Large-eddy simulation of crackle in heated supersonic jets

Crackle noise from heated supersonic jets is characterized by the presence of strong positive pressure impulses resulting in a strongly skewed far-field pressure signal (Ffowcs Williams et al., 1975). These strong positive pressure impulses are associated with N-shaped waveforms involving a shock-like compression, and thus is very annoying to observers when it occurs. In this talk, the origins of these N-shaped waveforms is investigated through high-fidelity large-eddy simulations (LES) applied to an over-expanded supersonic jet issuing from a faceted military-style nozzle. Two different levels of heating are considered. From the LES, we observe N-shaped waves associated with crackle to emerge directly from the jet turbulence. Furthermore, even at this extreme near-field location, we find that the emergent waves are already well-organized, having correlation over significant azimuthal distances.