Title: Turbulent Fluid Interfaces with Applications to Mixing and Aero-Optics

Speaker: Roberto C. Aguirre (MAE)

In turbulent flows, interfaces are highly irregular, both in spatial structure and temporal behavior across a wide range of scales, which consequently continue to present challenges with regard to physical modeling, computational simulations, and experimental examinations.  As part of this work, new physical-modeling methodologies are developed and flow-imaging experiments are designed with emphasis on capturing the behavior of fluid interfaces encountered in large-Reynolds-number flows in particular for turbulent mixing and aero-optics applications.  In this seminar, the main contributions of this research are summarized and results on the physical-thickness variations of mixed-fluid interfaces, resolution-scale effects on the mixing efficiency, and fluid-optical interactions of propagated laser beams are emphasized.

In particular, the variability of the physical thickness of fully-developed turbulent interfaces is examined using scalar measurements in the outer far-field regions of round jets at a Reynolds number of Re ~ 20,000 and Schmidt number of Sc ~ 2,000.  A database is generated that captures the whole-field 1000³ three-dimensional space-time interfacial behavior above the mixing transition.  Results on the intermittent thickness variations, lognormal conditional probability density distribution of the interfacial thickness, and self-similarity of the scale-local thickness density are presented.  In addition, the mixing efficiency, or mixture fraction, is quantified based on the volume of mixed fluid bounded by the outer interfaces which are observed to be dynamically confined near the unsteady large-scale flow boundaries.  The contributions of this research also include the development of a new pressurized flow facility and imaging techniques, which enable simultaneous measurements of refractive interfaces in turbulent flows and optical wavefronts of propagated laser beams.  Results obtained from experiments in separated compressible shear layers at large Reynolds number (Re ~ 1,000,000) and moderate convective Mach number (M_c ~ 0.4) together with a proposed interfacial-fluid-thickness approach indicate that the high-gradient refractive interfaces dominate the large-scale aero-optical interactions.