| Ocean energy has the features of wide distribution,abundant reserves,and renewable characteristics.Developing and utilizing ocean energy is feasible to deal with the traditional fossil fuel crisis and advance environmental sustainability.Offshore marine aquaculture keeps moving toward large-scale and intelligent operations,and its requirement for electrical power has surged.Additionally,fish and aquaculture facilities are more susceptible to damage because the marine ranching is located in deep-sea areas with strong wind,high waves,and turbulent flow.Therefore,considering the power consumption and protection requirements of the intelligent offshore farming platform,the ocean renewable energy technology is integrated to study the energy capture and flow reduction characteristics of the Savonius hydrokinetic turbine(SHT).The Savonius has been widely used in tidal energy as a classical drag-type rotor because of its simple design structure,good self-starting ability,and minimal environmental impact.However,the current study rarely investigates the effect of reducing the flow in the wake field of the Savonius turbine.In addition,the Savonius turbine extracting energy from the orbiting particles,as a new concept cyclical type wave energy converter(WEC),gradually draws increased attention due to low installation and maintenance costs and more minor energy transfer losses than conventional WECs.Due to wave-induced water particle motion features,which differ from stream flow,the operational environment of Savonius turbine harvesting wave energy is significantly more complicated than capturing tidal energy.However,most research is still in the early experimental stages,which leads to its hydrodynamic characteristic,surrounding flow field variation laws,and the impact of fundamental operational parameters that have not been well investigated.Thus,based on the Savonius turbine wakefield reduction characteristics and hydrodynamic properties of tidal and wave energy capture,the influence rules and mechanisms of the structure design and related operating parameters are analyzed and revealed.The research is conducted in this thesis as follows:(1)Based on the Betz momentum theory,the energy capture limit of the Savonius rotor was analyzed,and according to the active control research method,an experimental system was built in the physical drag pool to monitor the torque output and flow velocity variation at different points in the wake field of the SHT operating under different tip speed ratios.The experimental results were conducted to evaluate the tidal energy capture performance of the turbine and downstream velocity penalty.Considering the limitations of the experiment,based on the dense RANS turbulence simulation method,the Realizable model was introduced to establish a two-dimensional numerical tidal calculation model.It is used to visually analyze the instantaneous flow structure around the turbine and wakefield,summarize the distribution rule of the velocity contours,and reveal the flow reduction and energy harvesting mechanism.Through quantitative analysis and comparison of the velocity attenuation of the wakefield under the condition of the limit tip speed ratio,a basis for quantitatively judging the flow reduction intensity and range of SHT is given to evaluate its flow reduction effect accurately.(2)Based on the given flow criterion,they are combined with torque and power coefficient to study the effect of SHT parameters(tip speed ratio,blade overlap rate,number of blades,and blade curvature)on the characteristics of the tidal energy capture and flow reduction.Meanwhile,according to the mechanism of energy capture and flow reduction,the influence mechanism of the parameters is clarified by comparing the flow features of the field around the SHT,the wake,and also the hydrodynamic.The correlation of different parameters about how to impact the two characteristics is summarized through comparative analysis.The optimal operating range of the tip speed ratio is determined.Based above,a parameter design method of SHT that considers the tidal energy capture and flow reduction is further proposed.(3)The linear wave theory gives the movement of water particles induced by wave evolution under the pull-type and Euclidean-type descriptions.The coupling variation characteristics of relevant operating parameters are analyzed based on the turbine working principle.A simplified mathematical model is established,and a calculation method for the wave energy capture efficiency of a single-bladed SHT,which satisfies the Airy wave theory,is given.The experimental systems are set up in a flume equipped with a shaking plate wavemaker.A test is carried out on the SHT with two semicircle blades placed at four different depths to verify the feasibility of wave energy harvesting and also figure out the optimum submerged level.Three different operating strategies of SHT capturing wave energy are proposed according to the variation rule of coupling dynamic phase angle.The turbine is forced to rotate with the wave evolution in the numerical wave flume established based on the RANS-VOF model.The effect of phase angle,initial phase angle,and relative rotation frequency on the performance of SHT operating under three different strategies in the 5thorder Stocks wave is systematically and deeply analyzed to find the optimum operating strategy.(4)In the numerical wave flume,the effects of wave parameters,turbine diameter,blade number,blade curvature,and blade thickness on the wave energy capture characteristics of the SHT operating under the optimal strategy(locking the initial phase angle)were continued to be explored.The overall property is estimated by torque,output power,and also energy conversion efficiency.Besides,in order to clarify its hydrodynamic characteristics and reveal the influence mechanism of different parameters,the instantaneous flow structure around the turbine was analyzed,the pressure distribution law on the blade surface was summarized,and the various characteristics of the time-history curve of dynamic was illustrated.The study provides a theoretical review for further structural optimization.Additionally,an optimal design method of turbine diameter which can be widely applied to improve the SHT-WEC performance,is proposed based on the dual coupling effects of tip speed ratio and "relative short wavelength impact."... |
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