Towards Understanding the Mixing Characteristics of Turbulent Buoyant Flows.

摘要

This work proposes a new simulation methodology in which variable density turbulent flows can be studied in the context of a mixing layer with or without the presence of gravity. This methodology is developed to probe the nature of non-buoyantly-driven or buoyantly-driven mixing inside a mixing layer. Numerical forcing methods are incorporated into the velocity and scalar fields, extending the length of time over which mixing physics can be studied. The simulation framework is designed to allow for independent variation of four non-dimensional parameters, including the Reynolds, Richardson, Atwood, and Schmidt numbers. The governing equations are integrated in such a way to allow for the relative magnitude of buoyant energy production and non-buoyant energy production to be varied.;The computational requirements needed to implement the proposed configuration are presented. Key features of turbulent buoyant flows are reproduced as validation of the proposed methodology. These features include the recovery of isotropic Kolmogorov scales under buoyant and non-buoyant conditions, the recovery of anisotropic one-dimensional energy spectra under buoyant conditions, and the preservation of known statistical distributions in the scalar field, as found in other DNS studies.;This simulation method is used to perform a parametric study of turbulent buoyant flows to discern the effects of varying the Reynolds, Richardson, and Atwood numbers on mixing. The effects of the Reynolds and Atwood numbers are isolated by examining two energy dissipation rate conditions under non-buoyant (variable density) and constant density conditions. The effects of Richardson number are isolated by varying the ratio of buoyant energy production to total energy production from zero (non-buoyant) to one (entirely buoyant) under constant Atwood number, Schmidt number, and energy dissipation rate conditions. It is found that the primary differences between non-buoyant and buoyant turbulent flows are contained in the transfer spectrum and longitudinal structure functions, while all other metrics are largely similar. However, the scalar field dynamics are found to be similar whether the velocity field is subjected to buoyancy forces or not. Hence, the mixing dynamics in the scalar field are insensitive to the source of turbulent kinetic energy production (non-buoyant vs. buoyant).