Flap side-edge noise mechanism and shape modification noise reduction design

Flap side-edge noise is a significant noise source for airplane at takeoff and landing stages. The generation mechanism of flap side-edge noise is analyzed by numerical simulation on unsteady flow field using Very Large Eddy Simulation(VLES). Two kinds of flap side-edge shape modifications are proposed, and their frequency spectrum and directivity of far-field noise are compared with the baseline configuration using permeable integral surface Ffowcs Williams and Hawkings(FW-H) acoustic analogy method to investigate their effects on noise reduction. Via the numerical simulation of flow field and acoustic field, it proves that the flap side-edge noise is broadband noise in nature. The different shapes of flap side-edge change the pattern of flow field, vortex structures and the development of vortex, thus having influences on noise source distributions and characteristics of far-field noise. The result shows that at the given 5° angle of attack, the proposed flap side-edge shape modifications can reduce the overall sound pressure level(OASPL) by 1 to 2 dB without decreasing the lift and drag aerodynamic performances.     

Numerical prediction of envelope line-spectrum intensity in underwater acoustic waveguides

In order to predict the detectible range and region of passive sonar in underwater channel,the attenuation causes of envelope line-spectrum height during vessel noise propagation are analyzed and an approach of numerical prediction is proposed.In the paper a model for vessel radiated noise is established by a periodically locally stationary random process,two formulae of the envelope line-spectrum height with and without background-noise are deduced, therefore the attenuation rule of the envelope line-spectrum height is obtained.It is shown that the transmission loss of the sound level of the envelope line-spectrum is same as the sound level of the stationary spectrum,but the decrease of envelope line-spectrum height depends on a modified scale of amplitude modulation depth which is variable with the ratio of signal to noise. An approach of numerical prediction for envelope line-spectrum height is as follows:first,the transmission loss of the stationary radiated noise is derived using the numerical approaches of normal modes or wavenumber integration or PE etc.,then the ratio of signal to noise on sound field is calculated,finally the decrease of envelope line-spectrum height is obtained according to the modified scale,and the envelope line-spectrum height in sound field is predicted.The theory and the numerical prediction approach possess certain innovation,practicality,simplicity and suitability for engineering.     

Prediction of underwater target strength

1 IntroductionTarget Strength of underwater objects is an importallt parameter of the sonar equations.A method for the calculation of target strength using Lighthill's acoustic analogy approach ispresellted. Farassat's solution to the FW-H equation for the calculation of discrete noise ofsubsonic propeller in the free field is used. For a rigid bodys the suxface sound pressure inducedby incident sound wave could be obtained by united aerodynamics and aeroacoustics approach,which have been su…     

Active control of radiated sound using nearfield pressure sensing

Based on nearfield sound pressure sensing to pick up error information, an approach for constructing active acoustic structure to effectively reduce radiated sound power at low frequency is proposed. The idea is that a nearfield pressure after active control is used as error signals and transformed into an objective function in adaptive active control process.Firstly sound power expression using near-field pressure radiated from a flexible structure is derived, and then three kind of nearfield pressure based active control strategies,i.e.minimization of radiated sound power, minimization of sound power for dominant radiation modes and minimization of sound power for modified dominant radiation modes are respectively presented and applied to active control of radiated single and broadband noise. Finally computer simulations on sound power reduction under three strategies are conducted and it is shown that the proposed active control strategies are invalid and considerable reduction in radiated sound power can be achieved.     

Field acoustic measurements of high-speed train sound along BTIR

In this paper,single-point field measurements of noise radiated from high-speed trains were performed at two sites along Beijing-Tianjin intercity railway(BTIR),aiming at acquiring the realistic acoustic data for validation and verification of physical model and computational prediction.The measurements showed that A-weighted sound pressure levels(SPLs) were between 80 and 87 dBA as trains passed.The maximum noise occurred at the moment when the pantograph arrived,suggesting that pantograph noise was one of the most significant sources.Sound radiated from high-speed trains of BTIR was a typical broadband spectrum with most acoustic power restricted in the range of medium-high frequency from about 400 Hz to 5 kHz.Aerodynamic noise was shown to be the dominant one over other acoustic sources for high-speed trains.     

Recovery of the free field in noisy environment by using the spherical wave superposition method

To remove the scattering effect of the disturbing sound on the target source when implementing nearfield acoustic holography in a non-free field, a free field recovery technique based on the spherical wave superposition method is proposed. In the method, the sound field separation technique based on the spherical wave superposition method is first used to separate the incoming and outgoing fields, and a further step for separating the radiated and scattered fields is performed by utilizing the surface admittance of the target source as the boundary condition. The technique makes it possible to correctly identify noise sources in a non-free sound field. The basic principle of the technique is described firstly, a method for choosing the optimal number of spherical wave expansion terms is given, and two numerical simulations are used to demonstrate the validity of this technique. It is shown that, for the lower frequency, the scattering effect can be neglected, and the radiated field of the target source can be obtained by the sound field separation technique, however, as the increasing of the frequency, the scattering effect cannot be neglected, and the free field recovery technique has to be used to obtain the radiated field of the target source.     

The dynamics of low-frequency signal acoustic intensity vector vortex structure in shallow sea

Spatiotemporal vector and phase properties of interference field of low-frequency signalling tone between three local vortices in a real shallow sea wave-guide have been studied.It has been demonstrated that in the field of constructive interference,components of particle velocity field and acoustic pressure are coherent.As a consequence the transfer of signal energy alog the axis of a shallow sea wave-guide is accomplished with plane wave.Physical objects are detected in the field of destructive interference,which,according to known deterministic signs,can be defined as local vortices of the intensity vector.A large-scale vorticity with acoustic intensity vector curl,components different from zero originates in the vicinity of local vortices.Regular particle displacements of local vortices have been detected against combined receiving device phase centre along the axis of a wave-guide.It has been demonstrated that the structure of vortices depends on signal/noise ratio.Local vortices and vorticity form vortex structure of vector acoustic field.Signalling tone with frequency of 88 ± 1 Hz from near-surface moving sound source was taken into consideration.Introduced results of full-scale experiment expand our concepts of real fundamental properties of shallow sea acoustic field and are to be considered in theoretical models.     

Two ways of propeller recognition feature extraction

The analysis of the radiated noise of vessels given in this paper showssome strong superposed line components in low frequency speetrum below100Hz occurring at discrete frequencies which correspond with the rotationspeed of propeller shaft,or propeller blade frequency,or their harmonic fre-quencies.since the line components reflect propeller＇working characteristics,the propller＇s features can be extracted directly from low-frequency line com-ponents in addition to demodulated line component.So there are two ways toextract the features,one is direct way,the other is demodulation way.Detec-tion performance of the line component in background-noise is discussed inthis paper.The signal level is defined as the difrerence between the PDF＇s(Probability Density Function)mean of the peak of the line component andPDF＇s mean or the background-noise.In dircet way the signal level of the linecomponent is proportional to the signal noise ratio(S/N).In demodulationway the signal level of demodulated line component     

Patch nearfield acoustic holography combined with sound field separation technique applied to a non-free field

The conventional nearfield acoustic holography(NAH) is usually based on the assumption of free-field conditions, and it also requires that the measurement aperture should be larger than the actual source. This paper is to focus on the problem that neither of the above-mentioned requirements can be met, and to examine the feasibility of reconstructing the sound field radiated by partial source, based on double-layer pressure measurements made in a non-free field by using patch NAH combined with sound field separation technique. And also, the sensitivity of the reconstructed result to the measurement error is analyzed in detail. Two experiments involving two speakers in an exterior space and one speaker inside a car cabin are presented. The experimental results demonstrate that the patch NAH based on single-layer pressure measurement cannot obtain a satisfied result due to the influences of disturbing sources and reflections, while the patch NAH based on double-layer pressure measurements can successfully remove these influences and reconstruct the patch sound field effectively.