  High-fidelity large-eddy simulations (LES) are performed to investigate
aeroacoustic characteristics of jets issuing from twin rectangular nozzles with
an aspect ratio of 2:1 at two over-expanded conditions and the design
condition. For all three jet conditions simulated, LES predicts qualitatively
similar near-field flow statistics to those measured at the University of
Cincinnati. Using the Ffowcs Williams-Hawkings method, LES captures the
fundamental screech tone and its harmonics fairly well at multiple observer
locations in the far-field. Intense jet flapping motions in the near-field
along the minor axis, which are influenced by jet-to-jet interactions, are
found to correspond to those frequencies. Moreover, the predicted overall sound
pressure levels are within 1-2 dB of the experimental measurements. However,
the screech tones appear to be intermittent, as the twin-jet interaction
pattern varies irregularly. To extract dominant flow structures at the screech
frequencies and identify the twin-jet coupling modes, spectral proper
orthogonal decomposition (SPOD) analysis is used. SPOD analysis recovers
energetic peaks at the screech frequencies, and the corresponding leading modes
indicate strong upstream radiation originating from the fifth/sixth
shock-cells. For the two over-expanded conditions, the leading modes show
anti-symmetric coupling in the minor axis at the fundamental screech
frequencies. In contrast, the two jets behave symmetrically with respect to
each other in the major axis, in line with the absence of jet flapping in this
direction. Furthermore, the leading SPOD eigenvalues turn out to be, at least,
two orders of magnitude larger than higher-order eigenvalues, suggesting
potential of reduced-order models for the twin-jet screech.
