John T. Spyropoulos

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 source of crackle noise in heated supersonic jets

High-fidelity large eddy simulation (LES) is used to investigate the source of crackle noise generated heated supersonic jets. Crackle is associated with intermittent N-shaped acoustics waves consisting of a sudden compression followed by a more gradual expansion, and is extremely irritating to human auditory perception. Results from four different simulations of jets produced by an experimental military-style nozzle reveal that N-shaped waves are generated directly by the jet turbulence, and thus nonlinear propagation effects are not a necessary component of their formation. Different operating points for the simulations were chosen such that jet velocity and jet temperature were varied independently. While crackle levels are sensitive to jet velocity, full field pressure skewness results suggest that increasing temperature may also play an independent role in enhancing crackle. The structure of the crackle source in the most strongly crackling jet is investigated by a backtracking algorithm applied to the simulation resulting in conditional averages.

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

Toward Efficient Computational Aeroacoustic Analysis of High Speed Jets

Recent improvements in the algorithms and associated processes have resulted in significant improvements in the users’ ability to obtain aeroacoustics analysis of hot supersonic jets in a timely manner using only moderate computational resources. This paper discusses these approaches using the example of the Compressible High Order Parallel Acoustics (CHOPA) solver. The fundamental strategy looks to balance computational expense with the accuracy required to predict the most important effects. The objective is to resolve the larger eddies accurately, but exchanges the resolution of fine turbulent structures in return for decreased computational resource requirements. These strategies continue to be developed, along with the corresponding software. As the work progresses, the team expects additional improvements in the near future.Copyright

Proposed Standard 3-D Aircraft Flyover Noise Measurement and Analysis Methods

Noise from high performance military fighter aircraft impacts the hearing and performance of personnel working near these aircraft and can be a source of annoyance to people living near airbases, airports 1 , and ranges where these aircraft are operated. Developers and operators of these aircraft must report the noise produced in communities as part of the NEPA (National Environmental Policy Act of 1974) process. Additionally, overexposure of personnel to high levels/durations of noise can result in permanent hearing loss, the number one veteran disability in the United States, and negatively impact voice communication capability. Additionally, the widely varying techniques used to acquire aircraft noise data make comparisons of aircraft noise and noise reduction technology performance more difficult.