Direct Numerical Simulations of Flows with Phase Change

Abstract

Direct Numerical Simulations (DNS) of multiphase flows, where every continuum length and time scale are fully resolved, currently allow us to simulate flows of considerable complexity, such as the motion of several hundred bubbles or drops in turbulent flows, for sufficiently long time so that meaningful statistical quantities can be obtained. Additional physical processes such as heat transfer and phase change have also been included, although only for relatively small systems so far. After reviewing briefly recent studies of bubbles in turbulent channel flows, we discuss simulations of flows with phase change, focusing on bubble generation by boiling. The addition of new physics often results in new length and time scales that are shorter and faster than the dominant flow scales. Similarly, very small features such as thin films, filaments, and drops can also arise during coalescence and breakup of fluid blobs. The geometry of these features is usually simple, since surface tension effects are strong and inertia effects are relatively small and in isolation these features are often well described by analytical or semi-analytical models. Recent efforts to embed analytical and semi-analytical models to capture such features, in combination with direct numerical simulations of the rest of the flow, are discussed. We conclude by a short discussion of the use of DNS data for closure laws for model equations for the large scale flow.