Bryan Callender

Far-Field Acoustic Investigation into Chevron Nozzle Mechanisms and Trends

Chevron nozzles currently offer one of the most feasible methods of reducing jet exhaust noise in medium to high-bypass turbofan engines. Tests were conducted in the University of Cincinnati Nozzle Acoustic Test Facility, simulating a separate flow exhaust system to provide insight into some of the basic mechanisms and trends of this emerging technology. For this study, a baseline inner nozzle and three chevron nozzles were tested over a wide range of operating conditions, including dual and single flow. Chevrons with varying numbers of lobes and levels of penetration were selected for this study to provide insight into the impact of these geometric parameters on the noise level. Spectral and directivity results from heated, coaxial flow tests showed that the chevron nozzles are most effective at lower frequencies and at aft directivity angles. Reductions in overall sound pressure level (SPL) ranging from 3 to 6 dB were documented. Calculations of perceived noise level directivity also showed 4-6 dB reduction at aft angles. The data also illustrated clear and consistent trends with respect to the chevron geometric parameters. Specifically, the chevron penetration was determined to be a primary factor in controlling the tradeoff between low-frequency reduction and high-frequency SPL increases. Although slight differences were observed with varying chevron lobe numbers at a fixed penetration, it appears that the effect is less significant than the penetration. Finally, the data indicated clear dependence of the chevron benefit on the velocity difference between the inner and outer streams.

Near-Field Investigation of Chevron Nozzle Mechanisms

A detailed investigation into the effect of chevron nozzles on the near-field acoustics of a separate flow exhaust system was conducted at the University of Cincinnati Anechoic Test Facility. Chevrons with varying numbers of lobes and levels of penetration were selected to provide insight into the effects of these geometric parameters on the acoustic near field. Tests were conducted at two different nozzle operating conditions and the chevrons were shown to produce substantial modifications to the near field over a wide range of frequencies. The chevrons were most effective at lower frequencies where the peak noise region was reduced by 5-7 dB and dramatically reduced in size. At higher frequencies, the chevrons provided strong noise suppression downstream of approximately seven equivalent nozzle diameters with increases closer to the nozzle lip. The nozzle penetration was shown to have the most significant impact on the acoustic near field with more subtle differences being seen with respect to the number of chevron lobes.

A PIV Flow Field Investigation of Chevron Nozzle Mechanisms

Particle Imaging Velocimetry (PIV) measurements were performed to provide insight into the effect of core chevron nozzles on the flow field of a separate flow exhaust system. This study served as the follow on to previous investigations focusing on the chevron effect on the nozzle near and far field acoustics. Mean flow results showed that the chevron effectively redistributes energy from the high velocity primary stream outward to the lower velocity secondary stream by creating a series of high velocity lobes or secondary lateral jet structures. This leads to a more rapid decay of the peak jet velocity and a consequent reduction in the length of the jet potential core. Local increases of up to 65% in the fan stream velocity were also noted as a result of this increased mixing. The interaction of the high velocity secondary jets with the lower velocity fan stream also produces dramatic increases in turbulent kinetic energy (TKE) near the primary nozzle lip. At an axial distance of 2.5 equivalent diameters, TKE increases of nearly 50% were documented. Comparison of these flow field effects to the previously obtained acoustic results showed clear correlations and identified two primary physical mechanisms of the chevron nozzle, namely, reduced far field low frequency noise due to potential core shortening, and increased high frequency noise due to increased near field turbulence.

A Near -Field Investigation of Chevron Nozzle Mechanisms

A detailed investigation into the effect of chevron nozzles on the near -field acoustics of a separate flow exhaust system was conducted in the University of Cincinnati Anechoic Test Facility. The nozzle configurations included a baseline conic fan and core nozzle as well as three different chevron core nozzles. Chevrons with varying numbers of lobes and levels of penetration were selected to provide insight into the effects of these geometric parameters on the acoustic near -field. Tests were conducted at two different nozzle operating conditions and the chevrons were shown to produce substantial modifications to the near field over a range of frequencies. The chevrons were most effective at low frequencies where the peak noise region was reduced by 5 – 7dB an d dramatically reduced in size. The near -field showed relatively little sensitivity to the chevron geometry at the lower frequencies. At high frequencies, the chevrons were shown to generate increased noise near the nozzle lip, which is quite sensitive to the chevron geometry and nozzle operating condition. The nozzle penetration was shown to have the largest effect, particularly on the size and intensity of the increased noise region near the nozzle lip. More subtle differences were seen with respect to th e number of chevron lobes, with the largest differences being confined to the higher frequencies. All of these observations are consistent with trends seen in a previous study on the nozzle far field acoustics using the same nozzles. In addition to validat ing some of the conclusions from this previous study, the current near -field study was able to provide substantially more insight into the effects of the chevrons on the near -field sources and noise generation mechanisms.

Phased Array Measurements of Full-Scale Engine Inlet Noise

During the Quiet Technology Demonstrator 2 program, a GE90-115B engine was tested statically which provided the opportunity to demonstrate several advanced phased array systems for measuring inlet noise. In order of increasing complexity, these included a 10 element microphone arc array located in the acoustic near-field, a 100 element Kulite array mounted on the aeroacoustic bellmouth, and a 275 element Kulite array mounted on the turbulence control structure (TCS). This paper discusses the design objectives, implementation, and sample results from these arrays. The source localization capability of the relatively simple microphone arc array was demonstrated. The inlet noise was also decomposed into its modal content by both the bellmouth array and TCS array for a subsonic and a transonic fan tip relative Mach number. These phased array systems were successfully deployed to demonstrate the capabilities of the advanced measurements and provide useful insights into inlet radiated fan and compressor noise.

The Quiet Technology Demonstrator Program: Static Test of an Acoustically Smooth Inlet

The QTD2 static engine test sought to provide further insight into the noise reduction technologies demonstrated during the 2005 flight test program. One of the technologies demonstrated in the QTD2 program was the Acoustically Smooth Inlet. This technology represented a true zero splice design with the total treated area increased relative to a typical production design. During the flight test, this inlet demonstrated community noise reductions of over 2 EPNdB and significant reductions in forward cabin noise. The static test was successful in identifying key design features that led to these benefits. Multiple pure tone noise reduction was shown to be primarily attributable to increased acoustic area in the inlet. Reductions in BPF tone related noise was shown to be mostly due to the elimination of splices of the inlet. Finally, a detailed investigation into the effects of splices in the forward fan case region was completed.