Effect of tabs on transverse jet instabilities, structure, vorticity dynamics and mixing

Harris, Erica; Besnard, Andrea; Karagozian, Ann

doi:10.1017/jfm.2021.323

Cited by 8 publications

(52 citation statements)

References 51 publications

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“…For J = 7 in figure 10(b), the overall best molecular mixing tended to occur for the most active vortex collisions or interactions, especially for vortices with trailing columns, yet it is important to note that pulsewidth as well as pulse spacing contribute to these effects. It is also important to note that forcing conditions which resulted in the best molecular mixing did not usually correspond to conditions of maximum jet penetration, as observed in other forced as well as passively controlled transverse jet experiments [37,38,50]. Importantly, this observation contradicts long held assumptions that improvements in jet penetration correspond to improved molecular mixing [1].…”

Section: Mixing Quantificationmentioning

confidence: 94%

“…Each cavity produced light in the IR at 1064 nm, which was then passed through harmonic generators to produce light in the green at 532 nm and the UV at 266 nm, with the latter serving to excite the acetone tracer in the jet. Green laser light is utilized in separate experiments involving particle image velocimetry (PIV) [46,50].Triggering of the laser cavities was possible up to a maximum of 15 Hz, however the internal optics of the laser were optimized for the best UV energy output, and optimal UV to green signal-to-noise ratio (SNR), when operating at 5 Hz. At this operating condition each of the laser cavities was individually capable of producing 8 ns full width at half maximum pulses with UV energy levels of approximately 30-35 mJ.…”

Section: Optical Diagnosticsmentioning

confidence: 99%

“…More recent experiments by Gevorkyan et al [30] systematically examine several alternative mean and instantaneous mixing metrics in both centerplane and cross-sectional dimensions, and find that molecular mixing can accurately be quantified via the Unmixedness parameter, U, which examines statistics associated with the second moment or variance of the scalar field at a given location. This parameter has been used quite successfully in a number of recent studies on the JICF [37,38,50,53].…”

Section: E Mixing Metricsmentioning

confidence: 99%

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Effects of controlled vortex generation and interactions in transverse jets

et al. 2022

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This experimental study examined the effects of controlled vortex generation and interactions created by axisymmetric excitation of a transverse jet, with a focus on the structural and mixing characteristics of the flow. The excitation consisted of a novel double-pulse forcing waveform applied to the jet, where two distinct temporal square wave pulses were prescribed during a single forcing period. The two distinct pulses produced vortex rings of different strength and celerity, the strategic selection of which promoted vortex ring interactions or collisions in the nearfield to varying degrees. Jet flow conditions corresponding to a transitionally convectively/absolutely unstable upstream shear layer (USL) in the absence of forcing, at a jet-to-crossflow momentum flux ratio of J = 10, and to an absolutely unstable USL at J = 7, were explored for a jet Reynolds number of 1800. Acetone planar laser-induced fluorescence (PLIF) imaging was utilized to quantify the influence of different prescribed temporal waveforms. All forcing conditions enhanced the spread, penetration, and molecular mixing of the jet as compared to the unforced jet, though to differing degrees. Interestingly, when the jet was convectively unstable, forcing which promoted vortex collisions provided the greatest enhancement in molecular mixing, whereas the absolutely unstable jet produced the greatest enhancement in mixing when the vortex rings did not interact, with important implications for optimized jet control.

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Section: Mixing Quantificationmentioning

confidence: 94%

Section: Optical Diagnosticsmentioning

confidence: 99%

Section: E Mixing Metricsmentioning

confidence: 99%

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Effects of controlled vortex generation and interactions in transverse jets

et al. 2022

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“…The thickness of the tab is 0.2012D and the base width of the tab is 0.25D, resulting in a 2.3 % geometric blockage of the original circular jet. Note that in the experiments of Harris et al (2021), the nozzle length is extended by 0.19D to accommodate the tab template, which slightly weakened the shear layer compared with the original nozzle. We do not incorporate the same extension into the present simulations to permit direct comparisons with the non-tabbed results of Iyer & Mahesh (2016) and Regan & Mahesh (2017 for the original nozzle.…”

Section: Details Of the Geometry Grid And Computational Domainmentioning

confidence: 99%

“…In contrast, at higher R (corresponding to the naturally CU USL), both the upstream and downstream tabs improved mixing, and the asymmetry of the jet cross-section could be altered through asymmetric placement of the tab, aligning with the increased growth rates of asymmetric modes for R = 4 predicted by Regan & Mahesh (2019) and the sensitivity of these modes to actuation on either side of the upstream edge of the jet exit. Harris et al (2021) suggested that the contrast in USL spectral characteristics and the jet cross-section effects 'might imply greater changes than those actually observed in the jet structure'.…”

Section: Introductionmentioning

confidence: 99%

Effect of tabs on the shear layer dynamics of a jet in cross-flow

Morse

Mahesh²

2023

J. Fluid Mech.

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Direct numerical simulations (DNS) of a jet in cross-flow (JICF) with a triangular tab at two positions are performed at jet-to-cross-flow velocity ratios of $R = 2$ and $4$ with a jet Reynolds number of 2000 based on the jet's bulk velocity and exit diameter. The DNS and dynamic mode decomposition show the sensitivity of the tab's effect on the jet upstream shear layer (USL) structure and cross-section to $R$ , echoing the experimental discoveries of Harris et al. (J. Fluid Mech., vol. 918, 2021). Furthermore, DNS reveals that the presence of a tab placed on the upstream side of the nozzle significantly modifies the USL through production of streamwise vortices that curl around the spanwise vortex tubes originating from the primary instability of the USL. This provides an explanation for the improvement in mixing that has been associated with an upstream tab. The streamwise vortex structure shows remarkable similarities to the ‘strain-oriented vortex tubes’ observed for disturbed plane shear layers by Lasheras & Choi (J. Fluid Mech., vol. 189, 1988, pp. 53–86). For both $R$ cases, the USL instability is delayed, the jet penetration is reduced, and the jet cross-section is flattened, although the tab has a less pronounced effect on the USL structure at higher velocity ratios, where the formation of the streamwise vortices is delayed. In contrast, a tab placed 45 $^\circ$ from the upstream position produces significantly different effects compared with the upstream tab. At $R = 4$ , the jet cross-section is significantly skewed away from the tab and a tertiary vortex is formed, as observed in past studies of round JICFs at relatively high $R$ and low Reynolds numbers. The ability of the tab to produce a controllable steady-state tertiary vortex has implications for a variety of applications. The 45 $^\circ$ tab produces asymmetric effects in the wake of the jet at $R = 2$ , but the effect on the jet cross-section is much smaller, highlighting the sensitivity of jets at high $R$ to asymmetric perturbations.

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