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Aerospace 2018, 5(2), 49; https://doi.org/10.3390/aerospace5020049

Energy-Dynamics Resulting in Turbulent and Acoustic Phenomena in an Underexpanded Jet

Department of Mechanical and Aerospace Engineering, The Ohio State University, Columbus, OH 43210, USA
These authors contributed equally to this work.
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Received: 11 April 2018 / Revised: 23 April 2018 / Accepted: 25 April 2018 / Published: 1 May 2018
(This article belongs to the Special Issue Under-Expanded Jets)
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Abstract

Underexpanded jets exhibit interactions between turbulent shear layers and shock-cell trains that yield complex phenomena that are absent in the more commonly studied perfectly expanded jets. We quantitatively analyze these mechanisms by considering the interplay between hydrodynamic (turbulence) and acoustic modes, using a validated large-eddy simulation. Using momentum potential theory (MPT) to achieve energy segregation, the following observations are made. The sharp gradients in fluctuations introduced by the shock-cell structure are captured mostly in the hydrodynamic mode, whose amplitude is an order of magnitude larger than the acoustic mode. The acoustic mode has a relatively smoother distribution, exhibiting a compact wavepacket form. Proper orthogonal decomposition (POD) identifies the third-to-sixth cells as the most dynamic structures. The imprint of shock cells is discernible in the nearfield of the acoustic mode, primarily along the sideline direction. Energy interactions that feed the acoustic mode remain compact in nature, facilitating a simple propagation technique for farfield noise prediction. The farfield sound spectra show peak directivity at 30 to the downstream axis. The POD modes of the acoustic component also identify two main energetic components in the wavepacket: one representative of the periodic internal structure and the other of intermittent downstream lobes. The latter component occurs at exactly the same frequency as, and displays high correlation with, the farfield peak noise spectra, making the acoustic mode a better predictor of the dynamics than velocity fluctuations. View Full-Text
Keywords: underexpanded jets; shock-cell dynamics; acoustic mode underexpanded jets; shock-cell dynamics; acoustic mode
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Sasidharan Nair, U.; Goparaju, K.; Gaitonde, D. Energy-Dynamics Resulting in Turbulent and Acoustic Phenomena in an Underexpanded Jet. Aerospace 2018, 5, 49.

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