Multiscale Simulations of Multiphase Flows

直接数值模拟（DNS）可能是多相流研究中最大的新发展，这种模拟已经开始产生重大影响。随着其使用的增加，很明显，在许多情况下，形成小尺度的特征，如薄膜或液滴，需要过多的（通常是无法实现的）分辨率。在这里，它建议开发多尺度直接数值模拟，允许包括这样的小尺度现象的模拟，其他一切都完全解决。建议的工作有以下目标：发展的一般策略，包括多尺度描述的小尺度现象的多相流的动力学的数值模拟。该方法是基于观察到许多小尺度的功能（薄膜，线程，边界层，应变对流扩散反应层，非常小的液滴和气泡，等等）有一个相对简单的结构，因此可以相对准确地描述分析或半分析模型，同时演变的完全解决大尺度运动。这些挑战包括确定何时和何地使用这种描述，如何有效和准确地耦合小尺度描述和数值解析流，以及开发有效的数据结构以实现不同的描述，而不会压倒这些代码的开发人员。和化学反应来描述多流体和多相流直接数值模拟中的欠分辨率特征。在这些情况下，需要多尺度方法，因为非常薄的膜和线可以在多相流中自然形成，并且因为一方面流体运动的长度和时间尺度与另一方面质量传递和反应之间通常存在很大的差异。本文的工作分为小尺度流动特征的建模（膜和线）、传质问题中的薄边界层和反应层，将建立在目前正在开发的沸腾思想和一段时间前最初在扩散气体火焰建模的背景下开发的方法上，在那里，我们展示了我们可以用一个令人惊讶的简单近似策略来捕捉相当复杂的化学反应。数字方法将通过一个在线储存库提供，同时沿着关于方法和代码使用的详尽文件。虽然多相流的直接数值模拟（DNS）已经对我们对这种流动的理解产生了重大影响，并且目前现有方法的应用仍然存在许多机会，同样清楚的是，在许多情况下，尺度范围太大，即使使用自适应网格细化，也无法在相同的数值方法中处理。然而，多相流中的小尺度特征通常表现出相对简单的结构，可以通过分析或半分析来捕获。在目前的工作中，我们建议扩展DNS，包括这样的多尺度描述。这将大大扩展可以使用DNS研究的多相流的范围。特别是，它将使我们能够考虑反应系统的实际控制参数范围。拟议活动的更广泛影响：多相流在能源转换、材料加工、化学工业、大气过程和生命系统中至关重要。这些过程效率的逐步提高转化为数十亿美元的节省，新发现有可能改变整个行业。 计算研究将为多相系统的管理带来增量和变革。通过提供在线代码和文档来扩大用户社区将有助于实现这一目标。除了培养研究生和博士后，该项目还将为本科生提供研究机会。

Direct numerical simulations (DNS) are perhaps the biggest new development in studies of multiphase flows and such simulations are already starting to have a major impact. As their use has increased, it has become clear that in many situations the formation of small-scale features such as thin films or drops require excessive (and often unachievable) resolution. Here it is proposed to develop multi-scale direct numerical simulations to allow the inclusion of such small scale phenomenon in simulations where everything else is fully resolved. The proposed work has the following objectives:The development of a general strategy to include multi-scale description of small-scale phenomenon in numerical simulations of the dynamics of multiphase flows. The approach is based on the observation that many small-scale features (films, threads, boundary layers, strained advection-diffusion reaction layers, very small drops and bubbles, and so on) have a relatively simple structure and can therefore be described relatively accurately by analytical or semi-analytical models that are evolved concurrently with the fully resolved larger-scale motion. The challenges include identifying when and where to use such a description, how to efficiently and accurately couple the small-scale description and the numerically resolved flow, and the development of efficient data structures to implement the different descriptions in a way that does not overwhelm developers of such codes.The development of specific multi-scale descriptions for thin films and threads, mass transfer, and chemical reactions to describe under-resolved features in direct numerical simulations of multifluid and multiphase flows. The need for multi-scale approach in these situations arises both because very thin films and threads can form naturally in multiphase flows, and since there is usually a large discrepancy between the length and time scales of the fluid motion on the one hand and mass transfer and reactions on the other. This work, which can be divided into the modeling of small scale flow features (films and threads), thin boundary layers in mass transfer problems, and reaction layers, will build on ideas currently being developed for boiling and an approach originally developed some time ago in the context of modeling of diffusion gas flames, where we showed that we could capture reasonably complex chemical reactions using a surprisingly simple approximation strategy. The numerical methods will be made available through an online repository, along with a thorough documentation of the methodology and the use of the codes.The intellectual merit of the proposed activity: While direct numerical simulations (DNS) of multiphase flows have already had major impact on our understanding of such flow, and many opportunities still exist for applications of currently existing methods, it is also clear that in many cases the range of scales is too large to handle within the same numerical approach, even using adaptive grid refinement. Small-scale features in multiphase flows do, however, often exhibit a relatively simple structure that can be captured analytically or semi-analytically. In the present work we propose to extend DNS to include such multi-scale descriptions. This will greatly extend the range of multiphase flows that can be studied using DNS. It will, in particular, allow us to consider reacting systems for a realistic range of governing parameters.The broader impacts of the proposed activity: Multiphase flows are critical in energy conversion, material processing, the chemical industry, atmospheric processes, and living systems. Incremental improvement in the efficiency of such processes translates into billions of dollars in savings and new discoveries have the potential to transform whole industries. Computational studies will bring about both incremental and transformative changes in the management of multiphase systems. Enlarging the community of users by providing online codes and documentations will help make that happen. In addition to training graduate students and postdocs, this project will provide research opportunities for undergraduate students.