Wavepackets and trapped acoustic modes in a turbulent jet: coherent structure eduction and global stability
Abstract
Coherent features of a turbulent Mach 0.9, Reynolds number 10^6 jet are educed from a high-fidelity large eddy simulation. Besides the well-known Kelvin–Helmholtz instabilities of the shear layer, a new class of trapped acoustic waves is identified in the potential core. A global linear stability analysis based on the turbulent mean flow is conducted. The trapped acoustic waves form branches of discrete eigenvalues in the global spectrum, and the corresponding global modes accurately match the educed structures. Discrete trapped acoustic modes occur in a hierarchy determined by their radial and axial order. A local dispersion relation is constructed from the global modes and found to agree favourably with an empirical dispersion relation educed from the simulation data. The product between direct and adjoint modes is then used to isolate the trapped waves. Under certain conditions, resonance in the form of a beating occurs between trapped acoustic waves of positive and negative group velocities. This resonance explains why the trapped modes are prominently observed in the simulation and as tones in previous experimental studies. In the past, these tones were attributed to external factors. Here, we show that they are an intrinsic feature of high-subsonic jets that can be unambiguously identified by a global linear stability analysis.
Additional Information
© 2017 Cambridge University Press. Received 12 May 2016; revised 21 February 2017; accepted 7 June 2017; first published online 27 July 2017. O.T.S. gratefully acknowledges support by DFG grant no. 3114/1-1. We gratefully acknowledge support from the Office of Naval Research under contract N0014-11-1-0753 and N00014-16-1-2445. A.V.G.C. and P.J. acknowledge support from the Science Without Borders program (project number A073/2013). We are grateful to S. Piantanida and R. Kari for conducting the 'Bruit and Vent' measurements. The LES study was supported by NAVAIR SBIR project, under the supervision of Dr J. T. Spyropoulos. The main LES calculations were carried out on CRAY XE6 machines at DoD HPC facilities in ERDC DSRC.Additional details
- Eprint ID
- 80599
- DOI
- 10.1017/jfm.2017.407
- Resolver ID
- CaltechAUTHORS:20170818-083639884
- Deutsche Forschungsgemeinschaft (DFG)
- 3114/1-1
- Office of Naval Research (ONR)
- N0014-11-1-0753
- Office of Naval Research (ONR)
- N00014-16-1-2445
- Science Without Borders
- A073/2013
- Naval Air Systems Command (NAVAIR)
- Created
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2017-08-18Created from EPrint's datestamp field
- Updated
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2021-11-15Created from EPrint's last_modified field