Tretola Giovanni, Vogiatzaki Konstantina
Department of Engineering, King's College London, Strand, London, WC2R 2ND, UK.
Sci Rep. 2022 May 6;12(1):7416. doi: 10.1038/s41598-022-11188-7.
The impact of a liquid droplet onto a solid surface is a phenomenon present in a wide range of natural processes and technological applications. In this study, we focus on impact conditions characterised by ultra high velocities (up to 500 m/s), to investigate-for the first time-how the impact dynamics change when the compressibility of the liquid in the droplet is no longer negligible. A water droplet impacting a dry substrate at four different velocities, from 50 to 500 m/s, is simulated. Such conditions are particularly relevant to aviation as well as industrial gas turbine engine risk management. Thus, numerical investigations as the one we present here provide a powerful tool to analyse the process. We find that increasing the impact velocity changes the flow field within and outside the droplet the moment that the compressibility can no longer be neglected, with the rise of pressure fronts in both regions. Increasing the impact velocity, the compressibility affects also the lamella formed and changes its ejection velocity observed over time (and thus the wetting behaviour) when the region shift from incompressible to compressible. Moreover, it is found that the maximum pressure observed at the wall during the impact is located at the corner of the impact, where the lamella is ejected, not in the centre, and it is influenced by the initial velocity. To predict the maximum pressure experienced by the surface during the high velocity impact, we propose a correlation based on the initial Weber and Reynolds number of the droplet. The complexity and the scales of the dynamics involved in the ultra-high velocity impact is limiting the experimental and analytical studies. To the best of our knowledge there are no experimental data currently available at such conditions. In this study, through numerical simulations, new insights about the impact dynamics at such conditions are provided.
液滴撞击固体表面是广泛存在于自然过程和技术应用中的一种现象。在本研究中，我们聚焦于以超高速（高达500米/秒）为特征的撞击条件，首次研究当液滴中液体的可压缩性不再可忽略时撞击动力学如何变化。模拟了一个水滴以50至500米/秒的四种不同速度撞击干燥基底的情况。此类条件与航空以及工业燃气涡轮发动机风险管理特别相关。因此，像我们在此展示的数值研究为分析该过程提供了一个强大的工具。我们发现，当可压缩性不再可忽略时，增加撞击速度会改变液滴内外的流场，两个区域都会出现压力前沿的上升。随着撞击速度的增加，当区域从不可压缩转变为可压缩时，可压缩性还会影响形成的薄片，并改变其随时间观察到的喷射速度（进而影响润湿行为）。此外，还发现撞击过程中在壁上观察到的最大压力位于薄片喷射处的撞击角，而非中心，并且它受初始速度影响。为了预测高速撞击过程中表面所经历的最大压力，我们基于液滴的初始韦伯数和雷诺数提出了一种关联式。超高速撞击所涉及的动力学的复杂性和尺度限制了实验和分析研究。据我们所知，目前在这样的条件下没有可用的实验数据。在本研究中，通过数值模拟，提供了关于此类条件下撞击动力学的新见解。
