Direct Simulation of Multiphase Flows

Abstract

Direct numerical simulations using pseudo-spectral methods have proven to be of great value in elucidating single phase turbulent space flows. Recent applications to multi-phase systems containing dilute suspensions of particles indicate startling phenomena such as segregation and turbulence modulation even at very low concentrations. These effects appear to be related to particles being thrown out of vortex structures if their time constants are relatively short compared to the structure lifetime. Therefore heavier-than-fluid particles tend to accumulate in certain regions such as in the low speed streaks in boundary layers and result in high local concentrations. These in turn affect phenomena such as "bursting" and formation of streamwise vortices in shear layers. Bubbles have the opposite effect, segregating into the center of vortices and affecting dissipation rates. Experiments have verified some of the qualitative features of such simulations leading to considerable recent activity in the field. The paper will discuss two main approaches to simulations of turbulent flows containing a second dispersed phase. For characteristic sizes of the dispersed phase being very small, Stokes flow approximations can be used locally to account for the presence of the dispersed phase in direct simulations of continuous-phase motion. For finite length scales, however, such approximations break down as wakes develop behind the dispersed phase particles. The wakes can affect continuous..m-phase turbulence and such microstructural aspects must be taken into account int the simulations. A method has been developed recently to "freeze" in a computational sense, portions of the continuous-phase. In direct simulations, such frozen portions then behave like particles and wakes develop . behind them. The forces on such frozen portions such as drag, lift, torque, etc. are automatically calculated. Though the development of such simulations is at an early stage, the results are already of great interest and will be discussed.