Effects of surface roughness on splashing characteristics of large droplets with digital inline holographic imaging

Ice accretion on aircraft wings due to Supercooled Large Droplet (SLD) has a significant impact on flight safety. The dynamic behaviours of large droplet, such as deformation, breakup and splashing, have significant effects on icing accretion. The corresponding studies have been carried out experimentally and numerically, and empirical models to predict the dynamic behaviours of large droplets have been proposed. However, few empirical models involve surface roughness effects, especially its effects on the secondary droplets during icing accretion. In this paper, an experimental investigation on the impact of droplets onto surfaces with different roughness was conducted by using the high-speed Digital Inline Holography (DIH). The rapid splashing processes of droplets with Weber number around 570 and Mundo impact parameter K around 240 were captured and reconstructed based on a wavelet-based algorithm. Results show that DIH is suitable to diagnose the secondary droplets with a wide size range and record their velocities and locations. The roughness of the impact surface has a positive effect on the dynamics of splashing droplets, and the parameters including diameter, velocity, splash angle, splash height and mass-loss rate of splashing droplets rise with the increase of roughness. The new empirical correlations of the splashing characteristics of secondary droplets were fitted and developed, which could be used to update the current icing simulation models to take into account the effect of impact surface roughness on large droplets dynamics. This fundamental study may also be applied to the industrial applications including wind turbine icing, inkjets, sprays, as well as the erosion and formation of ice layers.