Title:
Understanding and Preventing the Howling of a Model-Scale Internally Mixed Nozzle
Understanding and Preventing the Howling of a Model-Scale Internally Mixed Nozzle
Author(s)
Ramsey, David
Advisor(s)
Ahuja, Krishan
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Abstract
Internally mixed nozzles are an attractive design choice for future supersonic civilian jet aircraft, offering thrust-specific jet-noise reductions that would be beneficial for noise certification at landing and take-off. Jets produced by a model-scale, axisymmetric internally mixed nozzle were studied at sonic and subsonic jet Mach numbers, involving a core jet and surrounding bypass stream routed into a round mixing duct before being expanded through a final nozzle. The nozzle was intended as a baseline for future studies using more sophisticated, non-axisymmetric nozzles, which offer superior mixing performance. The jets from this simple, internally mixed nozzle produced a loud howling at certain operating conditions, characterized by high-amplitude discrete tones measured above the broadband jet noise. This dissertation puts forth an understanding of the origins of this howling and how to suppress the howling entirely.
Two types of howling were observed. First, a howling occurred at high-subsonic jet Mach numbers and was determined to be due to a feedback phenomenon between a flow instability at the final nozzle's exit and a natural acoustic mode of the nozzle's interior. This type of howling is referred to as the shock-wave/boundary-layer interaction howl. Second, a howling was found to be rooted in the impingement of the core jet upon the final-nozzle lip. This impingement gave rise to a feedback phenomenon involving the core jet's instability waves and is referred to as the core-jet impingement howl. Both phenomena were suppressed upon applying a suitable boundary-layer trip at an appropriate location in the nozzle, suggesting that full-scale exhaust systems where boundary layers tend to be thicker and more turbulent are unlikely to be susceptible to the observed phenomena.
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Date Issued
2024-07-27
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Text
Resource Subtype
Dissertation