Steve Martens

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.

Crackle Noise in Heated Supersonic Jets

Crackle noise from heated supersonic jets is characterized by the presence of strong positive pressure impulses resulting in a strongly skewed far-field pressure signal. These strong positive pressure impulses are associated with N-shaped waveforms involving a shocklike compression and, thus, is very annoying to observers when it occurs. Unlike broadband shock-associated noise which dominates at upstream angles, crackle reaches a maximum at downstream angles associated with the peak jet noise directivity. Recent experiments (Martens et al., 2011, “The Effect of Chevrons on Crackle—Engine and Scale Model Results,” Proceedings of the ASME Turbo Expo, Paper No. GT2011-46417) have shown that the addition of chevrons to the nozzle lip can significantly reduce crackle, especially in full-scale high-power tests. Because of these observations, it was conjectured that crackle is associated with coherent large scale flow structures produced by the baseline nozzle and that the formation of these structures are interrupted by the presence of the chevrons, which leads to noise reduction. In particular, shocklets attached to large eddies are postulated as a possible aerodynamic mechanism for the formation of crackle. In this paper, we test this hypothesis through a high-fidelity large-eddy simulation (LES) of a hot supersonic jet of Mach number 1.56 and a total temperature ratio of 3.65. We use the LES solver CHARLES developed by Cascade Technologies, Inc., to capture the turbulent jet plume on fully-unstructured meshes.

Practical Jet Noise Reduction for Tactical Aircraft

GE and NAVAIR are working together to find and develop practical techniques to reduce jet noise on legacy tactical aircraft such as the F/A-18. Noise is an important issue for the Navy that has grown dramatically over the last number of years. The two most important issues are the hearing loss induced during operations of these aircraft on aircraft carriers and the impact to communities around Naval Air Bases and training sites. A near term noise reduction goal of 3 dB has been established by NAVAIR as the first step in a much longer term plan to significantly reduce the noise felt in both of these situations. A near term solution for noise reduction implies that it can be implemented in the existing fleet with relatively little impact to the current air vehicle and the way it is operated, deployed, maintained, and funded. These constraints quickly limit the magnitude and types of changes that can be made to legacy engines or exhaust systems. In 2009, a static acoustic test on an F404 engine demonstrated that chevrons are equally effective at reducing noise all the way to full afterburner conditions. This test also measured thrust and the chevrons were demonstrated to result in very minimal performance impact at sea level static conditions. These two results are very important, as this was the first demonstration at full scale of practical noise reduction at afterburner conditions with minimal thrust impact. This paper will report on this latest test.Copyright

The Effect of Chevrons on Crackle: Engine and Scale Model Results

GE and the USN continue to work together to find and develop practical techniques to reduce jet noise on tactical aircraft such as the F/A-18 E/F/G. Noise is an important issue for the Navy because of the harsh acoustic environment induced during operations of these aircraft on aircraft carriers and the impact to communities around Naval Air Bases and training sites. The noise generated by these systems is predominantly the noise generated by the exhaust plume due to the low bypass ratio of the engine and very high exhaust jet velocities. The main components of this jet noise are the jet mixing, shock and crackle noise. The present paper reports on progress, following Reference [1] with the F/A-18 E/F/G jet noise reduction program, which is currently focused on the USN near term goal of up to 3 dB reduction in the peak directivity direction. This goal also includes the reduction of the shock and crackle noise components. These goals are currently being pursued with nozzle plume mixing enhancement employing mechanical chevrons. These chevrons can be incorporated in the production version as a redesign of the F414 nozzle seals and do not involve the introduction of additional parts to the nozzle. This paper focuses on the effect of chevrons on the crackle noise component both in full scale on the F404 engine, and in small scale on the F414 engine nozzle in the twin configuration. The paper aims to make the case that this effect, which was first observed during ground engine testing of prototype chevrons, is a beneficial one in reducing/eliminating crackle which continues to be prevalent in high performance tactical aircraft engines today.Copyright

A PIV Flow Field Investigation of Medium Bypass Chevron Nozzles

Particle image v elocimetry tests were c onducted to assess the effect of core and fan nozzle chevrons on the flow field. A separate flow exhaust system was used to accurately simulate the discharge of a modern, medium bypass, turbo fan engine. Four nozzle configurations were tested; baseline (conical nozzles on both streams), core chevrons (chevrons on core stream and conical nozzle on fan stream), fan chevrons (chevrons on fan stream and conical nozzle on core stream), core and fan chevrons (chevron nozzles on both streams ). Far field acousti c measurements using the same configurations and cycle condition have been reported previously 1 . It was shown that configurations with chevrons on either core or fan nozzle reduce jet noise. It was also shown that a configuration with both core and fan n ozzle chevrons reduces jet noise to a greater degree. Additionally, that reduction was shown to be approximately additive when measured in decibel. That is, the reduction due to core chevrons plus the reduction due to fan chevrons was approximately equal to the reduction due to both chevrons. It was suggested that in general, chevrons modify the noise associated with the shear layer upon which they act; core nozzle chevrons affect the noise associated with the core/fan (inner) shear layer, fan nozzle che vrons affect the noise associated with the fan/free stream (outer) shear layer. The mixing region, which is created by the merging of the inner and outer shear layer, can be affected by either chevron nozzle. The total noise heard in the far field is thu s a superposition of the sources. In this way, jet noise is additive. Near -field acoustic measurements using the same configurations and cycle condition were also reported previously 2 . These results confirm this thesis. Core chevrons significantly redu ce the noise generation region that is typically associated with the inner shear layer. Fan chevrons reduce the noise generation region that is typically associated with the outer shear layer. Both chevrons reduce noise from the mixing region. These eff ects were observed when chevrons were used separately and when chevrons were used together. Results of the current tests demonstrate that in general, core chevrons modify the inner shear layer and the mixed region. Fan chevrons modify the outer shear lay er and mixing region. Variations in t urbulent kinetic energy have been correlated with variations in near field jet noise.

Near Field Acoustics of Core and Fan Chevron Nozzles

*† ‡ Tests were conducted to assess the effect of core and fan chevrons on near field jet acoustics. A separate flow exhaust system was used to accurately simulate the discharge of a modern, medium bypass, turbo fan engine. Four nozzle configurations were tested; baseline (conical nozzles on both streams), core chevrons (chevrons on core nozzle and conical fan nozzle), fan chevrons (chevrons on fan nozzle and conical core nozzle), core and fan chevrons (chevron nozzles on both nozzles). Far-field measurements using the same configurations and cycle condition were reported previously 1 . It was shown that configurations with either core or fan chevrons can reduce jet noise. It was also shown that a configuration with both core and fan nozzles chevrons reduces jet noise to a greater degree. Additionally, that reduction was shown to be approximately additive when measured in dB. That is, the reduction due to core chevrons plus the reduction due to fan chevrons was approximately equal to the reduction due to both chevrons. It was suggested that chevrons modify the noise associated with the shear layer upon which they act; core nozzle chevrons affect the noise associated with the core/fan (inner) shear layer, fan nozzle chevrons affect the noise associated with the fan/free stream (outer) shear layer. The mixing region, which is created by the merging of the inner and outer shear layer, can be affected by either chevron nozzle. The total far field noise is thus a superposition of the two sources. The current work seeks to explain these observations by examining the modification of the noise sources within the jet plume. Core chevrons significantly reduce the noise generation region that is typically associated with the inner shear layer. Fan chevrons reduce the noise generation region that is typically associated with the outer shear layer. Both chevrons reduce noise from the mixing region. These effects were observed when chevrons were used separately and when chevrons were used together.

Simultaneous Noise and Performance Measurements for High Speed Jet Noise Reduction Technologies: Part II—Near Field Array Calibration

Part I of this paper describes a methodology for assessing the far field jet noise from high speed exhaust nozzles using a microphone array in the near field of the exhaust plume. The near field noise measurement is mathematically propagated producing an estimate of the noise level at the new location. Outward propagation produces an estimate of the far field noise. Propagation toward the jet axis produces the source distribution. Part II described here provides a direct validation of this process using a generic CD nozzle in a facility where both the near field and the far field are measured simultaneously. Comparison of these data sets show good agreement over the typical operating range for this type of nozzle. The far field noise is characterized by two independent processes: Shock cell noise radiating in the forward quadrant is produced when the nozzle is operated at non-ideally expanded conditions. Mach wave radiation propagates into the aft quadrant when the exhaust temperature is elevated. Subsequent tests in an acoustically treated nozzle thrust stand demonstrate the value of the near field array allowing immediate feedback on the noise/performance tradeoff for high speed jet noise reduction technologies.Copyright