Sound generation mechanisms in a collapsible tube.

Collapsible tubes can be employed to study the sound generation mechanism in the human respiratory system. The goals of this work are (a) to determine the airflow characteristics connected to three different collapse states of a physiological tube and (b) to find a relation between the sound power radiated by the tube and its collapse state. The methodology is based on the implementation of computational fluid dynamics simulation on experimentally validated geometries. The flow is characterized by a radical change of behavior before and after the contact of the lumen. The maximum of the sound power radiated corresponds to the post-buckling configuration. The idea of an acoustic tube law is proposed. The presented results are relevant to the study of self-excited oscillations and wheezing sounds in the lungs.

The Effect of an Increasing Subglottal Stenosis Constriction That Extends From the Vocal Folds to the Inferior Border of the Cricoid Cartilage.

Acquired subglottal stenosis is an unpredicted complication that can occur in some patients who have undergone prolonged endotracheal intubation. It is a narrowing of the airway at the level of the cricoid cartilage that can restrict airflow and cause breathing difficulty. Stenosis is typically treated with endoscopic airway dilation, with some patients experiencing multiple recurrences. The study highlights the potential of computational fluid dynamics as a noninvasive method for monitoring subglottic stenosis, which can aid in early diagnosis and surgical planning. An anatomically accurate human laryngeal airway model was constructed from computerized tomography (CT) scans. The subglottis cross-sectional area was narrowed systematically using ≈10% decrements. A quadratic profile was used to interpolate the transformation of the airway geometry from its modified shape to the baseline geometry. The numerical results were validated by static pressure measurements conducted in a physical model. The results show that airway resistance follows a squared ratio that is inversely proportional to the size of the subglottal opening (R∝A-2). The study found that critical constriction occurs in the subglottal region at 70% stenosis (upper end of grade 2). Moreover, removing airway tissue below 40% stenosis during surgical intervention does not significantly decrease airway resistance.

On the Alignment of Acoustic and Coupled Mechanic-Acoustic Eigenmodes in Phonation by Supraglottal Duct Variations.

Sound generation in human phonation and the underlying fluid-structure-acoustic interaction that describes the sound production mechanism are not fully understood. A previous experimental study, with a silicone made vocal fold model connected to a straight vocal tract pipe of fixed length, showed that vibroacoustic coupling can cause a deviation in the vocal fold vibration frequency. This occurred when the fundamental frequency of the vocal fold motion was close to the lowest acoustic resonance frequency of the pipe. What is not fully understood is how the vibroacoustic coupling is influenced by a varying vocal tract length. Presuming that this effect is a pure coupling of the acoustical effects, a numerical simulation model is established based on the computation of the mechanical-acoustic eigenvalue. With varying pipe lengths, the lowest acoustic resonance frequency was adjusted in the experiments and so in the simulation setup. In doing so, the evolution of the vocal folds' coupled eigenvalues and eigenmodes is investigated, which confirms the experimental findings. Finally, it was shown that for normal phonation conditions, the mechanical mode is the most efficient vibration pattern whenever the acoustic resonance of the pipe (lowest formant) is far away from the vocal folds' vibration frequency. Whenever the lowest formant is slightly lower than the mechanical vocal fold eigenfrequency, the coupled vocal fold motion pattern at the formant frequency dominates.

Blood Speckle Imaging: An Emerging Method for Perioperative Evaluation of Subaortic and Aortic Valvar Repair.

BACKGROUND: This article presents the use of blood speckle Imaging (BSI) as an echocardiographic approach for the pre- and post-operative evaluation of subaortic membrane resection and aortic valve repair. METHOD: BSI, employing block-matching algorithms, provided detailed visualization of flow patterns and quantification of parameters from ultrasound data. The 9-year-old patient underwent subaortic membrane resection and peeling extensions of the membrane from under the ventricular-facing surface of all three aortic valve leaflets. RESULT: Post-operatively, BSI demonstrated improvements in hemodynamic patterns, where quantified changes in flow velocities showed no signs of stenosis and trivial regurgitation. The asymmetric jet with a shear layer and flow reversal on the posterior aspect of the aorta was corrected resulting in reduced wall shear stress on the anterior aspect and reduced oscillatory shear index, which is considered a contributing element in cellular alterations in the structure of the aortic wall. CONCLUSION: This proof-of-concept study demonstrates the potential of BSI as an emerging echocardiographic approach for evaluating subaortic and aortic valvar repair. BSI enhances the quantitative evaluation of the left ventricular outflow tract of immediate surgical outcomes beyond traditional echocardiographic parameters and aids in post-operative decision-making. However, larger studies are needed to validate these findings and establish standardized protocols for clinical implementation.

Machine Learning-Based Segmentation of the Thoracic Aorta with Congenital Valve Disease Using MRI.

Subjects with bicuspid aortic valves (BAV) are at risk of developing valve dysfunction and need regular clinical imaging surveillance. Management of BAV involves manual and time-consuming segmentation of the aorta for assessing left ventricular function, jet velocity, gradient, shear stress, and valve area with aortic valve stenosis. This paper aims to employ machine learning-based (ML) segmentation as a potential for improved BAV assessment and reducing manual bias. The focus is on quantifying the relationship between valve morphology and vortical structures, and analyzing how valve morphology influences the aorta's susceptibility to shear stress that may lead to valve incompetence. The ML-based segmentation that is employed is trained on whole-body Computed Tomography (CT). Magnetic Resonance Imaging (MRI) is acquired from six subjects, three with tricuspid aortic valves (TAV) and three functionally BAV, with right-left leaflet fusion. These are used for segmentation of the cardiovascular system and delineation of four-dimensional phase-contrast magnetic resonance imaging (4D-PCMRI) for quantification of vortical structures and wall shear stress. The ML-based segmentation model exhibits a high Dice score (0.86) for the heart organ, indicating a robust segmentation. However, the Dice score for the thoracic aorta is comparatively poor (0.72). It is found that wall shear stress is predominantly symmetric in TAVs. BAVs exhibit highly asymmetric wall shear stress, with the region opposite the fused coronary leaflets experiencing elevated tangential wall shear stress. This is due to the higher tangential velocity explained by helical flow, proximally of the sinutubal junction of the ascending aorta. ML-based segmentation not only reduces the runtime of assessing the hemodynamic effectiveness, but also identifies the significance of the tangential wall shear stress in addition to the axial wall shear stress that may lead to the progression of valve incompetence in BAVs, which could guide potential adjustments in surgical interventions.

Impact of Variation in Commissural Angle between Fused Leaflets in the Functionally Bicuspid Aortic Valve on Hemodynamics and Tissue Biomechanics.

In subjects with functionally bicuspid aortic valves (BAVs) with fusion between the coronary leaflets, there is a natural variation of the commissural angle. What is not fully understood is how this variation influences the hemodynamics and tissue biomechanics. These variables may influence valvar durability and function, both in the native valve and following repair, and influence ongoing aortic dilation. A 3D aortic valvar model was reconstructed from a patient with a normal trileaflet aortic valve using cardiac magnetic resonance (CMR) imaging. Fluid-structure interaction (FSI) simulations were used to compare the effects of the varying commissural angles between the non-coronary with its adjacent coronary leaflet. The results showed that the BAV with very asymmetric commissures (120∘ degree commissural angle) reduces the aortic opening area during peak systole and with a jet that impacts on the right posterior wall proximally of the ascending aorta, giving rise to elevated wall shear stress. This manifests in a shear layer with a retrograde flow and strong swirling towards the fused leaflet side. In contrast, a more symmetrical commissural angle (180∘ degree commissural angle) reduces the jet impact on the posterior wall and leads to a linear decrease in stress and strain levels in the non-fused non-coronary leaflet. These findings highlight the importance of considering the commissural angle in the progression of aortic valvar stenosis, the regional distribution of stresses and strain levels experienced by the leaflets which may predispose to valvar deterioration, and progression in thoracic aortic dilation in patients with functionally bicuspid aortic valves. Understanding the hemodynamics and biomechanics of the functionally bicuspid aortic valve and its variation in structure may provide insight into predicting the risk of aortic valve dysfunction and thoracic aortic dilation, which could inform clinical decision making and potentially lead to improved aortic valvar surgical outcomes.

Influence of aortic valve morphology on vortical structures and wall shear stress.

The aim of this paper is to assess the association between valve morphology and vortical structures quantitatively and to highlight the influence of valve morphology/orientation on aorta's susceptibility to shear stress, both proximal and distal. Four-dimensional phase-contrast magnetic resonance imaging (4D PCMRI) data of 6 subjects, 3 with tricuspid aortic valve (TAV) and 3 with functionally bicuspid aortic values (BAV) with right-left coronary leaflet fusion, were processed and analyzed for vorticity and wall shear stress trends. Computational fluid dynamics (CFD) has been used with moving TAV and BAV valve designs in patient-specific aortae to compare with in vivo shear stress data. Vorticity from 4D PCMRI data about the aortic centerline demonstrated that TAVs had a higher number of vortical flow structures than BAVs at peak systole. Coalescing of flow structures was shown to be possible in the arch region of all subjects. Wall shear stress (WSS) distribution from CFD results at the aortic root is predominantly symmetric for TAVs but highly asymmetric for BAVs with the region opposite the raphe (fusion location of underdeveloped leaflets) being subjected to higher WSS. Asymmetry in the size and number of leaflets in BAVs and TAVs significantly influence vortical structures and WSS in the proximal aorta for all valve types and distal aorta for certain valve orientations of BAV. Analysis of vortical structures using 4D PCMRI data (on the left side) and wall shear stress data using CFD (on the right side).

Fluid-Structure Interaction Analysis of Aerodynamic and Elasticity Forces During Vocal Fold Vibration.

The effect of the intraglottal vortices on the glottal flow waveform was explored using flow-structure-interaction (FSI) modeling. These vortices form near the superior aspect of the vocal folds during the closing phase of the folds' vibration. The geometry of the vocal fold was based on the well-known M5 model. The model did not include a vocal tract to remove its inertance effect on the glottal flow. Material properties for the cover and body layers of the folds were set using curve fit to experimental data of tissue elasticity. A commercially available FSI solver was used to perform simulations at low and high values of subglottal input pressure. Validation of the FSI results showed a good agreement for the glottal flow and the vocal fold displacement data with measurements taken in the excised canine larynx model. The simulations result further support the hypothesis that intraglottal vortices can affect the glottal flow waveform, specifically its maximum flow declination rate (MFDR). It showed that MFDR occurs at the same phase when the highest intraglottal vortical strength and the negative pressure occur. It also showed that when MFDR occurs, the magnitude of the aerodynamic force acting on the glottal wall is greater than the elastic recoil force predicted in the tissue. These findings are significant because nearly all theoretical and computational models that study the vocal fold vibrations mechanism do not consider the intraglottal negative pressure caused by the vortices as an additional closing force acting on the folds.

Computational Modeling of Nasal Drug Delivery Using Different Intranasal Corticosteroid Sprays for the Treatment of Eustachian Tube Dysfunction.

Eustachian tube dysfunction (ETD) is a common otolaryngologic condition associated with decreased quality of life. The first-line treatment of ETD is intranasal corticosteroid sprays (INCS). Computational fluid dynamics (CFD) was used to study particle deposition on the Eustachian tube (ET) using two commercial INCS (Flonase and Sensimist). Simulations also considered the effects of nostril side, insertion depth, insertion angle, cone spray angle, inhaling rates, wall impingement treatment, and fluid film. Flonase and Sensimist produced different particle size distributions and sizes. Sensimist droplets are smaller, less sensitive to asymmetry in nostrils anatomy and variation in insertion angle, and therefore can reach the posterior nasopharynx more readily. Flonase produces larger particles with greater inertia. Its particles deposition is more sensitive to intrasubject variation in nasal anatomy and insertion angles. The particle deposition on the ET was sensitive to the wall impingement model. The deposition on the ET was insignificant with adherence only <0.15% but increased up to 1-4% when including additional outcomes rebound and splash effects when droplets impact with the wall. The dose redistribution with the fluid film is significant but plays a secondary effect on the ET deposition. Flonase aligned parallel with the hard palate produced 4% deposition efficiency on the ET, but this decreased <0.14% at the higher insertion angle. INCS with larger droplet sizes with a small insertion angle may be more effective at targeting droplet deposition on the ET opening.

Change in aeroacoustic sound mechanism during sibilant sound with different velopharyngeal opening sizes.

The velopharyngeal valve regulates the opening between the nasal and oral cavities. The lack of complete closure is especially problematic in speech because inappropriate leakage of airflow and/or sound into the nasal cavity causes abnormal sound production and increased nasality. The purpose of this study is to use the large eddy simulation approach to examine changes in sound source mechanisms as the size of the opening changes during the production of a sibilant sound. The baseline geometry of the model is based on the pharyngeal airway of a subject having a small velopharyngeal opening while sustaining a sibilant sound. Modifications to the model are done by systematically widening or narrowing the opening (all else being equal). Results show that acoustic energy in the nasal cavity is directly related to the size of the velopharyngeal opening and that there is a critical size where the magnitude of Lighthill's acoustics source in the nasal cavity is maximized. The far-field acoustic energy and its correlation with the sound source mechanisms are also dependent on the size of the velopharyngeal opening. Patient-specific geometry with a velopharyngeal opening during a normal sibilant /s/ sound is shown to the left. Lighthill's acoustic source term is displayed on the right and varies depending on the size of the velopharyngeal opening.

Hemodynamics and tissue biomechanics of the thoracic aorta with a trileaflet aortic valve at different phases of valve opening.

In a normal cardiac cycle, the trileaflet aortic valve opening is progressive, which correlates with the phasic blood flow. Therefore, we aimed to determine the impact of including an anatomically accurate reconstructed trileaflet aortic valve within a fluid-structure interaction (FSI) simulation model and determine the cyclical hemodynamic forces imposed on the thoracic aortic walls from aortic valve opening to closure. A pediatric patient with a normal trileaflet valve was recruited. Using the Cardiac Magnetic Resonance Data (CMR), a 3D model of the aortic valve and thoracic aorta was reconstructed. FSI simulations were employed to assess the tissue stress during a cardiac cycle as the result of changes in the valve opening. The blood flow was simulated as a mixture of blood plasma and red blood cells to account for non-Newtonian effects. The computation was validated with phase-contrast CMR. Windkessel boundary conditions were employed to ensure physiological pressures during the cardiac cycle. The leaflets' dynamic motion during the cardiac cycle was defined with an analytic grid velocity function. At the beginning of the valve opening a thin jet is developing. From mid-open towards full opening the stress level increases where the jet impinges the convex wall. At peak systole two counter-rotating Dean-like vortex cores manifest in the ascending aorta, which correlates with increased integrated mean stress levels. An accurate trileaflet aortic valve is needed for capturing of both primary and secondary flow features that impact the forces on the thoracic aorta wall. Omitting the aortic valve underestimates the biomechanical response.

Effects of velopharyngeal openings on flow characteristics of nasal emission.

Nasal emission is a speech disorder where undesired airflow enters the nasal cavity during speech due to inadequate closure of the velopharyngeal valve. Nasal emission is typically inaudible with large velopharyngeal openings and very distorting with small openings. This study aims to understand how flow characteristics in the nasal cavity change as a function of velopharyngeal opening using computational fluid dynamics. The model is based on a subject who was diagnosed with distorting nasal emission and a small velopharyngeal opening. The baseline geometry was delineated from CT scans that were taken, while the subject was sustaining a sibilant sound. Modifications to the model were done by systematically widening or narrowing the velopharyngeal opening while keeping the geometry constant elsewhere. Results show that if the flow resistance across the velopharyngeal valve is smaller than resistance across the oral constriction, flow characteristics such as velocity and turbulence are inversely proportional to the size of the opening. If flow resistance is higher across the velopharyngeal valve than the oral constriction, turbulence in the nasal cavity will be reduced at a higher rate. These findings can be used to generalize that the area ratio of the velopharyngeal opening to the oral constriction is a factor that determines airflow characteristics and subsequently its sound during production of sibilant sound. It implies that the highest level of turbulence in the nasal cavity, and subsequently the sound that will likely be perceived as the most severe nasal emission is produced when the size of openings is equal.

Effects of Normal Variation in the Rotational Position of the Aortic Root on Hemodynamics and Tissue Biomechanics of the Thoracic Aorta.

PURPOSE: Variation in the rotational position of the aortic root relative to the left ventricle is present in normal trileaflet aortic valves. Its impact on the resulting fluid mechanics of blood flow in the thoracic aorta and structural mechanics in the aortic wall are unknown. We aimed to determine the regional hemodynamic and biomechanical differences in different rotational positions of the normal aortic root (clockwise, central, and counterclockwise positions). METHOD: Cardiac magnetic resonance imaging (CMR) data was acquired from a normal pediatric patient. These were used for reconstruction of the aortic valve and thoracic aorta 3D model. Fluid-structure interaction (FSI) simulations were employed to study the influence of the root rotation with a central position as compared to observed extreme variations. Patient-specific phase-encoding CMR data were used to assess the validity of computed blood flow. The 3D FSI model was coupled with Windkessel boundary conditions that were tuned for physiological pressures. A grid velocity function was adopted for the valve motion during the systolic period. RESULTS: The largest wall shear stress level is detected in the clockwise positioned aortic root at the sinutubular junction. Two counter-rotating vortex cores are formed within the aortic root of both the central and extreme root configurations, however, in the clockwise root the vortex system becomes more symmetric. This also coincides with more entrainment of the valve jet and more turbulence production along the shear layer. CONCLUSION: A clockwise rotational position of the aortic root imparts an increased wall shear stress at the sinutubular junction and proximal ascending aorta in comparison to other root rotation positions. This may pose increased risk for dilation of the sinutubular junction and ascending aorta in the patient with a clockwise positioned aortic root compared to other normal positional configurations.

Pharyngeal flow simulations during sibilant sound in a patient-specific model with velopharyngeal insufficiency.

Dysfunction of the velopharyngeal valve in the human airway causes speech disorders because there is no separation between the oral and nasal cavities during normal oral speech. The speech literature hypothesizes that undesired sound is formed by turbulent flow in the nasal cavity in cases of small velopharyngeal openings. The aim is to determine the flow behavior and the sound-generating mechanism in the vocal tract using computational fluid dynamics in two patient-specific models with small and large velopharyngeal openings and contrast it with cases of complete velopharyngeal closure. The geometry for the models was reconstructed from computed tomography scans that were taken while the patients were sustaining a sibilant sound. The results for the turbulence are correlated with the broadband acoustic models of Proudman and Curle. The models show that turbulence in the vocal tract increases downstream of a constriction and that sound may be generated from it. Furthermore, most of the sound due to turbulence in the nasal cavity is governed by a dipole source where turbulence interacts with the nasal cavity walls. The generated sound power by turbulence itself in the nasal cavity (the quadrupole source) is two orders of magnitude less than the dipole source.

Sound production mechanisms of audible nasal emission during the sibilant /s/.

Audible nasal emission is a speech disorder that involves undesired sound generated by airflow into the nasal cavity during production of oral sounds. This disorder is associated with small-to-medium sized velopharyngeal openings. These openings induce turbulence in the nasal cavity, which in turn produces sound. The purpose of this study is to examine the aeroacoustic mechanisms that generate turbulent sound during production of a sibilant /s/ with and without a small opening of the velopharyngeal valve. The models are based on two pediatric subjects who were diagnosed with severe audible nasal emission. The geometries were delineated from computed tomography scans taken while the subjects were sustaining a sibilant sound. Large eddy simulation with the Ffowcs Williams and Hawkings analogy was used to predict the flow behavior and its acoustic characterization. It shows that the majority of the acoustic energy is produced by surface loading, which is related to dipole sources that resonate in the nasal cavity. The quadrupole source term that is associated with the unsteady shear layers is seen to be less significant. It also shows that closure of the velopharyngeal valve changes the far-field spectrum significantly because aeroacoustic mechanisms in the nasal cavity are eliminated.

Generation Mechanisms of Rotating Stall and Surge in Centrifugal Compressors.

Flow instabilities such as Rotating Stall and Surge limit the operating range of centrifugal compressors at low mass-flow rates. Employing compressible Large Eddy Simulations (LES), their generation mechanisms are exposed. Toward low mass-flow rate operating conditions, flow reversal over the blade tips (generated by the back pressure) causes an inflection point of the inlet flow profile. There, a shear-layer induces vortical structures circulating at the compressor inlet. Traces of these flow structures are observed until far downstream in the radial diffuser. The tip leakage flow exhibits angular momentum imparted by the impeller, which deteriorates the incidence angles at the blade tips through an over imposed swirling component to the incoming flow. We show that the impeller is incapable to maintain constant efficiency at surge operating conditions due to the extreme alteration of the incidence angle. This induces unsteady flow momentum transfer downstream, which is reflected as compression wave at the compressor outlet traveling toward the impeller. There, the pressure oscillations govern the tip leakage flow and hence, the incidence angles at the impeller. When these individual self-exited processes occurs in-phase, a surge limit-cycle establishes.