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.

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.

Time-Dependent Fluid-Structure Interaction Simulations of a Simplified Human Soft Palate.

Obstructive Sleep Apnea Syndrome (OSAS) is a common sleep-related disorder. It is characterized by recurrent partial or total collapse of pharyngeal upper airway accompanied by induced vibrations of the soft tissues (e.g., soft palate). The knowledge of the tissue behavior subject to a particular airflow is relevant for realistic clinic applications. However, in-vivo measurements are usually impractical. The goal of the present study is to develop a 3D fluid-structure interaction model for the human uvulopalatal system relevant to OSA based on simplified geometries under physiological conditions. Numerical simulations are performed to assess the influence of the different breathing conditions on the vibrational dynamics of the flexible structure. Meanwhile, the fluid patterns are investigated for the coupled fluid-structure system as well. Increasing the respiratory flow rate is shown to induce larger structural deformation. Vortex shedding induced resonance is not observed due to the large discrepancy between the flow oscillatory frequency and the natural frequency of the structure. The large deformation for symmetric breathing case under intensive respiration is mainly because of the positive feedback from the pressure differences on the top and the bottom surfaces of the structure.

Buckling critical pressures in collapsible tubes relevant for biomedical flows.

The behaviour of collapsed or stenotic vessels in the human body can be studied by means of simplified geometries like a collapsible tube. The objective of this work is to determine the value of the buckling critical pressure of a collapsible tube by employing Landau's theory of phase transition. The methodology is based on the implementation of an experimentally validated 3D numerical model of a collapsible tube. The buckling critical pressure is estimated for different values of geometric parameters of the system by treating the relation between the intramural pressure and the area of the central cross-section as the order parameter function of the system. The results show the dependence of the buckling critical pressures on the geometric parameters of a collapsible tube. General non-dimensional equations for the buckling critical pressures are derived. The advantage of this method is that it does not require any geometric assumption, but it is solely based on the observation that the buckling of a collapsible tube can be treated as a second-order phase transition. The investigated geometric and elastic parameters are sensible for biomedical application, with particular interest to the study of the bronchial tree under pathophysiological conditions like asthma.

Analysis of the aerodynamic sound of speech through static vocal tract models of various glottal shapes.

The acoustic spectrum of our voice can be divided into harmonic and inharmonic sound components. While the harmonic components, generated by the oscillatory motion of the vocal folds, are well described by reduced-order speech models, the accurate computation of the inharmonic components requires high-order flow simulations, which predict the vortex shedding and turbulent structures present in the shear layers of the glottal jet. This study characterizes the dominant frequencies in the unsteady flow of the intra- and supraglottal region. A realistic vocal tract geometry obtained through magnetic resonance imaging (MRI) is applied for the numerical domain, which is locally modified to account for different convergent and divergent glottal angles. Both time-averaged and fluctuating values of the flow variables are computed and their distribution at various glottal shapes is compared. The impact of the registered modes in the unsteady flow on the acoustic far field is computed through direct compressible flow simulations. Furthermore, acoustic analogies are applied to localize the sources of the aerodynamically generated sound.

Compressible flow simulations of voiced speech using rigid vocal tract geometries acquired by MRI.

Voiced speech consists mainly of the source signal that is frequency weighted by the acoustic filtering of the upper airways and vortex-induced sound through perturbation in the flow field. This study investigates the flow instabilities leading to vortex shedding and the importance of coherent structures in the supraglottal region downstream of the vocal folds for the far-field sound signal. Large eddy simulations of the compressible airflow through the glottal constriction are performed in realistic geometries obtained from three-dimensional magnetic resonance imaging data. Intermittent flow separation through the glottis is shown to introduce unsteady surface pressure through impingement of vortices. Additionally, dominant flow instabilities develop in the shear layer associated with the glottal jet. The aerodynamic perturbations in the near field and the acoustic signal in the far field are examined by means of spatial and temporal Fourier analysis. Furthermore, the acoustic sources due to the unsteady supraglottal flow are identified with the aid of surface spectra, and critical regions of amplification of the dominant frequencies of the investigated vowel geometries are identified.

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.

Computational study of false vocal folds effects on unsteady airflows through static models of the human larynx.

Compressible large eddy simulation is employed to numerically investigate the laryngeal flow. Symmetric static models of the human larynx with a divergent glottis are considered, with the presence of false vocal folds (FVFs). The compressible study agrees well with that of the incompressible study. Due to the high enough Reynolds number, the flow is unsteady and develops asymmetric states downstream of the glottis. The glottal jet curvature decreases with the presence of FVFs or the ventricular folds. The gap between the FVFs stretches the flow structure and reduces the jet curvature. The presence of FVFs has a significant effect on the laryngeal flow resistance. The intra-glottal vortex structures are formed on the divergent wall of the glottis, immediately downstream of the separation point. The vortices are then convected downstream and characterized by a significant negative static pressure. The FVFs are a main factor in the generation of stronger vortices, and thus on the closure of the TVFs. The direct link between the FVFs geometry and the motion of the TVFs, and by extension to the voice production, is of interest for medical applications as well as future research works. The presence of the FVFs also changes the dominant frequencies in the velocity and pressure spectra.

Planning human upper airway surgery using computational fluid dynamics.

The study advances the idea of using computational fluid dynamics in the process of planning surgical treatment modalities for patients with obstructive airway disorders. It is hypothesized that the a priori knowledge of the functional outcome of surgical intervention on the flow and airway resistance can guide the surgeon in choosing an effective surgical strategy. Computed tomography images spanning the respiratory tract of an adult patient with a combined glottic and subglottic stenosis are used to reconstruct three-dimensional geometrical models of the airway. Computational fluid dynamics is used to obtain airway flow patterns during inspiration and expiration in these models. Numerical predictions about flow velocity, pressure distribution on the airway lumen, wall shear stress, and airway resistance are obtained so that the relevance of each individual stenotic level is quantified. Four different virtual surgeries in different combinations are assessed in order to remedy the constricted airway. The virtual surgery based airway models are evaluated by comparisons with the pre-treatment flow modeling results. The predicted numerical data revealed that the removal of the constriction at the level of the vocal folds will have the most significant effect on the airway resistance. The flow simulations offer a quantitative method of evaluating the airway resistance in patients with combined glottic and subglottic stenoses. Predictions of airway resistances and other numerical calculations from different virtual surgeries give additional inputs for the surgeon, in deciding the most appropriate surgery on a case-by-case basis.

Patterns in pharyngeal airflow associated with sleep-disordered breathing.

OBJECTIVE: To establish the feasibility of a noninvasive method to identify pharyngeal airflow characteristics in sleep-disordered breathing. METHODS: Four patients with sleep-disordered breathing who underwent surgery or used positive airway pressure devices and four normal healthy controls were studied. Three-dimensional CT imaging and computational fluid dynamics modeling with standard steady-state numerical formulation were used to characterize pharyngeal airflow behavior in normals and pre-and post-treatment in patients. Dynamic flow simulations using an unsteady approach were performed in one patient. RESULTS: The pre-treatment pharyngeal airway below the minimum cross-sectional area obstruction site showed airflow separation. This generated recirculation airflow regions and enhanced turbulence zones where vortices developed. This interaction induced large fluctuations in airflow variables and increased aerodynamic forces acting on the pharyngeal wall. At post-treatment, for the same volumetric flow rate, airflow field instabilities vanished and airflow characteristics improved. Mean maximum airflow velocity during inspiration reduced from 18.3±5.7 m/s pre-treatment to 6.3±4.5 m/s post-treatment (P=0.002), leading to a reduction in maximum wall shear stress from 4.8±1.7 Pa pre-treatment to 0.9±1.0 Pa post-treatment (P=0.01). The airway resistance improved from 4.3±1.4 Pa/L/min at pre-treatment to 0.7±0.7 Pa/L/min at post-treatment (P=0.004). Post-treatment airflow characteristics were not different from normal controls (all P ≥ 0.39). CONCLUSION: This study demonstrates that pharyngeal airflow variables may be derived from CT imaging and computational fluid dynamics modeling, resulting in high quality visualizations of airflow characteristics of axial velocity, static pressure, and wall shear stress in sleep-disordered breathing.

Large eddy simulation of the pharyngeal airflow associated with obstructive sleep apnea syndrome at pre and post-surgical treatment.

Obstructive Sleep Apnea Syndrome (OSAS) is the most common sleep-disordered breathing medical condition and a potentially life-threatening affliction. Not all the surgical or non-surgical OSAS therapies are successful for each patient, also in part because the primary factors involved in the etiology of this disorder are not completely understood. Thus, there is a need for improving both diagnostic and treatment modalities associated with OSAS. A verified and validated (in terms of mean velocity and pressure fields) Large Eddy Simulation approach is used to characterize the abnormal pharyngeal airflow associated with severe OSAS and its interaction with the airway wall in a subject who underwent surgical treatment. The analysis of the unsteady flow at pre- and post-treatment is used to illustrate the airflow dynamics in the airway associated with OSAS and to reveal as well, the changes in the flow variables after the treatment. At pre-treatment, large airflow velocity and wall shear stress values were found at the obstruction site in all cases. Downstream of obstruction, flow separation generated flow recirculation regions and enhanced the turbulence production in the jet-like shear layers. The interaction between the generated vortical structures and the pharyngeal airway wall induced large fluctuations in the pressure forces acting on the pharyngeal wall. After the surgery, the flow field instabilities vanished and both airway resistance and wall shear stress values were significantly reduced.

Unsteady laryngeal airflow simulations of the intra-glottal vortical structures.

The intra-glottal vortical structures developed in a static divergent glottis with continuous flow entering the glottis are characterized. Laryngeal airflow calculations are performed using the Large Eddy Simulation approach. It has been shown that intra-glottal vortices are formed on the divergent wall of the glottis, immediately downstream of the separation point. Even with non-pulsatile flow entering the glottis, the vortices are intermittently shed, producing unsteady flow at the glottal exit. The vortical structures are characterized by significant negative static pressure relative to the ambient pressure. These vortices increase in size and strength as they are convected downstream by the flow due to the entrained air from the supra-glottal region. The negative static pressures associated with the intra-glottal vortical structures suggest that the closing phase during phonation may be accelerated by such vortices. The intra-glottal negative pressures can affect both vocal fold vibration and voice production.

Influence of gender on pharyngeal airway length in obese adolescents.

OBJECTIVES: Although pharyngeal airway length has been implicated in an increased male predisposition for obstructive sleep apnea (OSA) in adults, data in obese children and adolescents are lacking. Our objective was to determine the influence of gender on pharyngeal airway length in obese adolescents, and to apply computational simulations to better understand the effect of pharyngeal airway length on the airway's predisposition to collapse in this select group. METHODS: Obese subjects without OSA were recruited from our Sleep Center. Their pharyngeal airway length was measured on midline sagittal magnetic resonance images as the distance between the hard palate and the base of the epiglottis. Computational fluid dynamics analysis was used to study the effect of pharyngeal airway length on airflow characteristics. The gender groups were compared for anthropometric measurements and pharyngeal airway length by an unpaired Student's t-test. RESULTS: Our study group included 18 female and 16 male obese adolescents with a mean (+/-SD) age of 14.7 +/- 2.3 years and a mean body mass index of 38.9 +/- 6.9 kg/m2. The groups did not differ in age, body weight, or normalized pharyngeal airway length (0.44 +/- 0.08 mm/cm in girls versus 0.44 +/- 0.11 mm/cm in boys; p = 0.9). The computational fluid dynamics simulation indicated that the 3-dimensional flow field and airway wall pressures were not significantly affected by pharyngeal airway lengthening of up to 10 mm. CONCLUSIONS: Our results indicate that in obese adolescents, there is no influence of gender on pharyngeal airway length, and pharyngeal airway length alone does not significantly affect the airway's predisposition to collapse. These findings suggest that pharyngeal airway length may not explain the increased male gender predisposition for OSA in this select group.

Validation of computational fluid dynamics methodology used for human upper airway flow simulations.

An anatomically accurate human upper airway model was constructed from multiple magnetic resonance imaging axial scans. This model was used to conduct detailed Computational Fluid Dynamics (CFD) simulations during expiration, to investigate the fluid flow in the airway regions where obstruction could occur. An identical physical model of the same airway was built using stereo lithography. Pressure and velocity measurements were conducted in the physical model. Both simulations and experiments were performed at a peak expiratory flow rate of 200 L/min. Several different numerical approaches within the FLUENT commercial software framework were used in the simulations; unsteady Large Eddy Simulation (LES), steady Reynolds-Averaged Navier-Stokes (RANS) with two-equation turbulence models (i.e. k-epsilon, standard k-omega, and k-omega Shear Stress Transport (SST)) and with one-equation Spalart-Allmaras model. The CFD predictions of the average wall static pressures at different locations along the airway wall were favorably compared with the experimental data. Among all the approaches, standard k-omega turbulence model resulted in the best agreement with the static pressure measurements, with an average error of approximately 20% over all ports. The highest positive pressures were observed in the retroglossal regions below the epiglottis, while the lowest negative pressures were recorded in the retropalatal region. The latter is a result of the airflow acceleration in the narrow retropalatal region. The largest pressure drop was observed at the tip of the soft palate. This location has the smallest cross section of the airway. The good agreement between the computations and the experimental results suggest that CFD simulations can be used to accurately compute aerodynamic flow characteristics of the upper airway.

Modeling flow in a compromised pediatric airway breathing air and heliox.

OBJECTIVES/HYPOTHESIS: The aim of this study was to perform computer simulations of flow within an accurate model of a pediatric airway with subglottic stenosis. It is believed that the airflow characteristics in a stenotic airway are strongly related to the sensation of dyspnea. METHODS: Computed tomography images through the respiratory tract of an infant with subglottic stenosis were used to construct the three-dimensional geometry of the airway. By using computational fluid dynamics (CFD) modeling to capture airway flow patterns during inspiration and expiration, we obtained information pertaining to flow velocity, static airway wall pressure, pressure drop across the stenosis, and wall shear stress. These simulations were performed with both air and heliox (helium-oxygen mixture). RESULTS: Unlike air, heliox maintained laminar flow through the stenosis. The calculated pressure drop over stenosis was lower for the heliox flow in contrast to the airflow case. This led to an approximately 40% decrease in airway resistance when using heliox and presumably causes a decrease in the level of effort required for breathing. CONCLUSIONS: CFD simulations offer a quantitative method of evaluating airway flow dynamics in patients with airway abnormalities. CFD modeling illustrated the flow features and quantified flow parameters within a pediatric airway with subglottic stenosis. Simulations with air and heliox conditions mirrored the known clinical benefits of heliox compared with air. We anticipate that computer simulation models will ultimately allow a better understanding of changes in flow caused by specific medical and surgical interventions in patients with conditions associated with dyspnea.

Modeling flow in a compromised pediatric airway breathing air and heliox.

OBJECTIVES/HYPOTHESIS: The aim of this study was to perform computer simulations of flow within an accurate model of a pediatric airway with subglottic stenosis. It is believed that the airflow characteristics in a stenotic airway are strongly related to the sensation of dyspnea. METHODOLOGY: Computed tomography images through the respiratory tract of an infant with subglottic stenosis, were used to construct the three-dimensional geometry of the airway. By using computational fluid dynamics (CFD) modeling to capture airway flow patterns during inspiration and expiration, we obtained information pertaining to flow velocity, static airway wall pressure, pressure drop across the stenosis, and wall shear stress. These simulations were performed with both air and heliox. RESULTS: Unlike air, heliox maintained laminar flow through the stenosis. The calculated pressure drop over stenosis was lower for the heliox flow, in contrast to the airflow case. This lead to an approximately 40% decrease in airway resistance when using heliox, and presumably causes a decrease in the level of effort required for breathing. CONCLUSIONS: CFD simulations offer a quantitative method of evaluating airway flow dynamics in patients with airway abnormalities. CFD modeling illustrated the flow features and quantified flow parameters within a pediatric airway with subglottic stenosis. Simulations with air and heliox conditions mirrored the known clinical benefits of heliox as compared with air. We anticipate that computer simulation models will ultimately allow a better understanding of changes in flow caused by specific medical and surgical interventions in patients with conditions associated with dyspnea.

Large Eddy Simulation and Reynolds-Averaged Navier-Stokes modeling of flow in a realistic pharyngeal airway model: an investigation of obstructive sleep apnea.

Computational fluid dynamics techniques employing primarily steady Reynolds-Averaged Navier-Stokes (RANS) methodology have been recently used to characterize the transitional/turbulent flow field in human airways. The use of RANS implies that flow phenomena are averaged over time, the flow dynamics not being captured. Further, RANS uses two-equation turbulence models that are not adequate for predicting anisotropic flows, flows with high streamline curvature, or flows where separation occurs. A more accurate approach for such flow situations that occur in the human airway is Large Eddy Simulation (LES). The paper considers flow modeling in a pharyngeal airway model reconstructed from cross-sectional magnetic resonance scans of a patient with obstructive sleep apnea. The airway model is characterized by a maximum narrowing at the site of retropalatal pharynx. Two flow-modeling strategies are employed: steady RANS and the LES approach. In the RANS modeling framework both k-epsilon and k-omega turbulence models are used. The paper discusses the differences between the airflow characteristics obtained from the RANS and LES calculations. The largest discrepancies were found in the axial velocity distributions downstream of the minimum cross-sectional area. This region is characterized by flow separation and large radial velocity gradients across the developed shear layers. The largest difference in static pressure distributions on the airway walls was found between the LES and the k-epsilon data at the site of maximum narrowing in the retropalatal pharynx.

Computational fluid dynamics analysis of upper airway reconstructed from magnetic resonance imaging data.

OBJECTIVES: We performed flow computations on an accurate upper airway model in a patient with obstructive sleep apnea and computed the velocity, static pressure, and wall shear stress distribution in the model. METHODS: Cartesian coordinates for airway boundaries were determined from cross-sectional magnetic resonance images, and a 3-dimensional computational model of the upper airway was constructed. Flow simulations were then performed within a FLUENT commercial software framework. Four different flow conditions were simulated during inspiration, assuming the steady-state condition. The results were analyzed from the perspectives of velocity, static pressure, and wall shear stress distribution. RESULTS: We observed that the highest axial velocity was at the site of minimum cross-sectional area (retropalatal pharynx) resulting in the lowest level of wall static pressure. The highest wall shear stresses were at the same location. The pressure drop was significantly larger for higher flow rates than for lower flow rates. CONCLUSIONS: Our results indicate that the presence of airway narrowing, through change in the flow characteristics that result in increased flow velocity and reduced static pressure, can itself increase airway collapsibility. Additionally, the effects of wall shear stress on airway walls may be an important factor in the progression over time of the severity of obstructive sleep apnea.

Computational modeling of upper airway before and after adenotonsillectomy for obstructive sleep apnea.

Adenotonsillectomy, the first-line surgical treatment for obstructive sleep apnea (OSA) in children, is successful in only 50% of obese children. Computational fluid dynamics tools, which have been applied to differentiate OSA patients from those without OSA based on the airway flow characteristics, can be potentially used to identify patients likely to benefit from surgical intervention. We present computational modeling of the upper airway before and after adenotonsillectomy in an obese female adolescent with OSA. The subject underwent upper airway imaging on a 1.5 Tesla magnetic resonance imaging (MRI) scanner, and three-dimensional airway models were constructed using airway boundary coordinates from cross-sectional MRI scans. Our results using computational simulations indicate that, in an obese child, the resolution of OSA after adenotonsillectomy is associated with changes in flow characteristics that result in decreased pressure differentials across the airway walls and thus lower compressive forces that predispose to airway collapse. Application of such findings to an obese child seeking surgical treatment for OSA can potentially lead to selection of the surgical procedure most likely to result in OSA resolution. Effective intervention for OSA in this high-risk group will result in reduction in morbidity and the public health concerns associated with OSA.