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Structure and Mixing Characterization of Variable Density Transverse Jet Flows


This dissertation describes an experimental study of the structural and mixing characteristics of transverse jets, or jets in crossflow (JICF). Hot-wire anemometry, stereo particle image velocimetry (PIV), and acetone planar laser-induced fluorescence (PLIF) measurements were utilized to illuminate and quantify the wind-ward (upstream) jet shear layer instability characteristics and their relationship to the velocity field evolution, as well as the effect of the overall velocity field on the scalar field distribution and resulting mixing

characteristics. Transverse jets of various jet-to-crossflow momentum flux ratios in the range 41 ≥ J ≥ 2, and jet-to-crossflow density ratios in the range 1.00 ≥ S ≥ 0.35, were generated using mixtures of helium and nitrogen in the jet fluid. Jets were injected from one of three different injectors explored: a convergent nozzle with circular geometry which was mounted flush with the wind tunnel floor, another convergent nozzle with circular geometry whose exit plane lies above the crossflow boundary layer, and a flush-mounted straight pipe injector with a circular orifice. Jet Reynolds number was kept constant for the majority of the mixing and structural exploration experiments at Rej = 1900, except when the effect of Reynolds number on cross-sectional jet structure was explored.

Previous hot-wire based measurements at UCLA suggest that the upstream jet shear layer transitions from convective instability to absolute instability, giving rise to self-excited nonlinear states, as either the momentum flux ratio is lowered below J ≈ 10, or the density ratio is lowered below S ≈ 0.45 for the JICF injected from the flush nozzle injector. A similar transition to absolute instability when lowering momentum flux ratio was found in this work for the flush-mounted pipe injector. Cross-sectional PLIF measurements in the present studies suggested clear correspondence between the formation of a symmetric counter-rotating vortex pair (CVP) and the generation of strong upstream shear layer instability. In contrast, weak, convectively unstable upstream shear layers corresponded with asymmetries in the jet cross-sectional shape and/or lack of a CVP structure. While momentum flux ratio J and density ratio S most significantly determined the strength of the instabilities and CVP structures, an additional dependence on jet Reynolds number for CVP formation was found, with significant increases in jet Reynolds number resulting in enhanced symmetry and CVP generation.

The mixing characteristics of Rej = 1900 jets of various J, S, and injector type were explored in detail in the present studies using jet centerplane and cross-sectional PLIF measurements. Various mixing metrics such as the jet fluid centerline concentration decay, Unmixedness, and Probability Density Function (PDF) were applied systematically using a novel method for comparing jets with different mass flux characteristics. It was found that when comparing mixing metrics along the jet trajectory, strengthening the upstream shear layer instability by reducing J, and achieving absolutely unstable conditions, enhanced overall mixing. Reducing density ratio S for larger J values, which under equidensity (S = 1.00) conditions would create a convectively unstable shear layer, was also observed to enhance mixing. On the other hand, reducing S for low J conditions, which are known to produce absolutely unstable upstream shear layers even for equidensity cases, was actually observed to reduce mixing, a result attributed to a reduction in crossflow fluid entrainment into shear layer vortex cores as jet density was reduced. Comparing injectors, the flush-mounted pipe was generally the best mixer, whereas the worst mixer was the nozzle that was elevated above the crossflow boundary layer due to upstream shear layer co-flow generated by the elevated nozzle contour; this co-flow was observed here and in prior studies to stabilize the shear layer.

The effect of the evolution of the velocity and vorticity fields on the scalar concentration

field was studied in more detail using simultaneous PLIF/PIV measurements of the jet centerplane. General correspondence between regions of high vorticity/strain and high scalar dissipation rate was found. Moreover, Proper Orthogonal Decomposition (POD) analysis of both scalar and velocity fields suggested that transition to absolute instability with J reduction dominated the scalar field evolution near the jet exit, consistent with the mixing results. However, time-varying and/or three-dimensional effects resulted in scalar diffusion layer-normal strain rates extracted from PIV measurements to be consistently higher than that calculated from PLIF-based extraction of scalar dissipation rate data for strained advection-diffusion layers in the scalar field. Nevertheless, trends in PIV-based strain field evolution and PLIF-based scalar dissipation rate evolution along the upstream shear layers were generally consistent with one another, and differences in the magnitudes of wind-ward and lee-side strain rates provided evidence for associated differences in jet wind-ward and lee-side ignition processes in the presence of a reaction.

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