Breaking Wave Characteristics and Breaking Wave Forces on Slender Cylinders

Abstract

Offshore wind farms have become an increasingly important source of clean and renewable energy. Most recent offshore wind farms are deployed close to the coast in shallow waters. One of the major factors influencing the initial investment of this technology is the design of the substructure and foundation. The physical processes associated with the non-linear shallow water hydrodynamics are rather complex since the wave motion is strongly influenced by the seabed. Breaking waves exert significant hydrodynamic loading on offshore wind turbine substructures and these impulsive loads of short duration can cause permanent structural damage. Wave impact force characteristics greatly depend on the evolution of free surface profiles and wave height, changes in velocities, and geometric properties associated with the breaking process. Understanding hydrodynamic loads from breaking waves has many design-related implications for structures employed in shallow and intermediate waters. Although extensive experimental, theoretical and numerical research has been carried out on modelling the breaking wave forces, the breaking mechanism and their wave impact characteristics are not yet fully understood due to many parameters involved in the complex physical processes. The main aim of the present research was to investigate wave breaking in shallow waters and breaking wave forces on slender cylinders. The open source CFD model REEF3D has been used for modelling wave breaking and computing wave breaking forces on slender cylinders in shallow waters. The model is based on the Reynolds-Averaged Navier-Stokes (RANS) equations together with the level set method for the free surface and the k − ω model for the turbulence. Numerical experiments on wave breaking on sloping sea beds and submerged structures are performed in a three-dimensional wave tank and breaking wave forces on slender cylinders are evaluated. Moreover, the numerical model is thoroughly validated against the experimental measurements for each case individually. First, the characteristics and geometric properties of wave breaking over slopes and submerged structures for different environmental parameters are examined. Comparison of the hydrodynamic characteristics and geometric properties of spilling and plunging breakers are also presented and discussed. Breaking wave forces on slender cylinders are evaluated for solitary and periodic waves. For both cases, the influence of the relative cylinder location with respect to the breaking point on the breaking wave forces is investigated for different incident wave characteristics. The numerical results for different cases are consistent with previous studies. A strong dependence of water depth, offshore wave steepness, and seabed slope on the breaking characteristics is observed for different slopes and submerged structures. Further, the evaluation of geometric properties of waves at breaking for different seabed conditions and wave characteristics suggests that the application of the wave steepness and asymmetry factors are appropriate for describing the breaker type and the wave profile at breaking. Analysis of breaking wave forces indicates that the relative cylinder location with respect to the breaking point has a large influence on the breaking wave force. It is seen from the results that the characteristics and geometric properties at breaking can be related to the wave impact forces from breaking waves. Moreover, the prominent flow features associated with breaking waves and their interaction with slender cylinders are reasonably well represented in the numerical simulation.