  In this work, we investigate both numerically and theoretically the sound
generated by entropy waves passing through sudden area expansions. The
numerical approach is based on a triple decomposition of the flow variables
into a steady mean, a small-amplitude coherent part, and a stochastic turbulent
part. The mean flow is obtained as the solution of the Reynolds-Averaged
Navier-Stokes (RANS) equations. The equations governing the coherent
perturbations are linearised and solved in the frequency domain. To account for
the effect of turbulence on the coherent perturbations, a frozen eddy viscosity
model is employed. When entropy fluctuations pass through the area expansion,
the generated entropy noise behaves as a low-pass filter. The numerical
predictions of the noise at low frequencies are compared to the predictions of
compact, quasi-one-dimensional, and isentropic theory and large discrepancies
are observed. An alternative model for the generated entropy noise tailored for
area expansions is then proposed. Such model is based on the conservation of
mass, momentum, and energy written in integral form. The model assumes zero
frequency and the one-dimensionality of the flow variables far upstream and
downstream of the expansion. The predictions of this model agree well with the
numerical simulations across a range of finite subsonic Mach numbers including
low, intermediate, and high Mach numbers.
