贯流风扇气动声场数值模拟及分析

针对贯流风扇气动噪声传播特性,采用基于非结构网格的CE/SE算法对其气动流场进行数值模拟,并模拟了气动噪声的传播.湍流模拟采用大涡模型,搭接式滑移网格模型处理动静干涉.噪声传播采用完全欧拉方程作为控制方程,远场采用无反射边界条件.将使用该方法得到的远场噪声频谱与实验数据进行对比分析,结果表明与实验测量得到宽带频谱不同,采用大涡模拟湍流模型进行声源模拟时,会加强部分离散频率上的声级,从而产生误差.     

波浪前缘降低后掠叶片前缘噪声数值模拟研究

采用大涡模拟(LES)结合Ffowcs Williams-Hawkings(FW-H)方程研究了波浪前缘对后掠叶片前缘宽频噪声的降噪效果。通过放置于叶片上游的圆柱产生各向异性的湍流,湍流与后掠叶片相互干涉产生宽频噪声。气流流速设置为40m/s,基于叶片弦长和基于圆柱直径的雷诺数约为400000和26000。在该研究中,研究对象为直前缘后掠叶片和波浪前缘后掠叶片。结果表明:当来流为各向异性的湍流时,波浪前缘可以降低叶片干涉噪声,并且在各个远场方位角下均能降低噪声总声压级而不改变其指向性,添加波浪前缘叶片降噪量约为2.3~3.4 dB。通过分析计算流场发现,波浪前缘可以改变后掠叶片前缘周围气流的流动方式,降低叶片前缘周围的压力脉动和叶片的不稳定载荷。     

二维平行桨涡干扰气动噪声特性分析

深入理解桨-涡干扰脉冲噪声特性对其噪声的控制具有重要意义。采用气动噪声直接法对低马赫数条件下的二维平行桨-涡干扰气动噪声进行了数值计算,分析了噪声的产生机理和传播、衰减规律。结果表明:当旋涡接近和经过翼型前缘时,翼型前缘附近压强发生强度不同的两次突变,导致翼型气动力变化的同时,向外辐射产生具有偶极子指向性的脉冲声波,其中较弱的一次压强突变能更有效率地辐射声波;通过对4种不同来流速度的声场进行分析,发现上、下远场声压峰值和传播距离成反比,和来流速度的三次方与升力系数波动幅值之积成正比,由此得出了远场声压峰值计算公式,为桨-涡干扰远场声压的预测提供了另一种途径。      

贯流风机内动静干扰发声机理的研究

本文通过实验与数值计算方法,探讨了贯流风机内动静干扰的现象及其诱导的噪声特性.文中给出了贯流风机流场的三维大涡数值计算结果,分析了声源场的特性,指出贯流风机流场具有较强的随机性。分别对直叶轮及斜叶轮气动性能及远场噪声进行实验测量,讨论了两种叶轮的优劣性。使用微压传感器对蜗舌出口处边界层压力脉动进行采集,给出了此处边界层辐射噪声源的相关分析.通过对实验结果分析指出声源脉动的宽频特性是被测风机噪声呈现宽频的主要原因。     

涡发生器结构对翼型绕流场的影响

为了研究涡发生器在风力机叶片上的应用,以进一步提高风力机气动效率,本文采用CFD数值模拟方法,分析涡发生器几何形状对其绕流场和翼型边界层特性的影响.涡发生器几何形状为同样高度的矩形、梯形和三角形。翼型为风力机专用翼型DU97-W-300。首先对数值模拟结果与实验值进行了对比,验证了数值方法的可信性。然后详细讨论了各种涡发生器所产生的集中涡涡量、翼型边界层特性、以及绕流场等沿流向的发展演变。总体上看,三角形涡发生器较适合用于风力机翼型的流动控制。     

襟翼侧缘噪声的理论模拟与分析

襟翼侧缘噪声是飞机机体重要的噪声源,作者应用平面传声器阵列测量技术对民用客机进场着陆过程中的襟翼侧缘噪声进行了测量。基于实验测量结果,本文应用气动声学的基本理论,对飞机襟翼侧缘噪声进行理论的分析和研究,目的是深入的理解襟翼侧缘噪声源声辐射的机理、建立可靠的噪声预测模型,并期望获得可行的飞机机体噪声抑制方法。研究结果表明,同简单的旋涡振荡模型相比,Howe的襟翼侧缘噪声模型更接近于实际襟翼侧缘宽频噪声物理过程。     

叶顶端翼提升主动襟翼叶型气动性能的数值研究

本文在振荡来流条件下,数值模拟叶顶端翼对加装主动Gurney襟翼的垂直轴风力机叶片非定常气动特性的影响。采用NACA0015翼型的直叶片,并在尾缘前6%弦长位置安装主动襟翼。在最大出力工况(折合频率为0.1)下,对比原型叶片,加装主动襟翼叶片的切向力系数提高了4.47%,安装有叶顶端翼的主动襟翼叶片的切向力系数提高21.18%。通过比较叶片端部涡结构分布,发现叶顶端翼不仅阻止了叶片压力面及吸力面的叶梢涡分支在尾缘处汇合,同时也隔断了主动襟翼产生的角涡与叶梢涡的融合,有效的降低了叶片端部损失,提升了风力机的整体性能。     

斜蜗舌对贯流风机内流的影响及降噪机制研究

本文提出采用一种新型的不等距蜗舌(这里统称为斜蜗舌)在提高风机的外特性性能的同时,相对降低噪声.通过试验与数值模拟,比较直叶片式,斜叶片式贯流风机分别用直蜗舌和斜蜗舌时内流、外特性及噪声频谱.采用三维Navier-Stokes方程和κ-ε两方程湍流模型对内部流场进行数值分析,发现与采用常规蜗舌的贯流风机不同的是斜蜗舌贯流风机偏心涡位置随着蜗舌间隙的变化而发生改变.本文旨在探讨斜蜗舌对贯流风机内流的影响及降噪的机制.     

射流冲击圆锥噪声特性实验及数值分析

为研究低马赫数条件下射流冲击圆锥形障碍物的气动声学特性,针对4个冲击距离下的射流声场进行了远场实验测量,并基于大涡模拟进行了数值模拟计算,结果显示声场测量与数值结果吻合较好。从数值模拟捕捉到的冲击射流涡结构产生、演化以及耗散的变化现象,分析了冲击距离与远场声压之间的影响因素及涡变化与声源分布关系,发现影响冲击射流的主要噪声源与冲击距离具有很强的相关性。研究表明:障碍物位于10D_e位置处时所接收到的声压级最大,影响冲击射流的主要噪声源为旋涡脱落引起的具有宽频带特性的偶极子噪声源;当冲击距离较远时,主要声源为具有低频特性的自由射流段涡环配对产生的四极子源。     

轴流式涡流管内三维流场的大涡模拟

根据涡流管内可压缩气体的强旋转运动是导致涡流管能量分离的根本原因,提出了在涡流管内加入一个"X"型导流片迫使气流产生强旋转运动,使其进气方式变为轴流式。利用计算流体动力学(CFD)的方法,建立了轴流式和切流式涡流管内部气体流动的三维大涡数值模型,对其内部气流流场进行了数值模拟。数值模拟结果表明:"X"型导流片可以改变轴向进入气体的运动方向,使气体产生高速旋转运动,得出"X"型导流片的叶片夹角为θ=120°时,气体的旋转效果最好;大涡模拟可以较好地模拟涡流管内气流的三维流场。     

Investigation of airframe noise for a large-scale wing model with high-lift devices

The acoustic characteristics of a large-scale model of a wing with high-lift devices in the landing configuration have been studied in the DNW-NWB wind tunnel with an anechoic test section. For the first time in domestic practice, data on airframe noise at high Reynolds numbers (1.1–1.8 × 10⁶) have been obtained, which can be used for assessment of wing noise levels in aircraft certification tests. The scaling factor for recalculating the measurement results to natural conditions has been determined from the condition of collapsing the dimensionless noise spectra obtained at various flow velocities. The beamforming technique has been used to obtain localization of noise sources and provide their ranking with respect to intensity. For flap side-edge noise, which is an important noise component, a noise reduction method has been proposed. The efficiency of this method has been confirmed in DNW-NWB experiments.     

Flap-edge aeroacoustic measurements and predictions

An aeroacoustic model test has been conducted to investigate the mechanisms of sound generation on high-lift wing configurations. This paper presents an analysis of flap side-edge noise, which is often the most dominant source. A model of a main element wing section with a half-span flap was tested at low speeds of up to a Mach number of 0.17, corresponding to a wing chord Reynolds number of approximately 1.7 million. Results are presented for flat (or blunt), flanged, and round flap-edge geometries, with and without boundary-layer tripping, deployed at both moderate and high flap angles. The acoustic database is obtained from a small aperture directional array (SADA) of microphones, which was constructed to electronically steer to different regions of the model and to obtain farfield noise spectra and directivity from these regions. The basic flap-edge aerodynamics is established by static surface pressure data, as well as by computational fluid dynamics (CFD) calculations and simplified edge flow analyses. Distributions of unsteady pressure sensors over the flap allow the noise source regions to be defined and quantified via cross-spectral diagnostics using the SADA output. It is found that shear layer instability and related pressure scatter is the primary noise mechanism. For the flat edge flap, two noise prediction methods based on unsteady-surface-pressure measurements are evaluated and compared to measured noise. One is a new causality spectral approach developed here. The other is a new application of an edge-noise scatter prediction method. The good comparisons for both approaches suggest that the prediction models capture much of the physics. Areas of disagreement appear to reveal when the assumed edge noise mechanism does not fully define the noise production. For the different edge conditions, extensive spectra and directivity are presented. The complexity of the directivity results demonstrate the strong role of edge source geometry and frequency in the noise radiation. Significantly, for each edge configuration, the spectra for different flow speeds, flap angles, and surface roughness were successfully scaled by utilizing aerodynamic performance and boundary-layer scaling methods developed herein.     

Optimal design of acoustic performance for automotive air-cleaner

The research on optimal design of acoustic performance for air-cleaner was presented and evaluated in this paper using a numerical simulation method and experimental method. A modified measurement was proposed in the paper to solve the problem of air flow noise caused by the original measurement layout. The modified measurement was proved to produce good results. A new study on the acoustic effect of filter was also developed here and noise reduction (NR) of the air-cleaner was predicted based on the study using the numerical simulation method. Actual intake noise was then predicted using the simulation method. Side branch silencers were designed to improve the acoustic capacity of the air-cleaner. The result shows that the newly optimized air-cleaner has a much better performance of noise elimination when engine operating.     

Helicopter noise in hover: Computational modelling and experimental validation

The aeroacoustic characteristics of a helicopter rotor are calculated by a new method, to assess its applicability in assessing rotor performance in hovering. Direct solution of the Euler equations in a noninertial coordinate system is used to calculate the near-field flow around the spinning rotor. The far-field noise field is calculated by the Ffowcs Williams–Hawkings (FW–H) method using permeable control surfaces that include the blade. For a multiblade rotor, the signal obtained is duplicated and shifted in phase for each successive blade. By that means, the spectral characteristics of the far-field noise may be obtained. To determine the integral aerodynamic characteristics of the rotor, software is written to calculate the thrust and torque characteristics from the near-field flow solution. The results of numerical simulation are compared with experimental acoustic and aerodynamic data for a large-scale model of a helicopter main rotor in an open test facility. Two- and four-blade configurations of the rotor are considered, in different hover conditions. The proposed method satisfactorily predicts the aerodynamic characteristics of the blades in such conditions and gives good estimates for the first harmonics of the noise. That permits the practical use of the proposed method, not only for hovering but also for forward flight.     

Vorticity dynamics and sound generation in two-dimensional fluid flow

An approximate solution to the two-dimensional incompressible fluid equations is constructed by expanding the vorticity field in a series of derivatives of a Gaussian vortex. The expansion is used to analyze the motion of a corotating Gaussian vortex pair, and the spatial rotation frequency of the vortex pair is derived directly from the fluid vorticity equation. The resulting rotation frequency includes the effects of finite vortex core size and viscosity and reduces, in the appropriate limit, to the rotation frequency of the Kirchhoff point vortex theory. The expansion is then used in the low Mach number Lighthill equation to derive the far-field acoustic pressure generated by the Gaussian vortex pair. This pressure amplitude is compared with that of a previous fully numerical simulation in which the Reynolds number is large and the vortex core size is significant compared to the vortex separation. The present analytic result for the far-field acoustic pressure is shown to be substantially more accurate than previous theoretical predictions. The given example suggests that the vorticity expansion is a useful tool for the prediction of sound generated by a general distributed vorticity field.     

Computational bluff body aerodynamic noise prediction using a statistical approach

A new method for calculating the aerodynamic noise generated by bluff bodies is presented in this paper. The methodology uses two-dimensional, unsteady Reynolds averaged Navier Stokes turbulent flow simulations to calculate the acoustic source terms. To account for turbulent flow effects that are not resolved by the flow simulation, a statistical approach has been developed and applied to introduce narrow band random noise. Spanwise de-correlation of flow information is accounted for using a correction method based on a de-correlation length scale. Curle’s compact acoustic analogy is used to calculate the far-field noise. To illustrate the effectiveness of the method, the turbulent flow and noise about two test cases are calculated and compared with experimental results from the literature.     

Effect of blade wrap angle on efficiency and noise of small radial fan impellers—A computational and experimental study

Radial impellers have several technical applications. Regarding their aerodynamic performance, they are well optimized nowadays, but this is in general not true regarding acoustics. This work was therefore concerned with analyzing the flow structures inside isolated radial impellers together with the far-field sound radiated from them in order to optimize the aerodynamic and acoustic performance. Both numerical and experimental techniques were applied in order to study the effect of varying wrap angle and otherwise identical geometric configuration on aerodynamics and acoustics of the radial impellers. The results give a detailed insight into the processes leading to sound generation in radial impellers. Measurements were performed using laser Doppler anemometry for the flow field and microphone measurements to analyze the radiated noise. In addition, unsteady aerodynamic simulations were carried out to calculate the compressible flow field. An acoustic analogy was employed to compute far-field noise. Finally, the phenomena responsible for tonal noise and the role of the wrap angle could be identified. Using this knowledge, design guidelines are given to optimize the impeller with respect to the radiated noise. This work shows that improved aerodynamic efficiency for isolated impellers does not automatically lead to a smaller flow-induced sound radiation.     

一种基于声相似律的旋转流体机械噪声声源分离方法

针对管道系统中的流体机械发声问题,提出了一种声源函数与声响应函数的频谱分离方法。该方法以声相似律为基础,分别采用最小均方值多重线性回归方法和高斯滤波获得声源函数和声响应函数。通过算例验证了该方法的有效性,辨识得到的声源函数和声响应函数的相对误差均远小于现有方法。利用该方法分析了自由空间和带管道两种安装条件下轴流风扇的辐射噪声,辨识得到了相应的声源函数、声响应函数和叶尖速度律指数,并对风扇的主要噪声成分进行了分析。      

Energy flow model considering near field energy for predictions of acoustic energy in low damping medium

The Acoustic Energy Flow Boundary Element Method (AEFBEM) is developed to predict the acoustic energy density and intensity of an engineering system. Up to now, the acoustic energy flow model has been used only for analysis of high frequencies or radiation noise because of plane wave and far-field assumptions. In this research, a new energy flow governing equation that can consider the near field acoustic energy term and spherical wave characteristics is derived successfully to predict the acoustic energy density and intensity of a system in the medium-to-high frequency range. A near field term of acoustic energy in spherical coordinate is added to the relationship between energy density and energy flow. But with the far-field assumption, this term vanishes, so the relationship between energy density and energy flow becomes the same as that of the plane wave. By considering the near field energy term without far-field assumption, the energy density at medium frequencies can be estimated. However, the governing equation has to be numerically manipulated for use in the analysis of complex structures; therefore, the Boundary Element Method (BEM) is implemented. AEFBEM is a numerical analysis method formulated by applying the boundary element method to an acoustic energy flow governing equation. It is very powerful in predicting the acoustic energy density and intensity of complex structures in medium-to-high frequency ranges, and can analyze interior noise and radiating sound. To verify its validity, several numerical results are provided. BEM and AEFBEM were compared with respect to energy density, and the results from both methods were similar.     

A study on generation of noise by high-speed pulsating jets

This paper describes the noise generation in an exhaust system of a reciprocating engine and focuses on the noise generated by shock/vortex interaction. The pulsating flow through the exhaust pipe consists of the compression and expansion wave, shock wave being generated by the non-linearity of the compression wave at its head. The jet noise is produced when the pulsating flow is discharged from the pipe end into atmosphere. The numerical simulation based on a finite difference method and experiment were made, the result of both of them being compared. First, the flow field in the pipe was obtained to easily discuss the characteristic of the pulsating jet in terms of the pressure history in the pipe. The jet structure was visualized by using schlieren and shadowgraph techniques. Sound pressure measurements at various locations were made in order to survey the directivity of the noise. The comparison between the result of numerical calculation and experiment showed a good agreement. A noise source related to shock/vortex interaction was confirmed by the numerical study clearly.