Gap resonance driven by linear and nonlinear excitations in random waves

• A high-fidelity numerical wave tank is developed in OpenFOAM to study gap resonance between fixed barges in irregular waves. • Both linear and nonlinear gap resonance mechanisms are investigated under random wave excitation. • Linear resonance exhibits a strong response dominated by the fundamental harmonic at the natural frequency, resulting in a single-peak trapped wave spectrum. • Nonlinear resonance leads to energy transfer to higher harmonics, forming a double-peak structure in the gap. • Second-order harmonic plays a significant role in the harmonic structure of the resonant waves under nonlinear excitation. This paper presents a numerical analysis of gap resonance driven by linear and nonlinear excitation in random waves. A numerical wave tank in the open-source toolbox OpenFOAM is set up to analyse the gap resonance between two identical fixed barges under unidirectional irregular wave conditions. The model solves the Reynolds-averaged Navier-Stokes (RANS) equations with the free surface tracked by the Volume of Fluid (VoF) scheme. Both linear and nonlinear gap resonances are considered in the present paper. The linear gap resonance is characterised by the close alignment of the peak frequency of the incident waves with the natural frequency of the gap resonance. The spectrum of the trapped waves in the gap exhibits a single-peak structure with a significantly enhanced peak. The response is predominantly linear and shows weak dependence on the steepness of the incident waves. However, the nonlinear gap resonance is excited through the nonlinear wave-wave interaction process, and energy is transferred to higher harmonics, which excite the gap resonance. The spectrum of the trapped wave evolves from a single-peak to a double-peak structure, and the second-order harmonic appears to be as important as the linear harmonic, with the third-order harmonics and above remaining negligible.