Study On Wave-Current Coupled Dynamic Responses Of A Horizontal Axis Tidal Turbine

To solve the contradiction between the continuous growth of energy consumption and the limitation of fossil energy consumption,it is imperative to develop renewable energy tech-nologies.Renewable energy has the advantages of limited environmental impacts and wide distribution of resources,compared to fossil energy.Ocean tidal energy,as a kind of marine renewable energy,converts the kinetic energy in ocean currents into electricity by energy con-version equipment.Compared with other kinds of renewable energy,ocean tidal energy has the advantages of high energy density,high predictability,stable inflow condition,and abundant resources.The horizontal axis tidal turbine rotates with the shaft in the same direction as the inflow to drive the generator by transmission device to generate electricity.The horizontal axis tidal turbine has the advantages of high energy efficiency,mature technology and strong self-starting capability.After decades of development,the horizontal axis tidal turbine has reached a power generation level of megawatts,and is considered to be the most promising large-scale application of tidal power generation.Tidal turbines are always exposed to harsh sea environments.Due to the unsteady conditions such as surface waves,sheared inflow and turbulence and so on,the unsteady loads on the turbine is very significant.The understanding of the unsteady loads is still very limited,and the prediction methods and models are also scarce.In this thesis,the unsteady loads of the tidal turbines,especially the wave induced loads,are studied by means of numerical simulation,theoretical analysis and model experiments.The coupled theory of blade element momentum theory and first-order wave-current theory is developed,and a wave load prediction model of the tidal turbines is established.This thesis proposes a 1MW,three-bladed standard horizontal axis tidal turbine.Based on the blade element momentum theory,the blade load predicting model based on the dynamic wake vortex model is established.The experiments is carried out in a towing tank.The interaction between the turbine and the free surface is also observed.Studies have shown that even if the tip of the blade is out of the water,there is still no obvious loss of power output on the turbine;however,when the submergence of the turbine is small,the wake induced wave can be clearly observed,and the wave propagates radially to the downstream.While the turbine is more close to the free surface,the effect of the wave is more obvious.Based on the blade element momentum theory,combined with dynamic stall model,dy-namic inflow model,first-order wave-current model and 1 degree-of-freedom turbine simulation model,a numerical model for wave-induced load is developed.The unsteady load with presence of surface waves is studied.Based on the study of the load characteristics under the regular wave,the blade load response characteristics under random wave conditions are further studied.The hydrodynamic load on the blade can be treated as combination of dynamic lift and drag,direct wave force on the blade,added mass force,and dynamic wake force.Linear waves do not affect the average load on the underwater turbine,but cause periodic fluctuations.Based on the long-wave assumption,a quasi-static formula for calculation of turbine load is given.For random waves,the response is dominated by the first-order wave-induced contribution,and the second-order response characteristics can be clearly observed in the response spectra of blade root blending moments.Although the energy content of the second-order load is small,the high-frequency load may be related to the blade,which may cause the blade to resonate.In this thesis,a simplified blade structure model is proposed,and an iterative solution for the blade eigenvalue problem considering added mass effect is given.The hydrodynamic loads of the turbine is combined with the elastic response of the blade to consider the hydro-elastic problem.A method for predicting the loads and deformation of elastic blades is established.In terms of the loads on the blade,this thesis considers that the results of hydro-elastic coupling are considered to be indistinguishable from the results with rigid blade.A conceptual design scheme of floating tidal energy turbine based on semi-submersible is proposed,and the design method is discussed.Based on the blade element momentum theory and the theory of floating body on waves,fully coupled floating tidal energy turbine loads and motion prediction methods is developed.The two response peaks of the turbine loads considering the six-degree-of-freedom floater motion are induced by the surge and pitch motion respectively;the floater motion induced loads has a certain phase relationship with the direct wave loads.The two effects may cancel out.The low-frequency drift motion of the floater causes a large displacement,but the motion velocity is slow due to the long motion period.Therefore the impact on the turbine loads are very limited.This thesis is a part of the research on the project of the National Natural Science Foundation of China.It uses theoretical analysis,numerical simulation,model experiments to study the dynamic responses of the unsteady loads on tidal turbines.Basic researches have been carried out,and numerical models including predictions of turbine direct wave load,turbine structure responses,floating body motion performance and mooring dynamics have been developed.The theoretical method is established,and the corresponding numerical predicting code is developed.The relevant research results are of great significance for the theoretical and technological progress of horizontal axis tidal current turbines.