Numerical simulation of wave-breaking and wave- structure interactions based on OpenFOAM

Abstract

Wave breaking and wave-structure interactions are quotidian phenomena in nearshore regions. The wave-breaking process contributes mainly to energy transfer in air-water interactions and energy dissipation due to turbulence. It also plays an important role in some coastal scenarios like sediment transport, and beach erosion. Wave-structure interactions, which occur when waves interact with coastal structures such as breakwaters, are very complex and involve wave transmission, reflection, breaking, and wave-driven motions of the structure. Studies of these phenomena are critically important in improving the safety and functional efficiency of offshore structures. In this thesis, numerical studies of wave-breaking and wave-structure interactions are conducted based on an open-source CFD library. A three-dimensional numerical wave tank is developed based on OpenFOAM. The propagation and breaking of waves with a moderate wavelength that could be generated in the laboratory are studied. The wave periods range from 0.5 to 1.79 s and wave steepness goes up to 0.05. The volume of fluid method is employed for interface capturing. The relaxation zone method is implemented for wave generation and absorption at the boundaries. Computationally significant factors like grid size, relaxation weight, and time step are investigated to find the optimal parameters of the numerical models for various wave regimes. A relationship between grid size and time step, similar to the wave dispersion relation, is found. The accuracy and performance of the numerical wave tank are assessed through the reflecting coefficient, wave damping, and phase distortion rates. Simulation conditions are monitored to achieve an error rate of less than 3%. To mitigate the effects of numerical errors on the calculation of the reflection coefficient, a modified version of Goda’s approach is proposed. Turbulent flows, especially for the breaking wave regime, are investigated by large eddy simulation. A one-equation sub-grid eddy viscosity model, with coefficients being derived from local flow properties, is employed for turbulence closure. A case study of wave breaking on a fixed bar is simulated. The validation of this numerical wave tank model is confirmed by comparing the numerical results with experimental data. The wave attenuation performance and motions of a type of floating breakwater are studied through numerical and experimental approaches. The motions of the floating breakwater are tracked by the six degrees of freedom model. A mooring system model is developed to simulate the constraints of the floating breakwater. A model floating breakwater with a scale of 1:20 is tested in both the experimental and numerical wave channels. Wave heights at the back/front of the floating breakwater and the constraint forces of the mooring wires are measured. The numerical models are validated by comparing the results with experimental measurements. The variations of transmission/reflection coefficients, energy dissipation rate, and maximum mooring force are calculated. Changes in the response amplitude operators with the ratio of floating breakwater width to wavelength (B/Lw) and wave steepness are analyzed. Wave steepness has a large influence on floating breakwater motions and the mooring system. The effect of Stokes drift is observed by the shift of position of the floating breakwater.