Added Sound Sources in Jets; Theory and Simulation

Abstract

The Large-Eddy Simulation (LES) system established over the last six years is reviewed in terms of its progress in accuracy and transition physics, and is then used to explore the transmission of noise from a known local source through a turbulent region, here the shear layer of a jet. For this, weak artificial monopole sources are added to an LES, and their sound tracked in detail both in the near-field and far-field. Sources are placed in the potential cores of the primary and secondary streams, as well as in the mixing layer and outside the jet with various locations relative to the observer, and different frequencies. Simple Ray Acoustics theory based on the mean flow field and assuming full conservation of wave action via the Blokhintsev equation is quite successful, both in terms of wave-fronts and sound level, even at a diameter Strouhal number St of only 0.5. The principal difference is that LES predicts a gradual cone of silence upstream, which theory does not. The abrupt downstream cones of silence agree. Thus, even crossing a mixing layer with a Mach 0.9 difference does not appear to alter the sound much. Cases with a dual nozzle and hot core stream return similar findings. This will be helpful when creating lower-order prediction tools, and correcting noise measurements made outside a co-flow.