Numerical study of wave-structure interactions with porous artificial reefs using Smoothed Particle Hydrodynamics

Porous artificial reefs are increasingly being used for nature-based coastal protection, given their ability to attenuate waves while providing habitat for marine species. The wave attenuation and ecological functions of porous artificial reefs depend on how wave-driven flows interact with the porous interior structure of a reef; however, these hydrodynamic processes are still relatively poorly understood. To overcome the challenges with resolving the detailed flow-structure interactions within porous artificial reefs at fine (order mm) spatial resolution, this study utilized a mesh-free Computational Fluid Dynamics modelling approach based on Smoothed Particle Hydrodynamics (SPH) using the DualSPHysics solver. The capability of the SPH model to accurately reproduce the reef hydrodynamics (including wave transformation, hydrodynamic forces acting on the structure, and drag and inertia coefficients) was first validated against three independent experimental datasets of wave-structure interactions. The model was then used in a two-dimensional (2D) numerical investigation of wave-structure interactions with porous artificial reefs, where the 3D geometric parameters of the reef structure were adjusted within the 2D model to properly account for the hydrodynamic forces within the reef (i.e., using a quasi-3D approach). The results reveal how the porous reefs modify the dynamics of wave-induced oscillatory flows within the reef structure that are responsible for generating horizontal and vertical drag forces, wave dissipation, turbulent kinetic energy, and mean currents. Drag coefficients decreased with the Keulegan-Carpenter number, with vertical drag coefficients typically larger than horizontal values. Wave dissipation across the porous reefs was due to a combination of horizontal drag forces and wave breaking, with vertical drag forces playing only a secondary role. Compared to less porous structures, the enhanced drag dissipation in porous artificial reefs enables them to attenuate waves more effectively over a greater range of water levels. Finally, the findings of this study underscore the potential for SPH models to be used as a cost-effective tool to support the design of porous artificial reefs for coastal protection.