Moderling Acoustic Transmission in Towed Array Sonars : Computations of the coupled acoustic and elastic wave equations

Sammanfattning: This thesis presents a method of solving the Helmholtz equation coupled with the Navier-Cauchy equation over a cross section of a Towed Array Sonar (TAS) modeled as a simplified cylindrical shell. The purpose of solving these equations is to estimate the transfer function of the pressure between the outside boundary and a position inside the TAS. Being able to find the transfer function in a sonar system is important since pressure that is not damped through the system is recorded as noise by acoustic sensors. This thesis focuses on the pressure generated at the surface of the TAS and how it propagates into the system. Other sources of acoustic pressure are neglected. The fluid pressure and solid material displacement over the TAS cross section are solved by utilizing the finite element method. Necessary equations are formulated on weak form and solved over a mesh depicting the cross section of interest. Pressure results inside the TAS are computed for three different frequencies of interest over several wavenumbers associated with the dimension along the TAS axis. The results show that the pressure inside the TAS is amplified mainly close to wave numbers associated with breathing waves that propagate along the TAS axis. Smaller peaks of amplification appear also for lower wave numbers, but as the wave number is increased the pressure inside the TAS is damped.