Abstract:
A test facility has been set up in the MIT Acoustics & Vibrations Laboratory with the intent of studying various modifications to shell structures which would facilitate control of membrane waves. Previous research on finite-length submerged shells has shown the importance of membrane waves in the mid-frequency, high aspect angle regimes. These shear and compressional waves are important because they are the principle determinants of the shell's radiation and scattering properties. Flexural waves are poorly coupled to the acoustic medium (water), so they serve mainly as modifiers of the membrane waves. MIT's preliminary test structure consists of a thin steel shell with conical endcaps. An axisymmetric source array is used to generate single modes and the resulting accelerations are measured with an axial line array. Array processing is used to separate the measured accelerations into the three wave types. Data showing shear, compressional, and flexural modes in both forward and backward propagation directions has been collected. Strong coupling between flexural modes was observed in the data, probably due to an axial weld seam along the length of the shell. The experimental dispersion curves closely match theoretical predictions. [Research sponsored by ONR.]