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In the actual marine environment, the hydrodynamic characteristics of floating counter-rotating tidal turbines (FCRTTs) are influenced by the motion responses of their carrier platforms. Therefore, accurately analyzing and predicting hydrodynamic loads under the motions of FCRTTs are crucial. In this paper, a fitting formula for hydrodynamic loads of FCRTTs applicable to rotational motion is derived. Then, the effects of surge amplitude, surge frequency, and tip speed ratio on the hydrodynamic loads of an FCRTT are also calculated. It is found that the instantaneous load fluctuation of the rear rotor is more severe than that of the front rotor. However, the average torque of both rotors is similar, which can effectively enhance the operational stability of the FCRTT. Additionally, the hydrodynamic loads are decomposed into average hydrodynamic force, damping force, and added mass force based on the least squares method. A fitting formula for the hydrodynamic loads applicable to different surge conditions is derived, incorporating 11 hydrodynamic coefficients. The results indicate that the damping coefficients n P 0 and n T 0 play a dominant role in the fluctuation amplitude of the hydrodynamic loads. Finally, an effective and fast prediction model for various hydrodynamic coefficients is successfully established using the three-dimensional radial basis function. The relative errors between the predicted peak values of all performance coefficients and the values calculated using the computational fluid dynamics (CFD) method are within −3.5%. This paper provides important guidance for engineering design and load prediction of FCRTTs. Moreover, the predictive methodology can be extended for application to other single-degree-of-freedom and couple motions. [ABSTRACT FROM AUTHOR]

Horizontal axis tidal turbines (HATTs) working in a complex flow environment will encounter unsteady streamwise flow conditions that affect their power generation and structural loads, where power fluctuations determine the quality of electricity generation, directly affecting the grid and reliability of the power transmission system; fatigue loads affect various structures and mechanical components of the turbine, directly determining the lifespan and reliability of the turbine. To gain insight into the generation mechanism and distribution of these excitations, a large eddy simulation is employed to analyze the inflow turbulence and unsteady forces excitations by a three-blade HATT. A spectral synthesizer was used to generate incoming turbulence flow. The strip method was applied on the HATT by dividing the blade into 20 strips. The thrust received by each strip and the flow velocity upstream and downstream of the blade's root, middle, and tip were monitored. The distribution of unsteady loads on the blades was analyzed, as well as the relationship between flow velocity upstream and downstream of the blade and the unsteady characteristics of the blades. The simulation results show that the unsteady hydrodynamic fluctuations of the HATT blades reach up to 57.44% under a turbulent intensity of 10%. Through intuitive analysis of flow separation on the suction surface of the blade at various moments under a low tip speed ratio, we can comprehend the variations in inflow velocity and flow separation on the blade surface. Analyzing the distribution of blade load from root to tip reveals that the maximum load values are concentrated in the 14th–16th strips, corresponding to the region from 0.7R to 0.8R. Moreover, the middle and tip sections of the blades predominantly contribute to the harmonics of the 3BPF (blade passing frequency) and broadband, with the middle section making a greater contribution. The tip section primarily contributes to harmonics above 3BPF. This research want to makes a valuable contribution to the comprehensive understanding of turbulence-induced exciting forces and the practical engineering design of HATT. [ABSTRACT FROM AUTHOR]

The coaxial rotor structure presents significant potential for the development of tidal turbines. In order to study the effect of the number of front and rear rotor blades on the hydrodynamic performance of counter-rotating horizontal-axis tidal turbines (CRHATTs), three models (CRHATT 3-2, CRHATT 3-3 and CRHATT 3–4) are established at the same tip speed ratio (TSR), and the accuracy of the numerical method is verified by experiments. The effect of blade number on the performance coefficients, velocity and pressure pulsations and vortex structure of CRHATTs are calculated based on CFD method and spectral analysis. The results show that the relationship of the power coefficient for the three models is: CRHATT 3–4 > CRHATT 3-3 > CRHATT 3-2. The turbulent kinetic energy in the flow field can be transferred more quickly from the large-scale vortex structure to the small-scale as the number of rear rotor blades increases. By weighing aspects such as load fluctuation and power coefficient, the CRHATT 3–4 can produce the largest power coefficients, the best stability during operation and the least vibration on the blades among the three models. This paper provides significant guidance for the engineering design and optimization research of CRHATTs. • The effect of the number of front and rear rotor blades on the hydrodynamic performance of counter-rotating horizontal-axis tidal turbines (CRHATTs) is studied in this paper, it supplements the optimization research on the influencing factor of leaf quantity in this field. • The reason for the oscillation of the performance coefficients and the relationship between the main peak frequency of power spectral density (PSD) and the number of rotor blades are given. • This paper combines spectral and vortex fields to jointly analyze the effect of the number of blades on the vortex structure and turbulent kinetic energy in the flow field. [ABSTRACT FROM AUTHOR]