Abstract
Flow Induced Vibrations (FIV) are conventionally destructive and should be suppressed. Since 2006, the Marine Renewable Energy Laboratory (MRELab) of the University of Michigan has been studying FIV of multiple cylinders to enhance their response for harnessing hydrokinetic power from ocean, river, and tidal currents. Interactions between multiple cylinders in FIV enable high power-to-volume ratio in a converter consisting of multiple oscillators of cylinders. This paper investigates experimentally the relation between oscillation patterns and frequency response of two cylinders in tandem. All experiments are conducted in the recirculating channel of the MRELab for 30,000<Re<120,000. Phase analysis reveals three dominant patterns of oscillation of two tandem cylinders by calculating the instantaneous phase difference between the two cylinders. This phase difference characterizes each major pattern. One is characterized by nearly 180° out of phase oscillations and one by small lead or lag of one cylinder over the other. In the third pattern, the instantaneous phase difference changes continuously from −180° to +180°. Using frequency spectra, oscillation characteristics of each cylinder are revealed in each flow speed range. Comparison of oscillation patterns and frequency spectra reveals that each oscillation pattern is related to a distinctly different frequency response.
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