### 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
**

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M. D. McCollum
**

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

*
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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.