Nonlinear time-domain simulation of pneumatic floating breakwater

Abstract

The performance of pneumatic-type floating breakwaters is studied using a numerical wave tank (NWT) simulation in time domain. The 2-dimensional fully nonlinear NWT is developed based on the potential theory, Boundary Element Method (BEM)/Constant Panel Method (CPM), Mixed Eulerian-Lagrangian (MEL)-nonlinear free-surface treatment, and Runge-Kutta 4th-order (RK4) time-marching scheme. The inner chamber of the pneumatic breakwater is modeled such that the time-dependent air pressure is linearly or quadratically proportional to the change of air volume at each time step, that is, the volume change results in airflow through an opening causing pneumatic damping. The air-chamber effect on wave-blocking performance is then assessed for various wave conditions and damping coefficients. Both fixed and floating breakwaters are considered. It is found that a significant enhancement of performance can be achieved by the damping effect if breakwaters are stationary. When the breakwater is floating against incident waves, the pneumatic damping effect becomes less significant in most wave frequencies considered, since the floating body tends to follow the vertical motion of incident waves and the resulting volume-change effect is small. However, near the resonance frequency, the air damping can play an important role by suppressing large motions. The fully nonlinear simulations with different wave heights are compared with linear ones. The energy conservation formula for pneumatic breakwaters is derived and confirmed by numerical simulations.