Generating intermittent breaking waves using a momentum source in two-fluid simulations

• A robust and effective numerical method for intermittently generating breaking waves. • A momentum-source approach is developed to generate linear, nonlinear, and breaking waves in two-fluid simulations. • A local-minimum-triggered source increases breaking probability from 5% to 14% without increasing forcing strength. • Three-dimensional results yield breaking statistics and effective steepness consistent with field observations. • The method reproduces the equilibrium low-wavenumber wave spectrum, capturing realistic breaking-wave dynamics. We propose a robust and effective numerical method for intermittently generating waves in two-fluid simulations. The method introduces a source term into the momentum equation to generate linear waves, whose magnitude is analytically derived to obtain a target wave amplitude and numerically verified using a computational two-fluid solver. Breaking waves are then produced by increasing the strength of the momentum source. Two-dimensional simulations show that imposing a large momentum source on linear wave consistently leads to wave breaking. In contrast, when the same momentum source is applied to a nonlinear wave field with a continuous spectrum, breaking does not occur sometimes. On the basis of an analysis of the evolution of the dominant wave steepness, we introduce a local-minimum-triggered source (LMSrc) method, where the momentum source is precisely applied when the dominant wave steepness reaches a local minimum. This adaptive strategy increases the probability of wave breaking from 5% to 14% without changing the source magnitude or forcing frequency. Finally, we extend the study to three-dimensional simulations and calibrate the wave-breaking probability P b and the effective breaking wave steepness ϵ b ˜ across various momentum source strengths. The obtained P b and ϵ b ˜ values fall within the ranges of field observations, and the averaged wave spectrum agrees with the equilibrium sea surface spectrum for wind-generated waves in the low-wavenumber range.