Computational methods for vibration analysis linked to noise from underwater structures

Sammanfattning

This master’s thesis investigates computational methods for vibroacoustic analysis of underwater structures, with the purpose of supporting the development of quieter underwater vehicles. The study addresses the connection between structural vibrations and acoustic radiation in water by evaluating how numerical simulations can be correlated with controlled physical experiments. A simplified box geometry was designed and dimensioned to generate measurable vibration and acoustic responses within the constraints of the available test environment. Physical impact tests were conducted in both dry and wet conditions using accelerometers, a microphone, and a hydrophone to capture structural response and acoustic pressure. The experimental results were then compared with LS-DYNA simulations using three acoustic solvers: BEM, Rayleigh and Kirchhoff. The results show that the transition from dry to wet conditions significantly affects the dynamic response of the structure, mainly through added mass and increased damping. In the dry condition, both BEM and Rayleigh showed good agreement with the experimental acoustic response, but Rayleigh at a lower computational cost. BEM was identified as the most reliable method for estimating acoustic pressure, where as the Rayleigh method proved to be an efficient alternative for simplified geometries. Kirchhoff was not reliable in the present implementation due to numerical instability. For the wet condition, the correlation between simulations and experiments was more challenging. Reflections in the test tank, fluid damping, and limitations in the explicit solid mechanics solver treatment of fluid interaction contributed to discrepancies between numerical and experimental results. The study concludes that accurate vibroacoustic prediction requires careful modelling of both the structural and fluid domains. Further work is needed to improve fluid interaction coupling in explicits imulations and to validate the workflow on more complex underwater structures