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We study coherent structures in subsonic turbulent jets subject to a flightstream. A thorough characterisation of the effects of a flight stream on theturbulent field was recently performed by Maia et al. (2023) and fluctuationenergy attenuations were observed over a broad range of frequencies andazimuthal wavenumbers. The Kelvin-Helmholtz, Orr and lift-up mecahnisms wereall shown to be weakened by the flight stream. Here we expand upon that studyand model the changes in the dynamics of jets in flight using global resolventanalysis. The resolvent model is found to correctly capture the main effects ofthe flight stream on the dynamics of coherent structures, which are educed froma large-eddy simulation database using spectral proper orthogonal decomposition(SPOD). Three modifications of note are: the damping of low-frequencystreaky/Orr structures that carry most of the fluctuation energy; a degradationof the low-rank behaviour of the jet in frequencies where modal instabilitymechanisms are dominant; and a rank decrease at very low Strouhal numbers. Thelatter effect is underpinned by larger gain separations predicted by theresolvent analysis, due to a reduction in the wavelength of associated flowstructures. This leads to a clearer relative dominance of streaky structuresgenerated by the lift-up mechanism, despite the fact that the lift-up mechanismhas been weakened with respect to the static jet.

The effect of flight on a turbulent jet: coherent structure eduction and resolvent analysis