Abstract
Buckling instabilities often produce the emission of sound, a typical example being aluminium cans under compression which produce a crushing sound during buckling. At a larger length-scale, axially compressed cylinders can generate loud sounds while navigating across post-buckled stable states.Post-buckled stable configurations are attractive as these can result from the large deformations produced by buckling, switching dynamically from an initial to an alternative configuration; a phenomenon that has increasingly been harnessed in novel morphing and deployable structures. Yet, the sound produced by buckling instabilities has very rarely been exploited, even though, nature has amazing examples of how to benefit from buckling sound production. For instance, it was recently discovered that ermine moths (Yponomeuta) possess wing tymbal organs (aeroelastic tymbals) that produce aposematic acoustic signals in the form of bursts of ultrasonic clicks to deter bats that prey upon them. However, no investigation about the buckling and sound radiation mechanisms was carried out prior to this study. This dissertation was therefore motivated by the challenge of explaining the source of aeroelastic sound production in ermine moths and the promise of taking advantage of buckling sound production to provide added functionality to buckling-driven morphing structures. Therefore, the aim was to explore and develop computational models inspired by aeroelastic tymbals. This was achieved by first collecting experimental evidence of the main features that allow the aeroelastic tymbal to buckle using high-speed videography, imaging techniques such as micro-computed tomography and confocal microscopy, and laser-Doppler vibrometry. Subsequently, bioinspired models of the aeroelastic tymbal at different fidelity levels were developed---from curved beams to doubly curved shells and origami mechanisms, which were analysed using nonlinear finite element methods and, where appropriate, the effect of structural-acoustic coupling.
The wing deformations occurring during flight were shown to be fundamental in the aeroelastic tymbal buckling. Specifically, the bursts of clicks being the result of a sequence of buckling instabilities, and a scale-free region of the wing serving as the main sound radiating structure. Imaging of the tymbal not only revealed an intricate shape but also graded material properties, and all bioinspired models were therefore a simplification of this elaborated morphology. Nevertheless, the models were able to capture and reproduce the fundamental physics driving the buckling sequence. However, raising the fidelity of the proposed models posed difficult challenges and therefore a different and novel approach based on origami principles was proposed to explain how a sequence of individual elastic instabilities could be triggered under a single actuation load. The computational approach outlined here comprised a first step on a multidisciplinary subject, where concepts rarely studied together, but intrinsically tied, could lead to future implementation in engineering systems, for example, as an acoustic sensing capability in multi-functional shape-changing or morphing structures.
| Date of Award | 24 Jan 2023 |
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| Original language | English |
| Awarding Institution |
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| Sponsors | CONACyT |
| Supervisor | Rainer Groh (Supervisor), Marc W Holderied (Supervisor) & Alberto Pirrera (Supervisor) |