Date on Master's Thesis/Doctoral Dissertation
Mechanical Engineering, PhD
Committee Co-Chair (if applicable)
Droplet; impact; CFD; multiphase; SLIP; LIS
Drop impact on different types of surfaces are important physical concepts that are routinely found in day-to-day life and such studies have immense application for various types of industries. One such important application of drop dynamics is in the field of aviation science which is concerned of very large freezing drizzle drops impacting on airplane wings. Such drops are known as supercooled large droplets (SLD), and they pose a great risk and have been long known to have caused notable accidents in the past. SLDs are liquid drops that can remain in the state of liquid phase and grow into ice after an impact onto a solid body. Sometimes such freezing drizzle can splash and break into multiple daughter or satellite drops, and sometimes they can bounce off the substrate on which the impingement occurs. Due to the importance in aviation safety, researchers over the past decade has studied SLDs, but most of the studies are experimental studies which produced empirical relationship and little numerical simulation that can effectively vary and optimize drop impingement parameters. In this study, numerical simulation is used to study the dynamics of water drops impacting on various types of substrates. The numerical simulation uses a very sharp interface reconstruction method known as moment-of-fluid method. At the interface between the solid-liquid and liquid-gas, lubricant-gas and lubricant-liquid, adaptive mesh refinement is used to correctly capture the moving interface curvatures and directions. To understand the importance of the underneath substrate surface, drop coalescence study has been done to show that merging drops can benefit from surface energy reduction to propel drops with higher kinetic energy, and the degree of curvature greatly affects the propelling behavior. For dry surface comparison, a drop impacting on a large micro-well cavity is studied and compared to a flat substrate. At different contact angles, and impact velocities, it has been shown that for certain range of speeds and wettability, the drops can only rebound from the micro-well cavity but not from the flat substrate. There has been found a notable difference in kinetic energy, spreading area, and wetting area ratios between the two cases. For the third study, a micro-well substrate is filled with lubricant, and drop impact cases at different velocities is studied. In this study we found that cloaking occurs when both lubricant and water interfacial tensions and impact speeds are low. Furthermore, we have observed that the thickness of the encapsulating lubricant layer changes over time. At moderate impact speeds, the lubricant layer is displaced, generating a lubricant-water jet, as we have demonstrated. However, at high impact speeds, a secondary impingement occurs, displacing a significant amount of lubricant and exposing the underlying substrate, which was not visible at lower impact speeds. Additionally, we conducted simulation on microwells infused with lubricant and observed that small spacing between the micro-well walls can limit lubricant drainage and displacement. The use of micro-wells also resulted in less splashing compared to substrates without micro-wells. Finally, we confirmed that microwells are more effective at preserving lubricant than substrates without micro-wells.
Nazrul Islam, Ahmed, "Drop impact on dry and liquid infused substrates with micro-wells." (2023). Electronic Theses and Dissertations. Paper 4045.