Flight Test Results for Uniquely Tailored Propulsion Airframe Aeroacoustic Chevrons: Shockcell Noise

Abstract

Azimuthally varying chevrons (AVC) which have been uniquely tailored to account for the asymmetric propulsion-airframe aeroacoustic interactions have recently shown significant reductions in jet-related community noise at low-speed take-off conditions in scale model tests of coaxial nozzles with high bypass ratio. There were indications that such AVCs may also provide shockcell noise reductions at high cruise speeds. This paper describes the flight test results when one such AVC concept, namely, the T-fan chevrons with enhanced mixing near the pylon, was tested at full-scale on a modern large twin-jet aircraft (777-300ER) with focus on shockcell noise at mid-cruise conditions. Shockcell noise is part of the interior cabin noise at cruise conditions and its reduction is useful from the viewpoint of passenger comfort. Noise reduction at the source, in the exhaust jet, especially, at low frequencies, is beneficial from the perspective of reduced fuselage sidewall acoustic lining. Results are shown in terms of unsteady pressure spectra both on the exterior surface of the fuselage at several axial stations and also microphone arrays placed inside the fuselage aft of the engine. The benefits of T-fan chevrons, with and without conventional chevrons on the core nozzle, are shown for several engine operating conditions at cruise involving supersonic fan stream and subsonic or supersonic core stream. The T-fan AVC alone provides up to 5 dB low-frequency noise reduction on the fuselage exterior skin and up to 2 dB reduction inside the cabin. Addition of core chevrons appears to increase the higher frequency noise. This flight test result with the previous model test observation that the Tfan AVCs have hardly any cruise thrust coefficient loss (< 0.05%) make them viable candidates for reducing interior cabin noise in high bypass ratio engines. Nomenclature

Understanding and Reduction of Cruise Jet Noise at Aircraft Level

“…This represents a very significant change brought to near-field jet noise physics. This peak reduction value is comparable in magnitude to the 5 dB gain mentioned in previous publications on cruise flight tests [21,22,23]. An important difference with results in these papers is the reduction of the high-frequency shock noise by the LINFaN chevrons.…”

“…Several fan nozzles are tested on engine 3 in an attempt to reduce shock-associated noise in cruise flight. Mengle et al [21,23] and Bultemeier et al [22] have shown recently that fan chevrons may reduce shock noise in cruise by up to 5 dB in the low frequencies on the external fuselage skin. They have also shown a sensitivity of reduction efficiency on the fan chevron design and the engine operating conditions.…”

“…• The beamforming used did not account for the refraction of sound waves in the boundary layer on the fuselage. Many experimentalists such as Mengle et al [21] have been using this assumption. Further, the complex influence of the boundary layer on measurements is a subject of current and future research.…”

“…Besides, the use of chevrons to reduce the BBSAN component in high-bypass-ratio jets is reported in several studies [19,20]. The noise reduction potential of different chevron concepts on fan and core nozzles has also been qualified in flight tests in cruise conditions [21,22,23]. The mechanisms by which chevrons affect BBSAN has not been fully clarified.…”

Flight Test Results for Uniquely Tailored Propulsion-Airframe Aeroacoustic Chevrons: Community Noise

“…This paper focuses on the flight tests results of this T-fan chevron nozzle with and without a conventional core chevron nozzle to determine the community noise results at take-off conditions. A companion paper 14 focuses on the acoustics results of these AVC nozzles for shock-cell noise reduction at cruise conditions important for interior cabin noise. In the next two sections we describe flight test configurations and the data analysis method, then we present the flight test results and conclude its applicability to high bypass ratio engines for more stringent noise restrictions.…”