Performance and fatigue analysis of a tidal turbine under wave–current interactions

Tidal current turbines operate in wave-affected currents where free-surface orbital kinematics and dynamic-pressure fluctuations introduce multi-scale unsteady loading. Although previous studies have reported that waves often have a limited impact on time-mean power and thrust, the underlying fatigue mechanisms and their direction-dependent characteristics remain insufficiently quantified. This work develops a blade-resolved computational fluid dynamics (CFD)–fatigue framework to address this gap. A two-phase Reynolds-averaged Navier–Stokes (RANS) solver with a volume-of-fluid (VOF) free-surface model is implemented in OpenFOAM, and rotor rotation is resolved using a sliding-mesh arbitrary mesh interface (AMI). Fatigue is assessed using rainflow counting and Miner’s rule, reported in terms of damage-equivalent load (DEL), and interpreted using a zero-phase spectral-notch band-removal attribution, viewed as a quantitative sensitivity metric rather than an additive damage partition, together with range-resolved rainflow decomposition. The numerical approach is validated against laboratory measurements, showing good agreement in free-surface elevation, mean thrust and torque, and wave-phase-locked oscillations. A systematic parametric study is then performed by varying mean current strength, tip-speed ratio, wave frequency, wave height, and wave–current propagation direction, expressed using standard nondimensional descriptors (depth-based Froude number, wave-to-rotation frequency ratio, and relative wave height). Across all cases, waves weakly influence mean performance, but strongly redistribute unsteady energy into wave-synchronous components, rotor harmonics and modulation sidebands that govern fatigue. Higher wave-to-rotation frequency ratios promote high-cycle fatigue through denser wave–rotation interactions, whereas lower ratios act primarily as slow envelope modulation and yield fewer damaging closed cycles. Increasing relative wave height amplifies wave-synchronous variability and shifts bending-fatigue dominance from the horizontal to the vertical direction. Critically, opposing waves can reduce the standard deviation (STD) of thrust and shaft torque while increasing DEL in transverse bending moments, because fatigue is carried by a heavier tail of moderate-to-large rainflow cycle ranges rather than by bulk variance. These results demonstrate that variance-based metrics are not reliable fatigue proxies and provide actionable guidance for operating and design strategies that maintain mean power while mitigating fatigue risk in realistic wave–current environments.