Kazuhide Ito

The influence of turbulence modeling and oral cavity geometry on cough jet predictions was investigated using Computational Fluid and Particle Dynamics. Three oral geometries were examined: two realistic geometries with teeth details and different mouth openings, plus a simplified circular opening. The Reynolds-Averaged Navier–Stokes Shear Stress Transport (SST) k–ω model and hybrid Stress-Blended Eddy Simulation (SBES) model were compared to assess droplet dispersion predictions. The SST k–ω model overpredicts droplet diffusion, producing approximately 24% greater lateral dispersion than SBES simulations. SBES resolved coherent vortex structures critical to realistic particle transport in cough clouds, balancing computational cost with accuracy. A realistic narrow oral opening generated concentrated jets with rapid dispersion at face level, while the wide cavity produced downward droplet trajectories. The simplified circular mouth produced unrealistic symmetric jets that overestimated spatial spread. The narrow oral cavity produced rapid expansion: the cough cloud volume increased from 0.0227 m3 at 0.5 s to 0.0657 m3 at 2.0 s while droplet number density decreased from 40.77 to 11.9 n/cm3. The wide oral cavity showed slower cloud growth and rapid droplet number density reduction due to fewer ejected particles. The simplified model significantly overestimated concentrations initially, with a 99.9% reduction in droplet density within the first second. Droplet vaporization analysis showed rapid diameter reduction within 0.14 s, highlighting how water fraction affects droplet lifetime. Both anatomical realism and turbulence modeling influenced respiratory flow simulations and accurate infection risk prediction.

Existing research on ventilation effectiveness (VE) has primarily focused on scalar concentration values (i.e., output metrics); however, discussions on the mechanisms governing their formation have been limited. To optimize ventilation systems, VE frameworks should provide insights into the indoor scalar heterogeneity and its formation mechanisms. Therefore, a computational fluid dynamics (CFD)-based VE framework is used to investigate the formation of indoor scalar concentration fields. By exploiting the linear nature of scalar transport, the steady-state scalar concentration field under a continuous scalar source condition, C∞, can be represented as the superposition of probability density functions (PDFs) describing scalar dispersion. Accordingly, by defining spatiotemporal PDFs from the time series of scalar distributions obtained through transient CFD simulation with an instantaneous source, their superposition successfully reconstructed the distribution of C∞. In other words, the formulation of spatiotemporal PDFs enabled the elucidation of indoor scalar heterogeneity and identification of its governing spectral characteristics. Additionally, the quantification results showed that dispersion processes during short travel times predominantly determine indoor scalar heterogeneity. However, transient scalar simulations are computationally expensive compared with steady-state analyses; therefore, repeated transient computations are impractical. To address this limitation, a method for identifying spatiotemporal PDFs was developed using only steady-state results. The proposed method successfully reproduced early-stage scalar dispersion, but its accuracy gradually deteriorated over time. The framework developed in this study demonstrates that indoor scalar heterogeneity is primarily governed by short-term dispersion processes and that understanding these processes is essential for the development of highly effective ventilation system

Children are vulnerable to exposure to airborne microplastics (MPs) because of their developing lungs and prolonged time spent indoors. Therefore, this study employed a computational fluid–particle dynamics framework to simulate the posture-dependent transport and deposition of MPs in a realistic airway model of a 2-year-old child extending to the 8th bronchial generation. A steady inhalation breathing flow rate of 5 L/min was used for both postures. Subsequently, a discrete-phase model was used to predict the transportation and deposition of MPs, which employed an appropriate drag coefficient model. MP configurations were selected from the field survey, identifying the diameters of 4.785, 9.797, and 15.731 µm with aspect ratios of 3, 5, and 10. The results showed that posture significantly altered the deposition patterns of small- and low-aspect-ratio MPs. Specifically, the total deposition fraction was higher in the upright position than in the supine position. Local deposition analyses revealed that hotspots of deposited MPs varied across the airways under the effects of posture, especially in the upper respiratory tract. These findings provide mechanistic insights into how posture shapes regional fiber deposition in children and highlight the need to consider body orientation in pediatric inhalation exposure assessments.

Indoor airborne microplastics, especially in fibrous form, pose significant respiratory health risks due to their widespread distribution and persistence. This study employed computational fluid–particle dynamics (CFPD) simulations to investigate the inhalation, transport, rotation, and deposition behaviors of fibrous microplastics in the human respiratory tract. Because full DEM simulations in realistic airways are computationally demanding, DEM was first applied in a simplified geometry to assess drag and torque closures for nonspherical particles, thereby supporting the accurate and efficient application of the DPM framework in the realistic airway model. This verification confirmed that the Tran‐Cong (TC) and Ganser (GS) drag laws accurately predict fiber motion. Subsequently, a comprehensive airway model, extending from the nostrils to the 16th bronchial bifurcation, was developed to quantify deposition patterns of fibrous microplastics with three diameters (4.785, 9.797, 15.731 μm) × three aspect ratios (AR = 3, 5, 10), yielding nine distinct size groups. The results revealed size‐dependent deposition behaviors, with fibers having an aerodynamic equivalent diameter larger than 20 μm predominantly depositing in the upper respiratory regions owing to inertial impaction, whereas smaller fibers (10–20 μm) penetrated deeper, notably accumulating around the 5th–7th bronchial generations, especially within the right lower lung lobe.