Investigation on the excitation forces and critical heights of artificially generated water waves via particle-based simulations and wave flume experiments

Abstract

With an increasing number of offshore structures for the exploration of marine resources and renewable energies, various experiments, analytical solutions, and numerical approaches have been performed to investigate the interaction of waves and structures to ensure the safety of the offshore structures. In particular, the classical wave transfer function and the Morison equation have been widely applied to predict and validate the accuracy of linear waves and the wave forces acting on structures, respectively. However, the former is limited to analyzing the wave height-to-stroke ratio of breaking waves caused by large wave steepness, and the latter does not accurately predict the force with short wavelength and the secondary load induced by higher harmonic force components. In this study, waves of different wave steepness are generated using an experimental wave flume with a piston-type wavemaker, and the forces exerted on the cylindrical structures of three different diameters satisfying a diameter-to-wavelength ratio of smaller than 0.2 are investigated. By introducing the piston stroke-to-water depth ratio, a breaking wave transfer function for piston-type wavemaker is developed, and the threshold of the maximum wave steepness at which the regular wave starts to break was analyzed. The force difference between the secondary load and wave force calculated using the Morison equation was empirically formulated using several dimensionless variables, and the proposed equation showed good agreement with the experimental results. Additionally, the obtained data were analyzed through comparison with theoretical solutions and used to verify the simulation model discretized by smoothed particles. With the use of a mass-weighted damping zone into the numerical wave tank, wave reflection is minimized, and resolution parameters for single-phase simulation and cohesive force of the modified equation of state for multi-phase simulation are optimized, respectively. The proposed equation is valid for predicting the breaking wave height and wave forces exerted on a vertical cylinder when the secondary load cycle occurs, and the optimized parameters can be employed as a guideline toward further studies including wave-structure interaction problems.