Ultra-thin arc-shaped conformal metasurface coating for broadband noise reduction in underwater pipeline

In this paper, a design method of arc-shaped conformal metasurface underwater muffling coating for noise reduction of fluid-filled pipeline is proposed. Combining Fabry-Pèrot(FP) channel and Helmholtz resonant cavity structure, a thin-layer metasurface of fan-shaped ultra-thin labyrinth channel is designed, which is conformally laid on the inner wall of the circular pipeline, and underwater low-frequency broadband noise reduction is realized by sound absorption. With the advantage of relatively loose size constraint in the length direction of the pipeline, we transfer the thickness dimension required for low-frequency broadband sound absorption to the length direction, which greatly reduces the thickness of the coating. Because the elasticity of solid structure is close to the compressibility of water medium, the coupling strength between structure and underwater sound wave is enhanced, and the loss ability of thin-layer structure to low-frequency sound wave is improved, which overcomes the contradiction between thin-layer structure and middle-low frequency noise reduction. By overlaying and combining structures with excellent performance in different noise reduction frequency bands in the axial direction and designing high-order sound absorption units, broadband muffling performance is achieved, and micro-perforated structures are used instead of macro-perforated structures to avoid the possibility that sand and gravel enter the structure and lead to the failure of sound absorption performance. The metasurface broadband muffling coating designed in this paper has a small thickness, which can shield the noise propagation while ensuring that the pipe can flow through the fluid, and solve the problem of the acoustic short circuit of the underwater pipeline, which is of great significance to the noise reduction of the underwater pipeline.