Direct numerical simulation (DNS) is performed to investigate the modes of shedding of the wake of a wall-mounted finite-length square cylinder with an aspect ratio (AR) of 7 for six different boundary layer thicknesses (0.0–0.30) at a Reynolds number of 250. For all the cases of wall boundary layer considered in this study, two modes of shedding, namely, anti-symmetric and symmetric modes of shedding, were found to coexist in the cylinder wake with symmetric one occurring intermittently for smaller time duration. The phase-averaged flow field revealed that the symmetric modes of shedding occur only during instances when the near wake experiences the maximum strength of upwash/downwash flow. The boundary layer thickness seems to have a significant effect on the area of dominance of both downwash and upwash flow in instantaneous and time-averaged flow field. It is observed that the near-wake topology and the total drag force acting on the cylinder are significantly affected by the bottom-wall boundary layer thickness. The overall drag coefficient is found to decrease with thickening of the wall boundary layer thickness.

We discuss the temporal evolution of a cough jet of an infected subject in the context of the spread of COVID-19. Computations were carried out using large eddy simulation, and, in particular, the effect of the co-flow (5% and 10% of maximum cough velocity) on the evolution of the jet was quantified. The Reynolds number (Re) of the cough jet, based on the mouth opening diameter ( D ) and the average cough velocity, is 13 002. The time-varying inlet velocity profile of the cough jet is represented as a combination of gamma-probability-distribution functions. Simulations reveal the detailed structure of cough jet with and without a co-flow for the first time, to the best of our knowledge. The cough jet temporal evolution is similar to that of a continuous free-jet and follows the same routes of instability, as documented for a free-jet. The convection velocity of the cough jet decays with time and distance, following a power-law variation. The cough jet is observed to travel a distance of approximately 1.1 m in half a second. However, in the presence of 10% co-flow, the cough jet travels faster and covers the similar distance in just 0.33 s. Therefore, in the presence of a co-flow, the probability of transmission of COVID-19 by airborne droplets and droplet nuclei increases, since they can travel a larger distance. The cough jet without the co-flow corresponds to a larger volume content compared to that with the co-flow and spreads more within the same range of distance. These simulations are significant as they help to reveal the intricate structure of the cough jet and show that the presence of a co-flow can significantly augment the risk of infection of COVID-19.

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The flow structure in any compound channel is a complicated process due to the transfer of momentum between the deep main channel section and the adjoining shallow floodplains. The boundary shear stress distribution in the main channel and floodplain greatly affects the momentum transfer. In the present work, the shear stress distributions across an assumed interface plane originating from the junction between the main channel and flood plain using the Divided Channel Method (DCM) are analyzed and tested for different compound channels and their flow conditions using global data. An improved equation to predict the boundary shear distribution in compound channels for different width ratios is derived that gives better results than other proposed models. Analyses are also done to suitably choose an appropriate interface plane for evaluation of stagedischarge relationship for compound channels having equal roughness in the channel beds and walls. The effectiveness of predicting the stage-discharge relationship using the apparent shear stress equation and boundary shear distribution models are discussed.

We numerically modeled the droplets’ size distribution of sneezing action from the COVID-19 patient, without considering the viral loading of droplets. Thus, we assumed the behavior of COVID-19 droplets same as that of standard water droplets . In this work, we evolved the initial Weibull distribution (typically used for representing the saliva droplets size distribution) using a non-dimensional droplet size distribution equation under extreme in-homogeneous conditions. We varied the environmental humidity contrast according to the range primarily encountered in the world’s major cities. We found bimodal size distribution of droplets for every humidity contrast, previously reported to be a function of Stoke’s number. This bimodal size distribution of droplets is a consistent event in in-homogeneous mixing. The high humidity contrast between the sneezing zone of influence to the environment will have a comparatively long tail of droplets. This long tail of droplets implies that the evaporation time scales will be highly variable and, consequently, significantly impact the transmission of the virus from a COVID-19 patient to a healthy human being. Essentially means that these cities of high humidity contrast will be more prone to high infections.