Boussinesq modelling of near-trapping in a four-cylinder array

In this paper, complex wave interactions with a four-cylinder array is considered, which has clear relevance to supporting columns of certain nearshore/offshore structures and supports so-called near-trapping phenomenon when subjected to water waves of appropriate period and direction. High free surface elevations and wave loading are expected and thus endanger structure safety. A Boussinesq model is established based on the cut-cell technique for dealing with complex geometries with curved surfaces. The model is validated against the experimental measurements for capturing the near-trapping by a four-cylinder structure in a square arrangement, and is extended to investigate the near-trapping for various scenarios. It is found that the wave periods at which the near-trapping occurs increase with the center-to-center spacings between two adjacent cylinders. Moreover, the re-reflection modes that the waves inside the cylinder array are reflected back and forth among neighboring cylinder pairs can be clearly seen from the flow field streamlines. The energy is trapped inside the array and decays slowly due to wave radiation. This trapped wave would eventually meet with the subsequent incident wave crest, leading to high free surface elevations at near-trapping periods. In view of this, two scenarios are designed to avoid/minimize the near-trapping by breaking the geometry symmetry, hence, the re-reflections modes.