Date of Award

Fall 12-15-2017

Level of Access

Campus-Only Thesis

Degree Name

Master of Science in Mechanical Engineering (MSME)

Department

Mechanical Engineering

Advisor

Xudong Zheng

Second Committee Member

Vincent Caccese

Third Committee Member

Senthil Vel

Additional Committee Members

Qian Xue

Abstract

It is a wonder that this world has sound – that every motion, however slight it may be, would leave its trace in the air/water and that animals have evolved sensory systems to perceive this trace to their advantages for better surviving. The most evolved form of sound production of animated beings is arguably vocalization since the motion involved (namely the vibration of vocal folds, be it human’s or animals’) is triggered only for the sake of the acoustic product. The locomotion sound, while a byproduct of locomotion, can also bear information and is commonly utilized by animals. For example, mosquitoes are reported to use the sound from flapping wings for sexual communication. Schooling fishes are hypothesized to sense the sound wave in water to make real-time adjustments for swimming in unison.

This thesis is composed of three studies that focus on different aspects of biological sound production. The first two chapters deal with human phonation. In particular, the effects (CHAPTER 1) and cause (CHAPTER 2) of the recently reported vertical stiffness variation of the vocal fold structure. Sound production of flapping wings is investigated in CHAPTER 3. The three chapters have been published as journal articles separately. They are assembled here with little modification.

In CHAPTER 1, a parametric study was conducted using the numerical technique that coupled a three-dimensional continuum vocal fold model with a one-dimensional Bernoulli flow model to investigate the effect of vocal fold vertical stiffness variation on voice production. Vertical stiffness gradient was defined as the ratio of the inferior-superior stiffness difference to the mean stiffness and was introduced in the cover layer. The results showed that increasing the vertical stiffness gradient would increase the peak flow rate and sound intensity and decrease the open quotient and threshold pressure. The effect was found to be more prominent at low subglottal pressures. The underlying mechanism might be that the reduced stiffness at the superior aspect of the vocal fold would allow a larger lateral displacement and result in a larger vibration. Increasing the vertical stiffness gradient was also found to increase the vertical phase difference and glottal divergent angle during the vocal fold vibration. Meanwhile, increasing the vertical stiffness variation only slightly increased the mean flow rate, which is important to maintaining the speech time between breaths.

In CHAPTER 2, a finite element method based numerical indentation technique was used to quantify the effect of the material stiffness variation and the subglottal convergence angle of the vocal fold on the vertical stiffness difference of the medial surface. It was found that the vertical stiffness difference increased with the increasing subglottal angle, and it tended to saturate beyond a subglottal angle of about 50 degrees. The material stiffness variation could be as important as the subglottal angle depending on the actual material properties.

In CHAPTER 3, the unsteady flow and acoustic characteristics of a three-dimensional (3D) flapping wing model of Tibicen linnei cicada at forward flight condition are numerically investigated. A single cicada wing is modelled as a membrane with prescribed motion reconstructed from high-speed videos of a live insect. The numerical solution takes a hydrodynamic/acoustic splitting approach: the flow field is solved with an incompressible Navier-Stokes flow solver based on immersed boundary method and the acoustic field is solved with linearized perturbed compressible equations (LPCEs). The 3D simulation allows examination of both directivity and frequency composition of the flapping wing sound in the full space. Along with the real wing model, other synthetic models that are constructed from principle kinematic modes are also simulated to investigate the effect of wing flexibility. The simulation results show that the flapping sound is directional; the dominant frequency varies around the wing. The first and second harmonics show different radiation patterns which are demonstrated to be highly associated with wing loading. The rotation and deformation in the flexible wings are found to help lower the sound strength due to lower aerodynamic forces, which is found to scale with the dynamic pressure force defined as the integral over the wing area of the dynamic pressure calculated from the normal component of the relative velocity.

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