Phononic-Crystal-Inspired Design of a Gradient-Porosity Multi-Row Pile Breakwater for Enhanced Wave Attenuation

Inspired by the concepts of phononic crystals and spatially graded wave control, this study proposes a gradient-porosity distributed multi-row pile breakwater (GPDMPB), in which porosity decreases stepwise from 0.30 to 0.10 along the wave-propagation direction. A three-dimensional numerical wave tank was established using OpenFOAM and OlaFlow. Reflection, transmission, and energy dissipation characteristics were evaluated for different porosity distributions, pile-row spacings, and wave steepnesses. Under the investigated regular-wave conditions, compared with the conventional permeable breakwaters considered, the GPDMPB achieved lower reflection and transmission and higher energy dissipation; the conventional configurations exhibited reflection and transmission coefficients 1.16–2.94 and 1.16–3.01 times those of the GPDMPB, while their dissipation coefficients were only 0.35–0.87 times as large. Compared with a uniform-porosity array (P = 0.20) having the same total blockage, the GPDMPB exhibited lower transmission, higher energy dissipation, and generally lower or comparable reflection. Flow-field results revealed successive flow contraction, separation, and recirculation through the pile rows, indicating progressive attenuation through distributed wave–structure interactions. Pile-row spacing mainly affected reflection, whereas increasing wave steepness increased reflection and transmission and reduced dissipation. These results highlight the potential of the proposed spatially graded configuration to improve wave attenuation in multi-row pile breakwaters under the investigated regular-wave conditions.