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Coherent turbulent wave-packet structures in a jet at Reynolds number 460000and Mach number 0.4 are extracted from experimental measurements and aremodeled as linear fluctuations around the mean flow. The linear model is basedon harmonic optimal forcing structures and their associated flow response atindividual Strouhal numbers, obtained from analysis of the global linearresolvent operator. These forcing-response wave packets ("resolvent modes") arefirst discussed with regard to relevant physical mechanisms that provide energygain of flow perturbations in the jet. Modal shear instability and the nonmodalOrr mechanism are identified as dominant elements, cleanly separated betweenthe optimal and suboptimal forcing-response pairs. A theoretical development inthe framework of spectral covariance dynamics then explicates the link betweenlinear harmonic forcing-response structures and the cross-spectral density(CSD) of stochastic turbulent fluctuations. A low-rank model of the CSD atgiven Strouhal number is formulated from a truncated set of linear resolventmodes. Corresponding experimental CSD matrices are constructed from extensivetwo-point velocity measurements. Their eigenmodes (spectral proper orthogonalor SPOD modes) represent coherent wave-packet structures, and these arecompared to their counterparts obtained from the linear model. Close agreementis demonstrated in the range of "preferred mode" Strouhal numbers, around avalue of 0.4, between the leading coherent wave-packet structures as educedfrom the experiment and from the linear resolvent-based model.

Resolvent-based modelling of coherent wavepackets in a turbulent jet