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