In significantly underexpanded jets, screech inherently ceases
to exist.
This paper
studies screech cessation in a supersonic rectangular jet and provides
an explanation
for its occurrence. Experimental data are presented for fully expanded
Mach numbers,
Mj, ranging from 1.1 to 1.9. Screech becomes
unsteady
beyond Mj=1.65 and ceases
to exist beyond Mj=1.75. The reason for this
cessation
has remained a mystery, and
this paper examines three suspects: (i) the theory of a frequency mismatch
between
screech tones and the band of the most-amplified jet instability waves,
(ii)
the notion
that Mach disk formation disrupts the shock-cell structure and weakens
the
screech-producing shocks, and (iii) the idea that acoustic feedback and
receptivity diminish at
high levels of underexpansion. A thorough interrogation of experimental
data shows
that (i) is not the main cause of screech cessation here, (ii) plays an
insignificant role,
and (iii) appears to have been largely responsible for screech cessation.
Cessation
occurs because feedback to the jet lip is diminished due to excessive expansion
of the jet boundary. Further, since the jet lip now reflects and scatters
low
intensity sound, the
end result is poor receptivity at the initial shear layer. This theory
is
substantiated by
the re-activation of screech when the nozzle lip thickness is made larger
than the
expanded jet boundary. Finally, increasing lip thickness is seen to produce
a
systematic shift (to higher Mj) of the onset
of
cessation. The results of this study are of direct
relevance to the sonic fatigue problem in aircraft structures, because
understanding screech helps prevent such damage.
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.
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