Progress in the observation and modeling of turbulent multi-phase flows

Abstract

Multi-phase flows of environmental import involve the interactions of solid, gas, and/or liquid phases. In the last decades, important advances have taken place in the observation and modeling of multi-phase flows. New accurate instruments to measure the air concentration in air-water flows and to analyse the motion of solid particles in water and gas; new theories to address the interactions of the diverse turbulence scales of the flow; and more powerful computers have all allowed researchers to study multi-phase flows from previously unexplored points of view. In Nature, multi-phase flows are mostly turbulent and, therefore, they are extremely complicated, with a broad range of relevant length and time scales [1]. For example, the time scales range from less than 1ms for the turbulence dissipation in a small stream to about 24 h and 50min for a tidal cycle in coastal zones (Fig. 1), and to more than 50 years for the currents controlling the balances between oxygen and carbon dioxide. Several recent papers have remarked that the difficulties in understanding multi-phase flows stem from two basic facts: the phases do not distribute uniformly, and the small-scale interactions may have profound effects on large-scale behaviours. These difficulties become important for the prediction of the behaviour of flows in many environmentalproblems and applications. The aim of this Special Issue is to present a group of papers that: (a) summarize thestate-of-the-art in the knowledge about multi-phase flows in environmental applications; (b)report recent results of research (of experimental, numerical and/or theoretical nature) on environmental multi-phase flows; and (c) suggest novel pathways of analysis in the area.