Abstract
The rich structures arising from the impingement dynamics of water drops onto solid substrates at high velocities are investigated numerically. Current methodologies in the aircraft industry estimating water collection on aircraft surfaces are based on particle trajectory calculations and empirical extensions thereof in order to approximate the complex fluid-structure interactions. We perform direct numerical simulations (DNS) using the volume-of-fluid method in three dimensions, for a collection of drop sizes and impingement angles. The high speed background air flow is coupled with the motion of the liquid in the framework of oblique stagnation-point flow. Qualitative and quantitative features are studied in both pre- and post-impact stages. One-to-one comparisons are made with experimental data available from the investigations of Sor and García-Magariño (2015), while the main body of results is created using parameters relevant to flight conditions with droplet sizes in the ranges from tens to several hundreds of microns, as presented by Papadakis et al. (2004). Drop deformation, collision, coalescence and microdrop ejection and dynamics, all typically neglected or empirically modelled, are accurately accounted for. In particular, we identify new morphological features in regimes below the splashing threshold in the modelled conditions. We then expand on the variation in the number and distribution of ejected microdrops as a function of the impacting drop size beyond this threshold. The presented drop impact model addresses key questions at a fundamental level, however the conclusions of the study extend towards the advancement of understanding of water dynamics on aircraft surfaces, which has important implications in terms of compliance to aircraft safety regulations. The proposed methodology may also be utilised and extended in the context of related industrial applications involving high speed drop impact such as inkjet printing and combustion.
Highlights
Since the days of Worthington (1876), the problem of droplet impact has offered the fluid dynamics research community exciting opportunities and challenges over the course of its history
In order to reach velocities beyond O(1) m/s it is necessary to have some form of ejection mechanism that ensures reproducibility of the shapes, and a stability of the dynamics in early stages as the drop travels through the quiescent air flow and may become immediately sheared and violently deformed and broken up
Therein, an experimental setup consisting of a monosize droplet dispenser, a rotating arm with a model wing fixed at its end, as well as associated motor and camera equipment are used to capture the drop dynamics as the solid body approaches the liquid droplets at velocities of up to 100 m/s
Summary
Since the days of Worthington (1876), the problem of droplet impact has offered the fluid dynamics research community exciting opportunities and challenges over the course of its history. In order to reach velocities beyond O(1) m/s it is necessary to have some form of ejection mechanism that ensures reproducibility of the shapes, and a stability of the dynamics in early stages as the drop travels through the quiescent air flow and may become immediately sheared and violently deformed and broken up. In an effort to reproduce the same type of air flow environment while preserving generality, we proposed an oblique-stagnation point flow model for the air flow, with the liquid drop being seeded sufficiently far away from the body on the dividing streamline of the flow. The reasons behind this choice are twofold: 1. The choice for stagnation-point background flow and the initial position of the drop ensures that the air flow undergoes only small changes until sufficiently close to the surface, which is when we expect the drop to start deforming in real life conditions
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