Multiscale Simulations of Multiphase Systems

在过去的几年里，多相流的直接数值模拟（DNS）已经取得了巨大的进展，其中所有连续尺度的流动都得到了充分的解决。计算机能力的提高和新的算法现在可以跟踪数百个颗粒（液滴，气泡和固体）的不稳定运动足够长的时间，以便计算流体混合物的有意义的平均值。然而，尽管DNS为相对复杂的流提供了大量的信息和理解，但真实的系统提供的挑战仍然限制了可以模拟的系统的范围，即使我们将研究限制在连续统理论所描述的系统上。正如人们所料，问题在于规模。从我们称之为“主导小尺度”的模拟开始，经常发现多相流也可以产生比主导流尺度小得多的特征，包括非常薄的薄膜，细丝和液滴。这些“特征”之间的尺度经常有明显的分离，通常惯性效应对局部演化的影响相对较小，孤立地说，这些特征通常可以通过分析模型很好地描述。这里提出了一种方法，将这种小规模的功能，模型到多相流的数值模拟。发展将包括一个理论框架和实施到一个数值方法。另一个问题，将在这里解决，虽然在一个更探索性的水平，是如何使用DNS的多相流的工程模型，这样的流动的结果。采用所谓的双流体模型对离散流动进行建模已经取得了相当大的进展，但是对于每一相都由连续部分和离散部分组成的流动，所取得的进展要少得多。一般来说，多相流的平均特性的建模远远落后于已经实现的均匀湍流，但最近的进展，在DNS的多相流，迫切需要推进这样的建模。这些方法将在通常表现出相当复杂性的物理系统的背景下开发，并且具有巨大的实际重要性。液体的雾化在许多应用中被发现，并且虽然喷雾的计算建模已经达到相当高的复杂程度，但是这些模型都依赖于用于初始雾化的非常粗略的近似。由于初始液滴尺寸分布通常对液滴的后续演变至关重要，因此对雾化的了解不足是喷雾计算的主要瓶颈。拟议活动的智力价值在于提高进行多相流数值模拟的能力，并利用这些能力大大提高对雾化的理解。 这里提出的研究的更广泛的影响是双重的。首先，它将有助于重新定义多相流研究的最新技术水平，展示如何使用计算来研究迄今为止一直在尝试的更复杂的问题。 其次，它还将有助于培训研究生在多相流中使用DNS。

ABSTRACT - 0522581Worcester Polytechnic InstituteDirect Numerical Simulations (DNS) of multiphase flows, where all continuum scales of the flow are fully resolved, have progressed enormously in the last few years. Increase in computer power and new algorithms now make it possible to follow the unsteady motion of several hundred particles (drops, bubbles and solids) for long enough times so that meaningful averages for the fluid mixture can be calculated. However, in spite of the enormous information and understanding that DNS are providing for relatively complex flows, real systems provide challenges that still limit the range of systems that can be simulated, even when we limit our studies to systems well described by continuum theories. The problem is, as one might expect, one of scale. Starting with simulations where what we might call the ''dominant small-scales'' are fully resolved, it is frequently found that multiphase flows also can generate features much smaller than the dominant flow scales, consisting of very thin films, filaments, and drops. Frequently there is a clear separation of scales between these ''features,'' usually inertia effects are relatively small for the local evolution, and in isolation these features are often well described by analytical models. It is proposed here to develop an approach to incorporate such small-scale features as models into numerical simulations of multiphase flows. The development will consist of both a theoretical framework and an implementation into a numerical method. The other problem that will be addressed here, although at a more exploratory level, is how to use the results of DNS of multiphase flows for engineering models of such flows. Considerable progress has been made in the modeling of disperse flows using the so-called two-fluid model, but much less has been achieved for flows with where each phase consists of both a continuous and a disperse part. In general, modeling of the averaged properties of multiphase flows lags far behind what has been achieved homogeneous turbulent flows, but the recent progress in DNS of multiphase flows makes it urgent to advance such modeling. These approaches will be developed in the context of a physical system that usually exhibits considerable complexity, and that is of immense practical importance. Atomization of liquids is found in a multitude of applications and while computational modeling of sprays has reached a fairly high degree of sophistication, such models all rely on very crude approximations for the initial atomization. As the initial droplet size distribution is generally critical for the subsequent evolution of the drops, the lack of understanding of the atomization is a major bottleneck for computations of sprays.The intellectual merit of the proposed activity lies in the advancement of the capabilities to conduct numerical simulations of multiphase flows and the use of these capabilities to provide greatly improved understanding of atomization. The broader impact of the study proposed here are two-fold. First of all, it will help redefine the state of the art in multiphase flow studies by demonstrating how computations can be used to examine much more complex problems that have been attempted until now. Secondly, it will also help train graduate students in the use of DNS for multiphase flows.