The focus of this research was to experimentally and computationally study air pressures and air flows through a model of the human larynx. The model, M6, was a symmetric, three-dimensional physical model. In this model, the transverse plane of the glottis was formed by half-sinusoidal arcs for each medial vocal fold surface, creating a maximum glottal width at the midcoronal section.
To study the effects of different glottal shapes, three glottal angles were used, namely, 10° convergent, 0° uniform, and 10° divergent, with the single diameter of 0.16 cm. In addition, to capture the effects of changing glottal diameters, three diameters of 0.16, 0.04, and 0.01 cm in the midcoronal plane were used, all with the single angle of 0°, (i.e., the uniform glottis). Inasmuch as the uniform case with maximum diameter (0.16 cm) was the common case in both groups, a total of five distinct pairs of modeled vocal folds were used.
Each case incorporated three rows of 14 pressure taps, located in the inferior-superior direction on the vocal fold surface at locations of the anterior (1/4), middle (1/2), and posterior (3/4) of the anterior-posterior span. This approach (i.e., empirically acquiring air pressure distributions at the three locations) has not been applied in prior studies. For each configuration, transglottal pressures of 0.294, 0.491, 0.981, 1.472, 1.962, and 2.453 kPa (i.e., 3, 5, 10, 15, 20, and 25 cm H2O) were used.
To consider the effects of the presence of the arytenoid cartilages on the intraglottal pressures, a simplified model of the cartilages was fabricated as a single structure based on available physiological data, and the intraglottal pressures were measured with and without its presence. With the arytenoid cartilages structure in place, the glottis is an eccentric orifice. The empirical pressures were compared to computational results obtained with the CFD software package FLUENT. Also, flow visualization using a laser sheet and seeded airflow was applied to study the flow patterns exiting the glottis. The false vocal folds were not included in this study.
The glottis with half-sinusoidal arcs makes a difference relative to intraglottal pressures at the anterior (1/4), middle, and posterior (3/4) planes for all cases. The amount of the pressure difference across the three locations varied based on the glottal angle and diameter; however, the maximum pressure differences did not rise above approximately 8% of the transglottal pressure, even in the presence of the arytenoid cartilages. There were pressure and velocity gradients in both the axial (upstream-downstream) and longitudinal (anterior-posterior) directions, with primary gradients axially and secondary gradients longitudinally. The flow in the M6 model was more stable than in the M5 model downstream of the vocal folds and it did not skew except for the smallest glottal diameter; however, in the M5 model, even for large glottal diameters, the flow skews randomly and creates two different pressure distributions. Flow contraction toward the midcoronal plane within and downstream of the glottis was a primary finding of this study, which was not seen in the rectangular models of the glottis. The arytenoid cartilages structure produced additional secondary flow only for the cases with the largest glottal diameter, which changed the intraglottal pressures along the longitudinal direction.
The results of this study present initial information about the relationship among intraglottal pressures, flow patterns, and the three-dimensionality of the glottis. This study suggests that the pressures and flows within the glottis are three-dimensional, and flow contraction in the sagittal plane is to be expected and considered in future phonatory modeling. Non-rectangular laryngeal geometries need to be accurately specified and are required in research programs of basic laryngeal function to establish benchmark empirical data.