Experimental investigation on synergistic flow-induced oscillation of three rough tandem-cylinders in hydrokinetic energy conversion

A very important underlying hydrodynamic factor affecting the harnessed power and efficiency of an FIO energy converter is the interference between cylinders and their wakes in synergistic FIO. Hydrokinetic energy harnessing by flow-induced oscillation (FIO) of tandem cylinders depends on several parameters including spacing ratio, spring stiffness, and damping. The center-to-center spacing is varied from 2.01 to 4.01 diameters. The harnessing damping ratio chosen is 0.00 ≤ ζharness ≤ 0.24 that is mandatory for a power take-off system. The spring stiffness considered is varied from 400 to 1000 N/m and Reynolds number is 30,000 ≤ Re ≤ 120,000. Experimental results for the oscillatory responses, harnessed power and efficiency, and oscillation patterns for critical case studies are included. Systematic experiments are carried out on three tandem cylinders with passive turbulence control to identify patterns resulting in optimal hydrokinetic energy conversion. The main conclusions are: (1) The highest efficiency reached in this set of tests is around 75% of the Betz limit for converted power and the harnessed power in the galloping is between three and four times that in the VIV. (2) In VIV, higher damping and stiffness contribute to higher harnessed power and in galloping, the optimal harnessed power tends to occur at softer spring stiffness and higher harnessed damping. (3) The synergy results show that the interactions between cylinders in close proximity have a positive effect in most cases. (4) In the optimal case, the oscillatory pattern of the three tandem cylinders matches the undulation mode of fish moving in the water. (5) For the global optima, the 1st cylinder and the 2nd cylinder oscillate in-phase and the 2nd cylinder and the 3rd cylinder oscillate out of phase.