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Mixing and Structural Characteristics of Unforced and Forced Jets in Crossflow


This dissertation describes an experimental exploration of structural and mixing characteristics of the unforced as well as forced jet in crossflow (JICF). For unforced experiments, variable jet-to-crossflow density ratios (0.35≤S≤1.00) and momentum flux ratios (5≤J≤41) were explored, for three alternative injectors with a fixed jet Reynolds number (Rej=1900). For forced jet experiments, an equidensity jet emanating from two alternative flush nozzles (Rej=1900; Rej=1500) were explored.

The interplay between scalar and velocity fields for the JICF was studied using simultaneous PLIF/stereo PIV measurements in the jet’s centerplane. POD analysis showed a transition from convective to absolutely instability in the upstream shear layer as J was reduced, consistent with hotwire measurements. Local strain rates were extracted from both PLIF and PIV data via the Howarth transformation.

Mixing characterization with variable mixing scale lengths was explored via centerplane PLIF images. Mixing associated with stirring as well as molecular mixing could be distinguished, apparent especially in vortical structures for flush pipe injection.

Sinusoidal forcing of the convectively unstable JICF demonstrated that relatively low amplitude forcing affected jet structures and mixing, especially close to the fundamental frequency. For the absolutely unstable JICF, a much higher forcing amplitude was required to alter jet characteristics, consistent with lock-in, leading to enhanced molecular mixing.

Single-pulse square wave forcing created deeply-penetrating vortical structures for specific stroke ratios (L/D), even for the absolutely unstable JICF. L/D producing the greatest jet spread and penetration decreased with decreasing J, but L/D for optimal molecular mixing corresponded to different values and trends in J.

Double-pulse square wave excitation, with two variable pulses within each temporal period, enabled the jet's nearfield vortex rings to interact and collide, with attendant effect on molecular mixing.

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