Filtered Azimuthal Correlations in the Acoustic Far Field of a Subsonic Jet

AIAA Journal ◽  
1979 ◽  
Vol 17 (1) ◽  
pp. 112-113 ◽  
Author(s):  
Daniel Juve ◽  
Michel Sunyach ◽  
Genevieve Comte-Bellot
Keyword(s):  
Far Field ◽  
Azimuthal Correlations ◽  
Subsonic Jet

Dynamics of an impinging jet. Part 2. The noise generation

1982 ◽  
Vol 116 ◽  
pp. 379-391 ◽  
Author(s):  
Nagy S. Nosseir ◽  
Chih-Ming Ho
Keyword(s):  
Coherent Structures ◽  
Near Field ◽  
Pressure Fluctuations ◽  
Aerodynamic Noise ◽  
Far Field ◽  
Noise Generation ◽  
Physical Mechanisms ◽  
Field Noise ◽  
Subsonic Jet ◽  
Jet Axis

The aerodynamic noise generated by a subsonic jet impinging on a flat plate is studied from measurements of near-field and surface-pressure fluctuations. The far-field noise measured at 90° to the jet axis is found to be generated by two different physical mechanisms. One mechanism is the impinging of the large coherent structures on the plate, and the other is associated with the initial instability of the shear layer. These two sources of noise radiate to the far field via different acoustical paths.

scholarly journals Near-field Wavepackets and the Far-field Sound of a Subsonic Jet

2013 ◽  
Author(s):  
David E. Breakey ◽  
Peter Jordan ◽  
Andre Cavalieri ◽  
Olivier Léon
Keyword(s):  
Near Field ◽  
Far Field ◽  
Field Sound ◽  
Subsonic Jet

Analysis of far-field coherence of subsonic jet noise

2018 ◽  
Author(s):  
José R. Sirotto ◽  
Julio A. Cordioli ◽  
André V. Cavalieri
Keyword(s):  
Jet Noise ◽  
Far Field ◽  
Field Coherence ◽  
Subsonic Jet

Cross-correlations of far field azimuthal modes in subsonic jet noise

2008 ◽  
Author(s):  
Victor Kopiev ◽  
Mikhail Zaitsev ◽  
Sergey Velichko ◽  
Anna Kotova ◽  
Ivan Belyaev
Keyword(s):  
Jet Noise ◽  
Far Field ◽  
Cross Correlations ◽  
Subsonic Jet

scholarly journals A coherence-matched linear source mechanism for subsonic jet noise

2015 ◽  
Vol 776 ◽  
pp. 235-267 ◽  
Author(s):  
Yamin B. Baqui ◽  
Anurag Agarwal ◽  
André V. G. Cavalieri ◽  
Samuel Sinayoko
Keyword(s):  
Linear Models ◽  
Hydrodynamic Instability ◽  
Near Field ◽  
Jet Noise ◽  
Turbulent Jets ◽  
Single Frequency ◽  
Linear Wave ◽  
Far Field ◽  
Potential Core ◽  
Subsonic Jet

We investigate source mechanisms for subsonic jet noise using experimentally obtained datasets of high-Reynolds-number Mach 0.4 and 0.6 turbulent jets. The focus is on the axisymmetric mode which dominates downstream sound radiation for low polar angles and the frequency range at which peak noise occurs. A linearized Euler equation (LEE) solver with an inflow boundary condition is used to generate single-frequency hydrodynamic instability waves, and the resulting near-field fluctuations and far-field acoustics are compared with those from experiments and linear parabolized stability equation (LPSE) computations. It is found that the near-field velocity fluctuations closely agree with experiments and LPSE computations up to the end of the potential core, downstream of which deviations occur, but the LEE results match experiments better than the LPSE results. Both the near-field wavepackets and the sound field are observed directly from LEE computations, but the far-field sound pressure levels (SPLs) obtained are more than an order of magnitude lower than experimental values despite close statistical agreement of the near hydrodynamic field up to the potential core region. We explore the possibility that this discrepancy is due to the mismatch between the decay of two-point coherence with increasing distance in experimental flow fluctuations and the perfect coherence in linear models. To match the near-field coherence, experimentally obtained coherence profiles are imposed on the two-point cross-spectral density (CSD) at cylindrical and conical surfaces that enclose near-field structures generated with LEEs. The surface pressure is propagated to the far field using boundary value formulations based on the linear wave equation. Coherence matching yields far-field SPLs which show improved agreement with experimental results, indicating that coherence decay is the main missing component in linear models. The CSD on the enclosing surfaces reveals that the application of a decaying coherence profile spreads the hydrodynamic component of the linear wavepacket source on to acoustic wavenumbers, resulting in a more efficient acoustic source.

scholarly journals Directivity of acoustic emissions from wave packets to the far field

2009 ◽  
Vol 640 ◽  
pp. 165-186 ◽  
Author(s):  
DOMINIK OBRIST
Keyword(s):  
Acoustic Emission ◽  
Wave Packet ◽  
Wave Packets ◽  
Acoustic Emissions ◽  
Spectral Domain ◽  
Far Field ◽  
Acoustic Analogy ◽  
Geometric Constructions ◽  
Subsonic Jet ◽  
Mach Waves

We investigate the acoustic emission from wave packets to the far field. To this end, we develop a theory for one- and two-dimensional source fields in the shape of wave packets with Gaussian envelopes. This theory is based on an approximation to Lighthill's acoustic analogy for distant observers. It is formulated in the spectral domain in which a Gaussian wave packet is represented again by a Gaussian. This allows us to determine the directivity of the acoustic emission (e.g. superdirectivity and Mach waves) by simple geometric constructions in the spectral domain. It is shown that the character of the acoustic emission is mainly governed by the aspect ratio and the Mach number of the wave packet source. To illustrate the relevance of this theory we use it to study two prominent problems in subsonic jet aeroacoustics.

Possible applications of fraunhofer holography in high resolution electron microscopy

1978 ◽  
Vol 36 (1) ◽  
pp. 222-223 ◽  
Author(s):  
N. Bonnet ◽  
M. Troyon ◽  
P. Gallion
Keyword(s):  
Electron Microscopy ◽  
High Resolution ◽  
Transfer Function ◽  
Radiation Damage ◽  
High Resolution Electron Microscopy ◽  
Far Field ◽  
Field Region ◽  
Crystalline Materials ◽  
Resolution Electron ◽  
The Ideal

Two main problems in high resolution electron microscopy are first, the existence of gaps in the transfer function, and then the difficulty to find complex amplitude of the diffracted wawe from registered intensity. The solution of this second problem is in most cases only intended by the realization of several micrographs in different conditions (defocusing distance, illuminating angle, complementary objective apertures…) which can lead to severe problems of contamination or radiation damage for certain specimens.Fraunhofer holography can in principle solve both problems stated above (1,2). The microscope objective is strongly defocused (far-field region) so that the two diffracted beams do not interfere. The ideal transfer function after reconstruction is then unity and the twin image do not overlap on the reconstructed one.We show some applications of the method and results of preliminary tests.Possible application to the study of cavitiesSmall voids (or gas-filled bubbles) created by irradiation in crystalline materials can be observed near the Scherzer focus, but it is then difficult to extract other informations than the approximated size.

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