Experimental investigations on the vortex-shedding from a highly tapered circular cylinder in smooth flow

Abstract

This paper presents the results of a wind tunnel test campaign conducted to investigate the aeroelastic and aerodynamic behaviour of a tapered circular cylinder in smooth flow conditions. The model under investigation is a single-mode aeroelastic one, whose taper ratio (in terms of diameter) is 8 %. Its global dynamic response was measured through the use of an internally-mounted piezo-electric accelerometer. The sensitivity of the vortex-induced response to different levels of structural damping was explored. The harmonic content of the wake was also measured through a hot-wire anemometer positioned downstream of the wind tunnel model at different heights: this allowed the detection of the portion of the model whose local resonance was accountable for the highest vortex-induced structural response. To further expand the findings of the wake measurements, four ‘rings’ of surface-mounted pressure sensors were installed on the wind tunnel model. The Reynolds number that was covered during the experiments ranged between 9.3×103 and 3.2×104.The experimental investigations revealed that the tapering has a beneficial effect with respect to vortex-induced oscillations, reducing their magnitudes when compared against the ones of a parallel-sided circular cylinder. On the other hand, the lock-in region was found to be wider, exhibiting three distinct peak regions for the lowest level of structural damping. The combined results from the accelerometer, the hot-wire anemometer and the pressure measurements, and the application of tailored time–frequency analyses based on the continuous wavelet transform, revealed that the maximum dynamic response is linked to the local resonance between the fundamental frequency and the bottom portion of the structure. Furthermore, the magnitudes of the local mean aerodynamic drag coefficient and the local standard deviation of the (vortex-induced) lift coefficient were found to be considerably lower than those of a parallel-sided circular cylinder.