Abstract Summary
Vortex-induced vibrations (VIV) is a phenomenon of self-exciting oscillations of elastic, or elastically mounted rigid bluff bodies exposed to in a gas or fluid flow. This sort of oscillations can be used to harvest electric energy from the kinetic energy of air or water flow. This fact determines the great practical significance in the study of this phenomenon. In this work we experimentally study a system, consisting of a finite-length circular cylinder elastically supported through a short splitter plate, which considered as a model of a wind generator. A support was a long rectangular steel plate 3x34 (mm). Its length, being a variable parameter, defines the structural bending and torsional frequencies. The plate is rigidly embedded into a massive base at one end and into a rigid cylinder with length 350 mm and external diameter 30 mm at the other end. Experiments were performed in a wind tunnel. As expected, we observed classical quasi-two-dimensional VIVs, at which the cylinder support experienced bending oscillations. However, at larger reduced speed, we discovered a new type of vortex-induced vibrations (VIV), which is caused by resonance with the torsional mode. It is suggested that during these fully three-dimensional torsional oscillations, von Karman vortex street, generated by upper and lower pieces of the cylinder, are shifted in phase by pi/2, and the aerodynamic transition from upper to lower segments occurs at the splitter plate, which prevents the vortex shedding at the cylinder center. For a torsion resonance state with beam length 170 mm, the characteristic reduced speed range is 7.2...7.9. The amplitude of torsional oscillations is investigated and turned out to be essentially larger than of classical quasi-two-dimensional VIVs, which suggests more effective energy harvesting from the torsional oscillations.
