scholarly journals Numerical Investigation on the Spectral Broadening of Acoustic Waves by a Turbulent Layer

2016 ◽  
Author(s):  
Vincent Clair ◽  
Gwenael Gabard
Keyword(s):  
Numerical Investigation ◽  
Acoustic Waves ◽  
Spectral Broadening ◽  
Turbulent Layer

scholarly journals Spectral broadening of acoustic waves by convected vortices

2018 ◽  
Vol 841 ◽  
pp. 50-80 ◽  
Author(s):  
Vincent Clair ◽  
Gwénaël Gabard
Keyword(s):  
Scattered Field ◽  
Acoustic Waves ◽  
Acoustic Scattering ◽  
Two Dimensions ◽  
Wind Tunnels ◽  
Spectral Broadening ◽  
Sound Fields ◽  
Physical Mechanisms ◽  
Doppler Shifts ◽  
Acoustic Spectra

The scattering of acoustic waves by a moving vortex is studied in two dimensions to bring further insight into the physical mechanisms responsible for the spectral broadening caused by a region of turbulence. When propagating through turbulence, a monochromatic sound wave will be scattered over a range of frequencies, resulting in typical spectra with broadband sidelobes on either side of the tone. This spectral broadening, also called ‘haystacking’, is of importance for noise radiation from jet exhausts and for acoustic measurements in open-jet wind tunnels. A semianalytical model is formulated for a plane wave scattered by a vortex, including the influence of the convection of the vortex. This allows us to perform a detailed parametric study of the properties and evolution of the scattered field. A time-domain numerical model for the linearised Euler equations is also used to consider more general sound fields, such as that radiated by a point source in a uniform flow. The spectral broadening stems from the combination of the spatial scattering of sound due to the refraction of waves propagating through the vortex, and two Doppler shifts induced by the motion of the vortex relative to the source and of the observer relative to the vortex. The fact that the spectrum exhibits sidebands is directly explained by the directivity of the scattered field which is composed of several beams radiating from the vortex. The evolution of the acoustic spectra with the parameters considered in this paper is compared with the trends observed in previous experimental work on acoustic scattering by a jet shear layer.

Numerical Investigation of a Non-Confined Spray Flame Exposed to Acoustic Forcing

2015 ◽  
Author(s):  
Virginia Fratalocchi ◽  
Jim B. W. Kok
Keyword(s):  
Velocity Field ◽  
Numerical Investigation ◽  
Acoustic Waves ◽  
Slip Velocity ◽  
Evaporation Rate ◽  
Momentum Exchange ◽  
Velocity Fluctuations ◽  
Spray Flame ◽  
Acoustic Forcing ◽  
The Impact

A numerical investigation of the interaction between a spray flame and an acoustic forcing of the velocity field is presented in this paper. The test-case which is the focus of this work is a non-confined flame1,2 burning at atmospheric pressure and therefore the velocity fluctuations play a key role. Acoustic waves will eventually affect the rate of combustion, and the oscillating fluctuation of the heat released by the flame might be increased by the evaporation process. The dynamic interaction between the evaporating fuel spray and the velocity fluctuations induced by an acoustic perturbation is investigated to understand the impact of the acoustic waves on the droplet dispersion and on the evaporation rate. The influence of the initial droplet diameter has been observed to be irrelevant, when two monodispersed sprays of 20 μm and 80 μm were numerically simulated. In this work the main question to address is how the interphase heat and mass transfer, and the momentum exchange are influenced, at low amplitude velocity fluctuations, by the forcing frequency, under two different imposed velocity profiles of the liquid fuel. A fast decay of the slip velocity is predicted under both steady and perturbed conditions. Thus, slip velocity fluctuations do not have a significant influence on the solved spray field. Finally, the impact of the forcing frequency and of the pilot are the main effects acting on the forced flame response. At low frequency, the entrainment of hot gases into the spray results in a clearly visible stretching of the flame which causes a high level of temperature fluctuation. At high frequency, despite the weak response of the gas velocity field, the dynamics of the combustion show a faster evaporation rate than the acoustic–free case.

An analytical and numerical investigation of ion acoustic waves in a two‐ion plasma

1994 ◽  
Vol 1 (11) ◽  
pp. 3542-3556 ◽  
Author(s):  
H. X. Vu ◽  
J. M. Wallace ◽  
B. Bezzerides
Keyword(s):  
Numerical Investigation ◽  
Acoustic Waves ◽  
Ion Acoustic Waves ◽  
Ion Plasma ◽  
Ion Acoustic

scholarly journals Theoretical investigation of alteration and radiation of large-scale structures due to jet impingement

2018 ◽  
Vol 18 (2-3) ◽  
pp. 231-257
Author(s):  
SAE Miller ◽  
Alexander N Carr
Keyword(s):  
Acoustic Waves ◽  
Large Scale ◽  
Acoustic Radiation ◽  
Jet Impingement ◽  
Far Field ◽  
Spectral Broadening ◽  
Aircraft Carrier ◽  
Instability Waves ◽  
Large Scale Structures ◽  
Scale Structures

Jet flows impinge on launch pad structures and aircraft carrier deck blast deflectors. Turbulent structures are deformed and acoustic radiation is reflected by the deflector. The coupling of reflected acoustic waves with the instability waves of the jet turbulence increases their amplitude and causes a feedback loop. Resultant far-field acoustic radiation is amplified. This amplification results in additional tones with significant spectral broadening occurring at frequencies corresponding to the constructive interference. We present a simple prediction methodology in the form of an acoustic analogy. The analogy accounts for reflected acoustic waves through a tailored Green’s function and models the large-scale structures as spatially and temporarily growing and decaying instability waves. The predictions are compared with two experimental datasets. Predictions compare favorably with measured frequencies and spectral broadening in the far-field.

Damping of nonlinear magneto-acoustic waves

1978 ◽  
Vol 20 (1) ◽  
pp. 125-136 ◽  
Author(s):  
G. L. Kalra ◽  
V. B. Bhatia
Keyword(s):  
Electrical Conductivity ◽  
Numerical Investigation ◽  
Acoustic Waves ◽  
Finite Amplitude ◽  
Turbulent Plasma ◽  
Mhd Equations ◽  
Space Plasmas ◽  
Fast Mode ◽  
Mode Propagation

Nonlinear magneto-acoustic waves in a turbulent plasma are simulated by collisional MHD equations. Damping of these waves due to electrical conductivity arising from micro-instabilities and collisional viscosity are analyzed. Numerical investigation of competing effects due to non-linearity and dissipation has been carried out. It is found that finite amplitude perturbation leads to the formation of a shock in both the slow and the fast mode propagation. Collisional viscosity plays an important role in the damping of nonlinear magneto-acoustic waves in the astrophysical and space plasmas.

Detailed numerical investigation of the interaction of longitudinal acoustic waves with fiber Bragg gratings in suspended-core fibers

2015 ◽  
Vol 344 ◽  
pp. 43-50 ◽  
Author(s):  
Ricardo E. Silva ◽  
Alexander Hartung ◽  
Manfred Rothhardt ◽  
Alexandre A.P. Pohl ◽  
Hartmut Bartelt
Keyword(s):  
Numerical Investigation ◽  
Acoustic Waves ◽  
Fiber Bragg Gratings ◽  
Bragg Gratings ◽  
Longitudinal Acoustic ◽  
Longitudinal Acoustic Waves
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