• Energy Research

Analysing the fluid dynamic performance of a bare rotor when succumb to yawed flow conditions has consistently presented a diminishment in mechanical power conversion efficacy. Introducing a duct along the rotor perimeter has been acknowledged to sustain performance, yet the causation behind this phenomenon is uncertain. This study puts forward an investigation into the hydrodynamic performance concerning a true-scale, ducted, high-solidity tidal turbine in yawed free-stream flows by utilising blade-resolved, unsteady computational fluid dynamics. Investigating the performance within an angular bearing range of 0 o to 45 o with the turbine axis, increases in mechanical rotational power and thrust are acknowledged within a limited range at distinct tip-speed ratio values. Through the multiple yaw iterations, the maximum attainment falls at an angle of 23.2 o , resulting in a power increase of 3.44% to a peak power coefficient of 0.35 at a nominal tip-speed ratio of 2.00, together with an extension of power development along higher tip-speed ratios. In verification of the power increase, the outcomes are analysed by means of linear momentum theory; by utilising area-averaged values of static pressure acquired from annular radial surfaces fore and aft of the rotor, an analogous relationship with the blade-integrated outcomes is attained. The analysis concludes that, at higher tip-speed ratios, the pressure drop across the rotor increases at limited flow bearings, enhancing the resultant axial force loading upon the blades, hence providing further performance augmentation of the ducted, high-solidity tidal turbine.

In this paper, we present numerical modelling for the investigation of dynamic responses of a floating offshore wind turbine subject to focused waves. The modelling was carried out using a Computational Fluid Dynamics (CFD) tool. We started with the generation of a focused wave in a numerical wave tank based on a first-order irregular wave theory, then validated the developed numerical method for wave-structure interaction via a study of floating production storage and offloading (FPSO) to focused wave. Subsequently, we investigated the wave-/wind-structure interaction of a fixed semi-submersible platform, a floating semi-submersible platform and a parked National Renewable Energy Laboratory (NREL) 5 MW floating offshore wind turbine. To understand the nonlinear effect, which usually occurs under severe sea states, we carried out a systematic study of the motion responses, hydrodynamic and mooring tension loads of floating offshore wind turbine (FOWT) over a range of wave steepness, and compared the results obtained from two potential flow theory tools with each other, i.e., Électricité de France (EDF) in-house code and NREL Fatigue, Aerodynamics, Structures, and Turbulence (FAST). We found that the nonlinearity of the hydrodynamic loading and motion responses increase with wave steepness, revealed by higher-order frequency response, leading to the appearance of discrepancies among different tools.

In this study, a three-dimensional VAWT with a spanwise passively deformable flexible blade has been modelled. The study mainly focuses on the analysis of blade structure characteristics associated with the bending and twist deflection. Two types of flexible blade material and two strut locations supporting H-type blades are being investigated. The unsteady external loads and energy efficiency of VAWT with such designed flexible blade are also being analysed. The simulation results show that the bending and twist deflection peak is positively correlated with the turbine tip speed ratio λ. For a flexible blade, an unevenly distributed structural stress along the blade with a high stress regime in the vicinity of strut location has also been observed. Due to the rotational motion of a VAWT, the centrifugal force acting on VAWT blade plays an important role on the blade structure characteristics. Reduction of the blade stiffness results in an increase of the blade stress. Changing the strut location from middle to tip will cause a large area under high stress. The results also indicate that the VAWT with a highly flexible blade is not an efficient energy extraction device when it is compared to a less flexible or a rigid blade.

Effects of effective angle of attack (AOA) profile on an oscillating foil thrust performance are studied using a computational method. The foil is subject to a combined pitching/plunging motion with effective AOA satisfying a harmonic cosine function. To achieve this, either the pitching or plunging motion is modified from the conventional harmonic sinusoids. Investigations are performed over a series of Strouhal numbers (St), three maximum effective angles of attack and three different phase angles between pitching and plunging. It is shown that the degradation of thrust force and efficiency with sinusoidal pitching/plunging oscillation, at higher St, is effectively alleviated or removed when the AOA is imposed as a cosine profile. The improvement is more significant for the phase angle being different from 90degrees. A better performance is obtained with the imposed modification on pitching motion. The stronger reversed Von Karman vortex wake associated with leading-edge vortex development is observed with the modified motions, which is believed to induce the improved thrust performance.

Abstract Most existing research related to a semi-submersible offshore floating platform focuses on the wave-structure interaction under either a regular or irregular wave condition. In order to numerically model the irregular wave impact on a semi-submersible platform hydrodynamic response with a low computational cost, in this study, a focused wave is utilized. The platform under this consideration is the DeepCwind semi-submersible platform. A high fidelity CFD numerical solver based on solving Navier-Stokes equations is adopted to estimate the dynamic response and the hydrodynamic loading of the platform. The focused wave is firstly generated based on a first order irregular wave theory in a numerical wave tank and validated against the linear wave theory results. Next, for CFD coding validation, the surface elevation of a fixed FPSO model associated with a focused wave is calculated and compared with the benchmark results. At last, the dynamic responses of the platform are numerically simulated under various focused wave parameters, and the results are compared with those obtained from an in-house potential flow theory tool at Électricité de France (EDF). It is found that the predicted CFD surge motion responses are close to those achieved with the second order potential theory while differ from the results obtained using linear potential theory. As to the pitch motion, differences are observed between two results, due to the different methods used for second order loads and viscous effects calculation. Turning to the results under different wave parameters, the surge and heave motion responses increase as the wave period goes up. However, the pitch motion is not affected significantly by varying wave periods. It may be due to the fact that the low-frequency effects have limited impact on the pitch motion. The strong nonlinearity at extremely large wave amplitude will be the task in our near future study.

The growing interest in renewable energy solutions for sustainable development has significantly advanced the design and analysis of floating offshore wind turbines (FOWTs). Modeling FOWTs presents challenges due to the considerable coupling between the turbine’s aerodynamics and the floating platform’s hydrodynamics. This review paper highlights the critical role of computational fluid dynamics (CFD) in enhancing the design and performance evaluation of FOWTs. It thoroughly evaluates various CFD approaches, including uncoupled, partially coupled, and fully coupled models, to address the intricate interactions between aerodynamics, hydrodynamics, and structural dynamics within FOWTs. Additionally, this paper reviews a range of software tools for FOWT numerical analysis. The research emphasizes the need to focus on the coupled aero-hydro-elastic models of FOWTs, especially in response to expanding rotor diameters. Further research should focus on developing nonlinear eddy viscosity models, refining grid techniques, and enhancing simulations for realistic sea states and wake interactions in floating wind farms. The research aims to familiarize new researchers with essential aspects of CFD simulations for FOWTs and to provide recommendations for addressing challenges.

Wells turbine concept depends on utilizing the oscillating air column generated over marine waves to drive a turbine. As a matter of fact, previous researches on the performance analysis of such turbine were based on the first law of thermodynamics only. Nonetheless, the actual useful energy loss cannot be completely justified by the first law because it does not distinguish between the quantity and the quality of energy. Therefore, the present work investigates the second law efficiency and entropy generation characteristics around different blades that are used in Wells turbine under oscillating flow conditions. The work is performed by using time-dependent CFD models of different NACA airfoils under sinusoidal flow boundary conditions. Numerical investigations are carried out for the incompressible viscous flow around the blades to obtain the entropy generation due to viscous dissipation. It is found that the value of second law efficiency of the NACA0015 airfoil blade is higher by approximately 1.5% than the second law efficiency of the NACA0012, NACA0020 and NACA0021 airfoils. Furthermore, it is found that the angle of attack radically affects the second law efficiency and such effect is quantified for NACA0015 for angle of attack ranging from -15° to 25°.