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

流体雾化描述了将紧凑的液体体积分解成许多小滴。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。在经验基础上设计和升级雾化器仍然是常见的做法。虽然预测模拟技术在计算流体动力学的其他领域中已经很好地建立，但是现有的雾化模型通常需要使用针对每个操作条件的详细测量进行校准。因此，喷雾特性通常是规定的，而不是预测的，因此没有充分利用现有技术的模拟方法的可能性。大涡模拟技术（LES）代表了一种计算湍流的模型，它有可能以良好的精度描述雾化过程。在推导LES过滤的两相流（液体/气体）方程时，出现了几个未封闭项。这些术语的可靠模型尚未建立，这是本提案第一部分的主题。然而，无法在所使用的流体体积方法中捕获直径为LES滤波器宽度量级的液滴，并且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-拉格朗日框架下开发完整雾化过程的闭合建模链，包括初级和次级破碎。该项目可分为三个主要部分。第一阶段将处理小于LES过滤器宽度的初始液滴的直接喷射的建模，而不经历二次破碎过程。在第二阶段，将在本项目的框架内使用和验证从文献中建立的二次破碎模型，以便可以考虑液滴碰撞，破碎和合并。这被称为四向耦合，而双向耦合描述了液滴对连续相的反馈效应。有很大的需要进行研究有关的双向耦合。这在本工作的上下文中特别适用，其中液滴直径可以是过滤器宽度的量级，因此两相的相互作用不能被忽略。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。本项目的成功验证将为雾化过程的大涡模拟提供一种实用的模拟工具。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进行验证，该压力雾化器已在文献中通过实验表征，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.