Controlled interactions in a forced axisymmetric jet. Part 1. The distortion of the mean flow

Long, Ted; Petersen, Robert A.

doi:10.1017/s0022112092001010

Cited by 37 publications

(26 citation statements)

References 13 publications

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“…The wave crests of modes at different frequencies or azimuthal wavenumbers travel at different streamwise velocities, which is not the case for the non-swirling jet. The strong dispersiveness in swirling jets hampers the use of effective flow control methods that utilize intermodal resonance principles (Long & Petersen 1992;Paschereit et al 1995).…”

Section: Discussionmentioning

confidence: 99%

“…For frequencies near neutral amplification, the instability waves are only weakly dispersive with c ph /V ≈ 0.55. The non-dispersiveness enables nonlinear modal interactions to occur, such as modes resonating with their subharmonics (Cohen & Wygnanski 1987a;Paschereit, Wygnanski & Fiedler 1995) or triad resonance between two spinning modes (Long & Petersen 1992). These mechanisms do not apply as rigourously to the swirling jet.…”

Section: Instabilities At the Nozzle Exitmentioning

confidence: 99%

“…With an array of eight actuators the highest azimuthal mode numbers that can be excited are m = ±4. A similar excitation device was successfully used by Long & Petersen (1992) to study instabilities in non-swirling jets and by Panda & McLaughlin (1994) for swirling jets. For the measurements of the impulse response, the flow was locally disturbed by using only one of the eight loudspeakers.…”

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confidence: 99%

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On the impact of swirl on the growth of coherent structures

Oberleithner

Paschereit

Wygnanski³

2014

J. Fluid Mech.

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Spatial linear stability analysis is applied to the mean flow of a turbulent swirling jet at swirl intensities below the onset of vortex breakdown. The aim of this work is to predict the dominant coherent flow structure, their driving instabilities and how they are affected by swirl. At the nozzle exit, the swirling jet promotes shear instabilities and, less unstable, centrifugal instabilities. The latter stabilize shortly downstream of the nozzle, contributing very little to the formation of coherent structures. The shear mode remains unstable throughout generating coherent structures that scale with the axial shear-layer thickness. The most amplified mode in the nearfield is a co-winding double-helical mode rotating slowly in counter-direction to the swirl. This gives rise to the formation of slowly rotating and stationary large-scale coherent structures, which explains the asymmetries in the mean flows often encountered in swirling jet experiments. The co-winding single-helical mode at high rotation rate dominates the farfield of the swirling jet in replacement of the co-and counter-winding bending modes dominating the non-swirling jet. Moreover, swirl is found to significantly affect the streamwise phase velocity of the helical modes rendering this flow as highly dispersive and insensitive to intermodal interactions, which explains the absence of vortex pairing observed in previous investigations. The stability analysis is validated through hot-wire measurements of the flow excited at a single helical mode and of the flow perturbed by a time-and space-discrete pulse. The experimental results confirm the predicted mode selection and corresponding streamwise growth rates and phase velocities.

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Section: Discussionmentioning

confidence: 99%

Section: Instabilities At the Nozzle Exitmentioning

confidence: 99%

mentioning

confidence: 99%

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On the impact of swirl on the growth of coherent structures

Oberleithner

Paschereit

Wygnanski³

2014

J. Fluid Mech.

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“…In the absence of temporal modulation, the mean-flow distortion due to the mutual interaction between a pair of helical modes induces a steady mean-flow distortion, which is of interest in its own right, and has been studied experimentally by several authors (Cohen & Wygnanski 1987b, Long & Petersen 1992.…”

Section: The Nonlinear Development Of Pairs Of Helical Modesmentioning

confidence: 99%

Low-frequency sound radiated by a nonlinearly modulated wavepacket of helical modes on a subsonic circular jet

Huerre

2009

J. Fluid Mech.

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A possible fundamental physical mechanism by which instability modes generate sound waves in subsonic jets is presented in the present paper. It involves a wavepacket of a pair of helical instability modes with nearly the same frequencies but opposite azimuthal wavenumbers. As the wavepacket undergoes simultaneous spatial-temporal development in a circular jet, the mutual interaction between the helical modes generates a strong three-dimensional, slowly modulating, 'mean-flow distortion'. It is demonstrated that this 'mean field' radiates sound waves to the far field. The emitted sound is of very low frequency, with characteristic time and length scales being comparable with those of the envelope of the wavepacket, which acts as a noncompact source. A matched-asymptotic-expansion approach is used to determine, in a self-consistent manner, the acoustic field in terms of the envelope of the wavepacket and a transfer factor characterising the refraction effect of the background base flow. For realistic jet spreading rates, the nonlinear development of the wavepacket is found to be influenced simultaneously by non-parallelism and non-equilibrium effects, and so a composite modulation equation including both effects is constructed in order to describe the entire growth-attenuation-decay cycle. Parametric studies pertaining to relevant experimental conditions indicate that the acoustic field is characterised by a single-lobed directivity pattern beamed at an angle about 45 o − 60 o to the jet axis, and a broadband spectrum centred at a Strouhal number St ≈ 0.07 − 0.2. As the nonlinear effect increases, the radiation becomes more efficient and the noise spectrum broadens, but the gross features of the acoustic field remain robust, and are broadly in agreement with experimental observations.

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“…of the normally circular mean velocity contours and, similarly, excitation of modes ± 2 results in a cos(40) distortion of the mean velocity contours (Long & Petersen 1991).…”

Section: Amplitude-modulated Excitationmentioning

confidence: 89%

Real-Time Adaptive Control of Mixing in a Plane Shear Layer

Glezer¹,

Champagne²

1993

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Controlled interactions in a forced axisymmetric jet. Part 1. The distortion of the mean flow

Cited by 37 publications

References 13 publications

On the impact of swirl on the growth of coherent structures

On the impact of swirl on the growth of coherent structures

Low-frequency sound radiated by a nonlinearly modulated wavepacket of helical modes on a subsonic circular jet

Real-Time Adaptive Control of Mixing in a Plane Shear Layer

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