Analysis of the volume of fluid (VOF) method for the simulation of the mucus clearance process with CFD

Oral configuration-dependent variability of the metrics of exhaled respiratory droplets during a consecutive coughing event

BACKGROUND AND OBJECTIVE

Coughing events are eruptive sources of virus-laden droplets/droplet nuclei. These increase the risk of infection in susceptible individuals during airborne transmission. The oral cavity functions as an exit route for exhaled droplets. Thus, its configuration contributes significantly to the metrics of exhaled droplets.

METHODS

In this study, two realistic numerical models were developed: the respiratory system from the throat to the second bifurcation and the oral cavity with different anatomical structures. A coupling of Eulerian Wall Film (EWF) - Discrete Phase Model (DPM) was employed to numerically describe the generation, absorption and exhalation properties. In addition, two sequential coughing episodes were considered with variegated profiles in the second cough.

RESULTS

As a result, the enlargement of the oral cavity caused considerable alterations in the original spatial distribution and total number concentration of exhaled droplets: these were reduced by approximately 30 %. Considering the consecutive second cough, maintaining or decreasing the flow rates resulted in a decrease in the total quantity of exhalation droplets by 25-90 %. The variations in the oral structure or coughing flow profile also reallocated the local spatial and proportional distribution of exhaled droplet. The expelled droplets/droplet nuclei's size remained approximately 0.25-20 μm range with varied development trends even though the peak concentration reserved unchanged at approximately 5 μm.

CONCLUSIONS

This study is a substantial work emphasizing the dependent variability of oral geometry and coughing physiology related to the properties of exhaled droplets. It emphasizes the uncertainties in the input parameters required for indoor transmission risk studies related to intersubject variability.

Nanoparticle Diffusion in Respiratory Mucus Influenced by Mucociliary Clearance: A Review of Mathematical Modeling

Background: Inhalation and deposition of particles in human airways have attracted considerable attention due to importance of particulate pollutants, transmission of infectious diseases, and therapeutic delivery of drugs at targeted areas. We summarize current state-of-the art research in particle deposition on airway surface liquid (ASL) influenced by mucociliary clearance (MCC) by identifying areas that need further investigation. Methodology: We aim to review focus on governing and constitutive equations describing MCC geometry followed by description of mathematical modeling of ciliary forces, mucus rheology properties, and numerical approaches to solve modified time-dependent Navier-Stokes equations. We also review mathematical modeling of particle deposition in ASL influenced by MCC, particle transport in ASL in terms of Eulerian and Lagrangian approaches, and discuss the corresponding mass transport issues in this layer. Whenever required, numerical predictions are contrasted with the pertinent experimental data. Results: Results indicate that mean mucus and periciliary liquid velocities are strongly influenced by mucus rheological characteristics as well as ciliary abnormalities. However, most of the currently available literature on mucus fiber spacing, ciliary beat frequency, and particle surface chemistry is based on particle deposition on ASL by considering a fixed value of ASL velocity. The effects of real ASL flow regimes on particle deposition in this layer are limited. In addition, no other study is available on modeling nonhomogeneous and viscoelastic characteristics of mucus layer on ASL drug delivery. Conclusion: Simplification of assumptions on governing equations of drug delivery in ASL influenced by MCC leads to imposing some limitations on numerical results.

Effect of Non-Newtonian Dynamics on the Clearance of Mucus From Bifurcating Lung Airway Models

Mucus hypersecretion is a common pathophysiological manifestation of several obstructive airway diseases in which the mucociliary clearance is impaired, and the airflow generated by a cough or a forced expiratory maneuver called the huff is primarily responsible for clearing mucus. This airflow driven clearance of mucus is a complex process that is affected by the mucus rheology, airflow rate, airway geometry, and gravity. This study examines the role of mucus rheology in the transport and distribution of mucus in idealized 3D airway geometries. The complex air-mucus interface was tracked by the volume-of-fluid (VOF) model, and the turbulence in the core airflow was modeled using the k-ω shear stress transport (SST) model. Mucus was modeled as a shear-thinning liquid by using a power-law model. The computational model was validated using in vitro experimental data available in the literature. Gravity-dominated eccentric core-annular flow was observed with the core biased toward the outer wall in the inclined daughter branches of the bifurcation models, which transitions into concentric core-annular flow in the trachea. The increase in tangential shear at the interface due to the secondary flow structures developed in the flow divider location resulted in a region of enhanced mucus clearance with reduced mucus layer thickness. Secondary flow developed due to the curvature in the airway geometry resulted in a local redistribution of mucus that reduced the eccentricity. The accumulation of mucus around the carinal ridges and the regions with reduced clearance are sites with the potential for microbial growth.