Nonlinear Analysis of Wave Energy Dissipation and Energy Transfer of Directional Breaking Waves in Deep Water

Abstract

Wave energy dissipation and energy transfer among wave components during the directional wave breaking are investigated experimentally. Directional breaking waves in deep water were simulated by focusing the multi-frequency and multi-directional wave components at a designed location based on constant wave amplitude and constant wave steepness frequency spectrum. The incipient and plunging breakers with the same spectral characteristics were generated by applying the different scale factors on wave amplitude. The time series of surface wave elevation were measured around the wave focusing point using a wave gauge array to examine the variation of directional spreading function. The free wave components of a directional wave train are separated from bound wave components by nonlinear decomposition based on directional hybrid wave model accurate up to second order. The local free wave components derived from nonlinear decomposition still include directional dispersion effect. A spatial variation of free wave packet due to directional dispersion is estimated by comparing incipient breaking wave packets at corresponding locations. When the bound wave components and directional dispersion effect are removed from the plunging breaking wave train, a variation of the directional wave spectrum of resultant free wave components before and after the wave breaking is solely responsible to wave energy dissipation and transfer between free wave components. By comparing free wave components of a plunging breaking wave packet before and after the wave breaking, the characteristics of energy dissipation and energy transfer caused by wave breaking are investigated and their dependences on frequency are analyzed. The breaking in deep water significantly dissipates wave energy in the upper region of peak-frequency band while enhances wave energy slightly in the low-frequency band by energy transfer.