Author(s): Tommaso Attili; Valentin Heller; Savvas Triantafyllou
Keywords: Foam-extend; Flexible structures; Long waves; Numerical modelling; Wave-structure interaction
Abstract: The physics governing the impact of waves on coastal structures, referred to as wave-structure interaction (WSI), pose a serious challenge for a range of coastal applications such as oil and gas rigs, offshore wind turbine platforms, breakwaters, flood protection systems and wave energy converters. These structures experience stresses due to waves and may undergo significant deformations. Meanwhile, the fluid flow is also affected by the structural deformations. This leads to a mutual interplay between the fluid and solid domains which may result in structural damage or even failure. An accurate understanding of WSI effects is still a major challenge. The present study focuses on the numerical investigation of linear and solitary waves impacting rigid and flexible plates with the open-source toolbox solids4foam, with the waves generated with the toolbox waves2Foam. Solids4foam solves fluid-solid interaction phenomena employing a Finite Volume Method discretisation with a partitioned coupling approach. The fluid governing equations are solved first. The fluid forces are then applied to the interface and the solid governing equations are solved. Consequently, the fluid mesh is updated based on the new solid velocities. The numerical model was validated with available experimental data for a solitary wave impacting a flexible plate. Overall, the comparison between the laboratory and numerical results reveals the capability of solids4foam to capture the main features of the phenomenon, apart from some discrepancies. These pertain to the size of the air pocket entrapped when the wave impacted the plate, which is smaller in the simulation, and the horizontal displacements of the plate that are slightly overestimated. Linear and solitary waves impacting rigid and flexible plates are then investigated, with the structures located offshore or onshore. The water surface elevations, wave forces and the plate responses, i.e. displacements and stresses, are analysed for a range of incident waves and plate configurations. Thicknesses, Young’s moduli and boundary conditions of the plates are varied to understand how the plate rigidity influences WSI. In the onshore tests, the waves propagate along the shore, impact and run-up at the plates. The forces on the plates show a first peak at initial impact followed by a second peak. This second peak is due to the collapse of the water column following the wave run-up and is up to 3.3 times larger than the first peak. The first peak was compared with theoretical predictions based on the velocity and depth of the overland flow. Semi-empirical correlations between the wave force on the plate and the total wave energy are proposed. Ongoing and future work aims to investigate a range of wave types impacting plates located offshore. This is hoped to improve our physical understanding of WSI to support the design of a range of coastal structures.