Splash: Understanding the Dynamics of High-Speed Drop Impact

毛细管驱动的自由表面流动的动力学，以及湍流行为，可以说，在流体动力学中最具挑战性的课题。它们涉及跨越几个长度和时间尺度的复杂流动。液滴形成和解体过程中涉及的动态流体过程是迷人的，但极其复杂，与时间相关的流体界面中断。撞击到表面（固体、液体或颗粒）上的液滴可导致简单的扩散、弹跳或飞溅。由此产生的动力学不仅取决于液体的性质和液滴的速度，而且还取决于各种其他参数，例如表面的粗糙度、硬度、化学性质和温度以及周围条件。虽然在某些工业应用中需要飞溅（例如冷却和燃烧），但在其他应用中应不惜一切代价避免飞溅（例如喷墨打印或预防传染病传播，如Covid-19大流行病所示）。在我的研究中，我目前正在使用先进的数学技术，结合最先进的计算能力，以及实验成像技术的技术进步，这使我能够以前所未有的细节和前所未有的速度观察和建模动态。飞溅是水滴领域最迷人的，尽管具有挑战性的主题之一。然而，触发飞溅的确切机制仍然难以捉摸。借助尖端的高速摄影、现代高分辨率数值模拟和渐近理论，拟议研究的主要目标是揭示飞溅背后的动力学和触发飞溅的动力学。特别是，我们的目标是：1-确定的参数导致飞溅时，液滴的影响池的可混溶和不可混溶的流体。这包括开发能够在这些剧烈条件下解析三相流的模型。2-揭示撞击时接触线（液滴/液池）的（垂直）速度。初步结果表明，一个复杂的关系之间的比率的密度和粘度之间的液体的下降和池在这里起着重要的作用。3-理解的贡献，由于纯粘性和粘弹性的影响的下降和目标。4-探索目标的曲率对所产生的动力学的影响。5-基于上述内容，探索抑制飞溅的技术。本研究的第一部分（1和2）已经进行了一系列系统的实验，对液滴在不同粘度的（不混溶的）基质上的影响进行了研究。目前正在进行流体体积模拟，这些模拟提供了实验中无法获得的细节（例如内部速度场），以更好地了解潜在的动力学以及如何导致飞溅。虽然上述研究领域主要涉及在液体对液体冲击的情况下液滴的运动，但池本身的运动也是非常感兴趣的，但经常被忽视，特别是在冲击的早期。虽然存在用于池运动的一些模型，但这些模型在很大程度上限于液滴和池流体是相同流体时，并且通常在它们通常不合适时使用。因此，另一个需要研究的领域是撞击前后水池的运动，以及水池运动如何受到相关流体特性的影响，以及水池运动如何影响飞溅。这些目标是主要的事业，并与R博士密切合作。这项研究福尔斯EPSRC的流体动力学和空气动力学、复杂流体和流变学以及制造未来的领域，因为在喷墨打印等工业应用中，上述研究具有重要意义。

The dynamics of capillary-driven free surface flows, together with turbulent behaviour are, arguably, some of the most challenging topics in fluid dynamics. They involve complex flows evolving across several length and time scales. The dynamic fluid processes involved during drop formation and disintegration are fascinating but extremely complicated, with time-dependent fluid interface disruptions. A drop impacting on to a surface (solid, liquid, or granular) can lead to simple spreading, bouncing, or splashing. The resulting dynamics depends not only on the liquid properties and speed of the drop, but on a variety of other parameters, such as the surface's roughness, stiffness, chemistry, and temperature, and surrounding conditions. While in some industrial applications splashing is desired (e.g. cooling and combustion), it is to be avoided at all cost in others (e.g. inkjet printing or in the prevention of the spreading of infectious diseases as the Covid-19 pandemic has shown). For my research, I am currently using advanced mathematical techniques combined with state-of-the-art computational power, and technological advances in experimental imaging techniques, which allow me to observe and model the dynamics in unprecedented detail and at unprecedented speeds.Splashing is one of the most fascinating, albeit challenging, topics in the field of drops. However, the exact mechanisms triggering a splash have remained elusive. With the help of cutting-edge ultrahigh speed photography, modern ultrahigh resolution numerical simulations and asymptotic theory the main objective of the proposed research is to reveal the dynamics underlying and triggering a splash. In particular, we aim at: 1- Identifying the parameters leading to a splash in when a droplet impacts a pool of miscible and immiscible fluids. This includes developing models capable of resolving three phase flows under these violent conditions.2- Reveal the (vertical) speed of the contact line (drop/pool) upon impact. Preliminary results demonstrate that a complex relationship between the ratio of densities and viscosities between the liquids of the drop and the pool play an important role here.3- Understand the contributions due to pure viscous and viscoelastic effects of both the drop and target. 4- Explore the influence that the curvature of the target has on the resulting dynamics.5- Based on the above, explore techniques to suppress splashing. The first part (1 and 2) of this this research has already been performed carrying out a series of systematic experiments of the impact of drops onto (immiscible) substrates of varying viscosity. Volume of Fluid simulations are currently being carried out which are providing detail not available from experiments (e.g. internal velocity fields) to enable greater understanding of the underlying dynamics and how this can lead to splashing. Whilst the aforementioned area of research is largely concerned with the motion of the droplet in the case of liquid on liquid impact the motion of the pool itself is also one of great interest but is often overlooked, especially in the early times upon impact. Whilst some models exist for the pool motion, these are largely limited to when the droplet and pool fluid are the same fluid and are often used when they are often not appropriate. For this reason, another area to be researched is the motion of the pool both before and after impact and how this pool motion is affected by the relevant fluid properties and how this pool motion can affect splashing. These objectives are major undertakings, and a close collaboration with Dr. R. Cimpeanu at the University of Warwick.The proposed research falls within the EPSRC areas of Fluid Dynamics and Aerodynamics, Complex Fluids and Rheology, as well as Manufacturing the Future for the importance identified above in industrial applications such as inkjet printing.