Uncertainty in Aspiration Efficiency Estimates from Torso Simplifications in Computational Fluid Dynamics Simulations

Abstract

Computational fluid dynamics (CFD) has been used to report particle inhalability in low velocity freestreams, where realistic faces but simplified, truncated, and cylindrical human torsos were used. When compared to wind tunnel velocity studies, the truncated models were found to underestimate the air's upward velocity near the humans, raising questions about aspiration estimation. This work compares aspiration efficiencies for particles ranging from 7 to 116 µm using three torso geometries: (i) a simplified truncated cylinder, (ii) a non-truncated cylinder, and (iii) an anthropometrically realistic humanoid body. The primary aim of this work is to (i) quantify the errors introduced by using a simplified geometry and (ii) determine the required level of detail to adequately represent a human form in CFD studies of aspiration efficiency. Fluid simulations used the standard k-epsilon turbulence models, with freestream velocities at 0.1, 0.2, and 0.4 m s −1 and breathing velocities at 1.81 and 12.11 m s −1 to represent at-rest and heavy breathing rates, respectively. Laminar particle trajectory simulations were used to determine the upstream area, also known as the critical area, where particles would be inhaled. These areas were used to compute aspiration efficiencies for facing the wind. Significant differences were found in both vertical velocity estimates and the location of the critical area between the three models. However, differences in aspiration efficiencies between the three forms were <8.8% over all particle sizes, indicating that there is little difference in aspiration efficiency between torso models.

Influence of secondary aspiration on human aspiration efficiency

“…The computational models used a humanoid geometry with realistic facial features but a simplified, truncated torso. Anthony & Anderson (2013) found that trends in aspiration efficiency agreed with those found in experimental wind tunnel studies. Good agreement was found with the linear inhalable particulate mass equation proposed by Aitken et al (1999) at 0.1 m s −1 freestream velocities.…”

“…However, simulations require an estimate of the CoR to determine how much energy results in a particle rebound versus deposition. Previous computational studies have been conducted investigating the effect of orientation on human oral (Anthony & Anderson, 2013) and nasal aspiration (Anderson & Anthony, 2014), the effect of torso complexity on aspiration (Anderson & Anthony, 2013) using similar geometries. The computational models used a humanoid geometry with realistic facial features but a simplified, truncated torso.…”

“…Furthermore they found that there appeared to be an upper size limit for aspiration efficiency for nose-breathing around 100 μm. Anderson and Anthony (2013) investigated the effect of torso complexity on aspiration efficiency and found that while increasing torso complexity changes the location where particles are inhaled, aspiration efficiency changed by less than 10%. As the focus of the previous work was on effects of orientation and torso geometry, the previous studies assumed a CoR of 0, ignoring the effect of particle bounce, and thus secondary aspiration.…”

An Empirical Model of Human Aspiration in Low-Velocity Air Using CFD Investigations

“…While a realistic head/neck was modeled, the torso was simplified into a simple geometry to improve simulation times: the effect of this simplification is negligible based on findings of earlier work indicating that while the torso shape affects the position of the critical area, it did not affect the resulting estimates of aspiration efficiency. (17) The time-dependent nature of breathing was simplified to continuous inhalation, reducing the simulation time to 10 days per combination of geometry and velocity settings: cyclical breathing would have required time-dependent simulations of airflow and particles which would have added to the complexity of the model and simulation time. The effect of this simplification has not been directly evaluated, but expired air is known to disrupt the airflow upstream of study mannequins (24) which likely disturbs the upstream concentration of aerosols, rendering the immediate area upstream of the mouth/nose no longer uniform.…”

Three-dimensional computational fluid dynamics modeling of particle uptake by an occupational air sampler using manually-scaled and adaptive grids

“…The human torso is simulated as a simplified truncated cylinder. The study by Anderson & Anthony (2013) determined that the differences in aspiration efficiencies between three torso geometries: (i) a simplified truncated cylinder, (ii) a non-truncated cylinder, and (iii) an anthropometrically realistic humanoid body, were <8.8% over all particle sizes, indicating that there is little difference in aspiration efficiency between torso models. Grid independence and convergence analysis can be applied in the same manner for an improved version that includes more realistic torso geometries.…”