| Tidal energy,as a form of renewable energy,possesses several advantages such as predictability,sustainability,and cleanliness.Floating tidal energy devices offer enhanced convenience and efficiency in installation and maintenance compared to seabed-mounted counterparts,enabling greater energy generation at the same location.However,the presence of waves induces wave-induced motions in the floating platforms,leading to multi-degreeof-freedom movements of the turbine relative to the platform.Consequently,the turbine experiences constantly changing flow velocities.If the turbine operates at a fixed rotational speed,it cannot achieve the optimal tip speed ratio,thereby reducing the overall power generation efficiency of the tidal energy system.Therefore,this study focuses on the investigation of the hydrodynamic load characteristics of a floating horizontal-axis turbine considering the influence of free surface and wave flow.It explores the rotational and pitching motions of the turbine and develops a methodology for predicting hydrodynamic loads on the turbine.This research provides valuable insights for the study of floating platform motion responses and the design of turbine control systems.Based on CFD theory,a numerical calculation method for hydrodynamic loads of horizontal-axis turbines under free liquid level is established,and the flow direction load coefficient,energy utilization ratio and pitching moment coefficient under different pitching cycles,pitching amplitudes and blade tip speed ratios are analyzed.On this basis,the hydrodynamic load decomposition model of the turbine is established,and based on the least square method,the damping coefficients and their variation laws under different pitching periods,amplitudes and tip-speed ratios are obtained.Considering the influence of pitching motion on relative inflow speed,the speed control law is put forward,the hydrodynamic numerical calculation method of variable speed rotation and pitching motion of the turbine under free liquid level is established,and the flow load coefficient,energy utilization ratio and pitching torque coefficient of the turbine under different pitching cycles and amplitudes are calculated and analyzed.Based on the hydrodynamic load decomposition model of fixed speed and pitching motion of the turbine and the variation law of damping coefficient,a fast forecasting method for flow load coefficient,energy utilization ratio and pitching moment coefficient of the turbine during variable speed rotation and pitching motion is established.The hydrodynamic load numerical calculation method for the fixed speed rotation and pitching motion of horizontal-axis turbine under wave-current condition was established,and the flow direction load coefficient,energy utilization ratio and pitching moment coefficient under different tip speed ratio,wave height,wave(pitching)period,pitching amplitude,initial phase and submergence depth of tip were calculated.On this basis,the hydrodynamic load decomposition model of the turbine at fixed speed and pitching motion under wave and current conditions is established.Based on the least square method,the variation laws of damping coefficient under different tip speed ratio,wave height,wave(pitching)period,pitching amplitude and submergence depth of blade tip are obtained.Based on the speed control law proposed,a numerical simulation method for variable speed rotation and pitching motion of horizontal-axis turbine under wave current condition is established to analyze the influence of different wave(pitching)cycles,pitching amplitude and initial phase on flow direction load coefficient,energy utilization ratio and pitching moment coefficient.Based on the hydrodynamic load decomposition model and damping coefficient variation law of turbine fixed speed speed and pitching motion under wave and current conditions,a fast forecasting method of flow load coefficient,energy utilization ratio and pitching moment coefficient is finally formed when turbine rotates and pitches under wave and current conditions. |
