  Three amplification mechanisms present in turbulent jets, namely lift-up,
Kelvin-Helmholtz, and Orr, are characterized via global resolvent analysis and
spectral proper orthogonal decomposition (SPOD) over a range of Mach numbers.
The lift-up mechanism was recently identified in turbulent jets via local
analysis by Nogueira et al. (J. Fluid Mech., vol. 873, 2019, pp. 211-237) at
low Strouhal number ($St$) and non-zero azimuthal wavenumbers ($m$). In these
limits, a global SPOD analysis of data from high-fidelity simulations reveals
streamwise vortices and streaks similar to those found in turbulent
wall-bounded flows. These structures are in qualitative agreement with the
global resolvent analysis, which shows that they are a response to upstream
forcing of streamwise vorticity near the nozzle exit. Analysis of mode shapes,
component-wise amplitudes, and sensitivity analysis distinguishes the three
mechanisms and the regions of frequency-wavenumber space where each dominates,
finding lift-up to be dominant as $St/m \rightarrow 0$. Finally, SPOD and
resolvent analyses of localized regions show that the lift-up mechanism is
present throughout the jet, with a dominant azimuthal wavenumber inversely
proportional to streamwise distance from the nozzle, with streaks of azimuthal
wavenumber exceeding five near the nozzle, and wavenumbers one and two most
energetic far downstream of the potential core.

Fluid Dynamics