Ph.D. Defense

David N. Ramsey

(Advisor: Prof. K. K. Ahuja)

"Understanding and Preventing the Howling of a Model-Scale, Internally Mixed Jet" 

Wednesday, July 17

1:00 p.m.

Montgomery Knight, Rm 325

Abstract

Internally mixed nozzles are an attractive design choice for future supersonic civilian jet aircraft in part because they may offer thrust-specific jet-noise reductions, which would be beneficial for noise certification at landing and take-off. Subsonic jets produced by a model-scale, axisymmetric internally mixed nozzle were studied in this dissertation, 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. 

Experimental jet-noise measurements revealed that the jets from this internally mixed nozzle produced a loud howling at certain operating conditions, characterized by high-amplitude discrete tones measured above the broadband jet noise. From a practical perspective, the howling limited the utility of the measured data for those interested in pure, broadband jet noise. Were the observed howling to be produced by a full-scale exhaust system, it would threaten an aircraft’s ability to meet noise regulations. From a basic research perspective, the howling presented itself as a phenomenon not yet understood. The goal of this dissertation was to understand the mechanisms responsible for this howling and how to suppress them.

A first type of observed 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 high-subsonic howl. A second type of howling observed was rooted in the impingement of the core jet upon the final-nozzle lip. This triggered 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 to 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 phenomena studied here.

Committee

• Dr. K. K. Ahuja, Advisor, School of Aerospace Engineering (Advisor)
• Dr. Tim Lieuwen, School of Aerospace Engineering
• Dr. Lakshmi Sankar, School of Aerospace Engineering
• Dr. Karim Sabra, School of Mechanical Engineering
• Dr. Joseph R. Gavin, Staff Scientist, Gulfstream Aerospace Corporation
• Dr. Jeff Mendoza, Senior Technical Fellow, RTX Technologies Research Center