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In the generation of discrete tones by subsonic impinging jets, there
exists a difference of opinion as how the feedback is achieved, i.e., the
path of the feedback acoustic waves is whether inside the jet or outside
the jet? The only available model (Tam and Ahuja model) for the prediction
of an average subsonic jet impingement tone frequency assumes that the
upstream part of the feedback loop is closed by an upstream propagating
neutral wave of the jet. But, there is no information about the plate
geometry in the model. The present study aims at understanding the effect
of the plate geometry (size and co-axial hole in the plate) on the
self-excitation process of subsonic impinging jets and the path of the
acoustic feedback to the nozzle exit. The present results show that there
is no effect of plate diameter on the frequency of the self-excitation. A
new type of tones is generated for plates with co-axial hole (hole diameter
is equal to nozzle exit diameter) for Mach numbers 0.9 and 0.95, in
addition to the axisymmetric and helical mode tones observed for plates
without co-axial hole. The stability results show that the Strouhal number
of the least dispersive upstream propagating neutral waves match with the
average Strouhal number of the new tones observed in the present
experiments. The present study extends the validity of the model of Tam and
Ahuja to a plate with co-axial hole (annular plate) and by doing so, we
indirectly confirmed that the major acoustic feedback path to the nozzle
exit is inside the jet.
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