Liran Oren

Flow Fields and Acoustics in a Unilateral Scarred Vocal Fold Model

Objectives: From prior work in an excised canine larynx model, it has been shown that intraglottal vortices form between the vocal folds during the latter part of closing. It has also been shown that the vortices generate a negative pressure between the folds, producing a suction force that causes sudden, rapid closing of the folds. This rapid closing will produce increased loudness and increased higher harmonics. We used a unilateral scarred excised canine larynx model to determine whether the intraglottal vortices and resulting acoustics were changed, compared to those of normal larynges. Methods: Acoustic, flow field, and high-speed imaging measurements from 5 normal and 5 unilaterally scarred canine larynges are presented in this report. Scarring was produced by complete resection of the vocal fold mucosa and superficial layer of the lamina propria on the right vocal fold only. Two months later, these dogs were painlessly sacrificed, and testing was done on the excised larynges during phonation. High-speed video imaging was then used to measure vocal fold displacement during different phases. Particle image velocimetry and acoustic measurements were used to describe possible acoustic effects of the vortices. Results: A higher phonation threshold was required to excite the motion of the vocal fold in scarred larynges. As the subglottal pressure increased, the strength of the vortices and the higher harmonics both consistently increased. However, it was seen that increasing the maximum displacement of the scarred fold did not consistently increase the higher harmonics. The improvements that result from increasing subglottal pressure may be due to a combination of increasing the strength of the intraglottal vortices and increasing the maximum displacement of the vocal fold; however, the data in this study suggest that the vortices play a much more important role. Conclusions: The current study indicates that higher subglottal pressures may excite higher harmonics and improve loudness for patients with unilateral vocal fold scarring. This finding implies that therapies that raise the subglottal pressure may be helpful in improving voice quality.

Intraglottal pressure distribution computed from empirical velocity data in canine larynx

Intraglottal velocity measurements were taken using particle image velocimetry and the corresponding estimates for the intraglottal pressure were computed using the pressure Poisson equation. Results from five canine larynges showed that when the flow separated from the divergent glottal walls during closing, the vortices that were formed in the separated region of the glottis created negative pressure near the superior aspect of the folds. The magnitude of the negative pressure was directly proportional to the subglottal pressure. At low subglottal pressure, negative pressures at the superior edge were not observed when the divergence angle of the wall was minimal and the glottal flow did not separate from the wall.

Intraglottal geometry and velocity measurements in canine larynges

Previous flow velocity measurements during phonation in canine larynges were done above the glottal exit. These studies found that vortical structures are present in the flow above the glottis at different phases of the glottal cycle. Some vortices were observed to leave the glottis during the closing phase and assumptions were proposed regarding their formation mechanism. In the current study, intraglottal velocity measurements are performed using PIV, and the intraglottal flow characteristics are determined. Results from five canine larynges show that at low subglottal pressure the glottis assumes a minimal divergence angle during closing and the flow separates at the glottal exit. Vortical structures are observed above the glottis but not inside. As the subglottal pressure is increased, the divergence angle between the folds during closing increases and the location of the flow separation moves upstream into the glottis. Entrainment flow enters the glottis to fill the void that is formed between the glottal jet and the fold. Vortical structures develop near the superior edge at medium and high subglottal pressures from the flow separation. The magnitude of their swirling strength changes as a function of the wall dynamics.

Role of Subglottal Shape in Turbulence Reduction

Objectives: In previous work, we found that airflow at the superior edge of the vocal folds, in the excised canine larynx, can be laminar even when the tracheal airflow is predominantly turbulent. Turbulent flow directly above the folds may lead to an irregular or “rough” voice. Thus, it is important to determine the mechanism of turbulence reduction. From fluid mechanics, it is known that a smoothly converging duct will reduce turbulence. In this study, we tested the hypothesis that the majority of the turbulence reduction is due to the smooth converging shape of the subglottis. Methods: In 3 excised canine larynges, hot-wire anemometry was used to measure the turbulence intensity (TI) below the cricoid cartilage and 2 to 3 mm above the superior edge of the vocal folds. Laminar flow was seen when the TI was approximately less than 2%. For our measurements, flow into the subglottis had an average TI of more than 20% (high turbulence) in the shear layer and a TI of more than 15% in the center of the jet. The larynges were tested under steady conditions (folds not phonating) with the vocal processes approximated. Results: For the center of the jet, there is moderate turbulence below the cricoid cartilage and laminar flow 2 to 3 mm above the folds. For the shear layer, there is very high turbulence below the cricoid cartilage and low turbulence 2 to 3 mm above the folds. Conclusions: The smooth converging shape of the subglottis can produce a significant reduction in turbulence. These findings may have important voice implications for operations that may change the subglottal shape (such as vocal fold medialization or airway reconstruction).

Characterization of the Vocal Fold Vertical Stiffness in a Canine Model

OBJECTIVES/HYPOTHESIS Characterizing the vertical stiffness gradient that exists between the superior and inferior aspects of the medial surface of the vocal fold. Characterization of this stiffness gradient could elucidate the mechanism behind the divergent glottal shape observed during closing. STUDY DESIGN Basic science. METHODS Indentation testing of the folds was done in a canine model. Stress-strain curves are generated using a customized load-cell and the differential Youngs modulus is calculated as a function of strain. RESULTS Results from 11 larynges show that stress increases as a function of strain more rapidly in the inferior aspect of the fold. The calculations for local Youngs modulus show that at high strain values, a stiffness gradient is formed between the superior and inferior aspects of the fold. CONCLUSIONS For small strain values, which are observed at low subglottal pressures, the stiffness of the tissue is similar in both the superior and inferior aspects of the vocal fold. Consequently, the lateral force that is applied by the glottal flow at both aspects results in almost identical displacements, yielding no divergence angle. Conversely, at higher strain values, which are measured in high subglottal pressure, the inferior aspect of the vocal fold is much stiffer than the superior edge; thus, any lateral force that is applied at both aspects will result in a much greater displacement of the superior edge, yielding a large divergence angle. The increased stiffness observed at the inferior edge could be due to the proximity of the conus elasticus.

Direct simultaneous measurement of intraglottal geometry and velocity fields in excised larynges

Current theories regarding the mechanisms of phonation are based on assumptions about the aerodynamics between the vocal folds during the closing phase of vocal fold vibration. However, many of these fundamental assumptions have never been validated in a tissue model. In this study, the main objective was to determine the aerodynamics (velocity fields) and the geometry of the medial surface of the vocal folds during the closing phase of vibration. The main hypothesis is that intraglottal vortices are produced during vocal fold closing when the glottal duct has a divergent shape and that these vortices are associated with negative pressures.

Flow Characteristics of Non Circular Synthetic Jets

Our study aimed to investigate the flow characteristics of circular and non-circular orifice configurations: a slot, a square, and an equilateral triangle, and to compare the results in order to investigate which orifice configuration is a better solution of synthetic jet for thermal control management. The synthetic jet actuators were tested at their resonance frequency where 500 PIV images were acquired randomly for each orifice configuration and were averaged to give the time-averaged flow fields. Velocity profiles along the axial and traverse directions where generated and analyzed. The results showed highly coherent structures for the circular and square configuration, and high spreading rate and mixing for the slot configuration which makes it most preferable configuration. Nomenclature Do Orifice diameter De Effective (hydraulic) diameter Ū Mean velocity in the streamwise direction V Mean velocity in the transverse direction Uo Mean velocity at the orifice exit in the streamwise direction UCL Centerline mean velocity u rms velocity in the streamwise direction x Streamwise distance from the orifice r Radial distance from the centerline

Intraglottal velocity and pressure measurements in a hemilarynx model

Determining the mechanisms of self-sustained oscillation of the vocal folds requires characterization of the pressures produced by intraglottal aerodynamics. Because most of the intraglottal aerodynamic forces cannot be measured in a tissue model of the larynx, current understanding of vocal fold vibration mechanism is derived from mechanical, analytical, and computational models. Previous studies have computed intraglottal pressures from measured intraglottal velocity fields and intraglottal geometry; however, this technique for determining pressures is not yet validated. In this study, intraglottal pressure measurements taken in a hemilarynx model are compared with pressure values that are computed from simultaneous velocity measurements. The results showed that significant negative pressure formed near the superior aspect of the folds during closing, which agrees with previous measurements in other hemilarynx models. Intraglottal velocity measurements show that the flow near the superior aspect separates from the glottal wall during closing and may develop into a vortex, which further augments the magnitude of negative pressure. Intraglottal pressure distributions, computed by solving the pressure Poisson equation, showed good agreement with pressure measurements. The match between the pressure computations and its measurements validates the current technique, which was previously used to estimate intraglottal pressure distribution in a full larynx model.

Direct measurement of planar flow rate in an excised canine larynx model

During phonation, skewing of the glottal flow waveform (Q) refers to a phenomenon that occurs when the flow decelerates more rapidly than it accelerates. This skewing is clinically important because it increases the glottal efficiency, which is defined by the acoustic intensity (sound pressure level) divided by the subglottal pressure. Current theoretical models predict that the only mechanism to cause skewing of Q involves changes in the vocal tract inertance. The purpose of the current work is to show that other factors at the vocal fold level can also cause skewing of Q and to determine if the acoustic intensity is correlated with maximum flow declination rate.

Turbulence Characteristics of Axisymmetric and Non-Circular Synthetic Jets

This paper discusses the turbulence and flow characteristics of axisymmetric and non-circular synthetic jets. The different orifice configurations, all having the same equivalent diameter, are: circular, triangular, square, and a slot. The deterministic component dominates the fluctuations of the flow near the orifice in the synthetic jet. Triple decomposition technique is used to extract the random fluctuations of the flow and it’s showed that their level is similar to the level in the conventional turbulent jet. POD modes are used to extract the coherent structures in the flow of the synthetic jets. The modes show the deformations of the coherent structures during the axis switching in the non-circular jets, and confirm the decline of the deterministic component in the fluctuations of the flow further downstream.