Spectral analysis of turbulence in jets
Professor Tim Colonius, California Institute of Technology
Large-eddy simulation and resolvent analysis are used to study the structure of turbulent jets at high Reynolds number. Simulations of several high-speed jets, validated in companion experiments, provide data from which structure is educed via spectral proper orthogonal decomposition (SPOD). Some recently established properties of SPOD are discussed. In particular, the close connection with resolvent analysis—the optimal forced response of the flow linearized about the turbulent mean—is exploited to examine the low-rank behavior of the large-scale, optimal gain structures. Consistent with past observations, there are a range of frequencies and azimuthal mode numbers at which the jet exhibits a dominant rank-one response; these structures, essentially Kelvin-Helmholtz instability wavepackets, are forced near the nozzle exit and consequently evolve, statistically speaking, in a (surprisingly) linear fashion. At other frequencies, a broadband response that is forced over an extensive region of the jet is dominant and corresponds to tilted Orr-like wavepackets. Some consequences of these observations for the radiated sound are explored, and the potential for predictive reduced-order models of free-shear turbulence is discussed.