The interactions between fluid flow and structural components of collapsible tubes are representative of those in several physiological systems. Although extensively studied, there exists a lack of characterization of the three-dimensionality in the structural deformations of the tube and its influence on the flow field. This experimental study investigates the spatio-temporal relationship between 3D tube geometry and the downstream flow field under conditions of fully open, closed, and slamming-type oscillating regimes. A methodology is implemented to simultaneously measure three-dimensional surface deformations in a collapsible tube and the corresponding downstream flow field. Stereophotogrammetry was used to measure tube deformations, and simultaneous flow field measurements included pressure and planar Particle Image Velocimetry (PIV) data downstream of the collapsible tube. The results indicate that the location of the largest collapse in the tube occurs close to the downstream end of the tube. In the oscillating regime, sections of the tube downstream of the largest mean collapse experience the largest oscillations in the entire tube that are completely coherent and in phase. At a certain streamwise distance upstream of the largest collapse, a switch in the direction of oscillations occurs with respect to those downstream. Physically, when the tube experiences constriction downstream of the location of the largest mean collapse, this causes the accumulation of fluid and build-up of pressure in the upstream regions and an expansion of these sections. Fluctuations in the downstream flow field are significantly influenced by tube fluctuations along the minor axes. The fluctuations in the downstream flowfield are influenced by the propagation of disturbances due to oscillations in tube geometry, through the advection of fluid through the tube. Further, the manifestation of the LU-type pressure fluctuations is found to be due to the variation in the propagation speed of the disturbances during the different stages within a period of oscillation of the tube.

We present a reduced-order model for fluid-structure interaction (FSI) simulation of vocal fold vibration during phonation. This model couples the three-dimensional (3D) tissue mechanics and a one-dimensional (1D) flow model that is derived from the momentum and mass conservation equations for the glottal airflow. The effects of glottal entrance and pressure loss in the glottis are incorporated in the flow model. We consider both idealized vocal fold geometries and subject-specific anatomical geometries segmented from the MRI images of rabbits. For the idealized vocal fold geometries, we compare the simulation results from the 1D/3D hybrid FSI model with those from the full 3D FSI simulation based on an immersed-boundary method. For the subject-specific geometries, we incorporate previously estimated tissue properties for individual samples and compare the results with those from the high-speed imaging experiment of in vivo phonation. In both setups, the comparison shows good agreement in the vibration frequency, amplitude, phase delay, and deformation pattern of the vocal fold, which suggests potential application of the present approach for future patient-specific modeling.

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.

Computational models of vocal fold (VF) vibration are becoming increasingly sophisticated, their utility currently transiting from exploratory research to predictive research. However, validation of such models has remained largely qualitative, raising questions over their applicability to interpret clinical situations. In this paper, a computational model with a segregated implementation is detailed. The model is used to predict the fluid-structure interaction (FSI) observed in a physical replica of the VFs when it is excited by airflow. Detailed quantitative comparisons are provided between the computational model and the corresponding experiment. First, the flow model is separately validated in the absence of VF motion. Then, in the presence of flow-induced VF motion, comparisons are made of the flow pressure on the VF walls and of the resulting VF displacements. Self-similarity of spatial distributions of flow pressure and VF displacements is highlighted. The self-similarity leads to normalized pressure and displacement profiles. It is shown that by using linear superposition of average and fluctuation components of normalized computed displacements, it is possible to determine displacements in the physical VF replica over a range of VF vibration conditions. Mechanical stresses in the VF interior are related to the VF displacements, thereby the computational model can also determine VF stresses over a range of phonation conditions.

The effect of subglottic stenosis on vocal fold vibration is investigated. An idealized stenosis is defined, parameterized, and incorporated into a two-dimensional, fully-coupled finite element model of the vocal folds and laryngeal airway. Flow-induced responses of the vocal fold model to varying severities of stenosis are compared. The model vibration was not appreciably affected by stenosis severities of up to 60% occlusion. Model vibration was altered by stenosis severities of 90% or greater, evidenced by decreased superior model displacement, glottal width amplitude, and flow rate amplitude. Predictions of vibration frequency and maximum flow declination rate were also altered by high stenosis severities. The observed changes became more pronounced with increasing stenosis severity and inlet pressure, and the trends correlated well with flow resistance calculations. Flow visualization was used to characterize subglottal flow patterns in the space between the stenosis and the vocal folds. Underlying mechanisms for the observed changes, possible implications for human voice production, and suggestions for future work are discussed.

An analysis is made of the nonlinear interactions between flow in the subglottal vocal tract and glottis, sound waves in the subglottal system and a mechanical model of the vocal folds. The mean flow through the system is produced by a nominally steady contraction of the lungs, and mechanical experiments frequently involve a 'lung cavity' coupled to an experimental subglottal tube of arbitrary or ill-defined effective length L, on the basis that the actual value of L has little or no influence on excitation of the vocal folds. A simple, self-exciting single mass mathematical model of the vocal folds is used to investigate the sound generated within the subglottal domain and the unsteady volume flux from the glottis for experiments where it is required to suppress feedback of sound from the supraglottal vocal tract. In experiments where the assumed absorption of sound within the sponge-like interior of the lungs is small, the influence of changes in L can be very significant: when the subglottal tube behaves as an open-ended resonator (when L is as large as half the acoustic wavelength) there is predicted to be a mild increase in volume flux magnitude and a small change in waveform. However, the strong appearance of second harmonics of the acoustic field is predicted at intermediate lengths, when L is roughly one quarter of the acoustic wavelength. In cases of large lung damping, however, only modest changes in the volume flux are predicted to occur with variations in L.

A method for the computation of low Reynolds number dynamic blood cell systems is presented. The specific system of interest here is interaction between cancer cells and white blood cells in an experimental flow system. Fluid dynamics, structural mechanics, six-degree-of freedom motion control and surface biochemistry analysis components are coupled in the context of adaptive octree-based grid generation. Analytical and numerical verification of the quasi-steady assumption for the fluid mechanics is presented. The capabilities of the technique are demonstrated by presenting several three-dimensional cell system simulations, including the collision/interaction between a cancer cell and an endothelium adherent polymorphonuclear leukocyte (PMN) cell in a shear flow.