Twin jet plumes on aircraft can couple, producing dynamic
pressures significant
enough to cause structural fatigue. For closely spaced jets with
a moderate aspect
ratio (e.g. 5), previous work has established that two coupling
modes (antisymmetric
and symmetric) are kinematically permissible. However, the
dynamics of twin-jet coupling have remained unexplored. In this
paper a more fundamental assessment of the
steady and unsteady aspects of twin-jet coupling is attempted.
While we document
and discuss the nozzle spacings and Mach numbers over which
phase-locked coupling occurs, our concentration is much more on
answering the following questions:
(a) What mechanism causes the jets to couple in
one mode or the other? (b) Why do
the jets switch from one mode to another? (c) Are the
two modes mutually exclusive
or do they overlap at the transition point? Our results reveal,
among many things,
the following. (i) For very closely spaced twin jets in the
side-by-side configuration
phased feedback based on source to nozzle exit distance of adjacent
jets does not
fully explain the coupling modes. However, the ‘null’
phase regions surrounding the
jets where the phase of an acoustic wavefront (arriving from
downstream) does not
vary appears to correlate well with the existence of the symmetric
mode. When the ‘null’ regions of adjacent jets do not overlap
antisymmetric coupling occurs and when
they do overlap the jets couple symmetrically. We provide a simple
correlation using a parameter (α) that can be used as a simple test
to
determine the mode of coupling.
(ii) The switch from the antisymmetric to the symmetric mode of
coupling appears to occur because of an abrupt shift in the effective
screech source from the third to
the fourth shock, which in turn causes the ‘null’
phase region surrounding the jets to
grow abruptly and overlap. (iii) The two modes are mutually
exclusive. Our results
provide considerable insight into the twin-jet coupling problem and
offer hope for
designing twin-jet configurations that minimize damage to aircraft
components.