Modeling of the human larynx with application to the influence of false vocal folds on the glottal flow

Human phonation is a complex phenomenon produced by multiphysics interaction of the fluid, tissue and acoustics fields. Despite recent advancement, little is known about the effect of false vocal folds on the fluid dynamics of the glottal flow. Recent investigations have hypothesized that this pair of tissue can affect the laryngeal flow during phonation. This hypothesis was tested both computationally and experimentally in this dissertation. The computations were performed using an incompressible solver developed in fixed Cartesian grid with a second order sharp immersed-boundary formulation while the experiments were carried out in a low-speed wind tunnel with physiologic speeds and dimensions. A parametric study was performed to understand the effect of false vocal folds geometry on the glottal flow dynamics and the flow structures in the laryngeal ventricle. The investigation was focused on three geometric features: the size of the false vocal fold gap, the height between the true and false vocal folds, and the width of the laryngeal ventricle. The computational simulations were used to study the flow structures of the glottal flow and pressure distribution on the surface of the larynx. The experimental pressure data served to validate the computational results and provided extended knowledge over a broad range of Reynolds numbers. It was found that the size of the false vocal fold gap has a significant effect on glottal flow aerodynamics; whereas the height between the true and false vocal folds and the width of the laryngeal ventricle were of lesser importance.

Due to lack of appreciation of the effect of real geometry of the larynx in the literature, a framework was discussed to extract the laryngeal geometry from the CT scan images. The image segmentation technique was utilized to extract the laryngeal geometries of a canine and a 45 years old female human larynx. Fully resolved three dimensional simulations of the laryngeal flow were conducted for physological Reynolds numbers in these realistic geometries to gain insight into the evolution of vortical structures in the larynx. It was shown that the glottal jet flow is highly three dimensional.

The two and three dimensional computational investigations revealed the presence of the rarely reported secondary vortices in the laryngeal ventricle known as rebound vortical structures. It was found that these vortical structures are formed due to the interaction between the starting vortex ring and the false vocal folds. Therefore, the small size of the false vocal folds gap was identified as an important factor in increasing the intensity of these vortical structures.

Finally, a novel high order Cartesian based moving least square finite volume solver was developed in this dissertation to model acoustic wave scattering at low Mach numbers flows. The computational aeroacoustic approach is based on incompressible viscous/acoustic splitting technique. In this solver, linearized perturbed compressible equations are solved on Cartesian grids and the boundaries are treated sharply using ghost fluid approach. The Cartesian grid framework is compatible with the incompressible solver and provides the flexibility of handling complex geometries. The acoustic solver was validated against several benchmark problems for which analytical solution is available.