Multiple stopbands and wavefield asymmetry of surface water waves in non-Bragg structures

In this paper, we use laboratory wave tank experiments to study the effect of asymmetry and defects on the bandgaps of surface water waves and use the finite element numerical method to validate our results. We demonstrate here that breaking the mirror symmetry around the midplane of a periodic structure introduces multiple bandgaps in the spectrum caused by the involvement of high-order transverse modes. The results show that the presence of a defect in the structure leads to the formation of a strong defect mode in the Bragg gap, which is localized around the defect element and a weak mode in the induced non-Bragg gap. The results show that the bandgap excited by symmetry breaking is much narrower due to the weak mode coupling. In addition, at the non-Bragg resonance frequency in the defect state, the transverse water surface wavefield distribution around the defect is asymmetric about the midplane of the channel. The multiple transmission modes in the spectrum of the structure can be applied in the design of Bragg reflection-based wave attenuation structures that can help protect shorelines and coastal infrastructure. The asymmetry of the surface wavefield in the non-Bragg gap can be applied in the development of energy harvesting technologies. Due to the generality of wave phenomena in periodic structures, the findings of this research provide a basis for more research in physical acoustics and optics and may lead to the development of cutting-edge appliances, such as bandpass filters.