  We present a two-point model to investigate the underlying source mechanisms
for broadband shock-associated noise (BBSAN) in shock-containing supersonic
jets. In the model presented, the generation of BBSAN is assumed to arise from
the non-linear interaction between downstream-propagating coherent structures
with the quasi-periodic shock cells in the jet plume. The turbulent
perturbations are represented as axially-extended wavepackets and the shock
cells are modelled as a set of stationary waveguide modes. Unlike previous
BBSAN models, the physical parameters describing the hydrodynamic components
are not scaled using the acoustic field. Instead, the characteristics of both
the turbulent and shock components are educed from large-eddy simulation and
particle image velocimetry datasets. Apart from using extracted data, a
reduced-order description of the wavepacket structure is obtained using
parabolised stability equations (PSE). The validity of the model is tested by
comparing far-field sound pressure level predictions to azimuthally-decomposed
experimental acoustic data from a cold Mach 1.5 underexpanded jet. At polar
angles and frequencies where BBSAN dominates, good agreement in spectral shape
and sound amplitude is observed for the first three azimuthal modes.
Encouraging comparisons of the radiated noise spectra, in both frequency and
amplitude, reinforce the suitability of using reduced-order linear wavepacket
sources for predicting BBSAN peaks. On the other hand, the mismatch in sound
amplitude at interpeak frequencies reveals the role of wavepacket jitter in the
underlying sound generating mechanism.
