Effect of a flight stream on subsonic turbulent jets

This study concerns a turbulent jet at Mach number Mi = 0.9, subject to a uniform external flow stream at Mf = 0.15. The analysis combines experimental and numerical databases, spectral proper orthogonal decomposition (SPOD), and linear modeling. The experiments involve time-resolved, stereo PIV measurements at different cross-sections of the jet. A companion large-eddy simulation was performed with the same operating conditions using the "CharLES" solver by Cascade Technologies to obtain a complete and highly resolved three-dimensional database. We assess the mechanisms that underpin the reduction in fluctuation energy that is known to occur when a jet is surrounded by a flight stream. We show that this energy reduction is spread over a broad region of the frequency-wave-number space and involves, apart from the known stabilization of the modal Kelvin-Helmholtz (KH) instability, the attenuation of flow structures associated with the nonmodal Orr and lift-up mechanisms. Streaky structures, associated with helical azimuthal wave numbers and very slow timescales, are the most strongly affected by the flight stream, in terms of energy attenuation and spatial distortion. The energy reductions are accompanied by a weakening of the low-rank behavior of the jet dynamics. These trends are found to be consistent, to a great extent, with results of a local linear model based on the modified mean flow in the flight-stream case.