Large Eddy Simulation of Primary and Secondary Breakup Using a Multiscale Euler-Lagrange Method

流体雾化是指将致密的液体体积分解成许多小液滴。它是许多技术过程的起点，在技术、医学和农业中有着广泛的应用。在经验的基础上设计和升级雾化器仍然是普遍的做法。虽然预测模拟技术在计算流体动力学的其他领域已经很好地建立起来，但现有的雾化模型通常需要使用每种操作条件的详细测量来校准。因此，喷雾特性通常是规定的，而不是预测的，因此，最先进的模拟方法的可能性没有得到充分利用。大涡模拟技术（LES）代表了一种计算湍流的模型，具有较好地描述雾化过程的潜力。在推导LES过滤后的两相流（液/气）方程时，会出现几个不闭合项。这些术语的可靠建模仍然没有建立，这是本提案第一部分的主题。然而，在使用的流体体积方法中无法捕获直径为LES滤波器宽度数量级的液滴，现在将在拉格朗日粒子跟踪的背景下考虑它们。因此，第二阶段的目标是在多尺度欧拉-拉格朗日框架的背景下，开发完整雾化过程的封闭建模链，包括初级和次级分解。本项目可分为三个主要部分。第一阶段将处理小于LES滤波器宽度而不经历二次破碎过程的初级液滴直接喷射的建模。在第二阶段，将使用文献中建立的二次破裂模型，并在本项目框架内进行验证，从而考虑液滴碰撞、破裂和合并。这被称为四向耦合，而双向耦合描述了连续相位上的滴的反馈效应。对双向耦合的研究是十分必要的。这在本工作的背景下尤其正确，其中液滴直径可以达到滤波器宽度的数量级，因此两相的相互作用不能忽略。在项目的最后一部分，将使用项目第一部分的DNS数据库和文献中的实验数据对整个模型进行校准和验证。通过成功验证，该项目结果将为雾化过程的LES模拟提供一个实用的工具。这将在最后一步通过计算具有复杂喷嘴几何形状的压力雾化器来验证，该雾化器已在文献中进行了实验表征，使用OpenFOAM结合开发的建模策略。

Fluid atomization describes the disintegration of a compact liquid volume into many small drops. It is the starting point for many technical processes with a wide range of applications in technology, medicine and agriculture. The design and upscaling of atomizers on an empirical basis is still common practice. While predictive simulation techniques are well established in other areas of computational fluid dynamics, existing atomization models typically require calibration using detailed measurements for each operating condition. As a consequence, the spray characteristic is often prescribed rather than predicted and hence the possibilities of state of the art simulation methods are not fully used. The large-eddy simulation technique (LES) represents a model for computing turbulent flows, which has the potential to describe the atomization process with good accuracy. When deriving the LES filtered two-phase flow (liquid / gas) equations, several unclosed terms occur. Reliable modeling of these terms is still not established and was the subject of the first part of this proposal. However, drops with diameter of the order of the LES filter width cannot be captured within the used Volume-of-Fluid methodology and they will now be considered in the context of Lagrangian particle tracking. The goal of this second phase is therefore the development of a closed modeling chain of the complete atomization process, consisting of primary and secondary breakup, in the context of a multiscale Euler-Lagrange framework.The project can be divided into three main parts. The first phase will deal with the modeling of the direct ejection of primary drops that are smaller than the LES filter width without undergoing a secondary breakup process. In the second phase, an established secondary breakup model from literature will be used and validated in the framework of this project, such that droplet collision, breakup and coalescence can be taken into account. This is called four-way coupling, while the two-way coupling describes the feedback effect of drops on the continuous phase. There is great need for research related to two-way coupling. This holds in particular true in the context of this work where the droplet diameter can be at the order of the filter width and hence the interaction of both phases cannot be ignored. In the last part of the project, the calibration and validation of the overall model will take place, using both the DNS database of the first part of the project and experimental data from literature. With successful validation the result of the project will be a practicable simulation tool for the LES of atomization processes. This is to be validated in a final step by calculating a pressure atomizer with complex nozzle geometry, which has been experimentally characterized in literature, using OpenFOAM in combination with the developed modelling strategy.