Propulsive jets and their acoustics.

The complex flow physics challenges and asks questions regarding these challenges a wide range of jet flows found in aerospace engineering. Hence, the daunting task facing Reynolds-averaged Navier-Stokes (RANS) technology, for which the time average of the turbulent flow field is solved, is set out. Despite the clear potential of large eddy simulation (LES)-related methods and hybrid forms involving some RANS modelling, numerous current deficiencies, mostly related to the limitations of computational resources, are identified. It is concluded that currently, these limitations make LES and hybrids most useful for understanding flow physics and refining RANS technology. The use of LES in conjunction with a ray-tracing model to elucidate the physics of acoustic wave transmission in jets and thus improved RANS technology is described. It is argued that, as a stopgap measure, pure RANS simulations can be a valuable part of the design process and can now predict acoustics spectra and directivity diagrams with useful accuracy. Ultimately, hybrid RANS-LES-type methods, and then pure LES, will dominate, but the time-scales for this transition suggests that improvements to RANS technology should not be ignored.