Session KA: Biofluid Dynamics X: Sounds

An Immersed-Boundary Method for Fluid-Structure Interaction in the Human Larynx

We describe a novel and accurate computational methodology for modeling the airflow and vocal fold dynamics in human larynx. The model is useful in helping us gain deeper insight into the complicated bio-physics of phonation, and may have potential clinical application in design and placement of synthetic implant in vocal fold surgery. The numerical solution of the airflow employs a previously developed immersed-boundary solver. However, in order to incorporate the vocal fold into the model, we have developed a new immersed-boundary method that can simulate the dynamics of the multi-layered, viscoelastic solids. In this method, a finite-difference scheme is used to approximate the derivatives and ghost cells are defined near the boundary. To impose the traction boundary condition, a third-order polynomial is obtained using the weighted least squares fitting to approximate the function locally. Like its analogue for the flow solver, this immersed-boundary method for the solids has the advantage of simple grid generation, and may be easily implemented on parallel computers. In the talk, we will present the simulation results on both the specified vocal fold motion and the flow-induced vocal fold vibration. Supported by NIDCD Grant R01 DC007125-01A1.   

Scaled-up in vitro experiments of vocal fold paralysis

Vocal fold paralysis is the inability of either one, or both vocal folds to open and close properly. Digital Particle Image Velocimetry (DPIV) measurements were taken to further understand the consequences paralyzed vocal folds have on the fluid dynamics downstream of the vocal folds during human phonation. The experiments were taken in a free-stream water tunnel using a simplified scaled-up model of human vocal folds. The Reynolds and Strouhal numbers ranged from 4500 to 10000, and 0.01 to 0.04, respectively. Various configuration setups were tested to emulate several types of vocal fold paralyses. These configurations include unilateral vocal fold immobility (UVFI), bilateral vocal fold immobility (BVFI) and the vocal folds operating at different oscillating frequencies. Data from these different conditions will be compared with an eye toward understanding the critical dynamics associated with this class of disease.   

Experimental study of the flow-induced vibration of a flexible duct

Experiments were conducted in a compliant, self-oscillating model of the glottis in a large free-surface water tunnel. The in vitro model was geometrically similar to the human vocal folds, allowing a greater understanding of fluid-solid coupling, but was not dynamically similar. The experimental measurement technique presented was developed to quantify the pertinent system parameters of the fully coupled vibration. DPIV imaging on the 2D mid-plane allowed the velocity vector field of the fluid surrounding of the model to be measured. Gradient-based image processing yields information regarding the shape and location of the structure. Characterizing the temporal variations of both these quantities is required to experimentally validate existing theories and increase our understanding of phonation. The model’s vibrational motion was shown to be periodic and asymmetric both temporally and spatially. Two separate modes of vibration were characterized using simplified measures of the model’s shape and spectral analysis. Additionally, the cyclic formation and advection of coherent vortices was shown to coincide with the model’s closure.   

Computational Analysis of Glottal Aerodynamics and Vocal Fold Vibrations during Phonation

Phonation is a complex biological phenomenon which results from a highly coupled biomechanical interaction between glottal aerodynamics and vocal fold tissue. During phonation, a self-sustained vocal fold vibration is observed and a turbulent jet is formed between the vibrating vocal folds. However, due to the complexity of human airway and nonlinearity of flow-tissue interaction, the physics of phonation is still not well understood. In this study, a high fidelity computational model is used to study this problem. Several key features are incorporated in the current study including (a) the use of modern computer graphic reconstruction technology for reconstructing the 3D human airway from CT-scan data, (b) accurate Immersed Boundary Method (IBM) for the glottal flow and (c) A three-layer finite-element anisotropic vocal fold tissue model for modeling the vocal fold vibration. Results from this study are presented with emphasis on the glottal aerodynamics and associated vocal-fold vibrations. This research is funded by NIH (NIDCD grant R01 DC007125-01A1)   

Computation of flow through a time-varying rectangular orifice

This talk will present results of a computation of the flow through a time-varying rectangular orifice.~ The geometry and motion of the walls are meant to produce a flow dynamically similar to that in the human glottis during phonation, and to experiments.~ The computations were performed using acuSolve, a 2nd-order accurate (time {\&} space), finite-element flow solver from acuSim Corporation of Mountain View, CA. Four cases are studied, corresponding to a reduced frequency of vibration ranging from 0.01 to 0.04, and a Reynolds number of 8000.~ The results are compared to experiments performed in the same geometry, Reynolds number, and reduced frequency.   

Starting vortex behavior in flow through a time-varying rectangular slit

The behavior of the starting vortex issuing from a time-varying rectangular slit with an imposed pressure gradient, representing the flow through the human glottis, is presented. The range of reduced frequency of vibration was 0.01-0.04 and the Reynolds number 8000. DPIV measurements of the velocity field on the plane of symmetry show that the starting vortex formation takes a longer fraction of the vibration period as the reduced frequency increases. The formation time and strength of the starting vortex are estimated from the velocity field measurements. In addition, the volume flow measurements allow the stroke ratio L/D to be estimated. The correlation L/D and pinch-off is also discussed.   

Vortex formation in a model glottal jet

This talk presents vortex formation timing in a model glottal jet, using DPIV measurements previously presented of the flow through a scaled-up model of the human glottis. The range of reduced frequency of vibration was 0.01-0.04 and the Reynolds number 8000. The jet issuing from the glottis is generally symmetrical and is characterized by the formation of vortex pairs along the jet. The data shows that the vortex formation time is not a strong function of reduced frequency of vibration, but that the vortex strength is. Furthermore, during periods of strong flow acceleration and deceleration, the timing is less regular.   

Flows Around Oscillating Grooved Spheroids

Fish can sense the frequency, amplitude, and direction of incident sounds using their ears, which contain a bony body (the otolith) overlaying an array of $O$(10$^{4})$ hair cells in a fluid-filled sac. The acoustically induced motion of the otolith relative to the surrounding fluid should generate flows that can be sensed by the hair cells. The otolith is typically an irregularly shaped body with a groove, the sulcus, where most of the hair cells are located. The acoustically induced flow for an otolith in a plane sound wave was modeled as a grooved spheroid oscillating sinusoidally at frequency $f$ and amplitude $s$ immersed in a viscous fluid. Experiments were performed at $Re=2\pi f\,L^2/\nu \approx 10-200$ and normalized oscillation amplitudes $\varepsilon =s/L\approx 0.05-0.2$, where the spheroid length scale $L$ is the product of the spheroid aspect ratio and its equivalent radius and $\nu $ is the fluid kinematic viscosity. Particle-image velocimetry and phase-locked pathline images of the steady streaming flow were obtained near the spheroid for grooves at different angular positions representing incident sound from different directions. More complicated sound fields were simulated by oscillating a spheroid at multiple frequencies along a single direction.