Author(s): Xingya Feng; Wei Bai; Dezhi Ning
Linked Author(s): Dezhi NING
Keywords: Focused wave group; Higher harmonics; Stokes expansion; Nonlinear simulation
Abstract: In this work we aim at extracting the higher harmonic wave profiles of nonlinear focused wave groups based on the assumption of a ‘Stokes-type’ harmonic structure. We present a methodology to generate high-quality nonlinear wave groups of a narrow-banded wave spectrum in a wave tank. The methodology manipulates the wave group phases by an active control of the wave paddle. Four phases with 90-degree shift each are prescribed – four runs for each case, in order to extract the harmonics up to fourth order. The input focused wave group is generated by the NewWave model, which represents an extreme wave in a random sea state. A fully nonlinear potential flow model is also employed to replicate the experiments carried out in the Kelvin Tank in Glasgow, UK. The results show clean separation of the first four harmonics in the elevation spectra. Comparisons between the experimental data and the fully nonlinear model show remarkably good agreements for the higher harmonics. We confirm the Stokes-type underlying nonlinear structure of the harmonic elevations in wave groups. This is concluded by simulating wave groups with varying wave steepness and calculating the corresponding elevation coefficients of the higher harmonics. The harmonic coefficients are found almost constant against varying steepness. We extract wave profiles of higher harmonics in space in the numerical simulation and the evolution of the higher harmonics is clearly demonstrated.