This paper describes the results of a study examining the flow and acoustic characteristics of an axisymmetric supersonic jet issuing from a Mach 1.5 converging-diverging (C-D) nozzle and impinging on a ground plane. A large diameter circular plate was attached at the nozzle exit to measure the forces generated on the plate due to jet impingement. The experimental results described in this paper include lift loss, Particle Image Velocimetry (PIV) and acoustic measurements. Suckdown forces as high as 60% of the primary jet thrust were measured when the ground plane was very close to the jet exit. The PIV measurements were used to explain the increase in suckdown forces due to high entrainment velocities. The self-sustained oscillatory frequencies of the impinging jet were well-predicted using a feedback loop that utilizes the measured convection velocities of the large scale coherent vortical structures in the jet shear layer. Near field acoustic measurements indicate that the presence of the ground plane increases the OASPL by approximately 8 dB relative to a corresponding free jet. For moderately under expanded jets, the influence of the shock cells on the important flow features was found to be negligible except for close proximity of the ground plane.

Flow field and noise characteristics of a supersonic impinging jet

The MARTEI. test rig of Centre National d'Etudes Spatiales, located in Poitiers, France, is designed to study the noise of highly supersonic hot jets under conditions approaching those of a launch vehicle at liftoff. The nonanechoic character of the site (concrete ground, large plate representing the launch pad) requires a specific measurement device in order to calculate the sound power radiated by the jets. The integration process defined by ONERA is discussed and is tested using a freejet noise model derived from the work of NASA, which has been extended to the jet-plate interaction by assuming that the sound power radiated by the jet remains unchanged in this case. First results concerning jets of various velocities seem to confirm the preceding hypothesis and the reliability of the method used.

Experimental Characterization of the Sound Power Radiated by Impinging Supersonic Jets

A number of aircraft and spacecraft configurations are being considered for future development. These include high-speed turboprop aircraft, advanced vertical take-off and landing fighter aircraft, and aerospace planes for hypersonic intercontinental cruise or flight to orbit and return. Review of the acoustic environment expected for these vehicles indicates levels high enough that acoustic fatigue must be considered. Unfortunately, the sonic fatique design technology used for current aircraft may not be adequate for these future vehicles. This has resulted in renewed emphasis on acoustic fatigue research at the NASA Langley Research Center. The overall objective of the Langley program is to develop methods and information for design of aerospace vehicles that will resist acoustic fatigue. The program includes definition of the acoustic loads acting on structures due to exhaust jets of boundary layers, and subsequent determination of the stresses within the structure due to these acoustic loads. Material fatigue associated with the high frequency structural stress reversal patterns resulting from acoustic loadings is considered to be an area requiring study, but no activity is currently underway.

Acoustic fatigue: Overview of activities at NASA Langley

This paper summarizes the efforts undertaken by the author to identify existing and future potential methods to reduce underwater sound levels created by nearly all oil and gas industry related activities, including sources associated with seismic exploration, construction, transport, drilling, and production. Information was collected from various sources including technical and trade journal articles, contacts with representatives of companies that make or own noise control products and technologies, contacts with members of academia, and data provided directly by members of the International Association of Oil and Gas Producers (OGP). A workshop was held on June 4-5, 2007 to discuss initial findings and further develop ideas for feasible sound reduction approaches and future methods. This effort was supported and financed by the by the Joint Industry Programme on E&P Sound and Marine Life (www.soundandmarinelife.org).

A Summary of Existing and Future Potential Treatments for Reducing Underwater Sounds from Oil and Gas Industry Activities

Day-night average sound level (DNL), first developed by the U.S. Environmental Protection Agency, is commonly used to quantify and assess environmental noise. A keystone to noise assessment is the dose-response relationship. However , the dose-response relationship is not an absolute; there is great scatter to the data on which it is based. In an attempt to reduce the scatter to the DNL data, the EPA suggested the use of ‘normalized’ DNL. Normalized DNL is the basic DNL value wit h a number of adjustments added to account for specific characteristics and factors of the sound. This article reviews and analyzes the concepts inherent in normalized DNL and provides an updated set of normalization factors that can re

On Normalizing DNL to Provide Better Correlation with Response

: This report establishes a consistent set of prediction methods which may be used to estimate near field noise levels for jet powered and rotor/propeller powered VTOL aircraft operating in close proximity to the ground. The methods provided utilize available theory augmented extensively by experimental data which was available or was obtained during the program. The noise prediction methods presented will allow an analysis of near field environments of VTOL aircraft using a simplified procedure suitable for manual calculations. The experimental effort carried out in the program consisted of noise measurements in the near field of a model propeller and a subsonic jet in a free and vertical configuration. Ground reflection correction factors for a simple source are tabulated along with the computer program listing. The methods described in this report are intended for use by aircraft designers for the prediction of the near field noise environment of VTOL aircraft as required in MIL A 8893 Airplane Strength and Rigidity, Sonic Fatigue Resistance.

Prediction methods for Near Field Noise Environments of VTOL Aircraft.

: This report considers the effectiveness of current noise regulations in Appendix F of FAR Part 36, examines the potential effectiveness of future technology to achieve further noise reduction, and evaluates a number of new concepts for noise certification procedures for propeller-driven small aircraft. The latter were based, in part, on results of a flight test program carried out with Cessna Aircraft Company, to evaluate the utility of takeoff noise tests and the possible use of sound exposure level as a suitable metric for noise certification of the subject aircraft. The study indicates that existing regulations have been effective in stimulating development of quieter propellers for the existing fleet of propeller-driven small aircraft. However, it does not appear economically feasible to achieve more noise reduction in most of this fleet using existing technology with the possible exceptions of some of the two-engine aircraft. However, application of future noise reduction technology, primarily for quieter propellers, should allow a reduction in current noise limits by about 6 dB and should reduce levels of the noisiest aircraft in the current fleet by as much as 10 dB. A takeoff test is appropriate for all propeller aircraft except those equipped with cruise-optimized fixed-pitch propellers. For this test, the current level flyover test appears to represent the noisiest operating condition.

Evaluation of noise control technology and alternative noise certification procedures for propeller-driven small airplanes

Various studies were conducted by NASA and others on the practical limitations of sonic boom signature shaping/minimization for the High-Speed Civil Transport (HSCT) and on the effects of these shaped boom signatures on perceived loudness. This current effort is a further part of this research with emphasis on examining shaped boom signatures which are representative of the most recent investigations of practical limitations on sonic boom minimization, and on examining and comparing the expected response to these signatures when experienced indoors and outdoors.

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Evaluation of outdoor-to-indoor response to minimized sonic booms

The preferred descriptor to define the spectral content of sonic booms is the Sound Exposure Spectrum Level, LE(f) This descriptor represents the spectral content of the basic noise descriptors used for describing any single event--the Sound Exposure Level, LE The latter is equal to ten times the logarithms, to the base ten, of the integral, over the duration of the event, of the square of the instantaneous acoustic pressure, divided by the square of the reference pressure, 20 micro-Pa When applied to the evaluation of community response to sonic booms, it is customary to use the so-called C-Weighted Sound Exposure Level, LCE, for which the frequency content of the instantaneous acoustic pressure is modified by the C-Weighting curve

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Sonic boom (human response and atmospheric effects) outdoor-to-indoor response to minimized sonic booms

: This report examines three particular problem areas associated with the measurement of noise levels of propeller-driven small airplanes required to comply with FAR Part 36, Appendix F. These problems are directly related to effects of atmospheric conditions on (1) the performance capabilities of an airplane when tested at various barometric pressures and altitude densities; (2) the noise signature generated at such conditions; and (3) the propagation of sound from the airplane to the measuring station. The first two of these have been addressed by deriving potential correction procedures which could be applied to noise levels obtained by tests not in strict compliance with Appendix F. These corrections are based on aircraft operations conducted within a specific margin of power setting. The corresponding measured noise levels are subsequently corrected for variation of propeller tip speed, forward velocity and barometric pressure relative to predetermined reference conditions. The third effect, that of sound propagation, si examined by applying SAE ARP 866A to a typical propeller airplane noise spectrum and demonstrating the range of A-weight noise levels that would result within a range of ambient conditions. (Author)

Louis C. Sutherland

Papers

Prediction methods for Near Field Noise Environments of VTOL Aircraft.

Evaluation of noise control technology and alternative noise certification procedures for propeller-driven small airplanes

Evaluation of outdoor-to-indoor response to minimized sonic booms

Sonic boom (human response and atmospheric effects) outdoor-to-indoor response to minimized sonic booms

Correction procedures for aircraft noise data: volume 5 : propeller aircraft noise