| The analysis of acoustic radiation and scattering from submerged elastic structures is often hampered by the fact that the acoustic pressure field can be very sensitive to structural detail. In this study, the nature of this sensitivity is examined. Initially, basic procedures for obtaining the first-order sensitivity of a structural-acoustic system to structural perturbations are presented. The types of perturbations are limited to those that do not alter the geometry of the wetted surface, but are otherwise completely general in nature. These techniques are applied to acoustic scattering from a cylindrical shell and are found to be generally effective for small perturbations but inaccurate at some frequencies. The nature of this sensitivity is then examined using a very simple impedance model, which illustrates how system resonances cause high sensitivity and can render first-order sensitivity data to be insufficient. It is also shown that the use of higher order terms does not reliably improve accuracy since nearby resonances can cause the Taylor series expansion to diverge for large perturbations. However, it is found that good approximations for large perturbations can be made if the Taylor series is used to construct a Padé, or rational polynomial, approximation. A procedure for creating this expansion using finite element techniques is presented and applied to several problems in which a large structural perturbation is introduced. The method is found to be very effective and computationally efficient. |
