Aeroacoustic source prediction using material surfaces bounding the flow.

This article presents an extension of Liepmann's characterization of an aeroacoustic source in terms of the motion of a bounding surface containing the source region. Rather than using an arbitrary surface, we express the problem in terms of bounding material surfaces, identified by Lagrangian Coherent Structures (LCS), which demarcate flow into regions with distinct dynamics. The sound generation of the flow is written in terms of the motion of these material surfaces using the Kirchhoff integral equation, so that the flow noise problem now appears like that of a deforming body. This approach provides a natural connection between the flow topology, as revealed through LCS analysis, and sound generation mechanisms. As examples, we examine two-dimensional cases of co-rotating vortices and leap-frogging vortex pairs and compare estimated sound sources to vortex sound theory.

Phase-averaged and cycle-to-cycle analysis of jet dynamics in a scaled up vocal-fold model.

Phase-averaged and cycle-to-cycle analysis of key contributors to sound production in phonation is examined in a scaled-up vocal-fold model. Simultaneous temporally and spatially resolved pressure and velocity measurements permitted examination of each term in the streamwise integral momentum equation. The relative sizes of these terms were used to address the issue of whether transglottal pressure is a surrogate for vocal-fold drag, a quantity directly related to sound production. Further, time traces of transglottal pressure and volume flow rate provided insight into the role of cycle-to-cycle variations in voiced sound production which affect voice quality. Experiments were conducted using a 10× scaled-up model in a free-surface water tunnel. Two-dimensional vocal-fold models with semi-circular ends inside a square duct were driven with constant opening and closing speeds. The time from opening to closed, To , was half the oscillation period. Time-resolved digital particle image velocimetry (DPIV) and pressure measurements along the duct centreline were made for 3650 ≤ Re ≤ 8100 and equivalent life frequencies from 52.5 to 97.5 Hz. Results showed that transglottal pressure does serve as a surrogate for the vocal-fold drag. However, smaller but non-negligible momentum flux and inertia terms, caused by the jet and vocal-fold motions, may also contribute to vocal-fold drag. Further, cycle-to-cycle variations including jet switching and modulation are inherent in flows of this type despite their high degrees of symmetry and repeatability. The origins of these variations and their potential role in sound production and voice quality are discussed.

Vortex Formation Times in the Glottal Jet, Measured in a Scaled-Up Model.

In this paper, the timing of vortex formation on the glottal jet is studied using previously published velocity measurements of flow through a scaled-up model of the human vocal folds. The relative timing of the pulsatile glottal jet and the instability vortices are acoustically important since they determine the harmonic and broadband content of the voice signal. Glottis exit jet velocity time series were extracted from time-resolved planar DPIV measurements. These measurements were acquired at four glottal flow speeds (u SS = 16.1-38 cm/s) and four glottis open times (T o = 5.67-23.7 s), providing a Reynolds number range Re = 4100-9700 and reduced vibration frequency f* = 0.01-0.06. Exit velocity waveforms showed temporal behavior on two time scales, one that correlates to the period of vibration and another characterized by short, sharp velocity peaks (which correlate to the passage of instability vortices through the glottis exit plane). The vortex formation time, estimated by computing the time difference between subsequent peaks, was shown to be not well-correlated from one vibration cycle to the next. The principal finding is that vortex formation time depends not only on cycle phase, but varies strongly with reduced frequency of vibration. In all cases, a strong high-frequency burst of vortex motion occurs near the end of the cycle, consistent with perceptual studies using synthesized speech.

Cycle-to-cycle flow variations in a square duct with a symmetrically oscillating constriction.

Spatially and temporally resolved Digital Particle Image Velocimetry (DPIV) measurements are presented of flow complexities in a nominally two-dimensional, symmetric, duct with an oscillating constriction. The motivation for this research lies in advancing the state-of-the-art in applying integral control volume analysis to modeling unsteady internal flows. The specific target is acoustic modeling of human phonation. The integral mass and momentum equations are directly coupled to the acoustic equations and provide quantitative insight into acoustic source strengths in addition to the dynamics of the fluid-structure interactions in the glottis. In this study, a square cross-section duct was constructed with symmetric, computer controlled, oscillating constrictions that incorporate both rocking as well as oscillatory open/close motions. Experiments were run in a free-surface water tunnel over a Strouhal number range, based on maximum jet speed and model length, of 0.012 - 0.048, for a fixed Reynolds number, based on maximum gap opening and maximum jet speed, of 8000. In this study, the constriction motions were continuous with one open-close cycle immediately following another. While the model and its motions were nominally two-dimensional and symmetric, flow asymmetries and oscillation frequency dependent cycle-to-cycle variations were observed. These are examined in the context of terms in the integral conservation equations.

Aeroacoustic source characterization in a physical model of phonation.

This paper presents measurements conducted in a physical model of the adult human airway. The goals of this work are to (1) benchmark the physical model to excised larynx models in the literature and (2) empirically demonstrate the relationship between vocal fold drag and sound production. Results from the airway model are first benchmarked to published time-averaged behavior of excised larynx models. The airway model in this work exhibited higher glottal volume flow, lower glottal resistance, and less fundamental frequency variation than excised larynx models. Next, concurrent measurements of source behavior and radiated sound were compared. Unsteady transglottal pressure (a surrogate measure for vocal fold drag) and radiated sound, measured at the mouth, showed good correlation. In particular, the standard deviation and the ratio of the power of the first and second harmonics of the transglottal and mouth pressures were strongly correlated. This empirical result supports the assertion that vocal fold drag is the principal source of sound in phonation.

Modeling of slightly-compressible isentropic flows and its compressibility effects on fluid-structure interactions.

In this study, an aeroacoustic fluid model for slightly-compressible isentropic flows is developed and evaluated for its compressibility effects in the context of fluid-structure interactions. This model considers computational feasibility and accuracy by adding compressibility terms directly on the incompressible form of Navier-Stokes equation. Rather than solving for the full compressible form, our slightly-compressible form significantly reduces the complications in establishing stabilization and implementation of its finite element procedure, and yet still captures the fluctuating acoustic waves expected in the compressible form. Using this approach, we demonstrate that generations and propagations of acoustic waves can be accurately captured, without the inclusion of a fully compressible representation of the fluid. Upon the successful verification of its accuracy against analytical and known solutions, we then evaluate the fluid compressibility effect on fluid-structure interactions. Our results show that comparing to an incompressible fluid, a deformable solid generates sound waves while it is driven by the flow and vibrates in the fluid. A periodic volume change in the fluid is also observed, which can be considered as a sound source.

Formation processes for large ejecta and interactions with melt pool formation in powder bed fusion additive manufacturing.

Ejecta with a size much larger than the mean particle size of feedstock powder have been observed in powder bed fusion additive manufacturing, both during post-process sieving and embedded within built components. However, their origin has not been adequately explained. Here, we test a hypothesis on the origin of large (much larger than the mass-median-diameter of feedstock powder) ejecta-that, in part, they result from stochastic, inelastic collisions of ejecta and coalescence of partially-sintered agglomerates. The hypothesis is tested using direct observation of ejecta behavior, via high-speed imaging, to identify interactions between ejecta and consequences on melt pool formation. We show that stochastic collisions occur both between particles which are nearly-simultaneously expelled from the laser interaction zone and between particles ejected from distant locations. Ejecta are also shown to perturb melt pool geometry, which is argued to be a potential cause of lack-of-fusion flaws.

The Perfectly Matched Layer absorbing boundary for fluid-structure interactions using the Immersed Finite Element Method.

In this work, a non-reflective boundary condition, the Perfectly Matched Layer (PML) technique, is adapted and implemented in a fluid-structure interaction numerical framework to demonstrate that proper boundary conditions are not only necessary to capture correct wave propagations in a flow field, but also its interacted solid behavior and responses. While most research on the topics of the non-reflective boundary conditions are focused on fluids, little effort has been done in a fluid-structure interaction setting. In this study, the effectiveness of the PML is closely examined in both pure fluid and fluid-structure interaction settings upon incorporating the PML algorithm in a fully-coupled fluid-structure interaction framework, the Immersed Finite Element Method. The performance of the PML boundary condition is evaluated and compared to reference solutions with a variety of benchmark test cases including known and expected solutions of aeroacoustic wave propagation as well as vortex shedding and advection. The application of the PML in numerical simulations of fluid-structure interaction is then investigated to demonstrate the efficacy and necessity of such boundary treatment in order to capture the correct solid deformation and flow field without the requirement of a significantly large computational domain.

Fully-coupled aeroelastic simulation with fluid compressibility - For application to vocal fold vibration.

In this study, a fully-coupled fluid-structure interaction model is developed for studying dynamic interactions between compressible fluid and aeroelastic structures. The technique is built based on the modified Immersed Finite Element Method (mIFEM), a robust numerical technique to simulate fluid-structure interactions that has capabilities to simulate high Reynolds number flows and handles large density disparities between the fluid and the solid. For accurate assessment of this intricate dynamic process between compressible fluid, such as air and aeroelastic structures, we included in the model the fluid compressibility in an isentropic process and a solid contact model. The accuracy of the compressible fluid solver is verified by examining acoustic wave propagations in a closed and an open duct, respectively. The fully-coupled fluid-structure interaction model is then used to simulate and analyze vocal folds vibrations using compressible air interacting with vocal folds that are represented as layered viscoelastic structures. Using physiological geometric and parametric setup, we are able to obtain a self-sustained vocal fold vibration with a constant inflow pressure. Parametric studies are also performed to study the effects of lung pressure and vocal fold tissue stiffness in vocal folds vibrations. All the case studies produce expected airflow behavior and a sustained vibration, which provide verification and confidence in our future studies of realistic acoustical studies of the phonation process.

Reconstruction and morphometric analysis of the nasal airway of the white-tailed deer (Odocoileus virginianus) and implications regarding respiratory and olfactory airflow.

Compared with other mammals (e.g., primates, rodents, and carnivores), the form and function of the ungulate nasal fossa, in particular the ethmoidal region, has been largely unexplored. Hence, the nasal anatomy of the largest prey species remains far less understood than that of their predators, rendering comparisons and evolutionary context unclear. Of the previous studies of nasal anatomy, none have investigated the detailed anatomy and functional morphology of the white-tailed deer (Odocoileus virginianus), a species that is ubiquitous throughout North and Central America and in northern regions of South America. Here, nasal form and function is quantitatively investigated in an adult white-tailed deer using high-resolution magnetic resonance imaging, combined with anatomical reconstruction and morphometric analysis techniques. The cross-sectional anatomy of the airway is shown and a three-dimensional anatomical model of the convoluted nasal fossa is reconstructed from the image data. A detailed morphometric analysis is presented that includes quantitative distributions of airway size and shape (e.g., airway perimeter, cross-sectional area, surface area) and the functional implications of these data regarding respiratory and olfactory airflow are investigated. The white-tailed deer is shown to possess a long, double scroll maxilloturbinal that occupies approximately half of the length of the nasal fossa and provides a large surface area for respiratory heat and moisture exchange. The ethmoidal region contains a convoluted arrangement of folded ethmoturbinals that appear to be morphologically distinct from the single and double scroll ethmoturbinals found in most other non-primates. This complex folding provides a large surface area in the limited space available for chemical sensing, due to the expansive maxilloturbinal. Morphologically, the white-tailed deer is shown to possess a dorsal meatus that leads to an olfactory recess, a nasal architecture that has been shown in other non-primate species to cause unique nasal airflow patterns to develop during sniffing that are optimized for odorant delivery to the sensory part of the nose. Additionally, we demonstrate that, during respiration, airflow in the nasal vestibule and the anterior maxilloturbinal region may be transitional or turbulent, in which case turbulent mixing is expected to enhance respiratory heat and moisture exchange, which could be an important contribution to thermoregulation and water conservation in the white-tailed deer.

Dynamics of temporal variations in phonatory flow.

This paper addresses the dynamic relevance of time variations of phonatory airflow, commonly neglected under the quasisteady phonatory flow assumption. In contrast to previous efforts, which relied on direct measurement of glottal impedance, this work uses spatially and temporally resolved measurements of the velocity field to estimate the unsteady and convective acceleration terms in the unsteady Bernoulli equation. Theoretical considerations suggest that phonatory flow is inherently unsteady when two related conditions apply: (1) that the unsteady and convective accelerations are commensurate, and (2) that the inertia of the glottal jet is non-negligible. Acceleration waveforms, computed from experimental data, show that unsteady and convective accelerations to be the same order of magnitude, throughout the cycle, and that the jet flow contributes significantly to the unsteady acceleration. In the middle of the cycle, however, jet inertia is negligible because the convective and unsteady accelerations nearly offset one another in the jet region. These results, consistent with previous findings treating quasisteady phonatory flow, emphasize that unsteady acceleration cannot be neglected during the final stages of the phonation cycle, during which voice sound power and spectral content are largely determined. Furthermore, glottal jet dynamics must be included in any model of phonatory airflow.

Unsteady behavior of flow in a scaled-up vocal folds model.

Measurements of the fluid flow through a scaled-up model of the human glottis are presented to determine whether glottal flow may be approximated as unsteady. Time- and space-resolved velocity vector fields from digital particle image velocimetry (DPIV) measurements of the flow through the gap between two moving, rigid walls are presented in four cases, over a range of Strouhal numbers: 0.010, 0.018, 0.035, 0.040, corresponding to life-scale f(0) of 30, 58, 109, and 126 Hz, respectively, at a Reynolds number of 8000. It is observed that (1) glottal flow onset is delayed after glottal opening and (2) glottal flow shutoff occurs prior to closure. A comparison between flow through a fully open, nonmoving glottis and that through the moving vocal folds shows a marked difference in spatial structure of the glottal jet. The following features of the flow are seen to exhibit strong dependence on cycle frequency: (a) glottal exit plane velocity, (b) volume flow, (c) vortex shedding rates, and (d) vortex amplitude. Vortex shedding appears to be a factor both in controlling flow resistance and in cycle-to-cycle volume flow variations. All these observations strongly suggest that glottal flow is inherently unsteady.

Theoretical assessment of unsteady aerodynamic effects in phonation.

This paper ranks the importance of unsteady aerodynamic mechanisms in glottal flow. Particular emphasis is given to separation point motion, acceleration of glottal airflow by vocal fold motion, and viscous blockage. How nondimensional parameters such as the Reynolds, Strouhal, and Womersley numbers help in this ranking is also addressed. An equation of motion is derived which includes terms explicitly describing the effects of interest, assuming (1) a symmetrical glottis, (2) zero pressure recovery downstream of the vocal folds, and (3) a quasisteady glottal jet. Estimating the order of magnitude of the terms in this equation, it is shown that the flow is characterized by two temporal regimes: (1) a flow initiation/shutoff regime where local unsteady acceleration and wall motion dominate, and (2) a "quasisteady" regime where the flow is dominated by convective acceleration. In the latter case, separation point motion and viscous blockage are shown to be out of phase with motion of the vocal folds, thereby impacting the shape of the glottal volume flow waveform. The analysis suggests that glottal flow may be considered quasisteady only insofar as traditional assumptions concerning glottal jet behavior can be confirmed.

Aeroacoustic production of low-frequency unvoiced speech sounds.

A theoretical approach to describing unvoiced speech sound production is outlined using the essentials of aerodynamics and aeroacoustics. The focus is on the character and role of nonacoustic air motion in the vocal tract. An idealized picture of speech sound production is presented showing that speech sound production involves the dynamics of a jet flow, characterized by vorticity. A formal expression is developed for the sound production by unsteady airflow in terms of jet vorticity and vocal-tract shape, and a scaling law for the aeroacoustic source power is derived. The generic features of internal jet flows such as those exhibited in speech sound production are discussed, particularly in terms of the vorticity field, and the relevant scales of motion are identified. An approximate description of a jet as a train of vortex rings, useful for sound-field prediction, is described using the scales both of motion and of vocal-tract geometry. It is shown that the aeroacoustic source may be expressed as the convolution of (1) the acoustic source time series due to a single vortex ring with (2) a function describing the arrival of vortex rings in the source region. It is shown that, in general, the characteristics of the aeroacoustic source are determined not only by the strength, spatial distribution, and convection speed of the jet vorticity field, but also the shape of the vocal tract through which the jet flow passes. For turbulent jets, such as those which occur in unvoiced sound production, however, vocal-tract shape is the dominant factor in determining the spectral content of the source.