  Coherent turbulent wave-packet structures in a jet at Reynolds number 460000
and Mach number 0.4 are extracted from experimental measurements and are
modeled as linear fluctuations around the mean flow. The linear model is based
on harmonic optimal forcing structures and their associated flow response at
individual Strouhal numbers, obtained from analysis of the global linear
resolvent operator. These forcing-response wave packets ("resolvent modes") are
first discussed with regard to relevant physical mechanisms that provide energy
gain of flow perturbations in the jet. Modal shear instability and the nonmodal
Orr mechanism are identified as dominant elements, cleanly separated between
the optimal and suboptimal forcing-response pairs. A theoretical development in
the framework of spectral covariance dynamics then explicates the link between
linear harmonic forcing-response structures and the cross-spectral density
(CSD) of stochastic turbulent fluctuations. A low-rank model of the CSD at
given Strouhal number is formulated from a truncated set of linear resolvent
modes. Corresponding experimental CSD matrices are constructed from extensive
two-point velocity measurements. Their eigenmodes (spectral proper orthogonal
or SPOD modes) represent coherent wave-packet structures, and these are
compared to their counterparts obtained from the linear model. Close agreement
is demonstrated in the range of "preferred mode" Strouhal numbers, around a
value of 0.4, between the leading coherent wave-packet structures as educed
from the experiment and from the linear resolvent-based model.
