Numerical investigation of drop impingement on dry and wet surfaces.

Abstract

In-flight icing due to supercooled large droplets (SLDs) imposes great danger on aviation safety. Because of the large size, SLDs have different characteristics than typical cloud droplets that most icing encounters involve. As a result, SLDs more likely hit the wing surface and move into areas not protected by de-icing devices to form ice, which can lead to loss of lift, increase in drag, altered controllability, and eventually stall and loss of control of the aircraft. The phenomenon of droplet splashing is considered as the most important aspect of the SLD icing accretion problem. Although previous studies have established splashing threshold and revealed most parameters affecting splashing, the splashing mechanism is still not fully understood. In this study, the impacts of droplets on both dry and wet surfaces were numerically investigated to understand the splashing mechanism. A Navier-Stokes solver was used to describe the flow field, the moment-of-fluid (MOF) method was used to capture the droplet interface evolution, the adaptive mesh refinement technique was employed to refine the mesh near the regions of interest, and the dynamic contact angle model was used to represent the wettability of the solid surface. Using the multiphase flow solver, the effect of ambient gas density on splashing was intensively studied and it was confirmed that lowering ambient gas density can suppress dry surface splashing while has no significant influence on wet surface splashing. Then, high-speed drop impact on thin liquid film with a focus on oblique impact was investigated. The numerical results showed that the tangential velocity can significantly alter impact phenomena: a higher tangential velocity leads to lower lamella height and radius on the side behind the advancing drop, the higher tangential velocity also leads to stronger vortices at the drop and film interface due to Kelvin-Helmholtz instability. Simulations of oblique impacts of two neighboring drops were also conducted for low-speed and high-speed impacts. Strong interaction occurred when the crowns formed by the two neighboring drops interfered with each other. For low-speed impact, droplets deposited on to the liquid film with short and thick crowns formed and the interaction region was superposition of crowns. For high-speed impact, crowns broken up to form splashing and the interaction behavior became complicated. Finally, the droplet impacts on a MS(1)-317 airfoil was studied and the water collection efficiency and impingement limit were investigated. Unlike most previous studies, the flow field and droplet behavior were simultaneously simulated using one multiphase flow solver. The results were compared with the ice accretion simulation code LEWICE and experimental data. The simulations showed that the calculated water collection efficiency of cloud-sized droplets matched the result of LEWICE, however, the calculated water collection efficiency of large droplets showed better agreement with the experimental data than LEWICE. The better agreement was attributed to the droplet trajectory calculation and the capture of droplet splashing.