Many technologies based on fluid-structure interaction mechanisms are being developed to harvest energy from geo-physical flows. The velocity of such flows is low, and so is their energy density. Large systems are therefore required to extract a significant amount of energy. The question of the efficiency of energy harvesting using VIV of cables is addressed in this paper, through the case of a hanging cable with a harvester at its upper extremity. An experimental analysis of the vortex-induced vibrations of a hanging cable with variable tension along its length is first presented. It is shown that standing waves develop and that the extracted mode shapes are self-similar. This self-similar behaviour of the spatial distribution of the vibrations along the hanging string is explained theoretically by performing a linear stability analysis of an adapted wake-oscillator model. The hanging cable is then combined with a localized harvester and its dynamics is measured. An appropriate reduced-order wake-oscillator model is also used to perform parametric studies of the impact of the harvesting parameters on the efficiency. An optimal set of parameters is identified and it is shown that the maximum efficiency is close to the value reached with an elastically-mounted rigid cylinder. The efficiency is found to be essentially driven by the occurrence of traveling wave vibrations.

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