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

## 2aSA7. Approximate closed-form analysis of modal coupling for radiation
and scattering from fluid loaded structures.

**Linda P. Franzoni
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*Mech. and Aerospace Eng., North Carolina State Univ., Raleigh, NC
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**Donald B. Bliss
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*Duke Univ., Durham, NC 27708
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Modal analysis is often used in problems involving radiation and
scattering from fluid loaded structures. The modal formulation leads to a
coupled system of equations for the modal amplitudes. Typically, structural
modes are coupled through the effects of the fluid. In practice, modal coupling
is sometimes assumed unimportant, enabling modal coefficients to be solved
independently in closed form. This approach is particularly appealing when
subsequent analytical treatment of expressions is desirable. The validity of
this approach, including the order of the error involved in neglecting coupling
is discussed. It is shown that the principal effects of weak modal coupling can
be incorporated in a simple way without solving the fully coupled system. An
approximate closed-form solution for weakly coupled systems of equations is
developed. The approximation is found to work well, even in systems where
coupling is fairly strong. The approximate form gives physical insight into the
dominant effects of modal coupling, and shows that neglecting coupling may lead
to significant errors. The effects of coupling are assessed by solution of
model problem involving acoustic scattering from a fluid loaded plate.
Solutions are compared for three methods: fully coupled, uncoupled, and the
approximate analysis. [Work supported by ONR.]