Exploring the link between nozzle dynamics and wavepackets in a mach 0.9 turbulent jet

Abstract

© 2017, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.It has been previously demonstrated in several works that the dynamics of jets important for sound radiation are dominated by low-energy and coherent azimuthal structures, wavepackets. However, the link between these and the nozzle dynamics has received less attention. It is not clear, for instance, if wavepacket amplitudes are determined by mechanisms upstream or downstream of the nozzle exit plane. In this work, a statistical analysis of a Mach 0.9 isothermal turbulent round jet is carried out with a focus on the nozzle dynamics. High-fidelity large eddy simulation data are used in the analysis. First, the azimuthal, axial and radial structures of fluctuations in the nozzle are presented. Distinct hydrodynamic and acoustic components are identified within the nozzle, and a model for the latter, based on duct acoustics, is explored. Two-point statistics of pressure and velocity fields are computed between the upstream and the downstream of the jet exit, with an aim to identify casual relation and coupling in these domains. It is seen that acoustic modes within the nozzle are linked with similar, acoustic disturbances downstream of the jet exit. Moreover, boundary-layer velocity fluctuations are shown to present significant cross-spectral densities with the downstream hydrodynamic wavepackets, suggesting that these boundary-layer disturbances excite the Kelvin-Helmholtz instability in the jet mixing layer.