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

流体雾化描述的是紧密的液体体积分解成许多小液滴的过程。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扩大雾化器的规模仍然是常见的做法。虽然预测模拟技术已经在计算流体力学的其他领域得到了很好的应用，但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，因此没有充分利用最先进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流动的模型，它具有很好的描述雾化过程的潜力。在推导大涡模拟过滤后的两相流(液/气)方程时，出现了几个非闭合项。这些术语的可靠模式仍未确定，这是本提案第一部分的主题。然而，在所用的流体体积方法中，直径约为LES滤光片宽度数量级的液滴不能被捕获，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，开发一个完整雾化过程的闭合模型链，包括一次和二次破碎。该项目可分为三个主要部分。第一阶段将在不经历二次破碎过程的情况下，对小于LES过滤器宽度的初级液滴的直接喷射进行建模。在第二阶段，将在本项目的框架内使用和验证文献中建立的二次破碎模型，以便可以考虑液滴的碰撞、破碎和合并。这称为四向耦合，而双向耦合描述的是液滴对连续相的反馈效应。因此，对双向耦合的相关研究非常必要。这一点在本工作的背景下尤其成立，其中液滴直径可以在过滤器宽度的数量级，因此两相的相互作用不能被忽略。在项目的最后部分，将使用项目第一部分的域名系统数据库和文献中的实验数据对整个模型进行校准和验证。验证成功后，该项目的结果将成为雾化过程大涡模拟的实用工具。这将在最后一步通过使用OpenFOAM结合开发的建模策略计算具有复杂喷嘴几何形状的压力雾化器来进行验证，文献中已经对其进行了实验表征。

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.