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http://hdl.handle.net/10985/17733
Effects of Surface Tension and Yield Stress on Mucus Plug Rupture: a Numerical Study
HU, Yingying; ROMANO, Francesco; GROTBERG, James B.
We study the effects of surface tension and yield stress on mucus plug rupture. A three-dimensional simplified configuration is employed to simulate mucus plug rupture in a collapsed lung airway of the 10 th generation. The Herschel-Bulkley model is used to take into account the non-Newtonian viscoplastic fluid properties of mucus. Results show that the maximum wall shear stress greatly changes right prior to the rupture of the mucus plug. The surface tension influences mainly the late stage of the rupture process when the plug deforms greatly and the curvature of the mucus-air interface becomes significant. High surface tension increases the wall shear stress and the time needed to rupture since it produces a resistance to the rupture, as well as strong stress and velocity gradients across the mucus-air interface. The yield stress effects are pronounced mainly at the beginning. High yield stress makes the plug take long time to yield and slows down the whole rupture process. When the effects induced by the surface tension and yield forces are comparable, dynamical quantities strongly depend on the ratio of the two forces. The pressure difference (the only driving in the study) contributes to wall shear stress much more than yield stress and surface tension per unit length. Wall shear stress is less sensitive to the variation in yield stress than that in surface tension. In general, wall shear stress can be effectively reduced by the smaller pressure difference and surface tension.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/10985/177332019-01-01T00:00:00ZHU, YingyingROMANO, FrancescoGROTBERG, James B.We study the effects of surface tension and yield stress on mucus plug rupture. A three-dimensional simplified configuration is employed to simulate mucus plug rupture in a collapsed lung airway of the 10 th generation. The Herschel-Bulkley model is used to take into account the non-Newtonian viscoplastic fluid properties of mucus. Results show that the maximum wall shear stress greatly changes right prior to the rupture of the mucus plug. The surface tension influences mainly the late stage of the rupture process when the plug deforms greatly and the curvature of the mucus-air interface becomes significant. High surface tension increases the wall shear stress and the time needed to rupture since it produces a resistance to the rupture, as well as strong stress and velocity gradients across the mucus-air interface. The yield stress effects are pronounced mainly at the beginning. High yield stress makes the plug take long time to yield and slows down the whole rupture process. When the effects induced by the surface tension and yield forces are comparable, dynamical quantities strongly depend on the ratio of the two forces. The pressure difference (the only driving in the study) contributes to wall shear stress much more than yield stress and surface tension per unit length. Wall shear stress is less sensitive to the variation in yield stress than that in surface tension. In general, wall shear stress can be effectively reduced by the smaller pressure difference and surface tension.Stability of thermocapillary flow in liquid bridges fully coupled to the gas phase
http://hdl.handle.net/10985/24324
Stability of thermocapillary flow in liquid bridges fully coupled to the gas phase
STOJANOVIĆ, Mario; ROMANO, Francesco; KUHLMANN, Hendrik C.
The linear stability of the axisymmetric steady thermocapillary flow in a liquid bridge made from 2 cSt silicone oil (Prandtl number 28) is investigated numerically in the framework of the Boussinesq approximation. The flow and temperature fields in the surrounding gas phase (air) are taken into account for a generic cylindrical container hosting the liquid bridge. The flows in the liquid and in the gas are fully coupled across the hydrostatically deformed liquid–gas interface, neglecting dynamic interface deformations. Originating from a common reference case, the linear stability boundary is computed varying the length of the liquid bridge (aspect ratio), its volume and the gravity level, providing accurate critical data. The qualitative dependence of the critical threshold on these parameters is explained in terms of the characteristics of the critical mode. The heat exchange between the ambient gas and the liquid bridge that is fully resolved has an important influence on the critical conditions.
Thu, 01 Sep 2022 00:00:00 GMThttp://hdl.handle.net/10985/243242022-09-01T00:00:00ZSTOJANOVIĆ, MarioROMANO, FrancescoKUHLMANN, Hendrik C.The linear stability of the axisymmetric steady thermocapillary flow in a liquid bridge made from 2 cSt silicone oil (Prandtl number 28) is investigated numerically in the framework of the Boussinesq approximation. The flow and temperature fields in the surrounding gas phase (air) are taken into account for a generic cylindrical container hosting the liquid bridge. The flows in the liquid and in the gas are fully coupled across the hydrostatically deformed liquid–gas interface, neglecting dynamic interface deformations. Originating from a common reference case, the linear stability boundary is computed varying the length of the liquid bridge (aspect ratio), its volume and the gravity level, providing accurate critical data. The qualitative dependence of the critical threshold on these parameters is explained in terms of the characteristics of the critical mode. The heat exchange between the ambient gas and the liquid bridge that is fully resolved has an important influence on the critical conditions.Attractors for the motion of a finite-size particle in a cuboidal lid-driven cavity
http://hdl.handle.net/10985/24322
Attractors for the motion of a finite-size particle in a cuboidal lid-driven cavity
WU, Haotian; ROMANO, Francesco; KUHLMANN, Hendrik C.
The motion of a finite-size particle in the cuboidal lid-driven cavity flow is investigated experimentally for Reynolds numbers 100 and 200 for which the flow is steady. These steady three-dimensional flows exhibit chaotic and regular streamlines, where the latter are confined to Kolmogorov–Arnold–Moser (KAM) tori. The interaction between the moving wall and the particle creates global particle attractors. For neutrally buoyant particles, these attractors are periodic or quasi-periodic, strongly attracting and located in or near KAM tori of the flow. As the density mismatch between particle and fluid increases, buoyancy and inertia become important, and the attractors evolve from those for neutrally buoyant particles, changing their shape, position and attraction rates.
Sun, 01 Jan 2023 00:00:00 GMThttp://hdl.handle.net/10985/243222023-01-01T00:00:00ZWU, HaotianROMANO, FrancescoKUHLMANN, Hendrik C.The motion of a finite-size particle in the cuboidal lid-driven cavity flow is investigated experimentally for Reynolds numbers 100 and 200 for which the flow is steady. These steady three-dimensional flows exhibit chaotic and regular streamlines, where the latter are confined to Kolmogorov–Arnold–Moser (KAM) tori. The interaction between the moving wall and the particle creates global particle attractors. For neutrally buoyant particles, these attractors are periodic or quasi-periodic, strongly attracting and located in or near KAM tori of the flow. As the density mismatch between particle and fluid increases, buoyancy and inertia become important, and the attractors evolve from those for neutrally buoyant particles, changing their shape, position and attraction rates.Instabilities identification based on a new centrifugal 3D impeller outflow model
http://hdl.handle.net/10985/24325
Instabilities identification based on a new centrifugal 3D impeller outflow model
FAN, Meng; DAZIN, Antoine; BOIS, Gérard; ROMANO, Francesco
Previous research works have shown that the inflow boundary conditions have a significant effect on the behavior of diffusers in a centrifugal machine. To better understand the vaneless diffuser instability mechanism and save computing resources, several numerical works are planned to be conducted for the solely vaneless diffuser, excluding the rest of the centrifugal machine from the flow domain to simulate. Previous reduced-order models used either two-dimensional approaches that focused exclusively on the core-flow instability or three-dimensional models tested for a few inflow conditions. To obtain the more realistic diffuser inlet boundary conditions, a modeling method is here developed by fitting the diffuser inflow velocity derived from numerical simulations of the entire machine. The classic fitting methods used to approximate inflow profiles by algebraic polynomials or Gaussian functions are observed to introduce numerical artifacts that significantly affect the flow and therefore its stability. The multi-stage scale-matching fitting approach developed in this study is designed as a robust successive-order approximation of the inflow conditions. Our objective is to demonstrate its robust capability of taking into account the main physical features of the inlet velocity profiles, which, in turn, allows us to significantly improve the prediction of the flow instability occurring in the pump diffuser. Firstly, the RANS and URANS simulations of the entire machine are carried out by OpenFOAM using the
SST turbulence model. The simulation results show that the RANS simulation is efficient in correctly capturing the diffuser inlet velocity profile except for developed stall conditions. The RANS simulations are carried out for cases with three different kinds of leakage configurations between the impeller and the diffuser. For each case, five flow rates were simulated to get the basic data for fitting the inlet profiles for a total of 125 simulations. The diffuser inlet velocity profiles are averaged in the azimuthal direction and fitted such to obtain an explicit function for the azimuthally-averaged velocity profile that varies with the flow rate Q. The fitting results are very close to the original data, and using our fits to predict the diffuser flow instabilities we show that our modeling approach compares well against the URANS simulations of the whole machine.
Fri, 01 Sep 2023 00:00:00 GMThttp://hdl.handle.net/10985/243252023-09-01T00:00:00ZFAN, MengDAZIN, AntoineBOIS, GérardROMANO, FrancescoPrevious research works have shown that the inflow boundary conditions have a significant effect on the behavior of diffusers in a centrifugal machine. To better understand the vaneless diffuser instability mechanism and save computing resources, several numerical works are planned to be conducted for the solely vaneless diffuser, excluding the rest of the centrifugal machine from the flow domain to simulate. Previous reduced-order models used either two-dimensional approaches that focused exclusively on the core-flow instability or three-dimensional models tested for a few inflow conditions. To obtain the more realistic diffuser inlet boundary conditions, a modeling method is here developed by fitting the diffuser inflow velocity derived from numerical simulations of the entire machine. The classic fitting methods used to approximate inflow profiles by algebraic polynomials or Gaussian functions are observed to introduce numerical artifacts that significantly affect the flow and therefore its stability. The multi-stage scale-matching fitting approach developed in this study is designed as a robust successive-order approximation of the inflow conditions. Our objective is to demonstrate its robust capability of taking into account the main physical features of the inlet velocity profiles, which, in turn, allows us to significantly improve the prediction of the flow instability occurring in the pump diffuser. Firstly, the RANS and URANS simulations of the entire machine are carried out by OpenFOAM using the
SST turbulence model. The simulation results show that the RANS simulation is efficient in correctly capturing the diffuser inlet velocity profile except for developed stall conditions. The RANS simulations are carried out for cases with three different kinds of leakage configurations between the impeller and the diffuser. For each case, five flow rates were simulated to get the basic data for fitting the inlet profiles for a total of 125 simulations. The diffuser inlet velocity profiles are averaged in the azimuthal direction and fitted such to obtain an explicit function for the azimuthally-averaged velocity profile that varies with the flow rate Q. The fitting results are very close to the original data, and using our fits to predict the diffuser flow instabilities we show that our modeling approach compares well against the URANS simulations of the whole machine.Computational pulmonary edema: A microvascular model of alveolar capillary and interstitial flow
http://hdl.handle.net/10985/24323
Computational pulmonary edema: A microvascular model of alveolar capillary and interstitial flow
GROTBERG, James B.; ROMANO, Francesco
We present a microvascular model of fluid transport in the alveolar septa related to pulmonary edema. It consists of a two-dimensional capillary sheet coursing by several alveoli. The alveolar epithelial membrane runs parallel to the capillary endothelial membrane with an interstitial layer in between, making one long septal tract. A coupled system of equations uses lubrication theory for the capillary blood, Darcy flow for the porous media of the interstitium, a passive alveolus, and the Starling equation at both membranes. Case examples include normal physiology, cardiogenic pulmonary edema, acute respiratory distress syndrome (ARDS), hypoalbuminemia, and effects of PEEP. COVID-19 has dramatically increased ARDS in the world population, raising the urgency for such a model to create an analytical framework. Under normal conditions fluid exits the alveolus, crosses the interstitium, and enters the capillary. For edema, this crossflow is reversed with fluid leaving the capillary and entering the alveolus. Because both the interstitial and capillary pressures decrease downstream, the reversal can occur within a single septal tract, with edema upstream and clearance downstream. Clinically useful solution forms are provided allowing calculation of interstitial fluid pressure, crossflows, and critical capillary pressures. Overall, the interstitial pressures are found to be significantly more positive than values used in the traditional physiological literature. That creates steep gradients near the upstream and downstream end outlets, driving significant flows toward the distant lymphatics. This new physiological flow provides an explanation to the puzzle, noted since 1896, of how pulmonary lymphatics can function so far from the alveoli: the interstitium is self-clearing.
Sat, 01 Jul 2023 00:00:00 GMThttp://hdl.handle.net/10985/243232023-07-01T00:00:00ZGROTBERG, James B.ROMANO, FrancescoWe present a microvascular model of fluid transport in the alveolar septa related to pulmonary edema. It consists of a two-dimensional capillary sheet coursing by several alveoli. The alveolar epithelial membrane runs parallel to the capillary endothelial membrane with an interstitial layer in between, making one long septal tract. A coupled system of equations uses lubrication theory for the capillary blood, Darcy flow for the porous media of the interstitium, a passive alveolus, and the Starling equation at both membranes. Case examples include normal physiology, cardiogenic pulmonary edema, acute respiratory distress syndrome (ARDS), hypoalbuminemia, and effects of PEEP. COVID-19 has dramatically increased ARDS in the world population, raising the urgency for such a model to create an analytical framework. Under normal conditions fluid exits the alveolus, crosses the interstitium, and enters the capillary. For edema, this crossflow is reversed with fluid leaving the capillary and entering the alveolus. Because both the interstitial and capillary pressures decrease downstream, the reversal can occur within a single septal tract, with edema upstream and clearance downstream. Clinically useful solution forms are provided allowing calculation of interstitial fluid pressure, crossflows, and critical capillary pressures. Overall, the interstitial pressures are found to be significantly more positive than values used in the traditional physiological literature. That creates steep gradients near the upstream and downstream end outlets, driving significant flows toward the distant lymphatics. This new physiological flow provides an explanation to the puzzle, noted since 1896, of how pulmonary lymphatics can function so far from the alveoli: the interstitium is self-clearing.Small-width wall-attached Coandǎ jets for flow control
http://hdl.handle.net/10985/24321
Small-width wall-attached Coandǎ jets for flow control
EL MOKKADEM, Oussama; CHEN, Xintong; PHAN, Charlene; DELVA, Jérôme; JOSEPH, Pierric; DAZIN, Antoine; ROMANO, Francesco
The flow dynamics of small-width wall-attached jets generated by a Coand-effect nozzle is investigated by unsteady Reynolds-averaged Navier–Stokes simulations. The data are validated by comparison with hot-wire velocity measurements performed on the same flow configurations. The jets exhibit a complex topology strongly influenced not only by the spanwise vorticity (as usually observed in wall jets) but also by a vorticity component normal to the wall and induced by the shear layer developing on the jet sides. This results in an original U-shaped jet whose characteristics are studied in detail for three different mass flow rates. The robustness of the flow topology on a larger range of injected mass flow rates is finally presented and discussed in terms of the injected momentum near the wall. The resulting flow profiles point out that our injector is expected to be a promising candidate for active flow control in gas-turbine compressors for aeronautical and energy applications.
Thu, 01 Jun 2023 00:00:00 GMThttp://hdl.handle.net/10985/243212023-06-01T00:00:00ZEL MOKKADEM, OussamaCHEN, XintongPHAN, CharleneDELVA, JérômeJOSEPH, PierricDAZIN, AntoineROMANO, FrancescoThe flow dynamics of small-width wall-attached jets generated by a Coand-effect nozzle is investigated by unsteady Reynolds-averaged Navier–Stokes simulations. The data are validated by comparison with hot-wire velocity measurements performed on the same flow configurations. The jets exhibit a complex topology strongly influenced not only by the spanwise vorticity (as usually observed in wall jets) but also by a vorticity component normal to the wall and induced by the shear layer developing on the jet sides. This results in an original U-shaped jet whose characteristics are studied in detail for three different mass flow rates. The robustness of the flow topology on a larger range of injected mass flow rates is finally presented and discussed in terms of the injected momentum near the wall. The resulting flow profiles point out that our injector is expected to be a promising candidate for active flow control in gas-turbine compressors for aeronautical and energy applications.Lagrangian chaos in steady three-dimensional lid-driven cavity flow
http://hdl.handle.net/10985/24424
Lagrangian chaos in steady three-dimensional lid-driven cavity flow
ROMANO, Francesco; TÜRKBAY, Tuǧçe; KUHLMANN, Hendrik C.
Steady three-dimensional flows in lid-driven cavities are investigated numerically using a high-order spectral-element solver for the incompressible Navier–Stokes equations. The focus is placed on critical points in the flow field, critical limit cycles, their heteroclinic connections, and on the existence, shape, and dependence on the Reynolds number of Kolmogorov–Arnold–Moser (KAM) tori. In finite-length cuboidal cavities at small Reynolds numbers, a thin layer of chaotic streamlines covers all walls. As the Reynolds number is increased, the chaotic layer widens and the complementary KAM tori shrink, eventually undergoing resonances, until they vanish. Accurate data for the location of closed streamlines and of KAM tori are provided, both of which reach very close to the moving lid. For steady periodic Taylor–Görtler vortices in spanwise infinitely extended cavities with a square cross section, chaotic streamlines occupy a large part of the flow domain immediately after the onset of Taylor–Görtler vortices. As the Reynolds number increases, the remaining KAM tori vanish from the Taylor–Görtler vortices, while KAM tori grow in the central region further away from the solid walls.
Wed, 01 Jul 2020 00:00:00 GMThttp://hdl.handle.net/10985/244242020-07-01T00:00:00ZROMANO, FrancescoTÜRKBAY, TuǧçeKUHLMANN, Hendrik C.Steady three-dimensional flows in lid-driven cavities are investigated numerically using a high-order spectral-element solver for the incompressible Navier–Stokes equations. The focus is placed on critical points in the flow field, critical limit cycles, their heteroclinic connections, and on the existence, shape, and dependence on the Reynolds number of Kolmogorov–Arnold–Moser (KAM) tori. In finite-length cuboidal cavities at small Reynolds numbers, a thin layer of chaotic streamlines covers all walls. As the Reynolds number is increased, the chaotic layer widens and the complementary KAM tori shrink, eventually undergoing resonances, until they vanish. Accurate data for the location of closed streamlines and of KAM tori are provided, both of which reach very close to the moving lid. For steady periodic Taylor–Görtler vortices in spanwise infinitely extended cavities with a square cross section, chaotic streamlines occupy a large part of the flow domain immediately after the onset of Taylor–Görtler vortices. As the Reynolds number increases, the remaining KAM tori vanish from the Taylor–Görtler vortices, while KAM tori grow in the central region further away from the solid walls.Forces and torques on a sphere moving near a dihedral corner in creeping flow
http://hdl.handle.net/10985/24421
Forces and torques on a sphere moving near a dihedral corner in creeping flow
ROMANO, Francesco; DES BOSCS, P.-E.; KUHLMANN, H.C.
The low-Reynolds-number flow past a sphere moving near a right dihedral corner made by a stationary and a tangentially sliding wall is considered. Using the superposition principle, the arbitrary motion of the sphere is decomposed into simple elementary motions. Fully-resolved spectral-element simulations are carried out in the frame of reference translating and rotating with the particle such that the velocity on the particle’s surface vanishes. Forces and torques on the sphere are obtained as functions of the particle position near the corner. The data obtained are fitted by closed-form expressions which take into account symmetries of the problem, exact solutions, and asymptotic solutions from lubrication theory. The correlations obtained can easily be implemented in larger-scale one-way-coupled particulate-flow simulations to correct the particle motion near dihedral corners where mere point-particle models break down.
Sun, 01 Nov 2020 00:00:00 GMThttp://hdl.handle.net/10985/244212020-11-01T00:00:00ZROMANO, FrancescoDES BOSCS, P.-E.KUHLMANN, H.C.The low-Reynolds-number flow past a sphere moving near a right dihedral corner made by a stationary and a tangentially sliding wall is considered. Using the superposition principle, the arbitrary motion of the sphere is decomposed into simple elementary motions. Fully-resolved spectral-element simulations are carried out in the frame of reference translating and rotating with the particle such that the velocity on the particle’s surface vanishes. Forces and torques on the sphere are obtained as functions of the particle position near the corner. The data obtained are fitted by closed-form expressions which take into account symmetries of the problem, exact solutions, and asymptotic solutions from lubrication theory. The correlations obtained can easily be implemented in larger-scale one-way-coupled particulate-flow simulations to correct the particle motion near dihedral corners where mere point-particle models break down.Finite-size coherent particle structures in high-Prandtl-number liquid bridges
http://hdl.handle.net/10985/24423
Finite-size coherent particle structures in high-Prandtl-number liquid bridges
BARMAK, Ilya; ROMANO, Francesco; KUHLMANN, Hendrik C.
The transport of liquid and of small rigid spherical particles in a high-Prandtl-number (Pr = 68) thermocapillary liquid bridge under zero gravity is studied by highly resolved numerical simulations when the flow arises as an azimuthally traveling hydrothermal wave with azimuthal wave number one. The Langrangian transport of fluid elements reveals the coexistence of regular and chaotic streamlines in the frame of reference rotating with the wave. The structure of the KAM (Kolmogorov-Arnold-Moser) tori is unraveled for several Reynolds numbers for which the flow is periodic in time and space. Based on the streamline topology the segregation of small rigid spherical particles of a dilute suspension into particle accumulation structures (PASs) is studied, based on the steric finite-particle-size effect when the particles moves close to the free surface. It is shown that the intricate KAM structures have their counterparts in a multitude of different attractors for the particle motion. Examples of PASs are provided, and their dependence on particle size, particle-to-fluid density ratio, and Reynolds number are discussed. A large parametric study reveals the most probable combinations of particle size and density ratio which lead to particle clustering.
Sun, 01 Aug 2021 00:00:00 GMThttp://hdl.handle.net/10985/244232021-08-01T00:00:00ZBARMAK, IlyaROMANO, FrancescoKUHLMANN, Hendrik C.The transport of liquid and of small rigid spherical particles in a high-Prandtl-number (Pr = 68) thermocapillary liquid bridge under zero gravity is studied by highly resolved numerical simulations when the flow arises as an azimuthally traveling hydrothermal wave with azimuthal wave number one. The Langrangian transport of fluid elements reveals the coexistence of regular and chaotic streamlines in the frame of reference rotating with the wave. The structure of the KAM (Kolmogorov-Arnold-Moser) tori is unraveled for several Reynolds numbers for which the flow is periodic in time and space. Based on the streamline topology the segregation of small rigid spherical particles of a dilute suspension into particle accumulation structures (PASs) is studied, based on the steric finite-particle-size effect when the particles moves close to the free surface. It is shown that the intricate KAM structures have their counterparts in a multitude of different attractors for the particle motion. Examples of PASs are provided, and their dependence on particle size, particle-to-fluid density ratio, and Reynolds number are discussed. A large parametric study reveals the most probable combinations of particle size and density ratio which lead to particle clustering.Effects of elastoviscoplastic properties of mucus on airway closure in healthy and pathological conditions
http://hdl.handle.net/10985/24425
Effects of elastoviscoplastic properties of mucus on airway closure in healthy and pathological conditions
ERKEN, O.; FAZLA, B.; MURADOGLU, M.; IZBASSAROV, D.; ROMANO, Francesco; GROTBERG, J. B.
Airway mucus is a complex material with both viscoelastic and viscoplastic properties that vary with healthy and pathological conditions of the lung. In this study, the effects of these conditions on airway closure are examined in a model problem, where an elastoviscoplastic (EVP) single liquid layer lines the inner wall of a rigid pipe and surrounds the air core. The EVP liquid layer is modelled using the Saramito-HB model. The parameters for the model are obtained for the mucus in healthy, asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis (CF) conditions by fitting the rheological model to the experimental data. Then the liquid plug formation is studied by varying the Laplace number and undisturbed liquid film thickness. Airway closure is a surface-tension-driven phenomenon that occurs when the ratio of the pulmonary liquid layer thickness to the airway radius exceeds a certain threshold. In previous studies, it has been found that airway epithelial cells can be lethally or sublethally damaged due to the high peak of the wall stresses and stress gradients during the liquid plug formation. Here we demonstrate that these stresses are also related to the EVP features of the liquid layer. Yielded zones of the liquid layer are investigated for the different mucus conditions, and it is found that the liquid layer is in a chiefly unyielded state before the closure, which indicates that this phase is dominated by the elastic behavior and solvent viscosity. This is further confirmed by showing that the elastic coefficient is one of the most critical parameters determining whether the closure occurs. This parameter also largely affects the closure time. The wall stresses are also investigated for the pathological and healthy cases. Their peaks for COPD and CF are found to be the highest due to the viscoelastic extra stress contribution. Contrary to the Newtonian case, the wall stresses for COPD and CF do not smoothly relax after closure, as they rather remain effectively almost as high as the Newtonian peak. Moreover, the local normal wall stress gradients are smaller for the COPD and CF liquid layer due to their higher stiffness causing a smaller curvature at the capillary wave. The local tangential wall stress gradients are also shown to be smaller for these cases because of the slower accumulation of the liquid at the bulge.
Mon, 01 May 2023 00:00:00 GMThttp://hdl.handle.net/10985/244252023-05-01T00:00:00ZERKEN, O.FAZLA, B.MURADOGLU, M.IZBASSAROV, D.ROMANO, FrancescoGROTBERG, J. B.Airway mucus is a complex material with both viscoelastic and viscoplastic properties that vary with healthy and pathological conditions of the lung. In this study, the effects of these conditions on airway closure are examined in a model problem, where an elastoviscoplastic (EVP) single liquid layer lines the inner wall of a rigid pipe and surrounds the air core. The EVP liquid layer is modelled using the Saramito-HB model. The parameters for the model are obtained for the mucus in healthy, asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis (CF) conditions by fitting the rheological model to the experimental data. Then the liquid plug formation is studied by varying the Laplace number and undisturbed liquid film thickness. Airway closure is a surface-tension-driven phenomenon that occurs when the ratio of the pulmonary liquid layer thickness to the airway radius exceeds a certain threshold. In previous studies, it has been found that airway epithelial cells can be lethally or sublethally damaged due to the high peak of the wall stresses and stress gradients during the liquid plug formation. Here we demonstrate that these stresses are also related to the EVP features of the liquid layer. Yielded zones of the liquid layer are investigated for the different mucus conditions, and it is found that the liquid layer is in a chiefly unyielded state before the closure, which indicates that this phase is dominated by the elastic behavior and solvent viscosity. This is further confirmed by showing that the elastic coefficient is one of the most critical parameters determining whether the closure occurs. This parameter also largely affects the closure time. The wall stresses are also investigated for the pathological and healthy cases. Their peaks for COPD and CF are found to be the highest due to the viscoelastic extra stress contribution. Contrary to the Newtonian case, the wall stresses for COPD and CF do not smoothly relax after closure, as they rather remain effectively almost as high as the Newtonian peak. Moreover, the local normal wall stress gradients are smaller for the COPD and CF liquid layer due to their higher stiffness causing a smaller curvature at the capillary wave. The local tangential wall stress gradients are also shown to be smaller for these cases because of the slower accumulation of the liquid at the bulge.