Acoustic scattering is used in both communication and detection of submerged structures such as submarines. A vessel's acoustic signature can determine not only the location but also the physical characteristics of the structure. Reducing the scattered field leads to the phenomenon of cloaking in which the structure appears invisible to the receiver. Naval applications of acoustic cloaking have great influence in developing future classes of stealth vessels.
Our research is focused on reducing the total scattering from a planar wave incident on a cylindrical elastic barrier by implementing multi-element multi-path (MEMP) design. This design technique, developed by our group, converts a single structural element into a system of elastically coupled elements which utilizes the inherent mechanical properties of the substructures rather than damping to tune its dynamic response. MEMP structures provide an innovative approach by increasing the number of wave transmission paths which allow for physical phenomena not evident in single element structures. Previous analytical and experimental work on vibration transmission in MEMP thin beams has proven successful in demonstrating substantial wideband attenuation.
Current work examines the 2D scattering from a submerged shell structure with a dry interior. In order to reduce scattering at a given frequency, the mechanical behavior or impedance of the submerged structure should match that of the displaced fluid. Wideband cloaking thus requires matching of numerous modal responses acting together and is unattainable with a singular isotropic structure. Through MEMP design, the structure is transformed into a multi-layered system of thin concentric cylindrical shells coupled with azimuthally continuous radial springs. Quasistatic cloaking is attained by matching the bulk modulus and density with that of the fluid, while novel resonance matching technique extends the reduction onto higher frequency ranges.
Selected References
Raudales, D and Bliss, DB. "Reduction of Low Frequency Scattering from a Cylindrical Elastic Structure Using a Multi-Element Multi-Path Design." The Journal of the Acoustical Society of America. 137, 2343 (2015).