• Energy Research

Marine current energy harnessed through vortex-induced vibration (VIV) offers a cost-effective solution for renewable energy generation, particularly in low-velocity flow environments. This study proposes a novel approach to enhance VIV energy harvesting efficiency by employing a multiple-cylinder configuration with a passive vortex generator, where the optimized diameter and spacing ratios of the fixed upstream cylinder are considered to enhance the vibration response of the cylinders positioned downstream. Initially, we analyze the impact of spacing ratios on the vibrations of the downstream tandem cylinders and identify the optimal spacing ratio. Subsequently, we explore the influence of damping ratios on the energy harvesting efficiency for the oscillating tandem cylinders. Through detailed examinations of the cylinders’ responses and forces in both frequency and time domains, we elucidate the strong interference between wakes and its impact on system dynamics. Our results demonstrate significant improvements in vibration amplitudes and energy efficiency for spacing ratios ranging from 5 to 20, with an optimal spacing ratio of 7 identified. Despite an increase in damping resulting in decreased vibration amplitudes, energy harvesting remains high for damping ratios larger than 0.2. The highest accumulated energy capture is observed at a damping ratio of 0.35 for the considered reduced velocity range of 3 to 13. Our findings suggest that VIV harvesters perform better in tandem configurations than a single cylinder configuration, offering a promising alternative for boosting energy harvesting efficiency from wake-induced vibration (WIV) in low-velocity water flows.

Marine current energy harnessed through vortex-induced vibration (VIV) offers a cost-effective solution for renewable energy generation, particularly in low-velocity flow environments. This study proposes a novel approach to enhance VIV energy harvesting efficiency by employing a multiple-cylinder configuration with a passive vortex generator, where the optimized diameter and spacing ratios of the fixed upstream cylinder are considered to enhance the vibration response of the cylinders positioned downstream. Initially, we analyze the impact of spacing ratios on the vibrations of the downstream tandem cylinders and identify the optimal spacing ratio. Subsequently, we explore the influence of damping ratios on the energy harvesting efficiency for the oscillating tandem cylinders. Through detailed examinations of the cylinders’ responses and forces in both frequency and time domains, we elucidate the strong interference between wakes and its impact on system dynamics. Our results demonstrate significant improvements in vibration amplitudes and energy efficiency for spacing ratios ranging from 5 to 20, with an optimal spacing ratio of 7 identified. Despite an increase in damping resulting in decreased vibration amplitudes, energy harvesting remains high for damping ratios larger than 0.2. The highest accumulated energy capture is observed at a damping ratio of 0.35 for the considered reduced velocity range of 3 to 13. Our findings suggest that VIV harvesters perform better in tandem configurations than a single cylinder configuration, offering a promising alternative for boosting energy harvesting efficiency from wake-induced vibration (WIV) in low-velocity water flows.

Vortex-induced vibration (VIV) of bluff bodies is one type of flow-induced vibration phenomenon, and the possibility of using it to harvest hydrokinetic energy from marine currents has recently been revealed. To develop an optimal harvester, various parameters such as mass ratio, structural stiffness, and inflow velocity need to be explored, resulting in a large number of test cases. This study primarily aims to examine the validity of a parameter optimization approach to maximize the energy capture efficiency using VIV. The Box–Behnken design response-surface method (RSM-BBD) applied in the present study, for an optimization purpose, allows for us to efficiently explore these parameters, consequently reducing the number of experiments. The proper combinations of these operating variables were then identified in this regard. Within this algorithm, the spring stiffness, the reduced velocity, and the vibrator diameter are set as level factors. Correspondingly, the energy conversion efficiency was taken as the observed value of the target. The predicted results were validated by comparing the optimized parameters to values collected from the literature, as well as to our simulations using a computational-fluid dynamics (CFD) model. Generally, the optimal operating conditions predicted using the response-surface method agreed well with those simulated using our CFD model. The number of experiments was successfully reduced somewhat, and the operating conditions that lead to the highest efficiency of energy harvesting using VIV were determined.

Vortex-induced vibration (VIV) of bluff bodies is one type of flow-induced vibration phenomenon, and the possibility of using it to harvest hydrokinetic energy from marine currents has recently been revealed. To develop an optimal harvester, various parameters such as mass ratio, structural stiffness, and inflow velocity need to be explored, resulting in a large number of test cases. This study primarily aims to examine the validity of a parameter optimization approach to maximize the energy capture efficiency using VIV. The Box–Behnken design response-surface method (RSM-BBD) applied in the present study, for an optimization purpose, allows for us to efficiently explore these parameters, consequently reducing the number of experiments. The proper combinations of these operating variables were then identified in this regard. Within this algorithm, the spring stiffness, the reduced velocity, and the vibrator diameter are set as level factors. Correspondingly, the energy conversion efficiency was taken as the observed value of the target. The predicted results were validated by comparing the optimized parameters to values collected from the literature, as well as to our simulations using a computational-fluid dynamics (CFD) model. Generally, the optimal operating conditions predicted using the response-surface method agreed well with those simulated using our CFD model. The number of experiments was successfully reduced somewhat, and the operating conditions that lead to the highest efficiency of energy harvesting using VIV were determined.

The hydrodynamic damping of a buoy stationed with three different mooring configurations was estimated using a Navier-Stokes (NS) equations solver coupled with a dynamic mooring model. The mooring configurations comprised a catenary system, a catenary system with sub floaters, and a catenary system with sub floaters and clump weights. Systematic simulations were achieved by adopting the overset grid scheme instead of the conventional morphing grid scheme, which required regenerating the entire mesh when the buoy’s initial position changed, thereby avoiding mesh distortions and numerical instabilities. Motion decay simulations in heave, pitch, and surge were conducted with and without various mooring systems. The analyzed results comprised decaying oscillating motions, natural periods, and associated amounts of damping. The mooring systems influenced not only restoring force characteristics, but also total damping of the moored buoy, which demonstrated the importance of considering mooring-induced damping when investigating moored offshore structures.

The hydrodynamic damping of a buoy stationed with three different mooring configurations was estimated using a Navier-Stokes (NS) equations solver coupled with a dynamic mooring model. The mooring configurations comprised a catenary system, a catenary system with sub floaters, and a catenary system with sub floaters and clump weights. Systematic simulations were achieved by adopting the overset grid scheme instead of the conventional morphing grid scheme, which required regenerating the entire mesh when the buoy’s initial position changed, thereby avoiding mesh distortions and numerical instabilities. Motion decay simulations in heave, pitch, and surge were conducted with and without various mooring systems. The analyzed results comprised decaying oscillating motions, natural periods, and associated amounts of damping. The mooring systems influenced not only restoring force characteristics, but also total damping of the moored buoy, which demonstrated the importance of considering mooring-induced damping when investigating moored offshore structures.

The development and utilization of wave energy, heralded as a potential leading source of clean energy worldwide, have garnered considerable attention from the global research community. Among the diverse array of wave energy converters (WECs), the raft-type WEC stands out for its potential to efficiently harness and utilize wave energy, offering high energy conversion rates and a broad frequency response range. This paper delves into the evaluation of a raft-type WEC’s performance in various mooring configurations under different wave conditions. Our analysis primarily focuses on the dynamics of the two-body WEC using a weakly nonlinear three-dimensional potential flow solver. The considered device comprises two interconnected floating barges, incorporating a power take-off system at the hinged connection point. This investigation involves the use of equivalent linear damping to model the power take-off (PTO) system. To validate the numerical simulations, we conduct physical model experiments with WECs. Additionally, the coupling of the raft-type WEC’s dynamics and its mooring dynamics was examined, highlighting the performance differences between various mooring systems through a comparative analysis.