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

Mech. and Aerospace Eng., North Carolina State Univ., Raleigh, NC

Donald B. Bliss

Duke Univ., Durham, NC 27708

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.]