The objective of this project is to quantify the multi-scale dynamics governing the mixing of a passive scalar quantity by turbulent fluid motion. Specifically, the time- and scale-dependent coupling between fluid turbulence and the scalar field within gas-phase shear flows will be investigated with simultaneous high-speed (> 10 kHz acquisition rate) 3D velocity and 2D conserved scalar imaging. Turbulent scalar mixing is ubiquitous in nature and engineering processes and has been a subject of research for more than sixty years; however, the underlying physics and governing mechanisms producing the observed phenomenology remain unclear. Turbulent flows are inherently intermittent, multi-dimensional phenomena, which create a highly dynamic system occurring on multiple length and time scales. In addition, scalar transport is likely coupled to the highly non-linear velocity field in a spatially and temporally complex manner. This not only leads to difficulties in developing tractable theoretical descriptions, but also to difficulties developing robust and predictive computational modeling capabilities. New measurement and analysis tools are needed to investigate, understand, and describe the multi-scale and multi-physics nature of the turbulent scalar mixing process. This project will be aided by recent technological advances in quantitative high-speed imaging and in particular, a new multi-kHz, high-energy laser system, which allows simultaneous time-resolved 3D velocity and 2D conserved scalar measurements in high-Reynolds number, gas-phase flows. The project will be transformative through characterization of the dynamic interaction between turbulent velocity and scalar fields in real time. Space- and time-correlation between fluid kinematics and scalar gradients will be quantified. The measurements will be used to investigate the underlying mechanisms governing the so-called "ramp-cliff" formation, which indicates the imprint of large-scale intermittency on smaller scales and persistent scalar anisotropy at all scales. Finally, the unique data sets will enable an investigation into the relative importance of advection and diffusion as a function of time, characterizing the level of intermittency of each process. Temporally based, joint velocity-scalar statistics will allow a new parameterization of the nature of velocity-mixing dynamics. In addition, it is proposed to experimentally determine new, multi-point, multi-time correlations, which are statistical quantities containing both spatial and temporal structural information that can be appropriately compared to both theory and time-dependent modeling results from large-eddy simulation (LES). Broader Impacts: A successful project will have significant impact on the fundamental understanding of scalar mixing in turbulent flows and in the field of turbulence in general. Since the performance of the majority of realistic combustion systems such as gas turbine and internal combustion engines are underpinned by turbulent mixing, a better understanding of the underlying physics can lead to cleaner and more efficient systems. The proposed research also will provide critical, new data to assess turbulence theory and to develop physically based LES models as well as their implementation into realistic turbulent environments. In terms of research-related education, a doctoral student will be supported by this project. Every effort will be made to include the participation of students traditionally classified as under-represented including women and minorities. Additional opportunities for broader impacts will be available through undergraduate honors projects, research seminars, the dissemination of results into the open literature, and through presentations at conferences. This research provides excellent opportunities student participants to work within (and significantly contribute to) a wide range of advanced topics including fluid dynamics and optical diagnostics.
