Sound radiation from a cylindrical shell with a multilayered resonant coating

The vibroacoustic response of a structurally excited cylindrical shell submerged in water is presented. The shell is coated with a soft elastic material embedded with circumferential arrays of resonant inclusions. The coating is modelled as a multilayered equivalent fluid composed of homogeneous layers of the host soft material and homogenised layers comprising voids or hard inclusions. The radiated acoustic pressure is analytically derived by assembling and solving continuity and kinematic conditions at the interfaces between the cylindrical shell and the multilayered coating. Coating designs with different combinations of homogenised layers are examined. Physical mechanisms governing acoustic performance of the various coating designs are described. We show that the material of the inclusions, tuning the local resonances of the inclusions and the distribution of the homogenised layers within the coating have a significant effect on the shell vibroacoustic response

PASSIVE AND ACTIVE CONTROL OF THE VIBROACOUSTIC RESPONSE OF A COATED CYLINDRICAL SHELL SUBMERGED IN A HEAVY FLUID

A major source of anthropogenic noise pollution in the marine environment is underwater radiated sound from marine vessels. This thesis investigates sound radiation from a coated cylindrical shell submerged in a heavy fluid. The shell is externally coated with a soft elastic material embedded with a regular distribution of resonant inclusions that are vacuous cavities or hard steel scatterers. The coating is treated as a multilayered equivalent fluid, in which the layers of inclusions are modelled as homogenised layers with effective material and geometric properties. The radiated acoustic pressure is analytically derived by assembling and solving continuity and kinematic conditions at the interfaces between the cylindrical shell, the multilayered coating, and the interior and exterior acoustic domains. Coating designs with different combinations of homogenised layers are examined and physical mechanisms governing acoustic performance of the coating designs are described. The vibroacoustic response of the locally resonant coated shell submerged in water of infinite extent, submerged at a finite depth near a free sea surface, and partially immersed in water is studied. The free sea surface is treated as a pressure release boundary. The image method is employed to capture the interaction between the free surface and the shell submerged at a finite depth. For the semi-immersed coated shell, a partial fluid loading condition is imposed on the wetted coating surface. A hybrid passive-active control system composed of an array of inertial actuators with tuned vibration absorbers is designed to further attenuate sound radiation from a coated shell with cavities. The passive components of the inertial actuators are tuned to shell circumferential resonances. The active component of the inertial actuators, driven using a feedforward linear-quadratic regulator algorithm, is utilised to suppress the spring-mass resonance of the coated shell. Using detailed understanding of the physical system responses, passive and active control of the radiated sound from a submerged coated shell over a broad frequency range is achieved

Acoustic performance of soft elastic media embedded with periodic inclusions

Elastic media comprising periodic inclusions are rationally designed composite materials which exhibit acoustic properties that go qualitatively and quantitatively beyond those of their bulk ingredients. A practical application of periodic scatterers in a soft rubber-like medium is in the design of acoustic coatings for maritime vehicles. Acoustic coatings are employed to reduce underwater noise pollution as well as to absorb external sound waves for stealth purposes. The aim of this thesis is to investigate the physical mechanisms governing the performance of underwater acoustic coatings comprising voided and/or hard inclusions embedded in a soft elastic medium. Analytical and numerical models are developed to study the transmission, reflection and absorption properties of the coatings. The analytical models are based on effective medium approximation theory whereby an inhomogeneous medium is modelled as layers of homogeneous media with effective geometric and material properties. The acoustic properties of the layered composites are then obtained using the transfer matrix method. The numerical models are developed using the finite element method whereby the exact geometry and material properties of the acoustic coatings are simulated. The coatings are submerged in water and the effects of water backing and steel-air backing on their acoustic performance are examined. The effects of different material and geometric properties of the coatings on the acoustic performance are also investigated. Different combinations of layers of voids and/or layers of hard inclusions are studied in detail and found to have notably different impacts on the acoustic performance of the coating

Aeronautical engineering: A special bibliography with indexes, supplement 49

The bibliography contains 368 abstract citations of reports, journal articles, and other documents concerned with the engineering and theoretical aspects of design, construction, evaluation, testing, operation, and performance of aircraft (including aircraft engines) and associated components, equipment, and systems. Research and development in aerodynamics, aeronautics, and ground support equipment are also treated. Subject, personal, and contract number indexes are included for ease of access

Space Resources and Space Settlements

The technical papers from the five tasks groups that took part in the 1977 Ames Summer Study on Space Settlements and Industrialization Using Nonterrestrial Materials are presented. The papers are presented under the following general topics: (1) research needs for regenerative life-support systems; (2) habitat design; (3) dynamics and design of electromagnetic mass drivers; (4) asteroids as resources for space manufacturing; and (5) processing of nonterrestrial materials