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，可以说是流体动力学中最具挑战性的一些课题。它们涉及在几个长度和时间尺度上演变的复杂流动。液滴形成和解体过程中涉及的动态流体过程令人着迷，但极其复杂，与时间相关的流体界面破裂。一滴落在表面(固体、液体或颗粒)上的液滴可能会导致简单的扩散、反弹或飞溅。由此产生的动力学不仅取决于液滴的性质和速度，还取决于各种其他参数，如表面的粗糙度、硬度、化学成分、温度和周围条件。虽然在一些工业应用中需要飞溅(如冷却和燃烧)，但在其他应用中应不惜一切代价避免飞溅(如喷墨打印或防止传染病传播，如新冠肺炎大流行所表明的那样)。在我的研究中，我目前正在使用先进的数学技术和最先进的计算能力，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动态并对其进行建模。飞溅是水滴领域最吸引人的、尽管具有挑战性的课题之一。然而，引发飞溅的确切机制仍然难以捉摸。借助前沿的超高速摄影、现代超高分辨率数值模拟和渐近理论，本研究的主要目的是揭示飞溅背后和触发飞溅的动力学。特别是，我们的目标是：1-确定当液滴撞击可混和不混相流体池时导致飞溅的参数。这包括开发能够在这些剧烈条件下分解三相流动的模型。2-揭示碰撞时接触线(水滴/水池)的(垂直)速度。初步结果表明，液滴和熔池液体之间的密度和粘度比之间的复杂关系在这里起到了重要作用。3-了解液滴和靶材的纯粘性和粘弹性效应所产生的贡献。4-探索目标的曲率对所产生的动力学的影响。5-在上述基础上，探索抑制飞溅的技术。这项研究的第一部分(1和2)已经进行了一系列关于液滴对不同粘度的(不相容)基材的影响的系统实验。目前正在进行大量的流体模拟，这些模拟提供了实验中无法获得的细节(例如内部速度场)，以使人们能够更好地了解潜在的动力学以及这如何导致飞溅。虽然上述研究领域主要涉及液体对液体撞击情况下液滴的运动，但水池本身的运动也是人们非常感兴趣的，但往往被忽视，特别是在撞击的早期。虽然存在一些用于水池运动的模型，但这些模型在很大程度上仅限于液滴和水池流体是相同的流体时，并且经常在它们经常不合适的情况下使用。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性质的影响，以及水池运动如何影响飞溅。这些目标是主要的任务，也是与华威大学的R.Cimpanu博士的密切合作。拟议的研究属于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，以实现上述在喷墨打印等工业应用中的重要性。

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.