Elucidating the Flow Structure and Addressing Modeling Issues in Turbulent Boundary Layers Based on Multiscale, 3D Velocity Measurements

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Katz0932941High Reynolds number turbulent boundary layers continue to pose major scientific and technological challenges due to their inherently complex couplings of dynamics across many length and time scales. Integration of recently introduced techniques enables 3D high-resolution volumetric velocity measurements across a range of scales, from very near the wall to the outer part of the boundary layer. The objective of this study is to experimentally investigate interactions among buffer layer vortices and outer (inertial) layer, larger-scale structures above a smooth wall at moderate to high Reynolds numbers. Data analysis will examine effects of mean flow acceleration and local instantaneous pressure gradients on the characteristics of buffer layer vortices and their effects on turbulence statistics. The study will also address fundamental Large-Eddy Simulation (LES) issues in wall-bounded flows, especially modeling of wall shear stresses in terms of large-scale features that are resolved in LES, and impact of unsteadiness, local pressure gradient, and instantaneous streamwise curvature on the wall stress. Finally, the PIs will develop a systematic method for determining the length-scale required for fully resolving the inner boundary layer flow, both experimentally and numerically. The multi-scale velocity measurements will be performed by simultaneously implementing two state-of-the-art, 3D flow measurement techniques: Relatively "coarse" measurements will be performed using tomographic particle image velocimetry (PIV) at a spatial resolution of 0.5mm. High-resolution velocity measurements near the wall within part of the coarse volume will be performed using digital holographic microscopy (DHM), at a spatial resolution of 20 um. Also, using recently introduced procedures, four-exposure DHM will measure the instantaneous distribution of material acceleration, and provide the local pressure gradients. Experiments will be performed in the optically index-matched facility at JHU that enables unobstructed near-wall measurements, even near rough walls and curved boundaries used for generating mean pressure gradients. In-line DHM, a high-resolution flow measurement technique recently developed in the PIs' laboratory, involves acquisition of in-line holograms of a seeded flow by a digital camera. Numerical reconstruction and particle tracking provide the 3D velocity distribution. The present optical setup and data analysis procedures will be further developed for simultaneous application with tomographic PIV. DHM extends the depth of field of a conventional microscope by 3 orders of magnitude, and may revolutionize microscopy in many other fields that require measurements of 3D dynamic processes as in swimming of bacterial suspensions. The PIs have been active in disseminating holographic microscopy by providing software, training and follow-up assistance to personnel in several academic laboratories, including Rutgers, LSU and VA Tech. The PIs will continue their long-term commitment to and record of success at involving undergraduate students in laboratory and field research, as part of the PIs' effort to motivate them to pursue graduate education. The PIs will also continue engaging high school students from Baltimore Polytechnic in an extensive, yearlong research experience as part of their Research Practicum.