Shanmugam Murugappan

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.

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 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.

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.

Optimal control of a swirl-stabilized spray combustor using system identification approach

An optimal controller using the system identification (SI) method was developed for a swirl-stabilized spray combustor operating between 30 and 114 kW. The efficacy of the controller was tested with two different nozzle configurations. The first consisted of a dual-feed nozzle whose primary fuel stream was utilized to sustain combustion, while the secondarystreamwasused for active control. The second configuration used a single-feed nozzle with two different swirling air streams. An LQG-LTR (linear quadratic Gaussian-loop transfer recovery) controller was designed using the SI-based model to determine the active control input, which was in turn used to modulate the secondary fuel stream. Using this controller, the thermoacoustic oscillations, which occurred under lean operating conditions, were reduced to the background noise level. A time-delay controller was also implemented for comparison purposes. The results showed that the LQG-LTR controller yielded an additional pressure reduction of 14 dB compared to the time-delay controller in both configurations. This improvement can be attributed to the added degrees of freedom of the LQG-LTR controller that allow an optimal shaping of the gain and phase of the controlled combustor over a range of frequencies in the neighborhood of the unstable mode. This leads to the extra reduction of the pressure amplitude at the unstable frequency while avoiding generation of secondary peaks.

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.

What can vortices tell us about vocal fold vibration and voice production.

Purpose of reviewMuch clinical research on laryngeal airflow has assumed that airflow is unidirectional. This review will summarize what additional knowledge can be obtained about vocal fold vibration and voice production by studying rotational motion, or vortices, in laryngeal airflow. Recent findingsRecent work suggests two types of vortices that may strongly contribute to voice quality. The first kind forms just above the vocal folds during glottal closing, and is formed by flow separation in the glottis; these flow separation vortices significantly contribute to rapid closing of the glottis, and hence, to producing loudness and high frequency harmonics in the acoustic spectrum. The second is a group of highly three-dimensional and coherent supraglottal vortices, which can produce sound by interaction with structures in the vocal tract. Present work is also described that suggests that certain laryngeal pathologies, such as asymmetric vocal fold tension, will significantly modify both types of vortices, with adverse impact on sound production: decreased rate of glottal closure, increased broadband noise, and a decreased signal to noise ratio. SummaryRecent research supports the hypothesis that glottal airflow contains certain vortical structures that significantly contribute to voice quality.

Using Particle Imaging Velocimetry to Measure Anterior-Posterior Velocity Gradients in the Excised Canine Larynx Model

Objectives To quantify the anterior-posterior velocity gradient, we studied the velocity flow fields above the vocal folds in both the midcoronal and midsagittal planes. It was also our purpose to use these fields to deduce the mechanisms that cause the anterior-posterior gradient and to determine whether the vortical structures are highly 3-dimensional. Methods Using the particle imaging velocimetry method for 5 excised canine larynges, we obtained phase-averaged velocity fields in the midcoronal and midsagittal planes for 30 phases of phonation. The velocity fields were determined synchronously with the vocal fold motion recorded by high-speed videography. Results The results show that immediately above the folds, there is no significant anterior-posterior velocity gradient. However, as the flow travels downstream, the laryngeal jet tends to narrow in width and skew toward the anterior commissure. Vortices are seen at the anterior and posterior edges of the flow. Conclusions The downstream narrowing in the midsagittal plane is consistent with and is probably due to a phenomenon known as axis switching. Axis switching also involves vortices in the sagittal and coronal planes bending in the axial plane. This results in highly 3-dimensional, complex vortical structures. However, there is remarkable cyclic repeatability of these vortices during a phonation cycle.

Role of vortices in voice production: normal versus asymmetric tension.

Decreasing the closing speed of the vocal folds can reduce loudness and energy in the higher frequency harmonics, resulting in reduced voice quality. Our aim was to study the correlation between higher frequencies and the intraglottal vorticity (which contributes to rapid closing by producing transient negative intraglottal pressures).

Control of penetration and mixing of an excited supersonic jet into a supersonic cross stream

Rayleigh/Mie scattering (from flow-field ice crystals) was used to study mixing and penetration of a forced supersonic jet in a supersonic Mach (M)-2 cross stream. Instantaneous images—using image planes along (side-view) and normal (end-view) to the flow axis—were used to study the dynamical structures in the jet whereas ensemble images provide information regarding the jet trajectory. Standard deviation images reveal information about the large-scale mixing/entrainment. Probability density functions were used to evaluate the mixing along the time-average jet interface. Forced cases indicate the presence of periodic formation of large-scale eddies in the jet/free stream interface. The eddies were bigger in size and more convoluted in the forced cases as compared to the baseline. These provided high penetration of the jet into the free stream. Forced cases also show a larger region involved in small scale and/or bulk mixing in both the side—and end-views. Different metrics such as total area contained in ...