Effect of polymeric foam core morphology and orthogrid stiffener design on the noise attenuation of fluid-immersed structures.

Sandwich cylindrical structures are widely used in various industries, including high-speed trains, automotive, and civil engineering applications. To improve their performance, integrating polymeric foam cores and various configurations of periodic stiffeners into sandwich cylindrical shells significantly enhances their noise-cancellation capabilities. This paper investigates, for the first time, the acoustic parameters of sound transmission loss (STL) and noise reduction (NR) in an infinitely long sandwich cylindrical shell reinforced with annular and axial stiffeners (rings and strings), considering the effects of open-cell and closed-cell foams. The cross-sectional architecture consists of three layers: a functionally graded (FG) outer layer, an FG polymeric foam core, and an isotropic inner layer. Fluid (air) fills the gaps between the layers, and the shell is submerged in an external fluid medium while subjected to excitation by a plane acoustic wave. The outer and inner layers are further strengthened by circumferential and axial stiffeners. The polymeric foam core plays a crucial role in absorbing and dissipating sound energy, significantly enhancing the structure's acoustic insulation properties. Meanwhile, the ring and string stiffeners improve overall mechanical stiffness, ensuring structural integrity and reducing vibrations. To address this problem, the motion equations of the shell are derived using the first-order shear deformation theory (FSDT) and Hamilton's principle. The Zener viscoelastic model, which accounts for the frequency-dependent variation of material properties, is employed to model the viscoelastic core. The acoustic parameters are calculated by incorporating boundary conditions and fluid-structure interaction effects. Among the various reinforcement configurations, the highest STL and NR are achieved when both the inner and outer layers are reinforced, significantly improving vibration damping and acoustic insulation. When only the outer layer is reinforced, the structure performs better in the mass-controlled region, while reinforcing only the inner layer leads to superior performance in the stiffness-controlled region. Furthermore, open-cell foam outperforms closed-cell foam at higher frequencies due to its superior damping properties.