实验流体力学

Contents

2024, 38(1)

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2024, 38(1): 1-1. doi: 10.11729/syltlx2024010

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A brief review on the numerical studies of the fundamental problems for the shock associated noise of the supersonic jets

ZHANG Shuhai, WU Conghai, LUO Yong, HAN Shuaibin, ZHANG Junlong

2024, 38(1): 1-27. doi: 10.11729/syltlx20230075

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The flow of the supersonic jets contains shock waves, vortices, turbulence and acoustic waves. The numerical simulation method and the mechanism of the shock associated noise of the supersonic jets have been topics of general interest. This paper contains two parts. Firstly, we briefly review the numerical studies on the fundamental problems of the shock associated noise of the supersonic jets. It includes the numerical methods for the shock associated noise, and the models of the supersonic jets, the axisymmetric and three dimensional supersonic jets. For the numerical method, we introduce the technique to reduce the non-physical oscillation and a criterion to design the smoothness indicator for high order shock capturing schemes. Secondly, we introduce our recent results based on the direct numerical simulations and experi-mental verification, including the localization of the axisymmetric modes, the development of trapped waves and the evolution of the flapping modes.

Noise control of serrated trailing edge airfoil under small incidence angle

HU Yasen, ZHANG Pengjunyi, ZHUANG Guohui, WAN Zhenhua, SUN Dejun

2024, 38(1): 28-36. doi: 10.11729/syltlx20230031

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Inspired by the silent flight capability of owls, the serrated trailing edge design is considered as an effective method to reduce the turbulent boundary layer-trailing edge interference noise. In this study, the near-field flow and noise characteristics of a NACA 0012 airfoil with additional serrated trailing edges are investigated in detail using an implicit large eddy simulation approach with Reynolds number Re = $9.6 \times {10^4}$, far-field Mach number Ma = 0.1631, and angle of attack $\alpha = {4^ \circ }$. The simulation adopts unstructured grids with 70 million degrees of freedom. In this particular calculation, a small sawtooth-shaped rough strip is added to the airfoil surface to facilitate the fast transition to turbulence for both straight and serrated trailing edge cases. At an angle of attack of 4°, an increase in noise radiation is observed with respect to that at an angle of attack of 0°, with a deflection of the primary radiation direction and a noise reduction of about 2.5 dB in this direction. The flow analysis shows that the sawtooth induces the regularly distributed vortex pair structures at its sides, which facilitates noise reduction in the far-field. The analysis of the wall pressure fluctuation shows that the sawtooth mainly changes the space-time correlation properties near the trailing edges, and the space-time correlation properties of the pressure cannot be described by the existing velocity-based Taylor or elliptical correlation models. In addition, the sawtooth suppresses the noise radiation while causing some loss to the aerodynamic performance of the airfoil.

Excitation and global calibration of duct modes with high orders and complex mixtures inside a cylindrical duct with flow

GAO Kang, KUAI Haoyu, HUANG Shichun, YU Liang, JIANG Weikang

2024, 38(1): 37-45. doi: 10.11729/syltlx20230057

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The spinning mode synthesizer is of great significance to the study of the propagation and radiation of aerodynamic noise in the cylindrical duct with flow as well as the evaluation of noise reduction with sound liners, working by controlling the array of loudspeakers and generating the specific distribution of the acoustical mode inside the duct. High-order circumferential modes, radial modes and their mixtures should be excited accurately with the manipulation of the multiple-rings array of loudspeakers. However, due to the systematic error of the loudspeaker, significant interference modes are generated simultaneously with the target mode, remarkably affecting the accuracy of mode excitation. A mode excitation method based on the least square and global calibration is proposed to motivate the target duct mode inside a flow duct and eliminate the influence of loudspeakers' system error on mode synthesizing. Through the global modeling of the system error of the loudspeaker sound source, the global calibration factor and flow field correction factor are introduced to convert the problem of solving the system error of each loudspeaker in the case of the flow field into the mode identification problem of a single loudspeaker in the case of no flow. The complex calibration factor of each loudspeaker is solved by matrix transformation and the least square method, after which the amplitude and phase excitation correction of the loudspeaker is realized. The presented method is applied to the spinning mode synthesizer in SJTU, whose experimental results indicate that the intensity of the interference modes is significantly suppressed, and the modal coefficient signal-to-noise ratio (SNRA) of the target excitation modes is greater than 15 dB within the operating frequency range.

A quadrupole correction model to suppress spurious sound with moving permeable integral surfaces

ZHOU Zhiteng, WANG Shizhao

2024, 38(1): 46-56. doi: 10.11729/syltlx20230072

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Ffowcs Williams–Hawkings (FW–H) equation is the extension of the Lighthill’s acoustic analogy equation for sound prediction with moving boundaries. However, the spurious sound often arises from vortex structures crossing through permeable FW–H surfaces. This work aims to approximate the contribution of the vortex structures to far-field sound using the Lighthill stress tensor flux and subtract the resulting spurious sound. Based on the frequency-domain Lighthill stress tensor quadrupole correction model, a quadrupole correction model is proposed to account for the effect of a moving integral surface on the spurious sound. Based on the frozen flow assumption and far-field approximation of the FW–H equation’s Green’s function, the proposed model incorporates the FW–H surface’s velocity into the integrand of the quadru-pole correction model by solving an algebraic equation of the quadrupole volume integral term. The proposed model is validated by the far-field sound prediction of flows over a circular cylinder and a convecting vortex.

Investigation of acoustic liner vibroacoustic response and its influence on impedance

QIU Shihao, CHEN Chao, LI Xiaodong

2024, 38(1): 57-66. doi: 10.11729/syltlx20230078

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The acoustic liners produce vibroacoustic response under the excitation of sound waves at high sound pressure level, and the rigid structure assumption is no longer applied. Their structural vibrations have a certain impact on sound absorption performance. The work presented here is an experimental study on the influence of panel vibrations on sound absorption and vibroacoustic response, and the influence law of vibroacoustic response on acoustic impedance under different perforated plate geometric parameters is obtained through parametric research. The experimental results show that the vibration of the perforated plate causes resistance to the generation of peaks or dips at the structural resonance frequency, and the sound absorption coefficient generates extra absorption peaks or dips that cannot be understood assuming rigid acoustic liners. The increase of the perforation rate and sound pressure level suppresses the influence of vibration, and there is a critical perforation rate. Perforated plate parameters affect the characteristics of resistance changes caused by structural vibration at high sound pressure levels. The phase difference between the small holes and the panel near the structure resonance frequency changes abruptly, resulting in an increase in the relative velocity and a change in the sound absorption performance.

Experimental study on the directivity and noise reduction of the blade leading-edge noise using Inverse Method SODIX based on microphone array

LIAN Jianxin, CHEN Weijie, QIAO Weiyang, DU Jun, LIU Yuanshi, LIU Bin

2024, 38(1): 67-78. doi: 10.11729/syltlx20230020

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Taking the NACA65(12)–10 blade as the object, a linear microphone array based on the SODIX (SOurce DIrectivity modeling in the cross-spectral matriX) method is used to study the leading-edge (LE) noise directivity of the baseline and the effect of the wavy LE on the LE noise directivity. First, a SODIX data processing program was developed, and the program was validated by numerical simulation. The validation results show that the data processing program has a good accuracy with an error less than 0.26 dB. Then, a linear array with 31 microphones is designed to identify the LE noise directivity of the baseline and the wavy LE blade experimentally in a semi-anechoic chamber. Within the measured degree range of 40°–142°, the directivity of LE noise shows a characteristic of typical dipole sound sources with a peak occurs at 130°. Besides, the higher the frequency is, the more obvious of the ‘lobe’ distribution of the LE noise directivity is. Further analysis shows that the wavy LE with various amplitudes and wavelengths especially with larger amplitudes can reduce LE noise in measured angle ranges especially among 90°–120°. And the maximum value is 7.71 dB for A30W20.

Analysis of aerodynamic noise mechanism and influencing factors at the skirt with grille under the vehicle at 400 km/h

ZHANG Zongfa, XIAO Xinbiao, HAN Jian, YANG Yi

2024, 38(1): 79-90. doi: 10.11729/syltlx20230065

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The grille located in the lower part of the train body is usually easy to form a grille-cavity structure with the equipment bay’s surface. The problem of flow-acoustic coupling resonance of this structure is more prominent when the train runs at high speed. It is necessary to further analyze the flow-acoustic coupling mechanism of the structure. Therefore, the skirt plate with the grille, which is located in the lower part of the train body and can be simplified to a grilling-cavity structure, is taken as an example. And the Delayed Detached Eddy Simulation (DDES) is used to analyze the grille-cavity structure’s aerodynamic noise generation mechanism, flow field, and sound field. The results show that the shear oscillation at the opening of the grille-cavity structure is more intense when the train is running at 400 km/h, especially near the impact edge of the cavity. From the spatial and frequency domain distribution of the global sound pressure level and the wave number spectrum of the turbulent pressure, it is found that the flow field of the square grille-cavity is always in a transition state of self-excited oscillation and the amplitude of oscillation in the global sound pressure level and wave number domain at different positions is always lower than that of the V-shaped and semi-circular grille-cavity. Considering the effect of the air outlet on the semi-circular grille cavity currently used, it is observed that the evolution of vortex clusters inside the cavity slows down significantly, which directly causes the global sound pressure level near the grille to drop by about 15 dB. It can be considered that the effect of air outlet has a significant effect on the reduction of near-field noise of the skirt plate with the grille.

Singularity distribution entropy analysis of impulsive acoustic signals

WANG Lu, CHEN Zhifei, CHEN Xi, ZHAO Qijun, BAO Ming

2024, 38(1): 91-102. doi: 10.11729/syltlx20230037

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In order to effectively analyze and calibrate the singularity difference of impulsive acoustic signals in complex environment with low signal-to-noise ratio, a singularity distribution entropy features analysis model based on the mode maximum theory is proposed. Firstly, the impulsive signal is normalized and wavelet transform is carried out to calculate the mode maximum and its specific distribution at each scale, which can reflect the family of mode maximum curves with singular differences. In order to describe the difference quantitatively, entropy is used to describe the distribution of the maximum points which constitute the family of modal maximum curves, and a singular distribution entropy feature model which can effectively analyze the singularity difference of impulsive signals is constructed. The model can describe the singularity difference of signals at low signal-to-noise ratio. Experiments show that the accuracy of 89.25% and 87.63% of typical helicopter impulsive signals (blade-vortex interaction signals and high-speed impulsive signals) can be obtained when the signal to noise ratio is −6dB.

Propagation characteristics of dynamic feature in transonic cavity shear layer

ZHOU Fangqi, WANG Xiansheng, YANG Dangguo, WU Jifei, YANG Ke, DONG Bin

2024, 38(1): 103-108. doi: 10.11729/syltlx20230066

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In the shear layer of the open cavity flow, the vortex interacts with the pre-transmission sound wave, causing self-sustaining oscillation. For a cavity model with a length-to-depth ratio of 7, the dynamic characteristics of the shear layer in the cavity were tested under the incoming flow condition of Mach number 0.9 by the pulsating pressure measurement technology, and the propagation law of the modal noise in the shear layer is revealed by the spectrum analysis and cross-correlation analysis. The results show that the superposition of the monotonically increasing broadband noise and cosine-like modal noise in the shear layer causes the wave-rise characteristics of the overall dynamic of the shear layer. The modal noise propagates in the reverse flow direction, its velocity is also cosine-like, and the change trend is consistent with the modal noise amplitude. Combined with the Rossiter mode estimation theory, it is revealed that the interaction between modal sound waves and vortices of the same frequency produces a standing wave-like phenomenon, resulting in periodic changes in the power spectrum density and propagation velocity of the modal noise along the flow direction.

Current Issue 2024, Volume 38, Issue 1

Current Issue
2024, Volume 38, Issue 1