Boundary Element Method And Experimental Investigation Of Hydrodynamics Of Floating Point Absorbers

In recent years,with the development of ocean engineering technologies,floating point absorbers(FPA)are more widely used in reality.This paper made theoretical and numerical investigations of wave-body interaction problems using boundary element method(BEM)based on the linear potential flow theory,in order to provide reasonable and valid prediction method for the hydrodynamic analyses of floating point absorbers and perforated structures.Besides,this paper also gave an experimental investigation of the floating point absorber and power-take-off(PTO)system.The primary research contents of the paper are as follows:1.This paper systematically discusses algorithms of 3D frequency domain free-surface Green function.For the Green function in infinite water depth,we divides the evaluated domain into three sub-domains.In each sub-domain,we calculate the Green function using the power series expansion or Gauss-Legendre integration.For the Green function in finite water depth,we transform the traditional series expression into a new form,which avoids numerical distortion at high frequencies,and for the integral expression,we use singularity removal method and Gauss-Laguerre integration to calculate the Green function.Through proof and comparison of the series and integral methods,this paper gives the application conditions of the two algorithms.2.This paper solves hydrodynamic problems of 3D body with arbitrary shape using BEM based on the 3D free-surface Green function in frequency domain.Based on the source-dipole distribution method,we describe all numerical techniques,including body surface distribution method,evaluation of the integral of the Green function over panels,wave excitation force and hydrodynamic coefficients,and develop a BEM code of frequency domain,which is able to deal with the quadrilateral and triangular panels.Through numerical cases,we compare the numerical results with the experimental data and values from other references and obtain good agreements,which prove that the present BEM code is valid.3.This paper introduces a two-body FPA model test conducted before and discusses rapid numerical method to predict the motion response and wave power absorption of the two-body FPA in waves.Considering the geometry of the floater,we introduce a quadratic drag term of the Morison's equation to evaluate the form drag force caused by viscous effects.This paper evaluates the wave load and hydrodynamic coefficients of the two-body floater using the present 3D BEM code and solve the nonlinear motion equation by iteration.Good agreements are obtained between the numerical results and experimental data,which prove that the present numerical method is valid.4.This paper selects a circular-cylinder with semi-sphere bottom as floater,and uses a linear electric motor as PTO simulator.Then we write a control code which can adjust PTO stiffness and damping,giving commands to the motor to generate PTO force acting to the floater.It is able to simulate the process that the FPA absorbs wave power from waves.Through the heave decay test in calm water,we evaluate the viscous damping and added mass,and give a viscous correction for the potential flow model.Based on this,we conduct the diffraction and free heave motion tests in regular waves,through the comparisons of the numerical results and experimental data,indicating that the evaluation of the viscous effects is valid.Furthermore,we adjust the PTO stiffness by using the control code,achieving the floater's velocity in phase control.In wave power absorption tests,through the phase control,we make the floater absorb wave power in resonance at all frequencies.The experimental data indicate that the PTO system and the FPA in phase control can absorb more wave power at frequencies far from the natural frequencies,compared to the traditional passive control schemes,and effectively broaden the wave power capture width of frequency.