Vortex excitation of rectangular cylinders with a long side normal to the flow

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Abstract
The vortex excitation of rectangular cylinders having side ratios of 0.2, 0.4 and 0.6, with a long side normal to the flow, in a mode of lateral translation, is investigated experimentally in a wind tunnel using free- and forced-oscillation methods. The range of reduced wind speeds investigated, 3-12, includes the vortex-resonance regime. The forced-oscillation experiment includes measurements of the fluctuating lift force at amplitudes up to 10% of the length of a long side. The experiments were also performed on cylinders with a long fixed splitter plate. The results of the measurements show that vortex excitation of a rectangular cylinder is strongly dependent on the side ratio. It is suggested that the critical change of the mean base pressure of an oscillating rectangular cylinder with increasing side ratio is closely correlated with the vortex-excitation characteristics. The concept of vortex excitation as aeroelastic flutter occurring in a fluid-body coupled system is proposed on the basis of the experimental results. The most essential feature of vortex excitation as a coupled flutter is that the fluid subsystem can be set into resonance by the body motion. A rapid phase-angle change in the lift force through vortex resonance produces a large out-of-phase force component which excites the motion of the body subsystem.