Nonlinear Interactions in Non-Resonant, Homogeneous Turbulent Jets

Abstract

Bispectral mode decomposition is used to investigate triadic interactions within a Mach 0.4 turbulent jet. We explore its potential to identify dominant triadic interactions and their associated spatial structures in an unforced turbulent jet. The bispectral measure is broadband in frequency for each azimuthal wavenumber triad. The [1,1,2] and [0,0,0] azimuthal wavenumber triads are dominant, emphasizing the importance of the self-interactions of the helical and axisymmetric components. Bispectral modes reveal that streaky structures are fed by the interaction of a Kelvin-Helmholtz wavepacket with its conjugate. Streaks are also observed in other frequency interactions, occurring in regions where the structures of these frequencies are spatially active. Furthermore, integral interaction maps and nonlinear transfer terms are computed to determine the direction of energy transfer and to pinpoint the spatial regions where nonlinearity is most active. As the shear layer develops, small scales interact nonlinearly, transferring energy to larger scales. Moving downstream, near the potential core closure, larger scales become more active, resulting in a forward energy cascade.

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