The modelling challenge
As outlined in the previous section, estimation of the sound radiation from a turbulent flow, using an acoustic analogy, requires the solution of a propagation equation given a corresponding source term. If the source is not known exactly (such exact knowledge implies knowledge of the full Navier Stokes solution) it must be modelled, and the question of how best to construct this model arises.
Regardless of the acoustic analogy used, the source is a function of the flow turbulence, and so the question of source modelling is inseparable from that of turbulence modelling. In this section we consider the turbulent jet, and the link between this and sound sources. The way turbulence is perceived and modelled has changed considerably in the last fifty years, as has, correspondingly, our understanding of the jet as a source of sound. We therefore briefly trace out these evolutions, providing examples of some recent developments where the source modelling question is concerned.