CAREER: Measurements of Subgrid-Scale Scalar Mixing to Study Large-Eddy Simulation of Turbulent Combustion

本文研究了在非预混湍流燃烧大涡模拟中起关键作用的守恒标量的亚网格尺度混合。基于Kolmogorovs假设，保守标量的混合通常被认为是一个大到小尺度的级联过程。当前的混合模型给出的结果在质量上是不正确的，其中叶栅过程应该占主导地位。同时，人们也认识到大尺度速度场和标量场对小尺度标量混合有重要的影响。本课题研究了大尺度结构和级联过程对守恒标量的过滤密度函数（SGS分布）的影响。在具有各向同性网格湍流的高雷诺数湍流射流中进行了速度和守恒标量的高分辨率测量。采用热丝阵列和冷丝阵列、粒子图像测速技术（PIV）和平面激光瑞利散射技术进行测量。研究了守恒标量的过滤密度函数和守恒标量的过滤联合密度函数及其梯度的建模问题。采用常规统计分析和基于条件采样和平均的统计分析，研究了湍流平流、湍流应变和旋转以及分子混合对过滤密度函数和其他变量的影响。

This is an investigation of the subgrid-scale (SGS) mixing of conserved scalars which play a critical role in large-eddy simulation (LES) of nonpremixed turbulent combustion. Mixing of conserved scalars is generally considered to be dominated by a large-to-small-scale cascade process based on Kolmogorovs hypothesis. Current mixing models give qualitatively incorrect results for flows where the cascade process should dominate. Also, it is now recognized that large-scale velocity and scalar fields have significant influence on small-scale scalar mixing. In this project, the effects of the large-scale structure and of the cascade process on the filtered density function (the SGS distribution) of a conserved scalar is studied. High-resolution measurements of velocity and of conserved scalars are made in high-Reynolds-number turbulent jets with isotropic grid turbulence. Hot-wire and cold-wire arrays, particle image velocimetry (PIV), and planar laser Rayleigh scattering are used for the measurements. Issues in modeling the filtered density function of a conserved scalar and the filtered joint density function of a conserved scalar and its gradient are investigated. Both conventional statistical analyses and those based on conditional sampling and averaging are performed to investigate the effects of turbulent advection, turbulent strain and rotation, and molecular mixing on the filtered density function and other variables.