Multiscale Simulations of Multiphase Systems

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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.