Intraglottal Flow Behavior in a CT-Based Laryngeal Model

Review Article

Austin J Otolaryngol. 2014;1(4): 7.

Intraglottal Flow Behavior in a CT-Based Laryngeal Model

Xue Q1*, Zheng X1 and Ye A2

1Department of Mechanical Engineering, University of Maine, USA

2Bangor High School STEM Program, Bangor High School, USA

*Corresponding author: Xue Q, Department of Mechanical Engineering, University of Maine, 5711 Boardman Hall, Room 219, Orono, ME, USA

Received: August 02, 2014; Accepted: October 25, 2014; Published: November 10, 2014


A direct numerical simulation of flow-structure interaction was carried out in a subject-specific larynx model to study human phonation under physiological conditions. The effect of the realistic shape of the vocal fold and airway lumen on intraglottal flow dynamics was explored. It was found that the complex shape of the larynx, especially the lateral confinement of the airway lumen in the supraglottal region and the anterior-posterior asymmetry of the laryngeal shape, has a profound effect on intraglottal flow dynamics. Several important new findings that have not been captured in past simplified models were reported.

Keywords: Phonation; Vocal fold; Flow-structure interaction; Realistic laryngeal model; Intraglottal flow


From a biomechanical point of view, voice production is intrinsically a multiphysics biological process resulting from complex nonlinear biomechanical interactions between glottal aerodynamics and vocal fold vibrations. The intraglottal region, which is also called the glottis, refers to the space/channel formed by the medial surface of the vocal folds. This is the region where the complex flow-structure interaction takes place and the primary sound sources are located. Considerable efforts have been undertaken to identify intraglottal flow behaviors and to ascertain their influence on the energy exchange during flow-structure interactions. Traditionally the intraglottal pressure was considered to be mainly governed by the Bernoulli’s effects [1-4]. The glottal flow was assumed to separate from the exit of the glottis and viscous loss was modeled as an empirical term obtained from the experimental data. Recently, more comprehensive experimental and numerical studies uncovered some more complex intraglottal flow behaviors and their important impacts on phonation [5-8]. During the glottal opening phase, a convergent shape of glottis is formed. Flow remains attached to the vocal fold wall within the glottis. A thin viscous boundary layer is developed and the intraglottal flow is mainly dominated by the Bernoulli’s effects. During the glottal closing phase, an adverse pressure gradient is induced by the divergent glottal shape and the boundary layer starts to separate from the vocal fold walls. Due to the inherent flow instability, the flow separation is asymmetric and the flow attaches to one side of the vocal fold walls until the glottal exit. The viscous “blockage effect” due to the boundary layer increases the flow impedance and alters the flow-pressure relationship [9]. Flow separation usually induces negative pressure (relative to the ambient pressure) around the superior portion of the divergent glottis [10]. This negative pressure determines the closing speed of the vocal fold, which has important implications for flow decline rate and vocal intensity [11-15]. Flow separation was also found to be a primary factor determining the phonation threshold pressure [16,17]. Furthermore, these viscous flow features, such as flow asymmetries, flow separations and vortical structure evolution, are highly unstable and constantly changing during a phonatory cycle. For instance, dynamic flow separation has been widely observed [18-20]. A recent experimental investigation showed that the development of intraglottal flow asymmetries is dependent on the acceleration of flow [7]. A more recent study observed that flow asymmetries in the dynamic models appear later in the cycle than in the static models [21].

While the aforementioned studies greatly improve the understanding of intraglottal flow behaviors, they were all conducted by using static and/or simplified larynx models. Therefore, their validity remains to be assessed by using a dynamic realistic laryngeal model. The larynx has a complex anatomical structure and the unsteady viscous flow is very sensitive to the geometry of larynx. For instance, intraglottal vorticity-velocity interaction, glottal jet structure and its transition to turbulence were all demonstrated to be highly three-dimensional [22,23,20]. The sub and supraglottal lumen in the human larynx has significant variations in the anterior-posterior direction which affects those flow behaviors [24]. Flow-induced vibration of vocal folds was also observed having strong anterior-posterior variations in high speed imaging studies [25].

In our previous study, we have successfully conducted a flow-structure interaction simulation in a subject-specific larynx model [24]. The obtained flow parameters and vibration pattern were found to be within the normal phonation range. The current study will extend this earlier study by comprehensively examining the intraglottal flow behaviors with the aim to provide additional insights into intraglottal flow dynamics under realistic physiological conditions.

Computational Model

The numerical algorithm and simulation set up have been reported in details in our previous study [24]. For the sake of completeness, the current paper describes concisely some salient features of the numerical methods, geometric model, contact model, boundary conditions, and material properties. The current study employed an explicitly coupled immersed-boundary-finite-element method based flow-structure interaction solver to model human phonation. Glottal airflow was governed by the 3D, unsteady, viscous, incompressible Navier-Stokes equations, and vocal fold dynamics was governed by the Navier equation. The coupling between the flow and solid solvers was implemented by tracking the aerodynamic load on the interface mesh as well as its deformed shape and velocity in a Lagrangian fashion.

The geometry of the airway lumen and vocal fold was reconstructed based on a CT scan of the larynx of a 30-year-old male subject by using the commercial medical image processing software, Mimics. It should be pointed out that the vibration part of the vocal fold was very blurry in the CT image. Therefore we manually adjusted the segmented model so that medial surface of the vocal fold aligns with the centerline of the glottis. Figure 1 (a) & (b) show the three-dimensional geometry of the airway lumen and vocal folds. The approximate dimension of each vocal fold was 0.9cm (thickness) × 1.0cm (depth) × 1.4 cm (length). Two artificial straight tubes were connected to both the subglottal trachea and the supraglottal pharynx to provide sufficient distance to apply uniform pressure boundary conditions. The model of the airway lumen and vocal folds were immersed into a 10cm (inferior-superior) × 2.9cm (medial-lateral) × 3.7cm (anterior-posterior) rectangular box, as shown in Figure 1(a). The vocal folds were located between the planes of y=2.5cm and y=3.5cm. Zero and 1.2kPa gauge pressure are applied at the outlet and inlet, respectively, yielding a typical 1.2kPa pressure drop across the vocal tract. Non-slip-non-penetration flow condition was applied on all of the lumen walls. To deal with the contact between two vocal folds during closed phase, we applied a kinematic constraint on the vocal folds to enforce a minimum glottal gap of 0.01 cm. This minimum gap is necessary for the success of the flow solver, but it also allows some “leakage” flow even when the vocal folds are considered completely closed.