The sound generated by vortex pairing in a two-dimensional
compressible mixing layer is investigated. Direct numerical
simulations (DNS) of the Navier–Stokes equations are used to
compute both the near-field region and a portion of the acoustic
field. The acoustic analogy due to Lilley (1974) is also solved with
acoustic sources determined from the near-field data of the DNS. It
is shown that several commonly made simplifications to the acoustic
sources can lead to erroneous predictions for the acoustic field.
Predictions based on the quadrupole form of the source terms derived
by Goldstein (1976a, 1984) are in excellent agreement with
the acoustic field from the DNS. However, despite the low Mach number
of the flow, the acoustic far field generated by the vortex pairings
cannot be described by considering compact quadrupole sources. The
acoustic sources have the form of modulated wave packets and the
acoustic far field is described by a superdirective model (Crighton
& Huerre 1990). The presence of flow–acoustic interactions
in the computed source terms causes the acoustic field predicted by
the acoustic analogy to be very sensitive to small changes in the
description of the source.
Characterization of equivalent acoustic sources to reproduce the acoustic field generated by engines on an aircraft fuselage