][5][6][7][8][9][10][11][12][13], and thus attempts to reduce the VIV of structures have been done. It is important to know that many object shapes may lead to VIV.…”

Towards developing an idealised city model with realistic aerodynamic features

“…Along with the persisting attempts to reduce the heat release from burning fossil fuels either by improving the efficiency of industrial equipment [1] or developing clean energy sources [2], scientific research targets planning modern cities of enhanced natural ventilation potentials and minimal solar absorption [3]. In addition to natural ventilation, the study of atmospheric flow through cities is important to evaluate the stresses [4] and vortex-induced vibrations [5] on buildings and structures due to turbulence coherent structures [6]. As computer capabilities improve, computational fluid dynamics (CFD) becomes more popular with its cost effectiveness and ability to impose a wide variety of conditions.…”