  Twin-jet configuration allows two different scenarios to close the screech
feedback. For each jet, there is one loop involving disturbances which
originate in that jet and arrive at its own receptivity point in-phase
(self-excitation). The other loop is associated with free-stream acoustic waves
that radiate from the other jet, reinforcing the self-excited screech
(cross-excitation). In this work, the role of the free-stream acoustic mode and
the guided jet mode as a closure mechanism for twin rectangular jet screech is
explored by identifying eligible points of return for each path, where upstream
waves propagating from such a point arrive at the receptivity location with an
appropriate phase relation. Screech tones generated by these jets are found to
be intermittent with an out-of-phase coupling as a dominant coupling mode.
Instantaneous phase difference between the twin jets computed by the Hilbert
transform suggests that a competition between out-of-phase and in-phase
coupling is responsible for the intermittency. To model wave components of the
screech feedback while ensuring perfect phase-locking, an ensemble average of
leading spectral proper orthogonal decomposition modes is obtained from several
segments of large-eddy simulations data that correspond to periods of invariant
phase difference between the two jets. Each mode is then extracted by retaining
relevant wavenumber components produced via a streamwise Fourier transform.
Spatial cross-correlation analysis of the resulting modes shows that most of
the identified points of return for the cross-excitation are synchronised with
the guided jet mode self-excitation, supporting that it is preferred in closing
rectangular twin-jet screech coupling.
