In the context of global carbon peaking and carbon neutrality goals, the development of clean energy technologies has become a strategic priority for the global community. As a significant source of renewable energy, tidal current energy is playing an increasingly important role in reshaping the global energy landscape. To address the critical scientific challenge presented by the wake effects induced by tidal turbines, this study introduces a high-precision Lagrangian dynamic (LD) sub-grid-scale model within the framework of large-eddy simulation (LES). By integrating the actuator-line model with the aforementioned LD–LES approach, a solver named JUST–FOAM–Turbine is developed, specifically tailored for high-fidelity wake simulations of horizontal-axis tidal turbines (HATTs). The solver's computational accuracy is validated against experimental data obtained from laboratory-scale HATT tests. Subsequently, the solver is employed to analyze the wake dynamics of the HATT under various tip speed ratios (TSRs). The findings are summarized as follows: (1) At low TSRs, the blade operates at a high angle of attack, increasing its susceptibility to stall and flow separation. At high TSRs, the blade experiences a low angle of attack, resulting in a reduced lift coefficient. In contrast, within the optimal TSR range, which corresponds to the blade's primary operational regime, the angle of attack is maintained near the critical value, thereby maximizing the turbine's power output. (2) As the TSR increases, the wake instability of the turbine intensifies, and the wake meandering phenomenon becomes more pronounced. The wake expansion downstream of the turbine is primarily driven by the blockage effect. Further downstream, the wake continues to widen laterally, resulting in an increased expansion width. (3) At a downstream distance of 6D, the wake effect begins to weaken due to momentum exchange between the low-speed region of the wake and the high-speed region of the surrounding water. The radial velocity gradient of the wake decreases significantly. When considering the turbine array layout, it is recommended to position additional turbines downstream beyond 6D. (4) As the TSR increases, the attached vortex on the turbine disk progressively disintegrates, forming small-scale, weaker vortex structures. The third-generation vortex identification method implemented in this study is threshold-independent and effectively captures both strong and weak vortex structures in the turbine wake simultaneously.

Recently, the output of Computational Fluid Dynamic (CFD) prediction on tidal stream turbine systems has been receiving great attention owing to the vast and untapped tidal stream potential. For several years, many publications have documented the accuracy of CFD methods for steady flows, but not for unsteady flows. A challenging area persisting in the computational field is its dependence on large computing resources, and the unsteady nature of usual tidal streams. To overcome this problem, researchers have proposed modelling simpler, representative devices or combined CFD with alternative predictive methods. Nevertheless, the turbulent modelling has been a critical issue in the CFD methods and great effort has been devoted to the study of turbulence and wave effect on the wake and functioning of the turbine. The present paper is a review of CFD application on tidal stream turbine performance in both steady and unsteady flows. The performance of the turbine predicted by actuator and blade resolved methods, has been in accordance with laboratory observations. Findings of the wake have been both consistent and inconsistent with measurements, arising from interpretation of the blade force, fluid-solving method and onset flow model, and downstream range distance. With regards to arrays, preliminary work shows turbine arrangement can have profound effects in the onset flow, and in consequence, the performance of adjacent turbine rows. The results reported appear to support the wind similarity assumption, such as wake Gaussian velocity distributions and fluctuating turbine output in unsteady flows. Under the influence of surface waves, the wake recovers faster than steady flow condition due to the flow’s convective acceleration, but the mean performance stays almost equal. The findings in this paper give a critical review and insight of important implications for future turbine research, such as experimental validation of the turbine prediction output in small and large arrays.

Hybrid AC/DC microgrid architecture is emerging as an interesting and attractive approach since it combines the prominent benefits of both AC and DC networks. In Hybrid Microgrids (HMGs), AC and DC networks are interconnected with each other through power converters. Therefore, power converters play a very important role in HMG operation, as their functionality is not only limited to interconnection but also provides power flow control and improves the power quality through different ancillary services. Moreover, due to intermittency of Renewable Energy Resources (RESs), synchronization, load variation, availability of both AC and DC networks, etc., the controllability of the power converters, power management, and protection become very complex in HMG. This paper initially provides a comprehensive review of different interconnection methods and then presents an overview of different control strategies for HMG and Inter-Linking Converters (ILCs). Moreover, power management in different operating modes, including mode transition, is discussed in detail. This paper also provides a critical assessment of the available protection challenges of HMG. Finally, a comprehensive review of different available protection schemes and devices is provided.

The misalignment between flow and rotor can significantly alter the efficiency of the tidal stream turbine (TST), and therefore, it is vital to predict the flow in the tidal field and the performance of the TST under yaw-offset conditions. First, this paper implements a high-precision Lagrangian dynamic sub-grid-scale model based on the large-eddy simulation (LES) method. A classical computational fluid dynamics benchmark case is selected to validate the accuracy of the dynamic LES (DLES) method. The results indicate that the newly implemented dynamic LES method exhibits reduced dissipation and effectively captures the local effects of non-uniform flow fields, including vortex structures. Second, an efficient high-fidelity numerical method (AL-DLES) for forecasting the TST wake is presented by integrating an actuator line (AL) code with the aforementioned DLES method based on the Lagrangian framework. After comparing the experimental results, it was discovered that the newly developed AL-DLES coupling approach, which addresses the issues of challenging turbine meshing, rapid wake dissipation, and insufficient flow field fidelity in previous methods, can accurately simulate the forces acting on the TST while also capturing detailed characteristics of the flow field. Furthermore, the study will be extended to investigate the TST wake dynamics under various yaw-offset conditions, exploring the mechanisms of instability evolution in wake meandering. Meanwhile, the latest third-generation (Ωnew) vortex identification program is implemented and successfully applied to the wake vortex visualization of the TST under yaw-offset conditions. Through a comparative analysis of three distinct vortex identification approaches, it was demonstrated that the Ωnew method exhibits superior accuracy in capturing the vortex system located behind the rotor, eliminating the need for manual threshold selection. In addition, it is capable of simultaneously capturing both strong and weak vortices, which is a vital aspect for future wake research.

This study investigates the impact of sidewall blockage effect on the hydrodynamic performance of twin-rotor vertical axis tidal stream turbine (TR-VATST) with varying solidities. A hydrodynamic analysis model for TR-VATST under sidewall blockage effect is developed using computational fluid dynamics. The hydrodynamic characteristics of TR-VATST in an unsteady flow field are numerically simulated, focusing on the influence of sidewall blockage effect on the energy extraction efficiency across different tip speed ratios and solidities. Base on these analysis, a modified model for sidewall blockage effect applicable to TR-VATST is proposed. The results indicate that: (1) the mean power coefficient of TR-VATST increases with the blockage ratio across different solidities, and the improvement is faster at higher solidity; (2) as the blockage ratio increases, the influence of sidewall blockage effect on TR-VATST intensifies, leading to an increase in the flow velocity near the blades, thereby enhancing energy from the incoming flow and improving the mean power coefficient; (3) The modified model for sidewall blockage effect accurately predict the mean power coefficient of TR-VATST within a specific range of tip speed ratios, solidities, and blockage ratios. Moreover, the model effectively eliminates the influence of laboratory flume sidewalls, enabling correction of experimental results to reflect offshore prototype performance in an unbounded flow environment. These findings provide valuable engineering insights for the performance prediction and optimization of TR-VATST systems.

A multiobjective optimization technique based on the computational fluid dynamics (CFD) simulations and the orthogonal test is proposed to reduce the pressure pulsation in this paper. Three levels of four well‐known performance factors L₉ (3⁴) were considered in the orthogonal test scheme: the number of blades, the blade setting angle, the hub ratio, and the distance between the blade and the guide vane. The evaluation indexes corresponded to the head, efficiency, shaft power, and pressure pulsation, respectively. An optimal configuration A₂B₁C₂D₃ was obtained by comprehensive frequency analysis method, after intuitive and range analysis. In comparison with the nonoptimized model, the new design’s head and efficiency increased by 17.8% and 4.26%, whilst the shaft power and the pressure pulsation coefficient reduced by 1.22% and 11%, respectively. Experiments conducted on the optimized pump were consistent with the CFD model. Six different rotational speed conditions in the optimal operating points were numerically calculated in order to explore the internal hydraulic characteristics of the optimized axial flow pump. It is verified that the comprehensive frequency analysis method based on the orthogonal test approach is effective for the multiobjective optimization of the axial flow pump.