On the Connection Between Near-Field and Far-Field Solutions of High-Speed Jet Noise

Model scale tests are conducted to assess the Noise/Performance trade for high speed jet noise reduction technologies. It is demonstrated that measuring the near field acoustic signature with a microphone array can be used to assess the far field noise using a procedure known as acoustic holography. 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. Tests are conducted on convergent/divergent nozzles with three different area ratios, and several different chevron geometries. 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. These results show good agreement with actual far field measurements from tests in the GE Cell 41 Acoustic Test Facility. Simultaneous performance measurement shows the change in thrust coefficient for different test conditions and configurations. Chevrons attached to the nozzle exit can reduce the noise by several dB at the expense of a minimal thrust loss.

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

Volume 1: Aircraft Engine; Ceramics; Coal, Biomass and Alternative Fuels; Wind Turbine Technology ◽

10.1115/gt2011-46656 ◽

2011 ◽

Author(s):

Dean Long ◽

Steve Martens

Keyword(s):

Noise Reduction ◽

High Speed ◽

Near Field ◽

Jet Noise ◽

Source Distribution ◽

Wave Radiation ◽

Far Field ◽

Performance Measurements ◽

Shock Cell ◽

Field Noise

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.

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Jet-Surface Interaction Test: Far-Field Noise Results

Journal of Engineering for Gas Turbines and Power ◽

10.1115/1.4023605 ◽

2013 ◽

Vol 135 (7) ◽

Cited By ~ 44

Author(s):

Clifford A. Brown

Keyword(s):

High Speed ◽

Radial Distance ◽

Jet Noise ◽

Surface Interaction ◽

Shielding Effect ◽

Noise Prediction ◽

Far Field ◽

Wide Range ◽

Field Noise ◽

Surface Length

Many configurations proposed for the next generation of aircraft rely on the wing or other aircraft surfaces to shield the engine noise from the observers on the ground. However, the ability to predict the shielding effect and any new noise sources that arise from the high-speed jet flow interacting with a hard surface is currently limited. Furthermore, quality experimental data from jets with surfaces nearby suitable for developing and validating noise prediction methods are usually tied to a particular vehicle concept and, therefore, very complicated. The Jet-Surface Interaction Tests are intended to supply a high quality set of data covering a wide range of surface geometries and positions and jet flows to researchers developing aircraft noise prediction tools. The initial goal is to measure the noise of a jet near a simple planar surface while varying the surface length and location in order to: (1) validate noise prediction schemes when the surface is acting only as a jet noise shield and when the jet-surface interaction is creating additional noise, and (2) determine regions of interest for future, more detailed, tests. To meet these objectives, a flat plate was mounted on a two-axis traverse in two distinct configurations: (1) as a shield between the jet and the observer and (2) as a reflecting surface on the opposite side of the jet from the observer. The surface length was varied between 2 and 20 jet diameters downstream of the nozzle exit. Similarly, the radial distance from the jet centerline to the surface face was varied between 1 and 16 jet diameters. Far-field and phased array noise data were acquired at each combination of surface length and radial location using two nozzles operating at jet exit conditions across several flow regimes: subsonic cold, subsonic hot, underexpanded, ideally expanded, and overexpanded supersonic. The far-field noise results, discussed here, show where the jet noise is partially shielded by the surface and where jet-surface interaction noise dominates the low frequency spectrum as a surface extends downstream and approaches the jet plume.

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The sources of jet noise: experimental evidence

Journal of Fluid Mechanics ◽

10.1017/s0022112008003704 ◽

2008 ◽

Vol 615 ◽

pp. 253-292 ◽

Cited By ~ 334

Author(s):

CHRISTOPHER K. W. TAM ◽

K. VISWANATHAN ◽

K. K. AHUJA ◽

J. PANDA

Keyword(s):

High Speed ◽

Noise Source ◽

Jet Noise ◽

Polar Angle ◽

Sound Field ◽

Source Distribution ◽

Far Field ◽

Noise Sources ◽

Directional Information ◽

Jet Mixing

The primary objective of this investigation is to determine experimentally the sources of jet mixing noise. In the present study, four different approaches are used. It is reasonable to assume that the characteristics of the noise sources are imprinted on their radiation fields. Under this assumption, it becomes possible to analyse the characteristics of the far-field sound and then infer back to the characteristics of the sources. The first approach is to make use of the spectral and directional information measured by a single microphone in the far field. A detailed analysis of a large collection of far-field noise data has been carried out. The purpose is to identify special characteristics that can be linked directly to those of the sources. The second approach is to measure the coherence of the sound field using two microphones. The autocorrelations and cross-correlations of these measurements offer not only valuable information on the spatial structure of the noise field in the radial and polar angle directions, but also on the sources inside the jet. The third approach involves measuring the correlation between turbulence fluctuations inside a jet and the radiated noise in the far field. This is the most direct and unambiguous way of identifying the sources of jet noise. In the fourth approach, a mirror microphone is used to measure the noise source distribution along the lengths of high-speed jets. Features and trends observed in noise source strength distributions are expected to shed light on the source mechanisms. It will be shown that all four types of data indicate clearly the existence of two distinct noise sources in jets. One source of noise is the fine-scale turbulence and the other source is the large turbulence structures of the jet flow. Some of the salient features of the sound field associated with the two noise sources are reported in this paper.

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Experimental Study of the Properties of Near-Field and Far-Field Jet Noise

12th AIAA/CEAS Aeroacoustics Conference (27th AIAA Aeroacoustics Conference) ◽

10.2514/6.2006-2649 ◽

2006 ◽

Cited By ~ 5

Author(s):

Sébastien Barré ◽

Vincent Fleury ◽

Christophe Bogey ◽

Christophe Bailly ◽

Daniel Juve

Keyword(s):

Experimental Study ◽

Near Field ◽

Jet Noise ◽

Far Field

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HELICOPTER NOISE PREDICTIONS USING KIRCHHOFF METHODS

Journal of Computational Acoustics ◽

10.1142/s0218396x96000106 ◽

1996 ◽

Vol 04 (03) ◽

pp. 321-339 ◽

Cited By ~ 17

Author(s):

ROGER C. STRAWN ◽

RUPAK BISWAS ◽

ANASTASIOS S. LYRINTZIS

Keyword(s):

Experimental Data ◽

Finite Difference ◽

High Speed ◽

Impulsive Noise ◽

Near Field ◽

Rotor Blades ◽

Far Field ◽

Computationally Efficient ◽

Efficient Manner ◽

Helicopter Noise

This paper presents two methods for predicting the noise from helicopter rotors in forward flight. Aerodynamic and acoustic solutions in the near field are computed with a finite-difference solver for the Euler equations. Two different Kirchhoff acoustics methods are then used to propagate the acoustic signals to the far field in a computationally-efficient manner. One of the methods uses a Kirchhoff surface that rotates with the rotor blades. The other uses a nonrotating Kirchhoff surface. Results from both methods are compared to experimental data for both high-speed impulsive noise and blade-vortex interaction noise. Agreement between experimental data and computational results is excellent for both cases. The rotating and nonrotating Kirchhoff methods are also compared for accuracy and efficiency. Both offer high accuracy with reasonable computer resource requirements. The Kirchhoff integrations efficiently extend the near-field finite-difference results to predict the far field helicopter noise.

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Near-Field and Acoustic Far-Field Response of a High-Speed Jet to Excitation

AIAA Journal ◽

10.2514/1.j053581 ◽

2015 ◽

Vol 53 (7) ◽

pp. 1894-1909 ◽

Cited By ~ 13

Author(s):

M. Crawley ◽

A. Sinha ◽

M. Samimy

Keyword(s):

High Speed ◽

Near Field ◽

Far Field ◽

Field Response

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Experimental Study of the Spectral Properties of Near-Field and Far-Field Jet Noise

International Journal of Aeroacoustics ◽

10.1260/147547207781041868 ◽

2007 ◽

Vol 6 (2) ◽

pp. 73-92 ◽

Cited By ~ 60

Author(s):

Christophe Bogey ◽

Sébastien Barré ◽

Vincent Fleury ◽

Christophe Bailly ◽

Daniel Juvé

Keyword(s):

Experimental Study ◽

Spectral Properties ◽

Near Field ◽

Jet Noise ◽

Far Field

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An Innovative Package EMC Solution Using a Highly Cost-Effective Sputtered Conformal Shield

Additional Conferences (Device Packaging HiTEC HiTEN & CICMT) ◽

10.4071/2016dpc-tha42 ◽

2016 ◽

Vol 2016 (DPC) ◽

pp. 002152-002181 ◽

Cited By ~ 1

Author(s):

Nozad Karim ◽

Rong Zhou ◽

Jun Fan

Keyword(s):

High Speed ◽

Metal Layer ◽

Near Field ◽

Cost Effective ◽

Circuit Board ◽

System Level ◽

Normal Operation ◽

Far Field ◽

Wide Range ◽

The Cost

High-speed digital and wireless devices radiate undesired electromagnetic noises that affect the normal operation of other devices causing electromagnetic interference (EMI) problems. Printed circuit board (PCB) and system-level shielding may alleviate inter-system EMI between the PCB board and the outside environment, but does not prevent intra-system EMI within the shielding enclosure. Package and System in Package (SiP) level shielding is often used to minimize intra-system EMI issues. An external metal lid is traditionally employed to prevent noise emission from a device, but the cost and size of this technique makes it unattractive for modern electronics. Conformal shielding is gaining momentum due to its size and height advantages. However, high cost and complexity of the sprayed coating shield prevents it from being used for a wide range of low cost commercial applications. In this paper, an innovative shielding technology with sputtered metal conformal shield is investigated using a specially designed test vehicle. By sputtering a conductive material onto a package, a very thin (typically a few μm) metal layer is constructed on the top and around four sides of the package. This thin sputtered metal layer adds virtually zero penalty to the package size. The cost and complexity of the sputtering process is significantly lower compared to a spraying process. Several types of shielded and unshielded modules were built and extensively tested for both far-field and near-field shielding effectiveness (SE) in a semi-anechoic chamber. The performance of the sputtered conformal shield is compared to that of an unshielded module and the sprayed shield. The measured results show that the sputtered shield performs equally well to a sprayed shield, in far field test, with most measurements better than 40 dB of SE. In near field testing, sputtered shields mostly outperform the sprayed shield, especially when compared in the entire scanned region. A well-designed sputtered conformal shield can, therefore, be a very cost-effective EMI solution for a wide range of packages, such as SiP. Also in the paper, a full wave 3D HFSS model is presented and simulated results for both far and near field are compared with measured data.

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