Splashing generation by water jet impinging on a horizontal plate

The presence of the SARS-CoV-2 virus in wastewater has raised concerns about airborne transmission during sewage transport. Prior research has shown that viruses can remain viable in aerosols, but the mechanisms of aerosol generation in sewer systems remain unclear. It was already known that water jets can produce droplets when impacting surfaces, but the extent and characteristics of these droplets were not fully understood. This gap motivated the need to investigate how water jets behave when they strike horizontal surfaces. No prior work had resolved the specific conditions under which splashing occurs or the size and velocity of the resulting droplets. Understanding these factors is important for assessing potential airborne transmission risks. The study focuses on the physical behavior of water jets and the droplets they produce. It aims to clarify the conditions that lead to different splashing regimes. This approach provides a foundation for evaluating how such processes might contribute to virus dispersion.

This study aimed to experimentally examine the behavior of water jets when they impinge on a horizontal plate. The specific problem addressed was the generation of tiny droplets and the factors influencing their formation. The motivation was to understand the potential for airborne virus transmission through wastewater systems. The researchers focused on the conditions under which splashing occurs and the characteristics of the resulting droplets. They sought to identify the regimes of splashing and quantify the splashing ratio. The study also aimed to determine the size, velocity, and ejection angle distributions of the droplets. These parameters are essential for modeling aerosol dispersion in real-world settings. The findings could inform risk assessments related to wastewater handling and transport.

The researchers conducted experiments using a water jet directed vertically onto a horizontal plate. They varied the jet conditions to observe different splashing behaviors. High-speed imaging and particle image velocimetry were used to capture droplet formation and motion. The jet was analyzed for whether it broke into primary drops before impact. Based on this, three splashing regimes were identified: non-splashing, jet-splashing, and drop-splashing. The splashing ratio was calculated as the portion of the jet flow rate that became splashing droplets. Droplet size and velocity distributions were measured using image analysis techniques. The ejection angles were determined from the direction of droplet movement relative to the impact surface. These methods allowed the researchers to quantify the physical properties of the generated droplets.

The splashing ratio ranged from 1% to 70% in the drop-splashing regime, while it remained below 2% in the jet-splashing regime. Droplet size and velocity distributions followed log-normal laws, indicating a wide range of sizes and speeds. The median diameter of splashing droplets was less than 0.1 times the impact jet or drop diameter. Their velocities mostly ranged from 0 to 3.0 times the impact velocity, with a median around 1.0. The medians of both the dimensionless diameter and velocity decreased with increasing impact Weber number. Ejection angles of splashing droplets followed a bell-shaped distribution, with the maximum around 70°. The median ejection angle ranged from 16° to 30°, depending on the regime. These results provide quantitative data on the behavior of droplets generated by water jet impingement.

The study identified three distinct splashing regimes based on whether the jet breaks into primary drops before impact. The drop-splashing regime produced the highest splashing ratio, ranging from 1% to 70%. Droplet size and velocity distributions followed log-normal patterns, with medians significantly smaller than the impact jet or drop. The researchers observed that both the median diameter and velocity of droplets decreased with increasing Weber number. Ejection angles were distributed in a bell-shaped pattern, peaking around 70°. The median ejection angle varied between 16° and 30°, depending on the regime. These findings provide a detailed characterization of droplet generation during water jet impingement. The results suggest that the physical properties of splashing droplets are influenced by the impact conditions and jet behavior.