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The effect of streamwise vortices on the aeroacoustics of a Mach 0.9 jet

Published online by Cambridge University Press:  26 April 2007

MEHMET B. ALKISLAR ,
A. KROTHAPALLI  and
G. W. BUTLER
MEHMET B. ALKISLAR*
Affiliation:
Department of Mechanical Engineering, 2525 Pottsdamer Street, Florida A&M University and Florida State University, Tallahassee, FL 32310, USA
A. KROTHAPALLI
Affiliation:
Department of Mechanical Engineering, 2525 Pottsdamer Street, Florida A&M University and Florida State University, Tallahassee, FL 32310, USA
G. W. BUTLER
Affiliation:
The Boeing Company, PO Box 3307 MC ML-67, Seattle, WA 98124, USA
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Abstract

The role of the streamwise vortices on the aeroacoustics of a Mach 0.9 axisymmetric jet is investigated using two different devices to generate streamwise vortices: microjets and chevrons. The resultant acoustic field is mapped by sideline microphones and a microphone phased array. The flow-field characteristics within the first few diameters of the nozzle exit are obtained using stereoscopic particle image velocimetry (PIV). The flow-field measurements reveal that the counter-rotating streamwise vortex pairs generated by microjets are located primarily at the high-speed side of the initial shear layer. In contrast, the chevrons generate vortices of greater strength that reside mostly on the low-speed side. Although the magnitude of the chevron's axial vorticity is initially higher, it decays more rapidly with downstream distance. As a result, their influence is confined to a smaller region of the jet. The axial vorticity generated by both devices produces an increase in local entrainment and mixing, increasing the near-field turbulence levels. It is argued that the increase in high-frequency sound pressure levels (SPL) commonly observed in the far-field noise spectrum is due to the increase in the turbulence levels close to the jet exit on the high-speed side of the shear layer. The greater persistence and lower strength of the streamwise vortices generated by microjets appear to shift the cross-over frequencies to higher values and minimize the high-frequency lift in the far-field spectrum. The measured overall sound pressure level (OASPL) shows that microjet injection provides relatively uniform noise suppression for a wider range of sound radiation angles when compared to that of a chevron nozzle.

Type
Papers
Information
Journal of Fluid Mechanics , Volume 578 , 10 May 2007 , pp. 139 - 169
Copyright
Copyright © Cambridge University Press 2007

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