Nonlinear Numerical Simulation Of Wave Interaction With Multi-Box System With Narrow Gaps

To efficiently make use of ocean space, large scale floating structures are used. For the convenience of manufacturing, transportation and installation, a large scale floating structure system usually comprises of several modules with narrow gaps among them and the gap will also exists in multi-hulled vessel in a side-by-side configuration. If the wave frequency is close to the resonance frequency of the narrow gaps among these modules, significant increasing of wave run-up in the gaps and wave forces are crucial threats to the structure safety. The strong water motion in the gaps will also have great effects on the seaward and backward wave heights. The marine environment in real sea is very complicated and extreme waves with great destructive power may appear in the coastal region. In order to study extreme wave in a simple and efficient way, the extreme wave is generally modeled as a solitary wave with a single crest and an infinite period. Solitary wave may pose great hazards to marine structures. Therefore, it is of practical importance to learn the interaction of wave and multi-box system with narrow gaps.To investigate the hydrodynamics around multiple rectangular-box structures close to each other in waves, a two-dimensional fully nonlinear time-domain numerical wave flume (NWF) is developed. In the numerical model, the incident regular waves are generated by the inner-domain sources such that the re-reflection at the input boundary can be avoided and a piston-type wave-maker is used to generate solitary waves. The fully nonlinear kinematic and dynamic boundary conditions are satisfied on the instantaneous free surface and an artificial damping is employed on the free surface in the gaps to approximate the viscous dissipation due to vortex motion and flow separation. The mixed Eulerian-Lagrangian method is adopted to track the transient water surface and the fourth order Runge-Kutta method is used to predict the velocity potential and wave elevation on the free surface. The acceleration potential technique is adopted to calculate the transient wave forces along the wetted object surface.Firstly, we simulate regular wave interaction with two-box system with a gap, three-box system with two gaps, and analyze the resonance frequency, resonance wave height and the wave forces on each box. In allusion to the interaction of solitary wave with a vertical wall, we analyze the run-up and wave force on the wall. The developed numerical model was verified by comparing the present numerical results with the published experimental and numerical data for regular wave interaction with two-box system, three-box system and solitary wave interaction with a vertical wall. Then, the extensive numerical experiments are performed to study the influences of the number of the boxes on the resonant frequency, resonant wave height, reflected wave height at the weather-side and transmitted wave height at the lee-side and wave forces on the boxes and analyze the hydrodynamic difference between marine structures with and without narrow gaps and the influences of the number of the gaps on the wave heights on both sides of the system and the wave loads on the system. At last, the interaction between double solitary waves with a time interval between them with a twin-box system is also investigated. The effects of the narrow gap and the size of each box on the wave run-ups at the two sides of the two-box system and in the narrow gap between two boxes, and the wave forces on the two boxes are also given.