Fully Nonlinear Investigation Of Overturning Slamming On An Oscillating Wave Surge Converter

As a new kind of nearshore wave energy converters,Oscillating Wave Surge Converters(OWSCs)are getting more and more attention in recent years.The oscillating flap of the device is hinged to the base,and continuously rotates about the hinge axis under the excitation of waves,driving the hydraulic cylinders to pump high pressure water to the electricity generators on the bank through pipes.When the device is subjected to rough or extreme sea states,the buoyant flap will firstly reach a maximum landward positition,then emerge from water due to the restoring moment,and finally hit on wave surface with a high velocity,resuling in a certain threat to device safety.Since the flap motion plays a decisive role in the formations of splash jet and slamming pressure,the issue of slamming on an OWSC is fundamentally different from the problems of wave impact on a fixed offshore platform or a seawall.Most of the present studies concerning the slamming on an OWSC are based on the typical conditions in the framework of regular wave theory in constant water depth,while the unique formation of waves and their irregular wave surface with strong non-linearity in extreme sea states have not been considered,and it is also short of the work corresponding to the slamming under the conditions of nearshore currents and uneven topography.To this end,this paper aims to bulid a fully nonlinear Higher Order Boundary Element Method(HOBEM)based on the potential flow theory to investigate the hydrodynamic slamming on an OWSC in time domain,focusing on the mechanism of wave slamming under the conditions of regular waves,extrem waves,uniform currents and uneven bottom.In this paper,the numerical model of OWSC hydrodynamic slamming under the regular wave theory framework is firstly established.The auxiliary function method is used to calculate the time derivative of the velocity potential on the surface of the floating flap,the pressure distribution is then obtained according to the Bernoulli equation,and on this basis,the mutual coupling between the movement of the floating flap and the fluid flow is finally decoupled.The free surface conditions are updated by adopting the fourth-order Runge-Kutta method and the rotating coordinate system based on the flap rotation,through which the possible numerical errors at the intersection between the body and the free surface are avoided.In view of the strong non-linear characteristics of slamming jet,non-uniform meshes are dispersed on the whole free surface to ensure both accuracy and efficiency.In order to overcome the numerical instability caused by the high oscillating pressure in the jet area on the body surfac,the linear distribution assumption is used to calculate the velocity potential of the corresponding element in the rotating coordinate system.When the splash jet detaches and the forms a secondary impact on the wave surface,the domain decomposition method based on the complex plane idea is applied to avoid the numerical errors caused by the complex connected area.The comparison against the existing experimental data and CFD numerical results verified the accuracy of the established numerical model to simulate the free-surface,motion and dynamic characteristics of the hydrodynamic slamming.After that,the parameteric studies concerning the flap and wave characteristics are carriesd out to investigate the causes and mechanisms of the irregular splash jet and high-magnitude slamming pressure.Secondly,two representative extreme waves are generated by the solitary wave theory and focused wave theory,and the numerical mode is then extended to the numerical investigation of hydrodynamic slamming on an OWSC under extreme conditions.Because of the special wave characteristics of those waves,effective numerical damping layers are respectively defined in free water surface conditions to eliminate the multiple reflections at the boundaries.In order to simultaneously simulate the extreme waves with strong nonlinearity and the irregular slamming jet,the non-uniform mesh scheme is further verified by convergence.The results show that the numerical method in this paper has a good performance for the formation and propagation of extreme waves.Afterwards,the effects of extreme wave parameters such as solitary wave height and water depth,focused wave height,water depth,frequency bandwidth and focused position on slamming are analyzed in detail.By comparing with the results of regular wave slamming,it has been found that unlike the regular wave,due to the greater horizontal acceleration of the fluid particles,the extreme waves will highlight the effect of the moment of rotational inertia and further enlarge the slamming pressure.Finally,the phenomenon and mechanism of the hydrodynamic slamming under the joint conditions of currents,extreme waves and uneven flume bottom are discussed.First,the iterative method is used to calculate the coupled dispersion equation of regular wave under in the presence of uniform currents,thereby generating the flow field where the regular wave and the uniform current coexist.Assuming that the flap impinges in a prescribed mode on the that flow field,the current effects on the slamming are accuractly obtained.Afterthat,the pressure auxiliary functions are further defined in the circumstance of uniform currents to decouple the multiple dependence among the flap motion,the waves and the currents.Based on the wave generation method using internal source,a flow field where extreme waves and currents coexist is generated.The obstacles of different specifications are set on the flume bottom to simulate the slamming of extreme waves and currents on an OWSC over uneven topography.In order to avoid the breaking waves caused by the coexistence of opposing currents,extreme waves and shallow water depth,the truncation method and the domain decomposition method combined with non-breaking conditions are used to ensure that the calculation process will not be terminated by the singular matrix before the expected result is obtained.After comparing the results of the interaction between the extreme waves and currents,and the interaction between the extremes waves and uneven bottom,the present method has been verified to have a good accuracy in the simulating of the near-shore flow field.The parametric studies focus on the current velocity,the incident wave height,the variation of water depth and the height and shape of bottom obstacles.The numerical results show that shallower water depth will significantly increase the nonlinearity of the wave surface and further enhance the slamming pressure on the flap.When there is a raised obstacle on the bottom,the incident solitary waves will obviously deform and diffuse,causing the flap to be affected by two slamming continuously.