A flow-dependent estimation of constriction size distribution in gap-graded soils: an integrated statistical approach

The clogging-unclogging process in gap-graded soils is a result of the migration of seepage-driven fines, which subsequently induces measurable changes in the local hydraulic gradients. This process can be temporally observed in the variations of Darcy's hydraulic conductivity (K). The current study proposes an integrated statistical Monte Carlo approach combining the discrete element method and 2D computational fluid dynamics simulations to estimate the flow-dependent constriction size distribution (CSD) for a gap-graded soil. The computational inferences were supported with experimental results using an internally stable soil, which was subjected to one-dimensional flow stimulating desired hydraulic loadings: a hydraulic gradient lower than the critical gradient applied as a multi-staged loading pattern. The 35th percentile size of the flow-dependent CSD (Dc35) for both internally stable and unstable gap-graded soils becomes approximately equal to Dc35 at steady-state. However, a greater variation of larger constrictions persists for the unstable soils. This pilot study has shown the applicability of the proposed method to estimate flow-dependent CSD for a wide range of experimentally observed K values.

Expiratory aerosol particle escape from surgical masks due to imperfect sealing

Wearing surgical masks or other similar face coverings can reduce the emission of expiratory particles produced via breathing, talking, coughing, or sneezing. Although it is well established that some fraction of the expiratory airflow leaks around the edges of the mask, it is unclear how these leakage airflows affect the overall efficiency with which masks block emission of expiratory aerosol particles. Here, we show experimentally that the aerosol particle concentrations in the leakage airflows around a surgical mask are reduced compared to no mask wearing, with the magnitude of reduction dependent on the direction of escape (out the top, the sides, or the bottom). Because the actual leakage flowrate in each direction is difficult to measure, we use a Monte Carlo approach to estimate flow-corrected particle emission rates for particles having diameters in the range 0.5–20 μm. in all orientations. From these, we derive a flow-weighted overall number-based particle removal efficiency for the mask. The overall mask efficiency, accounting both for air that passes through the mask and for leakage flows, is reduced compared to the through-mask filtration efficiency, from 93 to 70% for talking, but from only 94–90% for coughing. These results demonstrate that leakage flows due to imperfect sealing do decrease mask efficiencies for reducing emission of expiratory particles, but even with such leakage surgical masks provide substantial control.

Expiratory aerosol particle escape from surgical masks due to imperfect sealing

Wearing surgical masks or other similar face coverings can reduce the emission of expiratory particles produced via breathing, talking, coughing, or sneezing. Although it is well established that some fraction of the expiratory airflow leaks around the edges of the mask, it is unclear how these leakage airflows affect the overall mask efficiency at blocking emission of expiratory aerosol particles. Here, we show experimentally that the aerosol particle concentrations in the leakage airflows around a surgical mask are reduced compared to no mask wearing, with the magnitude of reduction dependent on the direction of escape (out the top, the sides, or the bottom). Because the actual leakage flowrate in each direction is difficult to measure, we use a Monte Carlo approach to estimate flow-corrected particle emission rates in all orientations, from which a flow-weighted overall efficiency is derived. The overall mask efficiency, accounting both for air that passes through the mask and for leakage flows, is reduced compared to the through-mask efficiency, from 93% to 70% for talking, but from only 94% to 90% for coughing. These results demonstrate that leakage flows due to imperfect sealing do decrease mask efficiencies for reducing emission of expiratory particles, but even with such leakage surgical masks provide substantial control.