Bispectral mode decomposition of axisymmetrically and non-axisymmetrically forced turbulent jets

Abstract

Large-eddy simulations (LES) of a turbulent unforced jet at Re = 50,000 and Mⱼ= 0.4, and jets forced at azimuthal wavenumbers m = 0, m =  1, m = 2 and m = 6 are performed. The objective of this study is to characterize the nonlinear interactions that are initiated by the forcing. To achieve this, we used the bispectral mode decomposition (BMD) technique, which is tailored to extract flow structures associated with triadic interactions. Azimuthal wavenumber triads are investigated using the cross-spectral variant of BMD. Axisymmetric forcing generates peaks at the forcing frequency and its harmonics in the m = 0 component only, whereas non-axisymmetric forcing generates peaks at different azimuthal wavenumbers. The latter aspect is investigated using cross-BMD. Forcing the jet at m =  1, the only odd-m forcing case considered, creates a cascade of triads that generates peaks at the forcing frequency and its odd harmonics at odd azimuthal wavenumbers and even harmonics at even azimuthal wavenumbers. Forcing the jet at m = m_f azimuthal wavenumbers produces peaks at the odd harmonics in the odd integer multiples of m_f and at even harmonic frequencies in the even integer multiples of m_f.

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