Status and future developments of Large-Eddy Simulation of turbulent multi-fluid flows (LEIS and LESS)

Current computational trends related to turbulent gas-liquid flow are discussed, together with the developments and open challenges needed to bring the discipline to a mature stage. The contribution presents the possibilities offered today by turbulent scale-resolving strategies (Large-Eddy Simulation, LES) to treat complex, multiphase flow topology in system components, and transcending more conventional kinetic energy dissipation-based models combined with phase-average approaches. The LES approach of turbulent gas-liquid flows introduced here under its sub-variants LESS and LEIS (Large-Eddy & Structures Simulation and Large-Eddy & Interface Simulation) is based on unifying the phase averaging concept and the turbulent-scale filtering operations into one single process. The paper is written in the spirit of a review, albeit it provides enough derivation details including the connection between the supergrid (resolved) and subgrid (unresolved) physics. A particular attention is paid here to the various attempts to model the underlying subgrid physics, including DNS-based model upscaling. A brief review of LEIS and LESS applications to phase-change heat transfer problems is provided, too. While the LESS variant based on the filtered multi-fluid equations is best suited for a range of problems in which one of the phases is dispersed in the other, LEIS provides further accuracy by directly predicting interface dynamics and turbulence motions down to the grid level. The paper addresses also the required developments for more complex multi-scale, multi-fluid flow problems, including a new approach termed as ARM, short for All-Regime Multiphase flow model.