Numerical Study On The Effects Of Wind And Current On The Nonlinear Characteristics Of Extreme Waves

Extreme waves(also called freak waves,focused waves,etc)are a strongly nonlinear wave,which possesses the characteristics of single wave crest,exceptionally large wave height,energy concentration,short duration and so on.Extreme waves have been considered as one of the most important reasons for the wreck of many offshore platforms and ships.Wind and current are not only an important factor in the generation of extreme waves,but also have great influence on the propagation of extreme waves.In this paper,the numerical models of wind and wave interaction and wave and uniform current interaction are established using the time domain higher-order boundary element method(HOBEM)based on potential theory.Boundary integral equations are developed by applying the Green's second identity to the fluid domain.The boundary surface is discretized into three-node element based on the quadratic shape functions.A 4th-order Runge-Kutta integration scheme is used to update the grid and the mixed Eulerian-Lagrange method was adopted to refresh the instantaneous free surface.The extreme waves trains are numerically generated through the superposition of different frequency wave components at a specific time and position by the sine motion of a piston wave-maker in the wave flume.A damping layer distributed at the end of the numerical flume is used to gradually absorb the outgoing wave.The boundary surfaces are re-gridded and updated at each time step to avoid numerical instability caused by the grid motion.Finally,the extreme waves generation are based on the spatial-temporal focusing.In this paper,the numerical model is established on Jeffreys' sheltering mechanism by introducing the wind pressure term and the current term into the fully nonlinear free surface dynamic boundary conditions.The developed numerical model was verified by comparing with both the published experimental results.The influences of wind and current on extreme waves are studied,including the focal position,peak crest value and symmetry with wind velocity,current speed,input wave and spectral width.The energy spectrum of wave propagation process is studied by using the Fast Fourier Transformation(FFT).Then the interaction of wind,extreme waves and vertical wall are studied,including the influence of wind speed,input amplitude and spectral bandwidths on the vertical wall run-up,hydrodynamic pressure and total force.Finally,the contribution of harmonic components to the total force is analyzed.