A new third-order solution for multi-directional irregular water waves in finite water depth is presented. The solution includes explicit expressions for the surface elevation, the amplitude dispersion and the vertical variation of the velocity potential. Expressions for the velocity potential at the free surface are also provided, and the formulation incorporates the effect of an ambient current with the option of specifying zero net volume flux. Harmonic resonance may occur at third order for certain combinations of frequencies and wavenumber vectors, and in this situation the perturbation theory breaks down due to singularities in the transfer functions. We analyse harmonic resonance for the case of a monochromatic short-crested wave interacting with a plane wave having a different frequency, and make long-term simulations with a high-order Boussinesq formulation in order to study the evolution of wave trains exposed to harmonic resonance.Key words: surface gravity waves, waves/free-surface flows
IntroductionNonlinear irregular multi-directional water waves are commonly described to second order using the formulation by Sharma & Dean (1981). This is based on a double summation over all possible pairs of wave components, utilizing a second-order solution for bi-directional bichromatic waves as the kernel in the summation. In this work, we present a third-order analytical solution for trichromatic tri-directional waves, which is then used as the kernel in a triple summation over all triplets, plus a double summation over the relevant doublets. The outcome of this work is thus a third-order theory for multi-directional irregular waves in finite water depth, including the effect of ambient or wave-induced currents. The formulation is an extension of the work by Zhang & Chen (1999) from deep water to arbitrary depth, and from collinear interactions to multi-directional interactions. It is also an extension of the work by from third-order bi-directional bichromatic waves to multi-directional irregular waves. demonstrated the importance of accounting for third-order effects in boundary conditions for monochromatic short-crested waves in connection with numerical or laboratory experiments. They made numerical simulations using a high-order Boussinesq-type formulation and performed an analysis