An experimental investigation on the surface water transport process over an airfoil by using a digital image projection technique

Icing is widely recognized as one of the most serious weather hazards to aircraft operations. Aircraft icing occurs when small, super-cooled, airborne water droplets, which make up clouds and fog, freeze upon impact with a surface, which results in the formation of ice (Mason 1971). The freezing can be complete or partial, depending on how rapidly the latent heat of fusion can be released into the ambient air. In a dry regime, all the water collected in the impingement area freezes on impact to form rime ice. For a wet regime, only a fraction of the collected water freezes in the impingement area, while the remaining water runs back and can freeze outside the impingement area to form glaze ice (Politovich 1989; Hu and Huang 2009; Hu and Jin 2010). Because of its wet nature, glaze ice can form much more complicated shapes, which are very difficult to accurately predict, and the resulting ice shapes tend to substantially deform the accreting surface with the formation of “horns” and larger “feathers” growing outward into the airflow (Vargas and Tsao 2007). Glaze ice is considered as the most dangerous type of ice. Glaze ice formation can severely decrease the airfoil aerodynamic performance by causing large-scale flow separations that produce dramatic increases in drag and decreases in lift (Politovich 1989; Gent et al. 2000). The transport behavior of unfrozen water prior to freezing has a direct impact on the shape of glaze ice because it Abstract In the present study, an experimental investigation was conducted to characterize the transient behavior of the surface water film and rivulet flows driven by boundary layer airflows over a NACA0012 airfoil in order to elucidate underlying physics of the important micro-physical processes pertinent to aircraft icing phenomena. A digital image projection (DIP) technique was developed to quantitatively measure the film thickness distribution of the surface water film/rivulet flows over the airfoil at different test conditions. The time-resolved DIP measurements reveal that micro-sized water droplets carried by the oncoming airflow impinged onto the airfoil surface, mainly in the region near the airfoil leading edge. After impingement, the water droplets formed thin water film that runs back over the airfoil surface, driven by the boundary layer airflow. As the water film advanced downstream, the contact line was found to bugle locally and developed into isolated water rivulets further downstream. The front lobes of the rivulets quickly advanced along the airfoil and then shed from the airfoil trailing edge, resulting in isolated water transport channels over the airfoil surface. The water channels were responsible for transporting the water mass impinging at the airfoil leading edge. Additionally, the transition location of the surface water transport process from film flows to rivulet flows was found to occur further upstream with increasing velocity of the oncoming airflow. The thickness of the water film/rivulet flows was found to increase