ASA 129th Meeting - Washington, DC - 1995 May 30 .. Jun 06

2pEA8. In-water modal analysis using a combined finite element/boundary element method.

Clementina M. Siders

M. D. McCollum

NRL, Orlando, 3909 Summerlin Ave., Orlando, FL 32856-8337

A combined finite element/boundary element modeling technique can be used to analyze fluid-loaded transducers to compute the displacements of the structure at specified frequencies. The problem with this technique is that there is no way of knowing a priori the in-water resonance frequency corresponding to a given in-air mode. A common approach is to ``guess'' a frequency range that will encompass the in-water resonance frequency by using a coarse frequency step size. A fluid-loaded analysis is then performed in this frequency range. By examining the resulting displacements versus frequency, it can be determined whether the resonance has occurred in the frequency range. If the in-water resonance frequency is located, a finer frequency step can be used over a smaller frequency range to determine the resonance to a better resolution. This paper describes a method to determine the in-water resonance frequency of a finite element/boundary element model using an iterative eigenvalue solution, thus avoiding the lengthy convergence process of a harmonic solution. The time savings between this new in-water modal solution and the traditional harmonic solution can be phenomenal. Several example problems are also presented.