Noisefield of Underexpanded Notched Circular-Slot Jets

The effect of notches on the flow and noise field of jets from circular-slots is experimentally investigated. Three types of slot geometries, namely, semicircular, triangular and square are studied. The results demonstrate that the presence of the notch introduces a slight aspect-ratio in the initial circular-slot geometry so that the notched jet exhibits characteristics, similar to jets exiting from non-circular geometries. At underexpanded condition, additional expansion and compression waves are observed to emanate from the region of notches that modifies the jet development and results in a reduction in the average shock-cell length, which is accompanied by a reduction in far-field shock associated noise. The acoustic spectrum of the radiated shock noise indicates that notch geometry variation strongly alters the acoustic emission characteristics of these jets both in the screech component and broadband shock associated noise.

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Impact of coherence decay on wavepacket models for broadband shock-associated noise in supersonic jets

Journal of Fluid Mechanics ◽

10.1017/jfm.2018.984 ◽

2019 ◽

Vol 863 ◽

pp. 969-993 ◽

Cited By ~ 5

Author(s):

Marcus H. Wong ◽

Peter Jordan ◽

Damon R. Honnery ◽

Daniel Edgington-Mitchell

Keyword(s):

Dynamic Models ◽

Critical Role ◽

Coherence Function ◽

Supersonic Jets ◽

Far Field ◽

Shock Cell ◽

Acoustic Spectrum ◽

Flow Mechanisms ◽

The Relationship ◽

Field Sound

Motivated by the success of wavepackets in modelling the noise from subsonic and perfectly expanded supersonic jets, we apply the wavepacket model to imperfectly expanded supersonic jets. Recent studies with subsonic jets have demonstrated the importance of capturing the ‘jitter’ of wavepackets in order to correctly predict the intensity of far-field sound. Wavepacket jitter may be statistically represented using a two-point coherence function; accurate prediction of noise requires identification of this coherence function. Following the analysis of Cavalieri & Agarwal (J. Fluid Mech., vol. 748, 2014. pp. 399–415), we extend their methodology to model the acoustic sources of broadband shock-associated noise in imperfectly expanded supersonic jets using cross-spectral densities of the turbulent and shock-cell quantities. The aim is to determine the relationship between wavepacket coherence-decay and far-field broadband shock-associated noise, using the model as a vehicle to explore the flow mechanisms at work. Unlike the subsonic case where inclusion of coherence decay amplifies the sound pressure level over the whole acoustic spectrum, we find that it does not play such a critical role in determining the peak sound amplitude for shock-cell noise. When higher-order shock-cell modes are used to reconstruct the acoustic spectrum at higher frequencies, however, the inclusion of a jittering wavepacket is necessary. These results suggest that the requirement for coherence decay identified in prior broadband shock-associated noise (BBSAN) models is in reality the statistical signature of jittering wavepackets. The results from this modelling approach suggest that nonlinear jittering effects of wavepackets need to be included in dynamic models for broadband shock-associated noise.

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Influence of Tsunami Aspect Ratio on Near and Far-Field Tsunami Amplitude

Geosciences ◽

10.3390/geosciences11040178 ◽

2021 ◽

Vol 11 (4) ◽

pp. 178

Author(s):

Natalia K. Sannikova ◽

Harvey Segur ◽

Diego Arcas

Keyword(s):

Aspect Ratio ◽

Near Field ◽

Decay Rates ◽

Far Field ◽

Present Evidence ◽

Positive Wave ◽

Initial Wave ◽

N Wave ◽

Tsunami Amplitude ◽

Wave Shapes

This study presents a numerical investigation of the source aspect ratio (AR) influence on tsunami decay characteristics with an emphasis in near and far-field differences for two initial wave shapes Pure Positive Wave and N-wave. It is shown that, when initial total energy for both tsunami types is kept the same, short-rupture tsunami with more concentrated energy are likely to be more destructive in the near-field, whereas long rupture tsunami are more dangerous in the far-field. The more elongated the source is, the stronger the directivity and the slower the amplitude decays in the intermediate- and far-fields. We present evidence of this behavior by comparing amplitude decay rates from idealized sources and showing their correlation with that observed in recent historical events of similar AR.

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