Multiscale Simulations of Multiphase Flows

直接数值模拟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。随着它们的使用越来越多，很明显，在许多情况下，形成薄膜或液滴等小尺寸特征需要过高的(而且往往无法实现的)分辨率。在这里，建议开发多尺度的直接数值模拟，以允许将这种小尺度现象包括在模拟中，而其他一切都完全解决了。这项拟议的工作有以下目标：开发一种通用策略，在多相流动力学的数值模拟中包括对小尺度现象的多尺度描述。这种方法是基于这样的观察，即许多小尺度特征(膜、线、边界层、应变平流-扩散反应层、非常小的液滴和气泡等)具有相对简单的结构，因此可以通过与完全分辨的大尺度运动同时演化的解析或半解析模型来相对准确地描述。挑战包括确定何时何地使用这种描述，如何有效和准确地耦合小规模描述和数值解析流动，以及开发高效的数据结构以实现不同描述的方式，而不会使这类代码的开发者不知所措。为薄膜和线程、传质和化学反应开发特定的多尺度描述，以描述多流体和多相流的直接数值模拟中未充分分辨的特征。在这些情况下需要采用多尺度方法，这既是因为在多相流中可以自然形成非常薄的薄膜和丝线，也因为一方面流体运动的长度和时间尺度与另一方面的传质和反应之间通常存在很大的差异。这项工作可分为小规模流动特征(薄膜和丝线)、传质问题中的薄边界层和反应层的建模，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，其中我们展示了我们可以使用令人惊讶的简单近似策略来捕获合理复杂的化学反应。数值方法将通过在线资源库提供，并提供方法和代码使用的完整文档。拟议活动的智力优势：虽然多相流的直接数值模拟(DNS)已经对我们对此类流动的理解产生了重大影响，目前现有方法的应用仍然存在许多机会，但很明显，在许多情况下，即使使用自适应网格细化，相同的数值方法的尺度范围也太大。然而，多相流中的小尺度特征往往表现出相对简单的结构，可以通过分析或半分析来捕捉。在目前的工作中，我们建议扩展域名系统以包括这样的多尺度描述。这将极大地扩大可用域名系统研究的多相流的范围。尤其是，它将使我们能够考虑反应系统的实际控制参数范围。拟议活动的更广泛的影响：多相流在能源转换、材料加工、化工、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。计算研究将给多相系统的管理带来增量和变革性的变化。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外，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.