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Open Access Publications from the University of California

Structure, Mixing, and Dynamics of Controlled Single and Coaxial Jets in Crossflow

  • Author(s): Harris, Elijah Weston
  • Advisor(s): Karagozian, Ann R
  • et al.

This dissertation describes an experimental study of the instability, structural, dynamical, and mixing characteristics of jets in crossflow (JICF). Constituent species of the jet fluid were nitrogen and helium, with additional seeding of tracer particulates for implementation in non-intrusive laser diagnostics of acetone planar laser induced fluorescence (PLIF) imaging, and particle image velocimetry (PIV). Various jet-to-crossflow momentum flux ratios in the range of $61\leq J\leq5$ were investigated for three alternative flush mounted circular contracting nozzle injector configurations: a small nozzle ($D=4.04~mm$) passively augmented by a small triangular tab placed about the jet exit periphery, a large nozzle ($D=7.59~mm$) actively augmented by axisymmetric excitation of the jet flow, and a coaxial nozzle ($D=3.81~mm$) with varying degrees of counterflow applied in the outer annulus. Hotwire anemometry was implemented for investigations of the instabilities along the upstream and downstream shear layers of the jet flow, in addition to more in depth analysis of the dynamics of the flow from application of time series analysis techniques to the signal. PLIF imaging served to characterize the JICF by providing qualitative visualizations of the flowfield, and quantitative measurements of the scalar field concentrations and mixing metrics, along both the centerplane and cross-sectional planes of the developing jet. PIV provided determination of the velocity and vorticity fields, along with further investigation of the flow dynamics from proper orthogonal decomposition (POD) analysis, again from both the centerplane and cross-sectional planes of the developing jet.

Flow conditions corresponding to a naturally absolutely unstable (AU) upstream shear layer (USL) ($J = 7$) and a convectively unstable (CU) condition ($J = 61$) with jet Reynolds numbers of $Re_j=2300$ were explored for PLIF imaging of the tabbed JICF. Tab location was seen in some cases to significantly alter shear layer instabilities, especially for the case with $J = 7$. Yet acetone planar laser induced fluorescence (PLIF) imaging revealed that more substantive structural changes could be realized with tab placement for the case where $J = 61$. Tab locations with the greatest influence appeared to be consistent with wavemaker regions predicted in numerical simulations of the round transverse jet by \cite{Regan}, providing evidence for the potential to tailor local shear layer rollup, jet structure, and mixing via simple passive geometrical alterations. For the PIV imaging different $J$ values were explored, ranging from naturally AU USL, for $J = 5$ and 8, to naturally CU conditions, for $J = 20$ and 41, with $Re_j=1900$. Placement of the tab at or near the upstream region of the jet exit caused a delay in shear layer rollup and, as quantified from the PIV, a reduction in USL vorticity associated with a thickening of the upstream jet momentum thickness. Tab placement was observed to have a symmetrizing influence on nearfield cross-sectional vorticity dynamics at high and low $J$ values, though specific tab locations had differing degrees of influence for different flow conditions. Proper orthogonal decomposition (POD) modes extracted from centerplane velocity field measurements showed significant influence of tab placement on jet upstream as well as wake structures, depending on $J$. Phase portraits extracted from POD mode coefficient plots produced periodic (circular) shapes for tab placement corresponding to conditions for which the USL was determined to be absolutely unstable.

Flow conditions corresponding to a naturally AU USL ($J = 7$) and a CU condition ($J = 10$) with $Re_j=1800$ were explored for PLIF imaging of the axisymmetrically excited JICF. Implementation of a novel double-pulse waveform demonstrated significant enhancement of the quantified jet mixing, where the most significant alterations was seen for forcing waveforms which generated nearfield vortical interactions and breakdown. The same forcing waveform yielded differences between the AU and CU jets resulting from changes to the formation number of the vortex rings as suggested by \cite{Sau_10}, which resulted in alterations in the celerity, circulation, and nearfield interaction of said vortex rings. Separate investigations treated a jet with a stronger AU USL ($J=6$) to comparing the novel double-pulse forcing with sinusoidal and square wave excitation of the jet. Synchronization analysis demonstrated dramatic improvement in the ability of the jet to lock-in to the forcing when a square or double-pulse waveform was implemented. Additional Van der Pol oscillator modeling of Fourier approximated square wave forcing suggested greater significance was seen in the proximity of the harmonics to the natural instability of the jet compared to the actual coherence of the waveform, analogoous to findings by \cite{Sau_10}. Interestingly, PIV-based POD further suggested the nearfield dynamics and efficacy of mixing were heavily dependent upon the vortex rings which were formed, in some cases quite independent of the state of synchronization to the applied forcing. Application of a quasiperiodic forcing significantly improved the mixing without significantly altering the jet structure.

For the coaxial JICF, a single flow condition corresponding to a CU USL with a naturally highly asymmetric cross-section ($J = 41$), at $Re_j=1900$, was explored with PLIF and PIV imaging. Suction was applied locally in the upstream and downstream edges of the jet in order to alter the jet shear layer instabilities and vortex dynamics. Indeed, hotwire based spectral characteristics along the USL demonstrated the jet transitioned to an AU flow with strong suction upstream. PIV-based POD also depicted significant enhancement of mode structures along the USL of the jet. Hotwire spectral measurements detected little alteration to the USL with suction applied in the downstream of the jet, where the jet remained CU even with strong suction. However, PIV-based POD dynamics depicted significant enhancement of structures downstream which resembled upright wake vortices, and appeared coupled with the vortex rollup along the downstream of the jet. Interestingly, the corresponding cross-sectional CVP structure was made quite symmetric with the suction applied both upstream or downstream of the jet, suggestive that the suction applied along the symmetry plane of the jet was able to overcome the jet's natural susceptibility to asymmetric perturbations (\cite{Alves_2}), and further supported the suggested wavemaker region for a CU jet purported by \cite{Regan}. Mixing metrics determined significant enhancement in mixing due to the applied suction, further establishing agreement in the correlation between the strength of the shear layer dynamics, symmetry of the cross-sectional CVP, and the resulting jet mixing.

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