## 5aSA8. Adaptation of a frequency domain structural acoustics finite element code to test the doubly asymptotic approximation.

### Session: Friday Morning, December 6

### Time: 10:30

**Author: Jerry H. Ginsberg**

**Location: School of Mech. Eng., Georgia Inst. of Technol., Atlanta, GA 30332-0405**

**Author: Henno Allik**

**Location: BBN, New London, CT 06320-6167**

**Abstract:**

The doubly asymptotic approximation is widely employed to model the
transient (time domain) response of structures that are partially or fully
immersed in water. It replaces the field equations for the fluid with an ad hoc
surface interaction law that was identified from analytical solutions for
spheres and infinitely long cylinders. Previously, the validity of the method
for slender hemi-capped cylindrical shells was investigated by comparing DAA in
the frequency domain to the surface variational principle, and then using FFT
theory to obtain alternative transient responses. That study was limited by its
use of Ritz series for displacement. The present work describes how SARA-2D,
which is a frequency domain structural acoustics computer code, may be employed
to assess the validity of DAA. SARA-2D uses finite elements for the structure,
while finite and infinite elements model the fluid. Because of the narrow
bandwidth of its matrices, it is more efficient than boundary elements for a
frequency sweep. However, internalization of the wet surface nodes within the
finite element mesh, the nonlocal nature of DAA, and the need to construct an
added mass matrix for DAA lead to interesting issues in deriving an equivalent
DAA model. [Work supported by ONR, Code 1222.]

ASA 132nd meeting - Hawaii, December 1996