Aeroacoustic Sources in Twin Turbulent Jets

Abstract

An understanding of the aeroacoustics of twin turbulent jets is essential for applications involving noise reduction in dual engine aircrafts and launch vehicles. The aeroacoustic dynamics of these jets are influenced by the spacing between the shear layer of the two jets as well as the spatio-temporal nature of the structures arising from the interaction between the two jets. In the present work, we construct reduced-order models of aeroacoustic sources for single and twin subsonic jets (Mj = 0.9, Re = 3600), with the individual jets being replicas of Freund (2001), with the goal of accurately recovering the far-field sound over a rather wide band of frequencies St = [0.07, 1.0] and directivity angles φ = [30◦, 120◦] within a subdecibel level accuracy. These models are designed as linear combinations of spatio-temporally coherent SPOD modes obtained in terms of the Lighthill’s stress tensor, which in turn is computed through large-eddy simulations (LES) of the turbulent jets. The present investigation involves two sets of twin subsonic jets of diameter D each, with spacings of 0.1D and 1D, where the jets merge upstream and downstream of breakdown, respectively. This is observed to alter the dynamics of twin jet evolution. The closely-spaced twin jet decays the slowest due to reduced turbulent stresses which are, however, more broadband due to early merging. Such jets also show strong shielding in the plane of jets, especially at shallow directivity angles where sound levels may drop below that of the single jet. The farther spaced twin jets have dynamics that are more akin to the constituent single jet with turbulent fluctuations peaking here at St = 0.34, but showing very little shielding, with their OASPL mostly linked to the nature of extra flow structures created during merging. Three-dimensional, energy-ranked, coherent structures (SPOD modes) for twin jets exhibit rather poor low-rank behaviour, especially, at the far-field spectral peak St = 0.14, unlike that of the single jet, which is indicative of spatio-temporally complicated structures arising from the merging of the turbulent merging of the twin jets. At St > 0.3, the SPOD wavepackets show strong visual coherence, resembling Kelvin–Helmholtz instability modes upstream of breakdown, while at the lower frequencies there is very little spatial coherence with wavepackets peaking downstream of breakdown. Despite the presence of low-rank modes with well-organized flow structures, the leading SPOD modes exhibit poor aeroacoustic radiating characteristics. The extent of this high rank behaviour is even more pronounced in the case of the closely-spaced twin jet which due to its greater hierarchy of spatio-temporal structures requires relatively more SPOD modes to reconstruct the far-field radiation. In order to explain the poor radiating characteristics of the dominant SPOD modes and to educe the flow structures responsible for far-field radiation, a SPOD formulation termed Lighthill-SPOD(L-SPOD) is constructed with the intent of maximizing the farfield acoustic power as opposed to the L2 norm of the near-field sources as performed previously. This decomposition method is demonstrated for the m = 0 azimuthal mode of the sources and the modal characteristics of these L-SPOD modes are found to be insightful in that, apart from exhibiting a strong low-rank behaviour, the spatiotemporal scales associated with these modes were very close to the spatio-temporal scales of an acoustic wave indicating that only sources, at least for the m = 0 azimuthal mode, whose spatio-temporal scales are very close to that of an acoustic wave are responsible for radiating to the far-field.