Engine Test Cell Aeroacoustics and Recommendations

Abstract

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Noise from a Jet Discharged into a Duct and its Suppression

“…In addition, the effect of three longitudinal fins ( Fig. 1d) was also explored with the D × L = 1″ × 2″ duct (producing super-resonance), following the suggestion in [8]. The fins were 0.625″ long and 0.25″ high and they were placed on the downstream end of the duct.…”

“…Instability characteristics of the jet and the acoustic resonance characteristics of the duct/collector must come into play. Apparently when there is a confluence of the two (i.e., matching of the frequencies and wavenumbers) a coupling occurs leading to the sharp resonance, referred to as 'super-resonance' in [8]. The problem is likely to be akin to such phenomena as 'whistles', ring-tones, etc.…”

“…However, as stated in [1], "…these changes have been made on a 'cut and try' basis with only limited understanding of the phenomena". Reference [8] suggested placement of longitudinal fins within the duct for suppression of the super-resonance with the following rationale. If the excited acoustic modes were spinning or flapping (m = ±1, first helical modes of opposite sense and combination), as inferred in [1], the fins might retard such motion and thus suppress the resonance.…”

A scale model study of an engine test facility for alleviating low-frequency howl

“…(The latter term was coined by Prof. Tam while investigating the AEDC howl issue). 4,5 This would be accompanied by a high intensity tone. A literature review on resonant tones in various flows was given in 3 that will not be repeated here.…”

“…The issue was followed up by model-scale experiments as well as computational and analytical studies. [4][5][6] A 1/48-scale model test at Georgia Institute of Technology as well as a 1/2-scale model test in the PSL at NASA GRC were conducted 6 ; however, it is apparent that these tests did not reproduce the intense tones noted in. 1,2 As it will be discussed in the following super-resonance may be a relatively uncommon occurrence, and in many cases high levels of low frequency broadband spectral components, even without a distinct tone, likely constitute the unwanted howling noise.…”