Nonlinear effects on wave elevation of a focused water wave group

Ocean extreme waves are of both scientific and engineering interests. These extreme waves pose great threats to the safety of coastal and offshore structures. They are highly nonlinear and transient in a real sea state. It is important to study their nonlinear effects for accurate prediction of them. In this study, a focused wave group is used to investigate the nonlinear properties in such transient extreme event experimentally and numerically. An experimental campaign is carried out in a large wave tank facility, and a fully nonlinear potential flow model is employed for numerical simulations. A phase-manipulation approach is applied on wave generation in order to extract clean higher harmonic wave elevations. Comparisons between the measurements and the numerical results show good agreements for higher harmonic elevations. The non-dimensional harmonic coefficients, based on a ‘Stokes-type’ harmonic structure, are found to increase gradually with the wave steepness. This suggests that the classical Stokes model might be used for nonlinear wave group prediction, given proper harmonic coefficients. To further investigate the nonlinear effects, the asymmetric properties of the wave profiles near focusing are explored by analysing the preceding and following wave crests. Clear nonlinearity is demonstrated by evaluating the wave envelope height and group bandwidth.