### ASA 128th Meeting - Austin, Texas - 1994 Nov 28 .. Dec 02

## 5aSA8. Implementation of infinite elements in adaptive multilevel
substructuring for structural acoustic calculations.

**Paul D. DSouza
**

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Jeffrey K. Bennighof
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**
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*Dept. of Aerospace Eng. and Eng. Mech., Univ. of Texas at Austin, Austin,
TX 78712-1085
*

*
*
A procedure is presented for incorporating infinite element modeling of
the external acoustic medium in structural acoustic calculations using adaptive
multilevel substructuring. The coupled structure--fluid system is modeled using
finite elements for the structure, and for the acoustic fluid external to the
structure and within a circumscribing prolate spheroid, and infinite elements
external to the prolate spheroid, following the formulation of Burnett (1994).
The adaptive multilevel substructuring approach transforms the structure model
from the original finite element one in terms of nodal displacements, to a much
more efficient one in terms of responses in approximate substructure vibration
modes, which are selectively included in the model to optimize accuracy per
degree of freedom. This approach is also used on the finite element portion of
the acoustic fluid model, and results in a high level of model reduction there.
Transformation of the infinite element portion of the acoustic fluid model is
not practical because of its frequency dependence. The transformation process
results in a linear system that is generally reduced in order, having a sparse
symmetric coefficient matrix amenable to the use of a profile solver. The
procedure is demonstrated on numerical examples in three dimensions. [Work
supported by ONR.]