Marine hydrokinetic energy in ocean currents can be harvested using Vortex Induced Vibration (VIV). The purpose of this paper is to investigate the VIV energy harvesting and maximize the power-to-volume density by arranging four staggered cylinders reasonably. Consequently, the effect of spacing between the cylinders is the focus considered throughout this paper. The in-flow spacing X varies from 1.2D to 10.0D, where D is the Diameter of the cylinder; the transverse spacing Y varies from 2.0D to 8.0D. A series of numerical simulations are conducted using two-dimensional Reynolds-Averaged Navier-Stokes equations in conjunction with the k-omega SST turbulence model. Results indicate that the interference between the tandem cylinders is stronger than between the side-by-side arranged cylinders. The multi-cylinder interference is divided into three regions: Synchronization Region, Blocking/Encouraged Region and Recovery Region. Interaction effects can induce much more vigorous VIV response of the downstream cylinder in Encouraged Region, where the maximum converted power ratio reaches up to 3.9 with in-flow spacing X of 2.0D and transverse spacing Y of 4.0D. Moreover, the average power-to-volume density will keep increasing as the spacing X decreases if there are no other physical limitations. These results lead the way for future layout optimization of multiple cylinders.
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