Acoustic scattering from an infinitely long cylindrical shell with an internal mass attached by multiple axisymmetrically distributed stiffeners

TL;DR

This paper derives a closed-form solution for acoustic scattering from an infinitely long cylindrical shell with internal mass and multiple axisymmetrically distributed stiffeners, revealing resonance behaviors and scattering characteristics.

Contribution

It provides a novel analytical model for shell scattering with discrete internal stiffeners, including resonance analysis and asymptotic scattering behavior for large numbers of stiffeners.

Findings

Resonances related to shell flexural waves are identified.

Scattering cross-section decreases at low frequencies with more stiffeners.

Sensitivity to incidence angle diminishes as the number of stiffeners increases.

Abstract

A thin infinitely long elastic shell is stiffened by $J$ in number identical lengthwise ribs distributed uniformly around the circumference and joined to a rod in the center. The 2D model of the substructure is a rigid central mass supported by $J$ axisymmetrically placed linear springs. The response of the shell-spring-mass system is quite different from a fluid filled shell or that of a solid cylinder due to the discrete number of contact points which couple the displacement of the shell at different locations. Exterior acoustic scattering due to normal plane wave incidence is solved in closed form for arbitrary $J$. The scattering matrix associated with the normal mode solution displays a simple structure, composed of distinct sub-matrices which decouple the incident and scattered fields into $J$ families. The presence of a springs-mass substructure causes resonances which are shown to be related to the subsonic shell flexural waves, and an approximate analytic expression is derived for the quasi-flexural resonance frequencies. Numerical simulations indicate that the new solution for $J\ge 3$ springs results in a complicated scattering response for plane wave incidence. As the number of springs becomes large enough, the total scattering cross-section is asymptotically zero at low frequencies and slightly increased compared to the empty shell at moderate frequencies due to the added stiffness and mass. It is also observed that the sensitivity to the angle of incidence diminishes as the number of springs is increased. This system can be tuned by selecting the shell thickness, spring stiffness and added mass to yield desired quasi-static effective properties making it a candidate element for graded index sonic crystals.