Download or read book Jet Shielding of Jet Noise written by John Charles Simonich and published by . This book was released on 1986 with total page 182 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Jet shielding of jet noise written by John Charles Simonich and published by . This book was released on 1986 with total page 177 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Jet Noise Shielding from Airframe Surfaces written by Salvador Mayoral and published by . This book was released on 2010 with total page 137 pages. Available in PDF, EPUB and Kindle. Book excerpt: This work investigates the potential suppression of jet noise by airframe surface shielding with application to the Hybrid Wing Body aircraft (HWB). Subscale experiments utilized a bypass-ratio-10 (BPR10) turbofan nozzle that operated at realistic cycle conditions using helium-air mixture jets. The shielding surface was a flat plate shaped in the form of a generic HWB and featured vertical fins and an adjustable elevator. Chevrons and porous wedge fan flow deflectors were incorporated into the baseline nozzle to restructure and alter the jet noise source distribution. Acoustic measurements of the unshielded and shielded jets were conducted inside an anechoic chamber using a 24-microphone apparatus composed of two polar arrays, each consisting of twelve condenser microphones. One polar array was mounted at an azimuth angle [straight phi] = 0° (downward) while the second array was mounted at [straight phi] = 60° (sideline). Noise source distribution measurements were performed by densely grouping twenty-four microphones on a linear array in the downward direction. Surveys of the jet mean flow were conducted with a fully automated three-dimensional Pitot rake traverse. The Pitot rake consists of five high-resolution Pitot probes that measure the local jet total pressure. The potential for noise suppression was quantified in terms of the estimated Effective Perceived Noise Level (EPNL), a noise metric used in aircraft certification. Acoustic results of the baseline BPR10 nozzle with the HWB shield yielded only marginal EPNL reductions, even when the engine was translated two fan diameters upstream of its nominal location. Using the estimated cumulative (downward plus sideline) EPNL reduction as a figure of merit, shielding of the baseline nozzle with the nominal shield configuration yielded a 2.5 dB reduction. Acoustic imaging of the baseline jet noise source revealed that the large fan diameter, inherent to the high bypass ratio, created a long noise source region. The integration of nozzle devices significantly improved EPNL reduction with the engine positioned at its nominal location. Application of the aggressive chevrons increased the reduction to 6.5 dB, while the best wedge configuration improved this value to 6.9 dB. Combination of wedge and aggressive chevrons yielded a benefit of 7.6 dB. Examination of high-definition noise source maps showed a direct link between insertion loss and axial location of the peak noise. Ultimately, the enhanced shielding effect from the nozzle devices resulted from translation of peak intensities upstream closer to the nozzle plug. Thus the compaction and/or redistribution of the noise source via nozzle devices are essential for effective jet noise shielding on the HWB. A proposed correlator for the noise reduction due to shielding is the average illumination angle. The average illumination angle is defined as the average angular sector between noise source and observer not intercepted by the shield. For average illumination angles less than 45 deg, the EPNL reduction exceeds 3 dB. A correlation between the acoustic and mean flow results reveal that the mean velocity field by itself cannot provide useful information for inferring the noise source location.

Download or read book Prediction of Jet Noise Shielding with Forward Flight Effects written by Salvador Mayoral and published by . This book was released on 2013 with total page 181 pages. Available in PDF, EPUB and Kindle. Book excerpt: Aircraft noise continues to be a major concern among airport-neighboring communities. A strong component of aircraft noise is the jet noise that is generated from the turbulent mixing between the jet exhaust and ambient medium. The hybrid wing body aircraft suppresses jet noise by mounting the engines over-the-wing so that the airframe may shield ground observers from jet noise sources. Subscale jet noise shielding measurements of a scaled-down turbofan nozzle and a model of the hybrid wing body planform are taken with two 12-microphone polar arrays. Chevrons and wedge-type fan flow deflectors are integrated into the baseline bypass ratio 10 (BPR10) nozzle to modify the mean flow and alter the noise source behavior. Acoustic results indicate that the baseline BPR10 nozzle produces a long noise source region that the airframe has difficulty shielding, even when the nozzle is translated two fan diameters upstream of its nominal position. The integration of either chevrons or fan flow deflectors into the nozzle is essential for jet noise shielding because they translate peak intensities upstream, closer to the fan exit plane. The numerical counterpart of this study transforms the system of equations governing the acoustic diffraction with forward flight into the wave equation. Two forward flight formulations are considered: uniform flow over slender body; and non-uniform potential flow at low Mach number. The wave equation is solved numerically in the frequency domain using the boundary element method. The equivalent jet noise source is modeled using the combination of a wavepacket and a monopole. The wavepacket is parameterized using the experimental far-field acoustic autospectra of the BPR10 jets and knowledge of their peak noise locations. It is shown that the noise source compacts with increasing Mach number and consequently there is an increase in shielding. An assessment of the error associated with the non-uniform formulation for forward flight shows that the error is low for Mach numbers less than or equal to 0.2, but can be on the same scale as the acoustic scatter field when the Mach number is 0.6.

Download or read book Fluid Shielding of High velocity Jet Noise written by Jack H. Goodykoontz and published by . This book was released on 1984 with total page 28 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Experimental and Analytical Studies of Shielding Concepts for Point Sources and Jet Noise written by R. L. M. Wong and published by . This book was released on 1983 with total page 189 pages. Available in PDF, EPUB and Kindle. Book excerpt: This analytical and experimental study explores concepts for jet noise shielding. Model experiments centre on solid planar shields, simulating engine-over-wing installations and 'sugar scoop' shields. Tradeoff on effective shielding length is set by interference 'edge noise' as the shield trailing edge approaches the spreading jet. Edge noise is minized by (1) hyperbolic cutouts which trim off the portions of most intense interference between the jet flow and the barrier and (2) hybrid shields - a thermal refractive extension (a flame); for (2) the tradeoff is combustion noise. In general, shielding attenuation increases steadily with frequency following low frequency enhancement by edge noise. Although broadband attenuation is typically only several decibels, the reduction of the subjectively weighted perceived noise levels is higher. In addition, calculated ground contours of peak PN dB (perceived noise level) show a substantial contraction due to shielding: this reaches 66% for one of the 'sugar scoop' shields for the 90 PN dB contour.

Download or read book Refraction and Shielding of Noise in Non Axisymmetric Jets written by Abbas Khavaran and published by . This book was released on 1996 with total page 18 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book The Acoustic Shielding of Noise by a Jet Wing written by Frederick Mark Sears and published by . This book was released on 1975 with total page 198 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book An Extension of the Lighthill Theory of Jet Noise to Encompass Refraction and Shielding written by National Aeronautics and Space Adm Nasa and published by Independently Published. This book was released on 2018-11 with total page 48 pages. Available in PDF, EPUB and Kindle. Book excerpt: A formalism for jet noise prediction is derived that includes the refractive 'cone of silence' and other effects; outside the cone it approximates the simple Lighthill format. A key step is deferral of the simplifying assumption of uniform density in the dominant 'source' term. The result is conversion to a convected wave equation retaining the basic Lighthill source term. The main effect is to amend the Lighthill solution to allow for refraction by mean flow gradients, achieved via a frequency-dependent directional factor. A general formula for power spectral density emitted from unit volume is developed as the Lighthill-based value multiplied by a squared 'normalized' Green's function (the directional factor), referred to a stationary point source. The convective motion of the sources, with its powerful amplifying effect, also directional, is already accounted for in the Lighthill format: wave convection and source convection are decoupled. The normalized Green's function appears to be near unity outside the refraction dominated 'cone of silence', this validates our long term practice of using Lighthill-based approaches outside the cone, with extension inside via the Green's function. The function is obtained either experimentally (injected 'point' source) or numerically (computational aeroacoustics). Approximation by unity seems adequate except near the cone and except when there are shrouding jets: in that case the difference from unity quantifies the shielding effect. Further extension yields dipole and monopole source terms (cf. Morfey, Mani, and others) when the mean flow possesses density gradients (e.g., hot jets). Ribner, Herbert S. Langley Research Center RTOP 505-59-52-01...

Download or read book Alleviation of Jet Aircraft Noise Near Airports written by United States. Office of Science and Technology and published by . This book was released on 1966 with total page 174 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Alleviation of Jet Aircraft Noise Near Airports written by Jet Aircraft Noise Panel (U.S.) and published by . This book was released on 1966 with total page 172 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Aeroacoustic Experiments with Twin Jets written by Richard F. Bozak and published by BiblioGov. This book was released on 2013-06 with total page 22 pages. Available in PDF, EPUB and Kindle. Book excerpt: While the noise produced by a single jet is azimuthally symmetric, multiple jets produce azimuthally varying far-field noise. The ability of one jet to shield another reduces the noise radiated in the plane of the jets, while often increasing the noise radiated out of the plane containing the jets. The present study investigates the shielding potential of twin jet configurations over subsonic and over-expanded supersonic jet conditions with simulated forward flight. The experiments were conducted with 2 in. throat diameter nozzles at four jet spacings from 2.6d to 5.5d in center-to-center distance, where d is the nozzle throat diameter. The current study found a maximum of 3 dB reduction in overall sound pressure level relative to two incoherent jets in the peak jet noise direction in the plane containing the jets. However, an increase of 3 dB was found perpendicular to the plane containing the jets. In the sideline direction, shielding is observed for all jet spacings in this study.

Download or read book High Velocity Jet Noise Source Location and Reduction written by T. F. Balsa and published by . This book was released on 1978 with total page 719 pages. Available in PDF, EPUB and Kindle. Book excerpt: This report presents the results of Task 2 conducted under the subject program over a period of 45 months. Task 2 was formulated as a fundamental theoretical and experimental study aimed at an understanding of the noise generation and suppression mechanisms of high velocity jets. The mechanisms investigated included changes in turbulence structure, fluid shielding, and alteration of convective amplification of jet noise sources. Several other areas such as physical shielding, shock associated noise, lip noise, effect of fluid/particle additives on jet noise, orderly structure in jets, ejector aeroacoustics, and flight effects on jet noise were also investigated. The most significant achievements of Task 2 were as follows. A unified aeroacoustic theory composed of three ingredients were developed: (1) the prediction of mean properties of the jet plume, (2) deduction of turbulence properties relevant to jet noise by similarity arguments, and (3) the prediction of far-field noise including the effects of fluid shielding. A semiempirical shock noise prediction procedure was also developed. Exhaustive theory-data comparisons for a wide range of nozzle configurations and velocity/temperature combinations were conducted and have confirmed the essential validity of this model as a prediction tool. A comprehensive series of experiments with simple suppressor elements (such as a single rectangular tube, twin jets, linear arrays of jets, circular arrays of jets) was carried out and revealed the importance of acoustic shielding by adjacent jets. A fundamental series of experiments, specifically tailored to reveal fluid shielding as a jet noise suppression mechanism, was successfully conducted.

Download or read book An Extension of the Lighthill Theory of Jet Noise to Encompass Refraction and Shielding written by Herbert S. Ribner and published by . This book was released on 1995 with total page 48 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Database of Inlet and Exhaust Noise Shielding for Wedge shaped Airframe written by Carl H. Gerhold and published by . This book was released on 2001 with total page 22 pages. Available in PDF, EPUB and Kindle. Book excerpt: An experiment to measure the noise shielding of the blended wing body design concept was developed using a simplified wedge-shaped airframe. The experimental study was conducted in the Langley Anechoic Noise Research facility. A wideband, omnidirective sound source in a simulated engine nacelle was held at locations representative of a range of engine locations above the wing. The sound field around the model was measured, with the airframe and source in place and with source alone, using an array of microphones on a rotating hoop that is also translated along an axis parallel to the airframe axis. The insertion loss was determined from the difference between the two resulting contours. Although no attempt was made to simulate the noise characteristics of a particular engine, the broadband noise source radiated sound over a range of scaled frequencies encompassing 1 and 2 times the blade passage frequency representative of a large, high-bypass-ratio turbofan engine.

Download or read book Jet Noise Suppression by an Impedance Shroud written by Ian S. F. Jones and published by . This book was released on 1970 with total page 26 pages. Available in PDF, EPUB and Kindle. Book excerpt: Jet noise can be attenuated by shielding the observer from the noise sources with a sheet of fluid having a different acoustic impedance to the undisturbed atmosphere. The attenuation due to a heated sheet of gas lying on an arc centered on the axis of a subsonic jet was measured. The maximum noise reduction in certain frequency bands in this experiment was 10 dB. (Author).

Download or read book Analytical Evaluation of Jet Noise Source Location Technique Utilizing an Acoustically Hard Baffle with Aperture written by Thomas D.. Norum and published by . This book was released on 1973 with total page 22 pages. Available in PDF, EPUB and Kindle. Book excerpt: An analytical investigation was conducted on the experimental technique that uses shielding by an acoustically hard baffle to determine the location of noise sources in a jet. The jet exhausts through a circular hole in the baffle and the radiated acoustic power is considered to be generated by a distributed source along the jet axis. It is found that the effect of the baffle on the radiated power can be neglected only when the size of the source is on the order of a wavelength of the emitted sound or larger. Since noise sources in a jet are compact, the experimental technique is insufficient to identify these sources.