Journal ArticleDOI
The near pressure field of co-axial subsonic jets
Charles E. Tinney,Peter Jordan +1 more
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In this paper, an analysis of the axial, temporal and azimuthal structure of the pressure field of a co-axial jet with and without serrations on the secondary nozzle lip is presented.Abstract:
Results are presented from pressure measurements performed in the irrotational near field of unbounded co-axial jets. Measurements were made for a variety of velocity and temperature ratios, and configurations both with and without serrations on the secondary nozzle lip. The principal objective of the study is to better understand the near pressure field of the jet, what it can tell us regarding the underlying turbulence structure, and in particular how it can be related to the source mechanisms of the flow.A preliminary analysis of the axial, temporal and azimuthal structure of the pressure field shows it to be highly organized, with axial spatial modes (obtained by proper orthogonal decomposition) which resemble Fourier modes. The effects of serrations on the pressure fluctuations comprise a global reduction in level, a change in the axial energy distribution, and a modification of the evolution of the characteristic time scales.A further analysis in frequency–wavenumber space is then performed, and a filtering operation is used to separate the convective and propagative footprints of the pressure field. This operation reveals two distinct signatures in the propagating component of the field: a low-frequency component which radiates at small angles to the flow axis and is characterized by extensive axial coherence, and a less-coherent high-frequency component which primarily radiates in sideline directions. The serrations are found to reduce the energy of the axially coherent propagating component, but its structure remains fundamentally unchanged; the high-frequency component is found to be enhanced. A further effect of the serrations involves a relative increase of the mean-square pressure level of the acoustic component – integrated over the measurement domain – with respect to the hydrodynamic component. The effect of increasing the velocity and temperature of the primary jet involves a relative increase in the acoustic component of the near field, while the hydrodynamic component remains relatively unchanged: this shows that the additional acoustic energy is generated by the mixing region which is produced by the interaction of the inner and the outer shear layers, whereas the hydrodynamic component of the near field is primarily driven by the outer shear layer.read more
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Supersonic Jet Noise Reduction with Novel Fluidic Injection Techniques
Abstract: Supersonic jets provide unique challenges in the aeroacoustic field due to very high jet velocities, shock associated noise components, flow dependence on jet expansion, and stringent performance requirements. Current noise suppression technology for commercial and military jet engines revolves around using chevrons or mechanical vortex generators to increase mixing near the nozzle exit, subsequently reducing peak turbulence levels in the mixing region. Passive noise control methods such as mechanical chevrons cause thrust loss throughout the flight envelope and performance can vary with the engine operating condition. Development of active noise control methods have the potential of improved performance throughout the flight envelope and the benefit of being deactivated when noise control is unnecessary. Fluidic injection of air into a supersonic jet is studied as an active control method with an emphasis on understanding the physics of the problem and identifying the controlling parameters. An experimental investigation with computational collaboration was conducted to understand the e↵ect of nozzle design on supersonic jet noise and to develop various fluidic injection techniques to control noise from a supersonic jet with a design Mach number of 1.56. The jet was studied at overexpanded, ideally expanded, and underexpanded conditions to evaluate the e↵ects throughout the operational envelope. As a passive noise control method, the internal contour of a realistic nozzle was modified to investigate the e↵ect on acoustics and performance. Thrust was improved up to 10% with no acoustic penalties through nozzle design, however it was found that the shock noise components were highly sensitive to the shock structure in the jet. Steady fluidic injection was used to generate vorticity at the trailing edge of the nozzle showing that noise reduction is achieved through vorticity generation, modification i of the shock structure, and interference with the screech feedback mechanism by decoupling the phase relationship between jet turbulence and shock spacing. Reduction of shock noise was found to be optimum at an intermediate injection pressure due to shock weakening from the fluidic injectors and injector interactions with the jet shock-expansion structure. Largescale mixing noise reduction was shown to depend on the vorticity strength and circulation. Unprecedented reduction of OASPL up to -8.5 dB were achieved at the peak noise direction through strong jet mixing and rapid collapse of the potential core. Pulsed fluidic injection was investigated to understand the acoustic benefits and drawbacks of unsteady injection. Valve frequency response up to 500 Hz was achieved but noise reduction dropped o↵ above 100 Hz due to poor flow response as verified by hot-wire and dynamic pressure measurements. At low pulse frequencies it was found that moderate noise reduction could be achieved with less flow than steady injection, but in general the mixing noise reduction scaled with the time integrated mass flow injection. It was discovered that the di↵erent components of supersonic jet noise had di↵erent characteristic response times to unsteady injection. Analysis of high speed shadowgraph images and acoustic spectra was used to identify time response of the jet during the unsteady injection cycle. Development of a code to apply the E↵ective Perceived Noise Level standard to laboratory jets was used to evaluate the various noise reduction methods. It was shown that EPNL was a↵ected most strongly by the sideline spectrum and the jet shock noise since EPNL is dominated by the maximum noise during a flight event, which almost always occurs when the aircraft passes nearest to the observer. The methods of noise reduction show potential for reduction of EPNL up to -4 dB and 50% reduction in the noise footprint area near the jet take-o↵ location. ii Copyright © 2013 Daniel Cuppoletti
Proceedings ArticleDOI
Characterization of Installation Effects for HBPR Engine, Part 4: Assessment of Jet Acoustics
TL;DR: In this paper, the authors characterised the installation of a typical high bypass ratio engine jet using numerical simulations and particle image velocimetry, and the global objectives are an improved understanding of the link between the jet ows and the generated noise, the validation of methods for computing the aerodynamic energy and the noise sources for industrial congurations.
Proceedings ArticleDOI
Intermittency of the azimuthal components of the sound radiated by subsonic jets
TL;DR: In this article, a continuous wavelet transform was applied to acoustic pressure and velocity field of an experimental Mach 0.6 jet in order to extract the intermittent bursts in the signals, and an intermittency measure based on this filter was used to quantify the significance of these events in the total acoustic intensity.
Proceedings ArticleDOI
Proper grid resolutions for the proper basis.
TL;DR: In this paper, the convergence of the proper orthogonal decomposition (POD) eigenvalues to the discretization of the measurement grid is discussed using a number of different experimental data sets, and the purpose is to determine the necessary conditions, based on an a prior understanding of the statistical properties of the turbulence field, for sufficiently obtaining a reduced order representation of the flow.
Journal ArticleDOI
Effect of Velocity Ratio on Noise Source Distribution of Coaxial Jets
TL;DR: In this article, the noise source distribution of coaxial jets with a diameter ratio of 1.6 and variable velocity ratio is investigated with a small-aperture microphone phased array.
References
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Journal ArticleDOI
The Proper Orthogonal Decomposition in the Analysis of Turbulent Flows
TL;DR: The Navier-Stokes equations are well-known to be a good model for turbulence as discussed by the authors, and the results of well over a century of increasingly sophisticated experiments are available at our disposal.
Journal ArticleDOI
The dynamics of coherent structures in the wall region of a turbulent boundary layer.
TL;DR: In this article, the wall region of a turbulent boundary layer is modelled by expanding the instantaneous field in so-called empirical eigenfunctions, as permitted by the proper orthogonal decomposition theorem.
Journal ArticleDOI
Noise sources in a low-Reynolds-number turbulent jet at Mach 0.9
TL;DR: In this article, the mechanisms of sound generation in a Mach 0.9, Reynolds number 3600 turbulent jet are investigated by direct numerical simulation and the results show that the phase velocities of significant components range from approximately 5% to 50% of the ambient sound speed.
Proceedings ArticleDOI
On the Two Components of Turbulent Mixing Noise from Supersonic Jets
TL;DR: In this paper, two similarity spectra, one for the noise from the large turbulence structures/instability waves of the jet flow, the other for the fine-scale turbulence, are identified.
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