Repetitive closure of the upper airway characterizes obstructive sleep apnea. It disrupts sleep causing excessive daytime drowsiness and is linked to hypertension and cardiovascular disease. Previous studies simulating the underlying fluid mechanics are based upon geometries, time-averaged over the respiratory cycle, obtained usually via MRI or CT scans. Here, we generate an anatomically correct geometry from data captured in vivo by an endoscopic optical technique. This allows quantitative real-time imaging of the internal cross section with minimal invasiveness. The steady inhalation flow field is computed using a k-ω shear-stress transport (SST) turbulence model. Simulations reveal flow mechanisms that produce low-pressure regions on the sidewalls of the pharynx and on the soft palate within the pharyngeal section of minimum area. Soft-palate displacement and side-wall deformations further reduce the pressures in these regions, thus creating forces that would tend to narrow the airway. These phenomena suggest a mechanism for airway closure in the lateral direction as clinically observed. Correlations between pressure and airway deformation indicate that quantitative prediction of the low-pressure regions for an individual are possible. The present predictions warrant and can guide clinical investigation to confirm the phenomenology and its quantification, while the overall approach represents an advancement toward patient-specific modeling.
SUMMARYAn improved approach for studying the stability of a cantilevered flexible plate positioned within twodimensional viscous channel flow is presented in the context of human upper-airway dynamics. Previous work has used constant inlet velocity conditions. Here we model a constant pressure drop that may better reflect inspiratory effort. Positioning of the flexible plate within the channel can also be varied. The constant pressure drop is imposed for each time step by computing appropriate inlet velocities. The Navier-Stokes equations are solved using an explicit finite-element method written specifically for the channel geometry within which the fully coupled plate moves. The motion of the plate, driven by the pressure-field, is modelled using classical thin-plate mechanics with the addition of the fluid shear-stress-induced tension term. The investigation focuses on low-amplitude motions of the flexible plate (soft-palate) that, when unstable, may be the precursors to snoring and airway blockage during sleep. We show that imposing constant inlet velocity conditions generates over-predictions of energy transfer between flow and flexible plate during inhalation. Finally, we show that offsetting the flexible plate within the channel leads to a reduction in oscillation frequency and a significant change to its energy interaction with the fluid flow.
A new approach for studying the stability of a cantilevered flexible plate positioned within a 2-D viscous channel flow is presented as a representation of the human upper airway. Previous work has used constant inlet velocity conditions, an unrealistic assumption when modelling inhalation. Here we model a constant pressure drop that reflects inspiratory effort. Positioning of the flexible plate within the channel can also be varied. The constant pressure drop is imposed for each time step by computing appropriate inlet velocities. The Navier-Stokes equations are solved using an explicit finite-element method written specifically for the channel geometry within which the fully coupled plate moves. The motion of the plate, driven by the pressure field, is modelled using classical thin-plate mechanics with the addition of a shear-stress induced tension term. The investigation focuses on the motion of the flexible plate (soft palate) as one of the contributors to airway blockage during sleep. It is found that the tension induced by the fluid shear-stress can be significant when the plate is sufficiently flexible. We also demonstrate that imposing constant inlet velocity generates over-predictions of energy transfer between flow and flexible plate. Finally, we show that offsetting the flexible plate within the channel leads to changes in oscillation frequency and significant change to its energy interaction with the fluid flow.
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