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1.
The acoustic analogy was introduced into acoustics by Lighthill in 1952 to understand and predict the noise generated by
the jet of an aircraft turbojet engine. The idea behind the acoustic analogy is simple but powerful. The entire noise generation
process is mathematically reduced to the study of wave propagation in a quiescent medium with the effect of flow replaced
by quadrupole sources. In jet noise theory, Lighthill was able to obtain significant and useful qualitative results from the
acoustic analogy. The acoustic analogy has influenced the theoretical and experimental research on jet noise since the early
1950s.
This paper, however, focuses on another area in which the acoustic analogy has had a significant impact, namely, the prediction
of the noise of rotating machinery. The governing equation for this problem was derived by Ffowcs Williams and Hawkings in
1969. This equation is a wave equation for perturbation density with three source terms, which have become known as thickness,
loading, and the quadrupole source terms, respectively. The Ffowcs Williams–Hawkings (FW–H) equation has been used for the
successful prediction of the noise of helicopter rotors, propellers, and fans. Several reasons account for the success and
popularity of the acoustic analogy. First, the problems of acoustics and aerodynamics are separated. Second, because the FW–H
equation is linear, powerful analytical methods from linear operator theory can be used to obtain closed-form solutions. Third,
advances in digital computers and computational fluid dynamics algorithms have resulted in high-resolution near-field aerodynamic
calculations that are suitable for noise prediction. We present some of the
mathematical results for noise prediction based on the FW–H equation, including examples for helicopter rotors. In particular,
we discuss the prediction of blade-vortex interaction noise and high-speed impulsive noise of helicopter rotors. For high-speed
propellers, we briefly discuss the derivation of a singularity-free solution of the FW–H equation for a supersonic panel on
a blade.
Received 24 January 1997 and accepted 30 May 1997 相似文献
2.
A three-dimensional (3D) hybrid LES-acoustic analogy method for computational aeroacoustics (CAA) is presented for the prediction
of open-cavity noise. The method uses large-eddy simulation (LES) to compute the acoustic source while the Ffowcs Williams-Hawkings
(FW-H) acoustic analogy is employed for the prediction of the far-field sound. As a comparison, a two-dimensional (2D) FW-H
analogy is also included. The hybrid method has been assessed in an open-cavity flow at a Mach number of 0.85 and a Reynolds
number of Re=1.36×10 6, where some experimental data are available for comparison. The study has identified some important technical issues in the
application of the FW-H acoustic analogy to cavity noise prediction and CAA in general, including the proper selection of
the integration period and the modes of sound sources in the frequency domain. The different nature of 2D and 3D wave propagation
is also highlighted, which calls for a matching acoustic solver for each problem. The developed hybrid method has shown promise
to be a feasible, accurate and computationally affordable approach for CAA. 相似文献
3.
In this paper, we study the acoustic emissions of the flow over a rectangular cavity. Especially, we investigate the possibility
of estimating the acoustic emission by analysis of PIV data. Such a possibility is appealing, since it would allow to directly
relate the flow behavior to the aerodynamic noise production. This will help considerably in understanding the noise production
mechanisms and to investigate the possible ways of reducing it. In this study, we consider an open cavity with an aspect ratio
between its length and depth of 2 at a Reynolds number of 2.4 × 10 4 and 3.0 × 10 4 based on the cavity length. The study is carried out combining high speed two-dimensional PIV, wall pressure measurements
and sound measurements. The pressure field is computed from the PIV data. Curle’s analogy is applied to obtain the acoustic
pressure field. The pressure measurements on the wall of the cavity and the sound measurements are then used to validate the
results obtained from PIV and check the range of validity of this approach. This study demonstrated that the technique is
able to quantify the acoustic emissions from the cavity and is promising especially for capturing the tonal components on
the sound emission. 相似文献
4.
The present paper describes a method to derive information about the acoustic emission of a flow using particle image velocimetry
(PIV) data. The advantage of the method is that it allows studying sound sources, the related flow phenomena and their acoustic
radiation into the far field, simultaneously. In a first step the time history of two-dimensional instantaneous pressure fields
is derived from planar PIV data. In a successive step the Curle’s acoustic analogy is applied to the pressure data to obtain
the acoustic radiation of the flow. The test cases studied here are two rectangular cavity flows at very low Mach number with
different aspect ratios L/ H. The main sound source is located at the cavity trailing edge and it is due to the impingement of vortices shed in the shear
layer. It is shown that the flow emits sound with a main directivity in the upstream direction for the smaller aspect ratio
and the directivity is more uniform for the larger aspect ratio. In the latter case the acoustic pressure spectra has a broader
character due to the impact of the downstream recirculation zone onto the shear layer instabilities, destroying their regular
pattern and alternating the main sound source. 相似文献
5.
This paper describes the three-dimensional flow structure in grooved channels with different cavity lengths at intermediate
Reynolds numbers. For steady flow, the three-dimensional effects are dominant near the side walls of the channel. However,
after the onset of self-sustained oscillatory flow due to Tollmien–Schlichting waves as the primary instability, a secondary
instability produces a three-dimensional flow with Taylor–Geortler-like vortical structure, at the bottom of the groove. This
trend becomes more significant as the cavity length increases. Furthermore, the reason for three-dimensional flow is discussed
using additional numerical analysis, and it is confirmed that the source of three-dimensional instability is the groove vortices
due to the presence of side walls, rather than the channel traveling wave.
Received: 7 September 1999/Accepted: 11 November 2000 相似文献
6.
This study investigates the noise radiated by a subsonic circular jet with a Mach number of 0.9 and a Reynolds number of 65000
computed by a compressible Large Eddy Simulation (LES). First, it demonstrates the feasibility of using LES to predict accurately
both the flow field and the sound radiation on a domain including the acoustic field. Mean flow parameters, turbulence intensities,
velocity spectra and integral length scales are in very good agreement with experimental data. The noise generated by the
jet, provided directly by the simulation, is also consistent with measurements in terms of sound pressure spectra, levels
and directivity. The apparent location of the sound sources is at the end of the potential core in accordance with some experimental
observations at similar Reynolds numbers and Mach numbers. Second, the noise generation mechanisms are discussed in an attempt
to connect the flow field with the acoustic field. This study shows that for the simulated moderate Reynolds number jet, the
predominant sound radiation in the downstream direction is associated with the breakdown of the shear layers in the central
jet zone.
Received 24 January 2002 and accepted 16 July 2002 Published online 3 December 2002
RID="*"
ID="*" A preliminary version of some of the results presented here was reported in AIAA Paper 2000–2009 presented at the 6th
AIAA/CEAS Aeroacoustics Conference in Lahaina, Hawaii, June 2000. Computing time was supplied by the Institut du Développement
et des Ressources en Informatique Scientifique (IDRIS – CNRS).
Communicated by T.B. Gatski 相似文献
7.
The process of vortex formation in a cavity with inclined walls, which has a moderate aspect ratio, is experimentally studied,
and the distribution of pressure coefficients is measured. The angle of inclination of the side walls ϕ is varied from 30
to 90°. It is found that the flow in the cavity becomes unstable in the range of inclination angles ϕ = 60–70°. Flow reconstruction
occurs, which substantially alters the surface-temperature and static-pressure distributions. Large changes in these characteristics
and their nonuniform distributions for these angles are observed across the cavity on its frontal wall and on the bottom.
For small angles (ϕ = 30 and 45°), the pressure on the rear wall drastically increases, which leads to a small increase in
pressure averaged over the entire cavity surface.
__________
Translated from PrikladnayaMekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 5, pp. 68–76, September–October, 2006. 相似文献
8.
The interaction of turbulent flows with the external structure of ground vehicles generates uncomfortable noise and a lot of attention is devoted to find new mechanisms for its suppression. The present work is concerned with open cavity flows, very often found in the automotive industry. A three-dimensional rectangular very wide open cavity with aspect ratio \(L/D=4\) at Reynolds number \(Re_D=5000\) and Mach number \(M=0.1\) is considered. The passive control technique is based on eight different geometrical modifications: the length of the cavity, the radius of the trailing, leading and bottom edges and the difference in heights between the left and right wall of the cavity. Wall-resolved Large Eddy Simulations (LES) are used to obtain the flow fields and a post-process based on Curle’s analogy is applied to evaluate the acoustic radiation and the effectiveness of the control mechanisms. The results show that the modifications on the trailing edge are the most effective to control the flow. They allow to reduce the pressure fluctuations produced by the recirculation confined inside the cavity and the abrupt ejection of the flow at the trailing edge. As a consequence, the overall sound pressure level can be decreased up to 9 dB. 相似文献
9.
A "swallowtail" cavity for the supersonic combustor was proposed to serve as an efficient flame holder for scramjets by enhancing the mass exchange between the cavity and the main flow. A numerical study on the "swallow- tail" cavity was conducted by solving the three-dimensional Reynolds-averaged Navier-Stokes equations implemented with a k-e turbulence model in a multi-block mesh. Turbu- lence model and numerical algorithms were validated first, and then test cases were calculated to investigate into the mechanism of cavity flows. Numerical results demonstrated that the certain mass in the supersonic main flow was sucked into the cavity and moved spirally toward the combustor walls. After that, the flow went out of the cavity at its lateral end, and finally was efficiently mixed with the main flow. The comparison between the "swallowtail" cavity and the conventional one showed that the mass exchanged between the cavity and the main flow was enhanced by the lateral flow that was induced due to the pressure gradient inside the cavity and was driven by the three-dimensional vortex ring generated from the "swallowtail" cavity structure. 相似文献
10.
The ability of Lighthill's analogy to predict the sound radiated by a transitional mixing layer is evaluated by means of direct
numerical simulation (DNS). The specific case of low Mach number flows with density variations is investigated. In order to
limit the global computational cost, the acoustic source information is based on numerical results where the sound waves have
been removed. It is shown that the low Mach number approximation coupled with the acoustic analogy can lead to very accurate
predictions for the radiated sound if the acoustic sources in Lighthill's equation are taken into account carefully. Results
for the acoustic intensity deduced from a repeated use of the Lighthill's analogy over a wide range of Mach numbers allow
us to discuss the adequacy of scaling laws proposed by previous authors (J. Sound Vib. 28(3), 563–585, 1973; 31(4), 391–397, 1973; 48(1), 95–111, 1976) for the prediction of noise from hot jets. 相似文献
11.
The flow and acoustic fields of subsonic turbulent hot jets exhausting from three divergent nozzles at a Mach number M=0.12 based on the nozzle exit velocity are conducted using a hybrid CFD-CAA method. The flow field is computed by highly resolved large-eddy simulations (LES) and the acoustic field is computed by solving the acoustic perturbation equations (APE) whose acoustic source terms are determined by the LES. The LES of the computational domain includes the interior of the nozzle geometry. Synthetic turbulence is prescribed at the inlet of the nozzle to mimic the exit conditions downstream of the last turbine stage. The LES is based on hierarchically refined Cartesian meshes, where the nozzle wall boundaries are resolved by a conservative cut-cell method. The APE solution is determined on a block structured mesh. Three nozzle geometries of increasing complexity are considered, i.e., the flow and acoustic fields of a clean geometry without any built-in components, a nozzle with a centerbody, and a nozzle with a centerbody plus struts are computed. Spectral distributions of the LES based turbulent fluctuated quantities inside the nozzle and further downstream are analyzed in detail. The noise sources in the near field are noticeably influenced by the nozzle built-in components. The centerbody nozzle increases the overall sound pressure level (OASPL) in the near field with respect to the clean nozzle and the centerbody-plus-strut nozzle reduces it compared to the centerbody nozzle due to the increased turbulent mixing. The centerbody perturbed nozzle configurations generate a remarkable spectral peak at S t=0.56 which also occurs in the APE findings in the near field region. This tone is generated by large scale vortical structures shed from the centerbody. The analysis of the individual noise sources shows that the entropy term possesses the highest acoustic contribution in the sideline direction whereas the vortex sound source dominates the downstream acoustics. 相似文献
12.
The Lighthill acoustic analogy combined with Reynolds-averaged Navier–Stokes flow computations are used to investigate the
ability of existing technology to predict the tonal noise generated by vortex shedding from a circular cylinder for a range
of Reynolds numbers (100 < Re < 5 million). Computed mean drag, mean coefficient of pressure, Strouhal number, and fluctuating lift are compared with experiment.
Two-dimensional calculations produce a Reynolds number trend similar to experiment but incorrectly predict many of the flow
quantities. Different turbulence models give inconsistent results in the critical Reynolds number range ( Re≈ 100000). The computed flow field is used as input for noise prediction. Two-dimensional inputs overpredict both noise amplitude
and frequency; however, if an appropriate correlation length is used, predicted noise amplitudes agree with experiment. Noise
levels and frequency content agree much better with experiment when three-dimensional flow computations are used as input
data.
Received 5 May 1998 and accepted 28 September 1998 相似文献
13.
The current study is focused on examining the effect of the cavity width and side walls on the self-sustained oscillation
in a low Mach number cavity flow with a turbulent boundary layer at separation. An axisymmetric cavity geometry is employed
in order to provide a reference condition that is free from any side-wall influence, which is not possible to obtain with
a rectangular cavity. The cavity could then be partially filled to form finite-width geometry. The unsteady surface pressure
is measured using microphone arrays that are deployed on the cavity floor along the streamwise direction and on the downstream
wall along the azimuthal direction. In addition, velocity measurements using two-component Laser Doppler Anemometer are performed
simultaneously with the array measurements in different azimuthal planes. The compiled data sets are used to investigate the
evolution of the coherent structures generating the pressure oscillation in the cavity using linear stochastic estimation
of the velocity field based on the wall-pressure signature on the cavity end wall. The results lead to the discovery of pronounced
harmonic pressure oscillations near the cavity’s side walls. These oscillations, which are absent in the axisymmetric cavity,
are linked to the establishment of a secondary mean streamwise circulating flow pattern near the side walls and the interaction
of this secondary flow with the shear layer above the cavity. 相似文献
14.
The initiation of narrow-band pressure fluctuations in the duct of a pipeline gas-compression station when the flow is turned
near a blind-ended cavity was studied in a wind tunnel. The flow pattern under consideration was estimated from visualization
of the flow in a water channel using the hydraulic analogy technique. It is believed that the high-intensity narrow-band pressure
fluctuations observed in the flow result from the instability and regular rearrangement of large-scale vortex structures in
the blind-ended cavity.
Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 2, pp. 104–111, March–April, 1998. 相似文献
15.
Convective heat transfer in a transverse cavity with a small aspect ratio, angle of wall inclination ϕ = 30–90°, and heated
bottom, frontal, and rear walls of the cavity is studied experimentally. Temperature distributions are measured in longitudinal
and transverse sections on three walls; temperature fields are measured over the entire heated surface. Local and mean heat-transfer
coefficients are calculated. The highest intensification of heat transfer is found to occur on the rear wall for low values
of ϕ Reconstruction of the one-cell structure to the two-cell structure of the primary vortex in the cavity leads to a drastic
decrease in heat transfer over the cavity span from the end faces toward the center in the case with ϕ = 60 and 70°. A certain
increase in the mean heat-transfer coefficient averaged over the entire heated surface is noted for ϕ = 60°.
__________
Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 4, pp. 23–29, July–August, 2007. 相似文献
16.
In this paper we will use Large Eddy Simulation (LES) to obtain the flow field of a turbulent round jet at a Reynolds number based on the jet orifice velocity of 11000. In the simulations it is assumed that the flow field is incompressible. The acoustic field of the jet is calculated with help of the Lighthill acoustic analogy. The coupling between the flow solver and the acoustic solver is discussed in detail. The Mach number used in the acoustic calculation was equal to 0.6. It is shown that the decay of the jet centerline velocity and centerline rms are in good agreement with experimental data of [12]. Furthermore, it is shown that the influence of the LES modeling on the acoustic field is very small, if the dynamic subgrid model is used. 相似文献
17.
The flows past a circular cylinder at Reynolds number 3900 are simulated using large-eddy simulation(LES) and the far-field sound is calculated from the LES results. A low dissipation energy-conserving finite volume scheme is used to discretize the incompressible Navier–Stokes equations. The dynamic global coefficient version of the Vreman's subgrid scale(SGS) model is used to compute the sub-grid stresses. Curle's integral of Lighthill's acoustic analogy is used to extract the sound radiated from the cylinder. The profiles of mean velocity and turbulent fluctuations obtained are consistent with the previous experimental and computational results. The sound radiation at far field exhibits the characteristic of a dipole and directivity. The sound spectra display the-5/3 power law. It is shown that Vreman's SGS model in company with dynamic procedure is suitable for LES of turbulence generated noise. 相似文献
18.
We present a finite element (FE) formulation of Lighthill's acoustic analogy for the hybrid computation of noise generated by turbulent flows. In the present approach, the flow field is computed using large eddy simulation and scale adaptive simulation turbulence models. The acoustic propagation is obtained by solving the variational formulation of Lighthill's acoustic analogy with the FE method. In order to preserve the acoustic energy, we compute the inhomogeneous part of Lighthill's wave equation by applying the FE formulation on the fine flow grid. The resulting acoustic nodal loads are then conservatively interpolated to the coarser acoustic grid. Subsequently, the radiated acoustic field can be solved in both time and frequency domains. In the latter case, an enhanced perfectly matched layer technique is employed, allowing one to truncate the computational domain in the acoustic near field, without compromising the numerical solution. Our hybrid approach is validated by comparing the numerical results of the acoustic field induced by a corotating vortex pair with the corresponding analytical solution. To demonstrate the applicability of our scheme, we present full 3D numerical results for the computed acoustic field generated by the turbulent flow around square cylinder geometries. The sound pressure levels obtained compare well with measured values. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
19.
The effects of material, geometry, length and position of the porous channels on energy transfer in air-filled enclosures
carrying a compressible pulsating wave are investigated. The pulsating fluid motion is created by an acoustic driver in a
resonant chamber. Three different porous materials (Corning Celcor, Reticulated Vitreous Carbon (RVC), and Mylar plastic),
three different geometries (square, open foam, and circular cross-section), six different lengths, “ L” (varying between 1 and 6.5 cm, L = 0.01–0.068 λ, where λ is the wavelength of the fundamental acoustic mode), and eight different positions (hot end of the
channel, varying between 0.5 and 8 cm) of the channels from the pressure anti-node is experimentally measured. The surface
temperature distribution on the channel wall and temperature difference generated across the channel walls are measured while
energy flow along the channel walls is calculated analytically. The experimental results are compared with a 1-D numerical
code and found excellent agreement. The material, geometry, length, and position of the porous channel strongly affect the
energy interactions between the porous channel and the working fluid. The temperature difference generated across the porous
RVC channel increases as the porosity increases form 20 to 80 PPI; but decreases if the porosity increases further. Corning
Celcor shows improved temperature difference generated across the channel as the length of the channel increases; but then
decreases if the length is further increased. The results of this study are applicable to the design of thermoacoustic devices. 相似文献
20.
In this paper we study the interaction of a shock wave with a cylindrical vortex. The objective of the study is to characterize
the shock and vortex deformations and the mechanism of sound generation. The approach relies on the solution of the two-dimensional
Euler equations by means of a high order finite volume weighted-ENO scheme. In order to provide some guidance into the analysis
we have also developed an acoustic analogy of the problem by formulating a wave equation for the pressure disturbance that
is solved analytically by means of Green's functions. A systematic study has been conducted by investigating the effects of
vortex intensity and shock strength. Specifically, we have determined the dependence of shock distortion and vortex compression,
and its subsequent nutation upon shock and vortex strengths. The acoustic field generated through shock--vortex interactions
has been found to evolve in three stages and to exhibit a three sound quadrupolar directivity. In the early stages of the
interaction the sound generated due to shock distortion shows a dipolar character, which then changes to a quadrupolar one
due to a restoring mechanism that acts during the interaction of the shock with the rear part of the vortex. In the third
stage secondary sounds are formed, which also show a quadrupolar directivity. The acoustic analogy applied to weak shock--vortex
interactions also confirms the numerically predicted sound generation mechanism.
Received 12 July 1999 and accepted 18 November 1999 相似文献
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