Several industrial operations involve droplet impactions on solid walls. This has attracted significant attention among many researchers who intend to fully understand the dynamical droplet impaction evolution. Alternatively, recent studies show that the shock waves occurring inside the droplets are as a result of the high-speed droplet impingement. Consequently, during the early stages of the droplet impaction, it has been pointed out that the liquid cavitation is attributed to the formation of the reflected waves. Unfortunately, the flow characteristics of the droplets are becoming more complicated thus rendering most of the currently used methods insufficient for their investigations. To this note, current researchers are adopting the use of computer simulation technologies for investigating the high-speed impaction in droplets. Specifically, this applies to water cylindrical droplets involving phase transitions.
As such, Tsinghua University scientists led by Professor Bing Wang from the School of Aerospace Engineering utilized numerical methods to investigate the complex flow characteristics, especially in high-speed cylindrical droplets impingement. In particular, they designed a compressible two-phase multi-component fluid model comprising of the phase transition procedure and utilized it in the solution of the desired two-phase hydrodynamics system. Their fascinating work is currently published in the research journal, Journal of Fluid Mechanics.
In brief, the research team commenced their work with a detailed cross-examination of the occurrence of the high-speed impingement of droplets on walls both naturally and in industries. Next, due to the missing information on the physical mechanisms of the complicated flow characteristics, a compressible two-phase fluid model was developed to describe the homogenous cavitation process and the Eulerian flow system. On the other hand, the phase transition was modeled based on the thermodynamic properties of the fluid. Eventually, both quantitative and qualitative analysis was carried out to determine the morphological and dynamic characteristics of the droplet processes.
The authors observed that a lower pressure value as compared to the threshold value was required to trigger the phase transition in the simulation process which paved way for detailed investigation of the cavitation evolution process in the droplet. In addition, the authors found out that the generation of the shock waves was as a result of the impact of the high-speed cylindrical droplet on the walls. Consequently, it was possible to reflect the shock waves on the curved surface of the droplet thus resulting in the formation of the cavitation zone. Furthermore, it was necessary to investigate the effects of the impact speeds on the cylindrical droplet deformation and evolution of the cavitation zone. It was worth noting that the initial impaction speed had got no effects on the focusing position that indicated the location of the cavitation core.
The study by Tsinghua University scientists successfully demonstrated the characteristic flow of the high-speed cylindrical droplets on a solid wall in which the effects of different impaction speeds on the cavitation processes and droplet dynamics were presented. For instance, it was noted that the strength of the produced shock waves increased with the increase of the initial impaction speeds. Therefore, by taking into consideration the effects of the surface tension and viscosity on the cavitation inception and collapse, the study will pave way for future analysis of droplets involving different liquids.
Wu, W., Xiang, G., & Wang, B. (2018). On high-speed impingement of cylindrical droplets upon solid wall considering cavitation effects. Journal of Fluid Mechanics, 857, 851-877.Go To Journal of Fluid Mechanics