A review of two-degree-of-freedom vortex-induced vibrations with hydrokinetic energy harvesting applications

A comprehensive review is provided on two-degree-of-freedom vortex-induced vibration systems (2DOF-VIV), considering both cross-flow and in-line directions of motion. While previous studies have primarily focused on one-degree-of-freedom (1DOF) systems, the additional degree of freedom (in-line) can significantly impact results. This paper investigates various parameters, including mass ratio, frequency ratio, vortex patterns, damping ratio, surface roughness, aspect ratio, energy harvesting mechanisms, cross-sectional shape, and Reynolds number, to understand the diverse performance of 2DOF oscillators. Key graphs include new Skop-Griffin plots derived from previous studies for high, moderate, and low mass ratios in both directions. Furthermore, it is observed that the ratio of natural frequencies between the two directions can influence vibration amplitudes. A comparative evaluation between 2DOF and 1DOF systems reveals that 2DOF oscillators exhibit higher displacement amplitudes, lift, drag coefficient, mechanical energy, and energy ratio in the cross-flow direction, particularly in VIV and energy harvesting scenarios. Therefore, the 2DOF-VIV systems have more energy extraction advantages because energy harvesting can be performed in both directions. While energy can be harvested effectively from the cross-flow direction of the 1DOF system, generally the 2DOF systems can harvest more energy, simultaneously, from both directions. Furthermore, comparisons between smooth and fouled systems imply that marine fouling reduces the disparities between 2DOF and 1DOF systems for the parameters including displacement amplitudes, lift, drag, mechanical energy, and energy ratio.