Influence of secondary aspiration on human aspiration efficiency

Abstract

Computational fluid dynamics (CFD) was used to evaluate the contribution of secondary aspiration to human aspiration efficiency estimates using a humanoid model with realistic facial features. This study applied coefficient of restitution (CoR) values for working-aged human facial skin to the facial regions on the humanoid CFD model. Aspiration efficiencies for particles ranging from 7 to 116 μm were estimated for bounce (allowing for secondary aspiration) and no-bounce (CoR=0) simulations. Fluid simulations used the standard k-epsilon turbulence model over a range of test conditions: three freestream velocities, two breathing modes (mouth and nose breathing, using constant inhalation), three breathing velocities, and five orientations relative to the oncoming wind. Laminar particle trajectory simulations were used to examine inhaled particle transport and estimate aspiration efficiencies. Aspiration efficiency for the realistic CoR simulations, for both mouth- and nose-breathing, decreased with increasing particle size, with aspiration around 50% for 116 μm particles. For the CoR=0 simulations, aspiration decreased more rapidly with increasing particle size and approached zero for 116 μm compared to realistic CoR models (differences ranged from 0% to 80% over the particle sizes and velocity conditions). Differences in aspiration efficiency were larger with increasing particle size (>52 μm) and increased with decreasing freestream velocity and decreasing breathing rate. Secondary aspiration was more important when the humanoid faced the wind, but these contributions to overall aspiration estimates decreased as the humanoid rotated through 90°. There were minimal differences in aspiration between uniform CoR values of 0.5, 0.8, 1.0 and realistic regionally-applied CoR values, indicating differences between mannequin surfaces and between mannequin and human skin will have negligible effect on aspiration for facing-the-wind orientation.

Keywords: Coefficient of restitution; Computational fluid dynamics; Human aspiration efficiency; Inhalability; Particle bounce.

Fig. 1

Simulation facial geometries. Areas of both mouth and nose geometries are given. Inhalation occurred either through the nose or mouth for a given simulation.

Fig. 2

Computational domain example for a humanoid at 0° to the oncoming wind (facing-the-wind). Large white arrows indicate direction of the flow, set at 0.1, 0.2, or 0.4 m s⁻¹, depending on the simulation underway. Origin is positioned at the center of the mouth.

Fig. 3

7 μm (a and b) and 100 μm (c and d) particle trajectories for 0.2 m s⁻¹ freestream velocity and moderate, mouth breathing inhalation at 15° orientation. Each image shows 25 particles released upstream at 0.01 m to the right of the mouth center (Y) over a vertical distance of 0.13 m (Z). Realistic CoR simulations are on left. CoR=0 simulations are on right.

Fig. 4

7 μm (a and b) and 100 μm (c and d) particle trajectories for 0.2 m s⁻¹ freestream velocity and moderate, nose breathing inhalation at 15° orientation. Each image shows 25 particles released upstream at 0.01 m to the right of the mouth center (Y) over a vertical distance of 0.13 m (Z). Realistic CoR simulations are on left. CoR=0 simulations are on right.

Fig. 5

Comparison of critical areas for realistic CoR and CoR=0 for 7 μm particles at 0.2 m s⁻¹ freestream velocity, moderate mouth-breathing inhalation. The black dashed line represents realistic CoR simulation and the gray line represent CoR=0 simulations.

Fig. 6

Comparison of critical areas for realistic CoR and CoR=0 for 7 μm particles at 0.2 m s⁻¹ freestream velocity, moderate mouth-breathing inhalation. The black line represents realistic CoR simulations and gray lines represents CoR=0 simulations.

Fig. 7

Comparison of orientation-averaged aspiration (fraction) for averaged over all simulation conditions for the realistic CoR simulations and CoR=0 simulations for mouth-breathing inhalation. The solid lines represent simulations with realistic CoR and the dashed lines represent CoR=0 simulations. Orientation-averaged over forward-facing orientations (0–90°). Standard deviations represent variability between velocity conditions.

Fig. 8

Comparison of orientation-averaged aspiration (fraction) for averaged over all simulation conditions for the realistic CoR simulations and CoR=0 simulations for nose-breathing inhalation. The solid lines represent simulations with realistic CoR and the dashed lines represent CoR=0 simulations. Orientation-averaged over forward-facing orientations (0–90°). Standard deviations represent variability between velocity conditions.

Fig. 9

Simulation aspiration efficiency for facing-the-wind orientation at 0.4 m s⁻¹ freestream velocity, moderate mouth-breathing compared to experimental facing-the-wind mouth-breathing aspiration data from Kennedy and Hinds (2002).