Reply to Comment by Sookhak Lari, K. and Davis, G. B. on “ ‘Large Eddy Simulation of Turbulence and Solute Transport in a Forested Headwater Stream’: Invalid Representation of Scalar Transport by the Act of Diffusion”

Abstract

Based on the molecular diffusion coefficient used in the large eddy simulation (LES) (equation (3) of the article), one can easily infer that a molecular Schmidt number of 700 was adopted for the solute. It should be acknowledged that in the published article we omitted to include the value used for the molecular diffusion and did not mention that is the reciprocal of the turbulent Schmidt number. Although these details should have been included, these omissions do not change well‐established facts that in turbulent flows the mixing of the solute by the molecular diffusion is many orders of magnitude less important than that induced by the turbulence. As we mentioned in our article, the viscous sublayer in the Eagle Creek stream was not resolved in our simulations as the first node off the solid boundaries in wall units was (on average) equal to 45 (see Table 3 of the article). This implies that all of the computational nodes are located well outside of the viscous sublayer, where molecular diffusion becomes dominant. Moreover, since we carry out LES, we do not resolve the Kolmogorov scales in the flow the rate of dissipation of which is governed by molecular viscosity. Rather, the subgrid scale eddy viscosity of the LES acts as the model for the effect of the unresolved scales of motion. Even though all these are well‐known facts from fundamental theory of turbulent mixing, we include in this response additional simulations to address the concerns raised in this commentary.

A computational study of expiratory particle transport and vortex dynamics during breathing with and without face masks

“…The saliva plume concentration in the dilute mixture of air and saliva is modeled as an active scalar. We compute the concentration of saliva plume using the following convection-diffusion equation: 39 where is the contravariant volume flux of saliva concentration in the vertical direction due to the settling velocity ( ) of the particles, is the laminar Schmidt number (= 1), 40 is the turbulent Schmidt number (= 0.75), 40 and is the eddy viscosity.…”

On the efficacy of facial masks to suppress the spreading of pathogen-carrying saliva particles during human respiratory events: Insights gained via high-fidelity numerical modeling

“…The following convection-diffusion equation governs the saliva particles’ concentration 71 : where the vertical contravariant volume flux of saliva concentration, induced by the particles’ settling velocity , is expressed as . Here, represents the eddy viscosity, while and respectively denote the laminar and turbulent Schmidt numbers, assigned values of 100 and 0.75 72 . The settling velocity of the particles is determined as such 60 : where is the diameter of the saliva particle.…”