Direct numerical simulation of aerodynamic fragmentation of liquid drops

由于快速惯性效应而破坏作为液滴分散在气体中的液相的气-液界面称为气动破碎或气动破碎。这种方法的技术应用广泛，涉及例如燃料、熔体、油漆、涂料或生物医学乳液的喷雾的二次破碎。工艺优化需要了解物理机制，并最终需要对气动破碎（AF）进行预测，而这两方面目前都受到严重限制。实验研究得出了重要的主要发现。然而，由于空间和时间尺度极小，很难获得定量的实验数据。单个液滴可以通过直接数值模拟（DNS，解决所有相关的空间和时间尺度的流动）进行研究，提供先进的高分辨率离散化方案和离散保守的界面模型。高效的计算策略需要量身定制的数据操作拆分算法和多分辨率表示。当气体中的液滴受到冲击时，冲击波衍射和反射，在液滴内外产生复杂的波型。压力梯度、斜压和表面张力效应使界面变形。沿着界面的剪切力沿着引发接触片不稳定性，而在AF期间的毛细管不稳定性是可以忽略的（表面张力本身不是），由于界面处的高声阻抗跳变而引起的Richtmyer-Meshkov不稳定性也是可以忽略的。通常，基于实验观察，三个典型的破碎场景被区分：（i）袋破碎，（ii）片剥离，（iii）catastrophic破碎，这强烈地依赖于数据和流动状态的不确定性。对不同参数范围内的普遍情况进行分类与应用高度相关。每一种破碎方案的特点是不同机制的相互作用，最重要的是：（一）瑞利-泰勒穿刺，（二）剪切诱导夹带，（三）剪切层不稳定性。本项目的目标有三个方面。首先，一个确定性的DNS方法，可压缩界面流，包括表面张力和界面摩擦的影响，需要适应和验证的应用程序在这个项目的基础上，一个可用的高分辨率模型。其次，将进行关于流行的解体机制的不同解体情景的现象学分析。一组数值模拟计划，其结果允许明确分配解体方案的参数范围和实验结果的资格。第三，在第二个供资期，我们将扩大马赫数范围，并评估其他相关机制。此外，不确定性传播工具将用于量化的信心。

The destruction of gas-liquid interfaces of a liquid phase dispersed as droplets in gas by fast inertial effects is called aerodynamic fragmentation or aero-breakup. Technical applications of such processes are widely spread, involve e.g. secondary fragmentation of sprays of fuels, melts, paints, coatings, or biomedical emulsions. Process optimization requires understanding of physical mechanisms and, eventually, a prediction capability of aerodynamic fragmentation (AF), which both currently are severely limited. Experimental research resulted in important main findings. Quantitative experimental data, however, are very difficult to obtain due to extremely small spatial and temporal scales. Individual drops can be investigated by direct numerical simulations (DNS, resolving all relevant spatial and temporal scales of the flow), provided advanced high-resolution discretization schemes and discretely conservative interface models are employed. Efficient computing strategies require tailored data-operation splitting algorithms and multi-resolution representations. When a liquid drop in a gas is impacted by a shock, the shock diffracts and reflects, giving rise to complex wave patterns inside and outside of the drop. Pressure-gradient, baroclinic and surface-tension effects deform the interface. Shear forces along the interface initiate contact-sheet instabilities, whereas capillary instabilities during AF are negligible (surface tension itself is not), as are Richtmyer-Meshkov instabilities due to the high acoustic impedance jump at the interface. Commonly, based on experimental observation, three typical breakup scenarios are distinguished: (i) bag breakup, (ii) sheet stripping, (iii) cata-strophic breakup, which strongly depend on data and flow state uncertainties. A classification of prevalent scenarios in different parameter ranges is highly relevant for applications. Each of the breakup scenarios is characterized by the interplay of different mechanisms, most importantly: (i) Rayleigh-Taylor piercing, (ii) shear-induced entrainment, and (iii) shear-layer instabilities. The objective of the current project is threefold. First, a deterministic DNS method, based on an available high-resolution model for compressible interfacial flows, including effects of surface tension and interfacial friction, needs to be adapted and validated for the applications in this project. Second, a phenomenological analysis of different breakup scenarios with respect to prevalent breakup mechanisms will be performed. A set of numerical simulations are planned whose results allow for clearly assigning breakup scenarios to pa-rameter ranges and for qualification of experimental findings. Third, in a second funding period, we will extend the Mach-number range and assess additional mechanisms that become relevant. Furthermore, uncertainty-propagation tools will be employed for quantifying the confidence.