Characteristics of Breaking Wave Forces on Piles over a Permeable Seabed

Most offshore wind turbines are installed in shallow water and exposed to breaking waves. Previous numerical studies focusing on breaking wave forces generally ignored the seabed permeability. In this paper, a numerical model based on Volume-Averaged Reynolds Averaged Navier–Stokes equations (VARANS) is employed to reveal the process of a solitary wave interacting with a rigid pile over a permeable slope. Through applying the Forchheimer saturated drag equation, effects of seabed permeability on fluid motions are simulated. The reliability of the present model is verified by comparisons between experimentally obtained data and the numerical results. Further, 190 cases are simulated and the effects of different parameters on breaking wave forces on the pile are studied systematically. Results indicate that over a permeable seabed, the maximum breaking wave forces can occur not only when waves break just before the pile, but also when a “secondary wave wall” slams against the pile, after wave breaking. With the initial wave height increasing, breaking wave forces will increase, but the growth can decrease as the slope angle and permeability increase. For inclined piles around the wave breaking point, the maximum breaking wave force usually occurs with an inclination angle of α = −22.5° or 0°.

Breaking Wave Interaction With a Group of Four Vertical Slender Cylinders in Two Square Arrangements

The main purpose of the study is to investigate the breaking wave interaction with a group of four circular cylinders. The physical process of wave breaking involves many parameters, and an accurate numerical modeling of breaking waves and the interaction with a structure remain a challenge. In the present study, the open-source computational fluid dynamics (CFD) model REEF3D is used to simulate the breaking wave interaction with multiple cylinders. The numerical model is based on the incompressible Reynolds-averaged Navier–Stokes (RANS) equations, the level set method for the free surface, and the k–ω model for turbulence. The numerical model is validated with experimental data of large-scale experiments for the free surface elevation and the breaking wave force on a single cylinder. A good agreement is obtained between the numerical results and experimental data. Two different configurations with four cylinders are examined: in-line square configuration and diamond square configuration. For both configurations, three different tank widths and four different spacings between the cylinders are investigated. The breaking wave forces on each cylinder in the group are computed for each case for the two configurations, and the results are compared with the breaking wave force on a single isolated cylinder. Furthermore, the study investigates the water surface elevations and the free surface flow features around the cylinders. For the closely spaced cylinders in a relatively narrower tank, the cylinders in both configurations experience the maximum forces lower than the maximum force on a single cylinder. But for the widely spaced cylinder in a relatively wider tank, the forces are higher and lower for the upstream cylinders and downstream cylinders, respectively, than the maximum force on a single isolated cylinder. The results of the present study show that the interference effects from the neighboring cylinders in a group strongly influence the kinematics around and the breaking wave forces on them.

Detailed Study on Breaking Wave Interactions With a Jacket Structure Based on Experimental Investigations

Wave breaking is one of the major concerns for offshore structures installed in shallow waters. Impulsive breaking wave forces sometimes govern the design of such structures, particularly in areas with a sloping sea bottom. Most of the existing offshore wind turbines were installed in shallow water regions. Among fixed-type support structures for offshore wind turbines, jacket structures have become popular in recent times as the water depth for fixed offshore wind structures increases. However, there are many uncertainties in estimating breaking wave forces on a jacket structure, as only a limited number of past studies have estimated these forces. Present study is based on the WaveSlam experiment carried out in 2013, in which a jacket structure of 1:8 scale was tested for several breaking wave conditions. The total and local wave slamming forces are obtained from the experimental measured forces, using two different filtering methods. The total wave slamming forces are filtered from the measured forces using the empirical mode decomposition (EMD) method, and local slamming forces are obtained by the frequency response function (FRF) method. From these results, the peak slamming forces and slamming coefficients on the jacket members are estimated. The breaking wave forces are found to be dependent on various breaking wave parameters such as breaking wave height, wave period, wave front asymmetry, and wave-breaking positions. These wave parameters are estimated from the wave gauge measurements taken during the experiment. The dependency of the wave slamming forces on these estimated wave parameters is also investigated.

Numerical Modelling of Breaking Wave Interaction With a Group of Four Vertical Slender Cylinders in Square Arrangements

The main purpose of the study is to investigate the breaking wave interaction with a group of four circular cylinders. The physical process of wave breaking involves many parameters and an accurate numerical modelling of breaking waves and the interaction with a structure remain a challenge. In the present study, the open-source (Computational Fluid Dynamics) CFD model REEF3D is used to simulate the breaking wave interaction with the multiple cylinders. The numerical model is based on the incompressible Reynolds Averaged Navier-Stokes (RANS) equations, the level set method for the free surface and the k–ω model for turbulence. The model uses a 5th-order conservative finite difference WENO scheme for the convective discretization and a 3rd-order Runge-Kutta scheme for time discretization. The numerical model is validated with experimental data of large-scale experiments for the free surface elevation and the breaking wave force on a single cylinder. A good agreement is seen between the numerical results and experimental data. Two different configurations with four cylinders are examined: in-line square configuration and diamond square configuration. The breaking wave forces on each cylinder in the group are computed for the two cases and the results are compared with the breaking wave force on a single isolated cylinder. Further, the study investigates the water surface elevations and the free surface flow features around the cylinders. In general, the cylinders in both configurations experience the maximum forces lower than the maximum force on a single cylinder. The results of the present study show that the interference effects from the neighbouring cylinders in a group strongly influence the kinematics around and the breaking wave forces on them.