The oscillating hydrofoil, a device used for collecting environmentally friendly tidal energy, is the focus of the study. The flexibility of the hydrofoil's trailing edge can impact its surface pressure distribution, lift, and moment characteristics. To improve the energy harvesting performance of oscillating hydrofoils, it is important to conduct thorough research on their energy harvesting mechanism. Therefore, numerical analysis is employed to develop a numerical model of the fully passive oscillating hydrofoil with the flexible trailing edge. The dynamic development behavior of surface vortices on hydrofoils is analyzed, demonstrating that the fluid–structure interaction between the hydrofoil and the surrounding fluid alters the hydrofoil's motion. The vortex patterns and pressure distribution on the hydrofoil surface are also affected, ultimately influencing the energy harvesting performance. By optimizing the flexibility coefficient of the fully passive oscillating hydrofoil with a flexible trailing edge, the energy harvesting performance of the oscillating hydrofoil is improved. When the maximum chord offset δm= 0.1c and the flexibility coefficient n= 2, the energy harvesting efficiency is 31.37%, and the average power coefficient is 1.17. Therefore, increasing the tail flexibility can be considered to enhance energy harvesting performance when designing the fully passive oscillating hydrofoil. The research provides a comprehensive analysis of energy harvesting performance, addressing the dynamic problem of the fully passive oscillating hydrofoils with flexible trailing edges. The findings of this study may provide guidance for the design and optimization of tidal energy harvesting devices with similar structures.

Dynamic response characteristics of the passive heave compensator with auxiliary gas bottles are investigated in this paper. A mathematical model of the passive heave compensator is developed which includes mechanics, hydraulics and pneumatics. The key innovation of the proposed model is that the thermodynamic model of gas exchange between the piston accumulator and the gas bottles is derived and discussed. Meanwhile, a one-dimensional model of the pipeline resistance effect is established to calculate the pressure drop across the oil pipeline. The proposed model is used to evaluate the different design parameters of the passive heave compensator for heavy lifting cranes. A study was conducted to investigate the influence of the design parameters on the effectiveness of the passive compensator to reduce the payload displacement. The simulation results indicated that substantial improvement may be possible by careful design parameter selection and optimization.

The oscillating hydrofoil represents a promising tidal energy harvesting device, but its hydrodynamic behavior and energy harvesting performance can be significantly affected by the wing-in-ground (WIG) effect under near-wall conditions. The present work establishes a two-dimensional numerical model that accounts for bilateral WIG effect to investigate the effect of three key parameters, namely the dimensionless wall distance ($H^{\ast }$), the Reynolds number ($Re$), and the reduced oscillation frequency ($f^{\ast }$) on energy harvesting performance. The formation, shedding, and reattachment of the leading-edge vortex (LEV) are analyzed to clarify the underlying hydrodynamic mechanisms. The results demonstrate that the WIG effect notably enhances the energy harvesting efficiency of the oscillating hydrofoil. At $Re=5\times {10}^5$ and $f^{\ast }=0.14$, the maximum energy harvesting efficiency reaches 69.6% at $H^{\ast }=2$, representing an improvement of 84.1% over the reference efficiency of 37.8% at the negligible WIG reference condition of $H^{\ast }=40$. As $H^{\ast }$ increases, the hydrodynamic performance gradually approaches that of the unconfined hydrofoil. Based on the predefined 5% efficiency-deviation criterion, the WIG effect becomes negligible at approximately $H^{\ast }=9$. The sensitivity to the WIG effect increases at higher oscillation frequencies, whereas its dependence on $Re$ is non-monotonic, decreasing up to $Re=4.5\times {10}^5$ and increasing thereafter. Finally, the four-component Gaussian mixture model (GMM) is employed to construct a prediction model for the wall distance at which the WIG effect becomes negligible under different combinations of the Reynolds number and the oscillation frequency. The model achieves an $RMSE$ of 1.991 and an $R^2$ of 0.667. These results quantify the effective range of the bilateral WIG effect and provide a practical criterion for reducing wall interference in relevant experiments.