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Dissertation
EXPLORING HIGH AND MID-FREQUENCY ELASTIC MECHANISMS IN ACOUSTICALLY ILLUMINATED TARGETS IN WATER
Washington State University
Doctor of Philosophy (PhD), Washington State University
01/2022
DOI:
https://doi.org/10.7273/000004416
Handle:
https://hdl.handle.net/2376/125348
Abstract
We explore some intriguing instances of surface-elastic waves (SEW) on insonified cylindricalshells of different materials in water. These serve as a continuation of similar studies in the past–for example, on spherical shells of comparable materials. The waves in question are a generalization of Lamb Waves on flat plates. In this simpler case, noting an overall symmetric and antisymmetric class of wave, each exists as a unique root of Lamb’s Characteristic Equations. Similarly, such waves on curved surfaces, in particular that of a cylindrical or spherical shell, will be the root of the appropriate shell equation. This solution, in turn, serves as a mathematical description of the phenomena under discussion. We focus on a few such roots, one of which is the (symmetric) s2b wave in the high frequency range, where we observe the unique case of a phase and group velocity moving in opposite directions. Though this wave has been noted in previous studies, we now explore the behavior with new targets and new materials. In the mid-frequency range, we have the 0th symmetric and antisymmetric roots, and look at the behaviors of these in the case of a "semi-thin" cylindrical shell. This study complements similar experiments done to observe such phenomena with both "thick" and "thin" shells. Among other challenges, the a0 wave transitions from supersonic (relative to surrounding water) to subsonic at a "coincidence frequency" whose position depends on the shell. In this case, mathematical models that assume either subsonic or supersonic conditions must be modified to accommodate a transitional zone between the two. Finally, we compare our semi-thin results to others previous using thick and thin shells. For the tilted semi-thin shell, prominent backscattering contributions are associated with guided helical subsonic waves, and with a meridional supersonic wave. Elastic waves also affect sonar images of the cylinder.
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Details
- Title
- EXPLORING HIGH AND MID-FREQUENCY ELASTIC MECHANISMS IN ACOUSTICALLY ILLUMINATED TARGETS IN WATER
- Creators
- Bernard Richard Hall
- Contributors
- Philip L. Marston (Advisor)Matthew Duez (Committee Member)Frederick Gittes (Committee Member)
- Awarding Institution
- Washington State University
- Academic Unit
- Department of Physics and Astronomy
- Theses and Dissertations
- Doctor of Philosophy (PhD), Washington State University
- Publisher
- Washington State University
- Number of pages
- 149
- Identifiers
- 99900883238901842
- Language
- English
- Resource Type
- Dissertation