Some Aspects of Turbulent Mixing of Jets in the Marine Environment

Prominent among environmental problems is the pollution of the coastal marine zone as a result of anthropogenic activities. On this point, while studies of jets in still water and in crossflows have been developed in many research centres, studies on jets interacting with waves are still rare. The present study analyses turbulent, non-buoyant water jets issued into a wave environment. A comparison of the time-averaged and phase-averaged velocity components has been carried out, in order to highlight the flow patterns in the two configurations. The experimental data have also been compared with others in the literature, such as the relationship between the dimensionless, longitudinal, time-averaged velocities of the jet mean axis and the distance from the source. Such comparisons reveal a good agreement. Furthermore, using the analogy between the equation of the turbulent transport of a solute concentration and the equation of the turbulent kinetic energy, the paper presents also estimates of the turbulence diffusion coefficients and advection terms of jets in a wave environment. The experimental results are compared with jets in still water. With the presence of waves, the turbulence length-scales in the streamwise direction vary, contributing to an increase in streamwise turbulent diffusion, relative to the condition of the same jet in still water. The analysis of the jet streamwise advection term reveals that it increases in the case of jets in a wave environment, as compared to no-wave conditions

Multi-phase simulation of infected respiratory cloud transmission in air

In the face of the increasing death toll of the COVID-19 global pandemic, countries around the world have instituted restrictive measures to mitigate the serious effects of the pandemic. Human-to-human transmission of COVID-19 occurs primarily through large droplets that are expelled with sufficient momentum to come in direct contact with the recipients' mouth. Therefore, the physics of flow is central to transmission of COVID-19. Respiratory infections increase the frequency of violent expiration, including coughing and sneezing that are particularly effective in dispersing virus-carrying droplets. Moreover, the high viral load in droplets of asymptomatic hosts that are expelled during respiratory activities is contributing to the rapid growth of the COVID-19 global pandemic. The present study uses 2D smoothed-particle-hydrodynamics multi-phase simulations of the fluid dynamics of violent expiratory events in order to obtain a deeper understanding of the multi-phase nature of respiratory clouds, which can help determine separation distances from an infected person needed to minimize respiratory transmission. Our results indicate that there are three phases of jet cloud flow: the first is dominated by no-buoyancy jet-like dynamics characterized by a high speed, the second is dominated by negative buoyancy, and the third is dominated by gravity that deflects the cloud downward. Moreover, two modes of jet behavior that differ in dilution have been identified to be a function of distance from the human mouth. This work is of direct relevance to studies on the spread of COVID-19 and similar outbreaks in the future

Tailwater Level Effects on Flow Conditions at an Abrupt Drop

This paper presents an experimental study of the transition from supercritical to subcritical flow at an abrupt drop. The paper reports a wide range of experimental conditions and the relevant regime charts. Long-term experimental results show that some flow configurations tend to behave quasiperiodically (i.e. oscillating flow patterns). The experimental results have direct implications on the design and construction of spillway stilling basins where abrupt drops may be used to stabilise the position of the jump. The present study proposes design guidelines taking into account the different flow types, for a range of tailwater flow conditions. These guidelines are essential for a safe and proper operation of stilling basins with abrupt drop