Significance: Airborne transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) or other pathogens is probably increased during indoor exercise, but data on the emission of aerosol particles by an exercising individual are lacking. Here, we report that aerosol particle emission increases on average 132-fold from 580 ± 489 particles/min at rest to 76,200 ± 48,000 particles/min during maximal exercise. Aerosol particle emission increases moderately up to an exercise intensity of ≈2 W/kg and exponentially at higher exercise intensities. These data not only explain SARS-CoV-2 transmissions during indoor group exercise but also can be used to design better targeted mitigation measures for physical activity indoors such as physical education in school, dance events during weddings, or high-intensity gym classes such as spinning.

Emulsion formation is of great interest in the chemical and food industry and droplet breakup is the key process. Droplet breakup in a quiet or laminar flow is well understood, however, actual in-dustrial processes are always in the turbulent flow regime, leading to more complex droplet breakup phenomena. Since high resolution optical measurements on microscopic scales are extremely dif-ficult to perform, many aspects of the turbulent droplet breakup are physically unclear. To over-come this problem, scaled experimental setups (with scaling factors of 5 and 50) are used in con-junction with an original scale setup for reference. In addition to the geometric scaling, other non-dimensional numbers such as the Reynolds number, the viscosity ratio and the density ratio were kept constant. The scaling allows observation of the phenomena on macroscopic scales, whereby the objective is to show that the scaling approach makes it possible to directly transfer the findings from the macro- to the micro-/original scale. In this paper, which follows Part I where the flow fields were compared and found to be similar, it is shown by breakup visualizations that the turbulent droplet breakup process is similar on all scales. This makes it possible to transfer the results of detailed parameter variations investigated on the macro scale to the micro scale. The evaluation and analysis of the results imply that the droplet breakup is triggered and strongly influenced by the intensity and scales of the turbulent flow motion.

Properties of emulsions such as stability, viscosity or color can be influenced by the droplet size distribution. High-pressure homogenization (HPH) is the method of choice for emulsions with a low to medium viscosity with a target mean droplet diameter of less than 1 µm. During HPH, the droplets of the emulsion are exposed to shear and extensional stresses, which cause them to break up. Ongoing work is focused on better understanding the mechanisms of droplet breakup and relevant parameters. Since the gap dimensions of the disruption unit (e.g., flat valve or orifice) are small (usually below 500 µm) and the droplet breakup also takes place on small spatial and time scales, the resolution limit of current measuring systems is reached. In addition, the high velocities impede time resolved measurements. Therefore, a five-fold and fifty-fold magnified optically accessible coaxial orifice were used in this study while maintaining the dimensionless numbers characteristic for the droplet breakup (Reynolds and Weber number, viscosity and density ratio). Three matching material systems are presented. In order to verify their similarity, the local velocity profiles of the emerging free jet were measured using both a microparticle image velocimetry (µ-PIV) and a particle image velocimetry (PIV) system. Furthermore, the influence of the outlet geometry on the velocity profiles is investigated. Similar relationships were found on all investigated scales. The areas with the highest velocity fluctuations were identified where droplets are exposed to the highest turbulent forces. The Reynolds number had no influence on the normalized velocity fluctuation field. The confinement of the jet started to influence the velocity field if the outlet channel diameter is smaller than 10 times the diameter of the orifice. In conclusion, the scaling approach offers advantages to study very fast processes on very small spatial scales in detail. The presented scaling approach also offers chances in the optimization of the geometry of the disruption unit. However, the results also show challenges of each size scale, which can come from the respective production, measurement technology or experimental design. Depending on the problem to be investigated, we recommend conducting experimental studies at different scales.

Overview: The future belongs to children and they need education to shape the future with foresight and intention. Children therefore have the right to education, according to Article 29 of the UN Convention on the Rights of the Child [1]. However, professional education is not everything, because children must also experience their strengths and weaknesses together and educate each other to be responsible and considerate people, so that they become socially valuable personalities. Only in this way can they shape the future in a peaceful and humane way. Therefore, attending school is essential. However, children also have the right to protection and care by their parents and the state, because the welfare of the child must also be given priority in accordance with Article 3 of the UN Convention on the Rights of the Child. The question is therefore how schooling in community schools can be realized during the SARS-CoV-2 pandemic without exposing children to an unnecessary risk of infection. It is not only about the children, because if the children are at risk, then so are their parents and grandparents and ultimately society as a whole. There are numerous concepts that promise safety in schools during the pandemic. When selecting concepts, the costs must of course be weighed against the benefits. People rightly expect an efficient use of resources. This means that either the set goal is achieved with the least possible resources or that the available resources are used to achieve the greatest possible approximation to the goal. In addition to the financial resources, however, the long-term consequences for the state, the economy, the population and the environment under the pressure of the pandemic must also be taken into account. Social cohesion and democracy must not be jeopardized either. Various protection concepts are currently under discussion. Often the advantages are overstated and the disadvantages concealed. Furthermore, some arguments are based on assumptions that are not true. The aim of this study is to provide a comparative assessment of the main protection concepts and to demonstrate, with the help of experimental analyses, the extent to which the protection concepts are effective. We will show that a comparatively high level of safety against infection in classrooms can be technically ensured without exposing children to masks. At the same time, the protection concept makes economic sense and the burden on the environment is comparatively low, so that infection prevention and climate protection do not have to be weighed against each other, because infection prevention and climate protection are political and social goals that have to be achieved together.

The droplet deformation in dispersing units of high-pressure homogenizers (HPH) is examined experimentally and numerically. Due to the small size of common homogenizer nozzles, the visual analysis of the transient droplet generation is usually not possible. Therefore, a scaled setup was used. The droplet deformation was determined quantitatively by using a shadow imaging technique. It is shown that the influence of transient stresses on the droplets caused by laminar extensional flow upstream the orifice is highly relevant for the droplet breakup behind the nozzle. Classical approaches based on an equilibrium assumption on the other side are not adequate to explain the observed droplet distributions. Based on the experimental results, a relationship from the literature with numerical simulations adopting different models are used to determine the transient droplet deformation during transition through orifices. It is shown that numerical and experimental results are in fairly good agreement at limited settings. It can be concluded that a scaled apparatus is well suited to estimate the transient droplet formation up to the outlet of the orifice.

Mobile air cleaners have emerged as a practical solution for reducing indoor aerosol concentrations, particularly in the absence of HVAC systems. Their efficiency is typically assessed under standardised conditions, but how turbulence influences the effective air exchange rate indoors is not well understood. In this study, we present a systematic investigation of the impact of enhanced turbulence on aerosol decay in two room sizes (50 m³ and 200 m³) using a mobile air cleaner combined with different fan configurations. Particle counter measurements were conducted simultaneously with particle image velocimetry ($PIV$), enabling direct comparison of air exchange rates and flow field properties. Our results show that specific fan arrangements significantly modify turbulent kinetic energy ($TKE$) distributions and, in turn, alter the effective air exchange rate. In the smaller room, configurations generating higher $TKE$ brought the measured exchange rates closer to theoretical predictions, while in large rooms other arrangements led to noticeable deviations. We anticipate that these findings provide a reference framework for understanding the role of turbulence in indoor air cleaning performance, with implications for optimizing the operation and placement of mobile air cleaners in practical environments.