Clustered scatterers: the effect on the mean acoustic field

2008 ◽  
Vol 123 (5) ◽  
pp. 3897-3897
Author(s):  
Thomas C. Weber
Keyword(s):  
Acoustic Field ◽  
The Mean

scholarly journals A note on acoustic turbulence

2019 ◽  
Vol 874 ◽  
Author(s):  
Erik Lindborg
Keyword(s):  
Structure Function ◽  
Entropy Production ◽  
Acoustic Field ◽  
Three Dimensional ◽  
Weak Shock ◽  
Separation Distance ◽  
Ideal Gas ◽  
Order Structure ◽  
Specific Heats ◽  
The Mean

We consider a three-dimensional acoustic field of an ideal gas in which all entropy production is confined to weak shocks and show that similar scaling relations hold for such a field as for forced Burgers turbulence, where the shock amplitude scales as $(\unicode[STIX]{x1D716}d)^{1/3}$ and the $p$th-order structure function scales as $(\unicode[STIX]{x1D716}d)^{p/3}r/d$, $\unicode[STIX]{x1D716}$ being the mean energy dissipation per unit mass, $d$ the mean distance between the shocks and $r$ the separation distance. However, for the acoustic field, $\unicode[STIX]{x1D716}$ should be replaced by $\unicode[STIX]{x1D716}+\unicode[STIX]{x1D712}$, where $\unicode[STIX]{x1D712}$ is associated with entropy production due to heat conduction. In particular, the third-order longitudinal structure function scales as $\langle \unicode[STIX]{x1D6FF}u_{r}^{3}\rangle =-C(\unicode[STIX]{x1D716}+\unicode[STIX]{x1D712})r$, where $C$ takes the value $12/5(\unicode[STIX]{x1D6FE}+1)$ in the weak shock limit, $\unicode[STIX]{x1D6FE}=c_{p}/c_{v}$ being the ratio between the specific heats at constant pressure and constant volume.

Noise generation by high-frequency gusts interacting with an airfoil in transonic flow

2000 ◽  
Vol 411 ◽  
pp. 91-130 ◽  
Author(s):  
I. EVERS ◽  
N. PEAKE
Keyword(s):  
Steady Flow ◽  
Acoustic Field ◽  
Transonic Flow ◽  
High Frequency ◽  
Power Series Expansion ◽  
Outer Region ◽  
Leading Edge ◽  
Mean Flow ◽  
Flow Distortion ◽  
The Mean

The method of matched asymptotic expansions is used to describe the sound generated by the interaction between a short-wavelength gust (reduced frequency k, with k [Gt ] 1) and an airfoil with small but non-zero thickness, camber and angle of attack (which are all assumed to be of typical size O(δ), with δ [Lt ] 1) in transonic flow. The mean-flow Mach number is taken to differ from unity by O(δ2/3), so that the steady flow past the airfoil is determined using the transonic small-disturbance equation. The unsteady analysis is based on a linearization of the Euler equations about the mean flow. High-frequency incident vortical and entropic disturbances are considered, and analogous to the subsonic counterpart of this problem, asymptotic regions around the airfoil highlight the mechanisms that produce sound. Notably, the inner region round the leading edge is of size O(k−1), and describes the interaction between the mean-flow gradients and the incident gust and the resulting acoustic waves. We consider the preferred limit in which kδ2/3 = O(1), and calculate the first two terms in the phase of the far-field radiation, while for the directivity we determine the first term (δ = 0), together with all higher-order terms which are at most O(δ2/3) smaller – in fact, this involves no fewer than ten terms, due to the slowly-decaying form of the power series expansion of the steady flow about the leading edge. Particular to transonic flow is the locally subsonic or supersonic region that accounts for the transition between the acoustic field downstream of a source and the possible acoustic field upstream of the source. In the outer region the sound propagation has a geometric-acoustics form and the primary influence of the mean-flow distortion appears in our preferred limit as an O(1) phase term, which is particularly significant in view of the complicated interference between leading- and trailing-edge fields. It is argued that weak mean- flow shocks have an influence on the sound generation that is small relative to the effects of the leading-edge singularity.

Simulation and Analysis of the Radiated Acoustic Field Measurement of the Underwater Vehicle Based on NAH

2012 ◽  
Vol 503-504 ◽  
pp. 1575-1579
Author(s):  
Shao Chun Ding ◽  
Lin Na Zhou ◽  
Jing Jun Lou ◽  
Shi Jian Zhu
Keyword(s):  
Acoustic Field ◽  
Sound Pressure ◽  
Power Level ◽  
Sound Intensity ◽  
Sound Power ◽  
Pressure Method ◽  
Sound Power Level ◽  
Measuring Surface ◽  
Source Level ◽  
The Mean

we use the NAH method for the simulation and analysis of sound power level and source level with the plan measuring surface under the same distance and size. The sound intensity integral method, the mean square sound pressure method and the NAH reversal method have been adopted in this paper. We also compare the sound power level between the plan measuring surface and the cylinder measuring surface, thus helps verifying the accuracy of the measurement of the radiated acoustic field based on the method of NAH. The conclusion we have drawn here can also provides dependable experimental basis for the choosing of measuring surfaces

Numerical Simulation of Internal Cavities Acoustic Trapped Modes With Mean Flow

2010 ◽  
Author(s):  
Kareem Aly ◽  
Samir Ziada
Keyword(s):  
Mach Number ◽  
Acoustic Field ◽  
Particle Velocity ◽  
Perturbation Equation ◽  
Duct System ◽  
Trapped Modes ◽  
Mean Flow ◽  
Phase Gradient ◽  
The Mean ◽  
Mach Numbers

Flow-excited acoustic resonance of trapped modes in ducts has been reported in different engineering applications. The excitation mechanism of these modes results from the interaction between the hydrodynamic flow field and the acoustic particle velocity, and is therefore dependent on the mode shape of the resonant acoustic field, including the amplitude and phase distributions of the acoustic particle velocity. For a cavity-duct system, the aerodynamic excitation of the trapped modes can generate strong pressure pulsations at moderate Mach numbers (M>0.1). This paper investigates numerically the effect of mean flow on the characteristics of the acoustic trapped modes for a cavity-duct system. Numerical simulations are performed for a two-dimensional planar configuration and different flow Mach numbers up to 0.3. A two-step numerical scheme is adopted in the investigation. A linearized acoustic perturbation equation is used to predict the acoustic field. The results show that as the Mach number is increased, the acoustic pressure distribution develops an axial phase gradient, but the shape of the amplitude distribution remains the same. Moreover, the amplitude and phase distributions of the acoustic particle velocity are found to change significantly near the cavity shear layer with the increase of the mean flow Mach number. These results demonstrate the importance of considering the effects of the mean flow on the flow-sound interaction mechanism.

Acoustic Field in Ducts With Sinusoidal Area Variation

2013 ◽  
Vol 136 (1) ◽  
Author(s):  
N. S. Vikramaditya ◽  
R. B. Kaligatla
Keyword(s):  
Differential Equation ◽  
Partial Differential Equation ◽  
Acoustic Field ◽  
Second Order ◽  
Ordinary Linear Differential Equation ◽  
Order Partial Differential Equation ◽  
Mean Flow ◽  
Area Variation ◽  
The Mean ◽  
Linear Differential

The purpose of this article is to provide an analytical solution for the acoustic field in a duct with sinusoidal area variation along the length. The equation describing the acoustic field in a variable area duct is a second-order partial differential equation. It is converted into a second-order ordinary linear differential equation, whose solution is dependent on the choice of area variation. The solution for the differential equation is obtained in terms of the area and is obtained neglecting the mean flow. Therefore, it is applicable in the absence of mean flow or in cases where the effects of mean flow are insignificant.

Photo-electric Hβ and uvby photometry

1966 ◽  
Vol 24 ◽  
pp. 170-180
Author(s):  
D. L. Crawford
Keyword(s):  
Narrow Band ◽  
Line Strength ◽  
Interference Filter ◽  
B Stars ◽  
Relative Line ◽  
The Mean ◽  
F Stars ◽  
Two Parameters ◽  
Filter Photometry ◽  
The Continuum

Early in the 1950's Strömgren (1, 2, 3, 4, 5) introduced medium to narrow-band interference filter photometry at the McDonald Observatory. He used six interference filters to obtain two parameters of astrophysical interest. These parameters he calledlandc, for line and continuum hydrogen absorption. The first measured empirically the absorption line strength of Hβby means of a filter of half width 35Å centered on Hβand compared to the mean of two filters situated in the continuum near Hβ. The second index measured empirically the Balmer discontinuity by means of a filter situated below the Balmer discontinuity and two above it. He showed that these two indices could accurately predict the spectral type and luminosity of both B stars and A and F stars. He later derived (6) an indexmfrom the same filters. This index was a measure of the relative line blanketing near 4100Å compared to two filters above 4500Å. These three indices confirmed earlier work by many people, including Lindblad and Becker. References to this earlier work and to the systems discussed today can be found in Strömgren's article inBasic Astronomical Data(7).

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