Effects of Expansion Ratio and Nozzle Asymmetry on Flowfield of Diamond Jets

Prasad, Anirudh Lakshmi Narasimha; Saleh, Yousef; Sellappan, Prabu; Unnikrishnan, S.; Alvi, Farrukh S.

doi:10.2514/1.j061624

Cited by 4 publications

(10 citation statements)

References 60 publications

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“…This is further established by the relative amplitudes of the azimuthally decomposed φ modes. The spatially localized growth and decay of the acoustic component is consistent with observations on energetic structures made by Arndt, Long & Glauser (1997) and Colonius & Freund (2002) for circular jets, Chakrabarti et al (2021) and Lakshmi Narasimha Prasad & Unnikrishnan (2023) for rectangular jets and Lakshmi Narasimha Prasad et al (2022a) for diamond jets. Due to the relatively low AR of the current configuration, most of the energy is present in the axisymmetric φ mode 0.…”

Section: Azimuthal Coherencesupporting

confidence: 87%

“…(2021) and Lakshmi Narasimha Prasad & Unnikrishnan (2023) for rectangular jets and Lakshmi Narasimha Prasad et al. (2022 a ) for diamond jets. Due to the relatively low AR of the current configuration, most of the energy is present in the axisymmetric mode 0.…”

Section: Impact On Radiating Characteristicsmentioning

confidence: 99%

“…The approximate node count in the coarser grid is about 40.60 × 10 6 while that of the finer grid is about 47.53 × 10 6 . The chosen grid spacing for the finer mesh is informed by previous simulations at similar Reynolds numbers, involving compressible jets exiting circular nozzles (Unnikrishnan & Gaitonde 2016), rectangular nozzles (Chakrabarti et al 2021) and diamond nozzles (Lakshmi Narasimha Prasad et al 2022a). The coarse mesh is around 10 % coarser in the streamwise direction, and around 20 % coarser along each of the transverse directions.…”

Section: Appendix Grid Independence Studymentioning

confidence: 99%

“…2021) and diamond nozzles (Lakshmi Narasimha Prasad et al. 2022 a ). The coarse mesh is around coarser in the streamwise direction, and around coarser along each of the transverse directions.…”

Section: Figure 26mentioning

confidence: 99%

“…For each plane considered, the term containing the planar spanwise gradient is the dominant production term (Lakshmi Narasimha Prasad et al. 2022 a , b ). This results from the large gradients across the shear layer, and strong correlation between streamwise and spanwise velocity fluctuations.…”

Section: Effect On Mean Flow and Turbulent Statisticsmentioning

confidence: 99%

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Effect of plasma actuator-based control on flow-field and acoustics of supersonic rectangular jets

Prasad

Unnikrishnan

2023

J. Fluid Mech.

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We perform a computational study on the effects of localized arc filament plasma actuator based control on the flow field and acoustics of a supersonic 2:1 aspect ratio rectangular jet. Post validation of the baseline jet, effects of control in the context of noise reduction are studied at experimentally guided forcing parameters, including frequencies $St=0.3, 1.0$ and $St=2.0$ with duty cycles of $20\,\%$ and $50\,\%$ . In general, high-frequency forcing reduces noise in the downstream direction, with the actuator signature appearing mostly in the sideline direction. Here $St=1$ , ${\rm DC}=50\,\%$ yields an optimum balance between peak noise reduction (of ${\sim }1.5$ dB) and actuator tones, with control being most effective on the major axis plane that bisects the shorter edges of the nozzle. Shear layer response to the most effective forcing includes generation of successive arrays of mutually interacting staggered lambda vortices, which eventually energize streamwise vortical elements. Causal mechanisms of noise mitigation are further elucidated as follows. First, the control reduces the energy within the supersonic phase speed regime of peak radiating frequencies by redistributing a part of it into a high-frequency band. Second, it enhances azimuthal percolation of energy into the first and second helical modes at frequencies where noise reduction is seen, thus weakening the radiatively efficient axisymmetric mode. Finally, sound-producing intermittent events in the jet are significantly reduced, thereby minimizing the high-intensity acoustic emissions. This small-perturbation-based control strategy results in only minor variations in the mean flow properties. However, reduced production and enhanced convection attenuate turbulent kinetic energy within the spreading shear layer in the controlled jet.

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Section: Azimuthal Coherencesupporting

confidence: 87%

Section: Impact On Radiating Characteristicsmentioning

confidence: 99%

Section: Appendix Grid Independence Studymentioning

confidence: 99%

Section: Figure 26mentioning

confidence: 99%

Section: Effect On Mean Flow and Turbulent Statisticsmentioning

confidence: 99%

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Effect of plasma actuator-based control on flow-field and acoustics of supersonic rectangular jets

Prasad

Unnikrishnan

2023

J. Fluid Mech.

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Noise mitigation in rectangular jets through plasma actuator-based shear layer control

Lakshmi Narasimha Prasad,

Unnikrishnan

2024

J. Fluid Mech.

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A computational analysis is performed to study the three-dimensional response of rectangular shear layers to plasma actuator-based control, in the context of sound mitigation of supersonic non-axisymmetric jets. A Mach $1.5$ rectangular jet with an aspect ratio $2:1$ is controlled using experimentally informed actuation patterns, referred to as M0, M1, M2, M3, M ${\rm \pi}$ and M+/ $-$ 1. While the first five progressively increase the phase difference between successive actuators thus enhancing three-dimensionality of the shear layer structures, the latter corresponds to the flapping mode of the jet. A preliminary linear analysis identifies that the frequency, $St\sim 1$ , has a relatively high overall amplification within the baseline shear layer, and is hence utilized for control in the subsequent nonlinear simulations. Each actuation reveals unique near-field vortical and acoustic responses that have a profound impact on far-field noise levels. The M0 actuation induces circumferentially interconnected strong streamwise vortices, while M1 actuation enhances the circumferential variability in the coherent structures. The M2 actuation encompasses both these effects, and along with a very low tonal impact of forcing, produces the most desirable far-field noise mitigation ( ${\sim }2.6\,{\rm dB}$ ), contributed by a broadband reduction around the column-mode peak of the baseline jet. Beyond M2 actuation, effectiveness of control saturates, particularly along the direction of peak noise radiation. Through a near-field analysis of the acoustic component, the efficacy of M2 actuation is attributed to the attenuation of the radiative efficiency of the jet, including reduced energy in the supersonic phase speeds, and redistribution of energy into the higher helical modes. Further, it curtails the nonlinear difference interactions in the plume that energize column-mode frequencies, which often appear as strong intermittent sound-producing events. While the shear layer turbulent kinetic energy decreases with actuation, the controlled jets show minimal variations in mean flow properties, particularly under M2 actuation, suggesting this to be a promising small-perturbation-based noise control strategy.

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Experimental Characterization of Supersonic Diamond Jet Acoustics

Saleh,

Sellappan,

Alvi

2023

AIAA Journal

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Acoustic emissions from supersonic jets are of interest due to their deleterious effects on nearby personnel and structures. As development of both military and commercial supersonic aircraft continues to grow, it is essential to combat increasing noise pollution. Axisymmetric jets are highly prevalent where their aeroacoustic characteristics have been well-documented; in contrast, relatively few studies have examined non-axisymmetric jets. In the present study, aeroacoustics of supersonic jets produced by a diamond-shaped Mach 2 nozzle are explored across a large operational envelope under different pressure ratios ranging from highly overexpanded to moderately underexpanded. Measurements of the far-field acoustics in an anechoic environment were used to characterize the acoustic flowfield and contrast the distribution of energy in the spectral domain between the characteristic major and minor axes of the nozzle. Overall, the diamond jet exhibits a highly directional acoustic field with a distinct local minimum for sideline noise. Furthermore, screech behavior was only observed in a narrow, overexpanded flow regime; this is in contrast to the documented behavior of axisymmetric and rectangular jets. Over the conditions examined there is no indication of distinct staging. Additionally, there exists a nozzle pressure ratio threshold beyond which peak noise switches axes.

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Effects of Expansion Ratio and Nozzle Asymmetry on Flowfield of Diamond Jets

Cited by 4 publications

References 60 publications

Effect of plasma actuator-based control on flow-field and acoustics of supersonic rectangular jets

Effect of plasma actuator-based control on flow-field and acoustics of supersonic rectangular jets

Noise mitigation in rectangular jets through plasma actuator-based shear layer control

Experimental Characterization of Supersonic Diamond Jet Acoustics

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