Theoretical Study On The Nested Oscil-Lating Water Column Wave Energy Con-Version Devices

Ocean covers almost 71%of the surface of the earth,filled with infinite power.With reason-able development and utilization,ocean surface associating with waves could provide a satisfactory solution to the serious environmental and climate problems in the world.To ensure the safety of human activities and engineering practices at nearshore or offshore areas,various types of break-waters are being developed and constructed.Meanwhile,to seek for an alternative of traditional energy resources,the converters for the renewable energy,such as wave energy,have received more and more attention.As a result,the coupling of breakwaters and wave energy converters be-comes the main contents of this thesis,which is benefit for saving construction cost and protecting marine environment.Based on the potential flow theory,the matched eigenfunction expansion method is employed to solve the wave-structure interaction problems.By ignoring the variation of seafloor topogra-phy,a two-dimensional Cartesian coordinates is established to investigate the hydrodynamic per-formances in different sub-domains.According to the singular distributions of the fluid flow in the vicinities of the lower sharp corners of the structures,sets of specific orthogonal polynomials are adopted to approximate the velocity distributions on the common interfaces among the sub-domains.In this thesis,the solving process,start,s with a simple wave diffraction problem by a fixed structure,and gradually covers the radiation problems due to both the response of the structures and the fluctuating air pressure inside the column.Firstly,by solving the interaction problem between the linear waves and T-typed fixed break-waters,the effects of geometrical parameters,such as draft and width of breakwater,are compre-hensively examined.The numerical results show that increasing the length of the thin vertical plate can significantly reduce the wave transmission;the vertical wave loading increases as the vertical plate moves from the seaward side to the leeside of the horizontal rectangular box.Secondly,the theory of the interaction of the linear wave,s and T-typed pitching breakwaters is developed.By examining different geometric parameters,the phenomena of the total wave reflection and transmission appear near the resonance frequencies;the thin vertical plate installed on the leeside of the horizontal rectangular box can help to enhance the stability of the system.Thirdly,an oscillating water column(OWC)wave energy device,associated with a front wall in surging motion under wave excitations,is amounted at the seaward side of a nearshore breakwater with a cliff-like wall.By the semi-analytical computations,it is found that the surging motion of the front wall can result in more energy entering the column,and then help to broaden the bandwidth of high-efficient frequencies;the thickness of the front wave,depth of immersion,width of the air chamber,and elastic coefficient of the spring have a significant impact in the performances of the OWC device.Finally,a dual OWCs device with a movable surging mid wall is proposed.Through a com-prehensive examination in the geometrical parameters of the structure,the bandwidth of the high-efficient frequencies is obviously wider than that of the fixed single oscillating water column,and the peak value increases by 80%;the high-efficient frequencies shift to the low frequency regions as the depth of immersion and the width of the structure increase.The innovations contain the following aspects:·The theories of T-typed fixed and pitching breakwaters are developed respectively.It is found that the geometrical parameters of the vertical plate have a significant influence in the wave loadings and the motion response of the system.· A shore-based OWC device with a surging front wall is proposed.It is found that the surging motion of the front wall can significantly widen the bandwidth of the high-efficient frequencies.· A dual OWCs device with a surging mid wall is proposed.It is shown that the peak value of the conversion efficiency is 80%higher than that of the single OWC device,and the high-efficient bandwidth is obviously wider.