Biomechanical Modeling of Glottal Aerodynamics and Vocal Fold Vibration During Phonation Open Access
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Phonation is a complex biological phenomenon which results from the coupled biomechanical interaction between glottal aerodynamics and vocal fold tissue. Due to the complexity and nonlinearity of flow-tissue interaction, reduced fidelity models such as inviscid/irrotational flow, lumped mass vocal fold models, stationary or specified vocal fold motion, etc. have been widely used in the past for modeling of phonation. While these models are able to capture some of the basic characteristics of phonation, they cannot provide quantitative results of a quality and fidelity required for clinical diagnosis or treatment. Furthermore, these methods provide only limited insights into the biophysics of phonation. In the current study, a new, high-fidelity computational tool has been developed for modeling the biophysics of phonation in all its complexity. While the immediate objective is to use this tool is to gain new insights into the biophysics of phonation, the long term goal of this project is develop a computer-based tool for laryngeal surgical planning. The key components of the tool include (a) an immersed boundary method (IBM) based Navier-Stokes solver for modeling the glottal flow, (b) a finite-element method (FEM) based solver for modeling the viscoelastic deformation of vocal-fold tissue, (c) a penalty coefficient method for modeling the vocal-fold contact and (d) a loosely coupled IBM-FEM methodology for modeling fluid-structure interaction in the human larynx. In addition, a high resolution CT scan has been used to provide guidance for construction of a realistic laryngeal model. Two- as well as three-dimensional simulations have been performed to gain insights into the biophysics of phonation. Self-sustained vocal fold vibrations with vibratory modes that correspond to physiological observations have been captured (Zemlin, 1988) and the vibration frequency is in the correct phonatory frequency range (Titze, 1994). The glottal flow has been shown to form a pulsatile turbulent jet and this jet flow is highly asymmetric. The effect of subglottal pressure on phonation onset and fundamental frequency has been investigated. The predicted phonation frequency shows a nonlinear increase at the lower end of the phonatory pressure range but settles to a nearly constant value at or above the normal frequency. The effect of false vocal folds on phonation has investigated through a systematic comparison of two models, one with false vocal folds and one without. It has been found that false-vocal folds tend to aid phonation by reducing the effort required to phonate and by increasing the sound intensity for a given effort. While similar effects have been noted in the past, a key contribution of the current study is to clearly delineate a physical mechanism for this effect - a reduction in viscous losses associated with reduced tendency of the glottal jet flapping in the presence of false vocal folds.The current study shows that while two-dimensional laryngeal models are amenable to comprehensive analysis through simulations, the computational requirements for three-dimensional models are significantly larger. This effectively puts such analysis using three-dimensional models out of reach unless significant advances can be made in the computational speed and efficiency. In order to achieve significant improvement in computational speeds, a new local-grid refinement approach that employs a hierarchical nested grid approach has been developed and applied to a sharp interface immersed boundary solver. The key feature of the methodology is that the structured grid approach is retained at all the refinement levels and this allows one to use powerful line-SOR schemes and a geometric multigrid method. A set of simulations of canonical flows have been conducted and these indicate that the solver accurately reproduces the key features of the flows and holds promise for phonation modeling with complex three-dimensional models.
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