关于气动声学数值计算的方法与进展

气动声学数值计算是近年才出现的研究领域。本文介绍了气动声学数值计算的方法和有关的问题、边界条件的处理以及计算非线性声波的数值方法和进展。讨论了计算气动声学(CAA)的特性及其与计算流体力学(CFD)的差异，指出气动声学数值方法的关键是建立能保持色散关系的差分方程和正确处理无反射边界条件。对于非线性声波传播的问题，为了得到正确的解，应注意提高差分格式对短波的分辨能力，同时发展能抑制“伪”振荡(短波)而对长波基本不起作用的数值方法。     

Time-domain Impedance Boundary Conditions for Computational Acoustics and Aeroacoustics

This paper reviews the short history, motivation, numerical and theoretical issues, and development of methods for treating a boundary as a reflective/absorptive surface for the time-domain computation of waves in general and acoustic waves in particular. It begins with the extension and implementation of the frequency-domain impedance to a time-domain impedance-equivalent boundary condition (TDIBC), and illustrates how the theoretical, numerical, and implementation issues are addressed and resolved for acoustic/aeroacoustic applications. Comments are also made on the extendibility and applicability of the concept and TDIBC to other fields and types of problems.     

空腔流激振荡发声的数值模拟研究

通过求解二维非定常雷诺平均Navier—Stokes方程,对亚音速情况下长宽比为L／D=2的空腔流动进行了计算气动声学数值模拟研究．湍流模型选取标准κ-ε模型,空间离散采用频散相关保持格式,时间积分采用低耗散低频散龙格库塔法,进出口远场边界采用以摄动解为基础的无反射边界条件,计算结果首先与Krishnamurty实验观测的密度场纹影图进行了对比,不论是在腔内流场分布还是腔外辐射指向性上,都与实验结果符合较好;然后与Rossiter公式进行了频率对比,其结果不但较为准确地捕捉到了振荡低阶模态的频率,而且也捕捉到了振荡高阶模态的频率特性,在此基础上研究了边界层厚度对振荡性质的影响,研究表明边界层厚度对振荡幅值影响较大,而且使振荡频率发生小幅偏移,最后探讨了振荡的发声机理,分析了振荡发声及声反馈的过程,发现了空腔前缘的二次发声现象。     

Development of numerical techniques for near‐field aeroacoustic computations

This work is concerned with the development of a numerical scheme capable of producing accurate simulations of sound propagation in the presence of a mean flow field. The method is based on the concept of variable decomposition, which leads to two separate sets of equations. These equations are the linearised Euler equations and the Reynolds‐averaged Navier–Stokes equations. This paper concentrates on the development of numerical schemes for the linearised Euler equations that leads to a computational aeroacoustics (CAA) code. The resulting CAA code is a non‐diffusive, time‐ and space‐staggered finite volume code for the acoustic perturbation, and it is validated against analytic results for pure 1D sound propagation and 2D benchmark problems involving sound scattering from a cylindrical obstacle. Predictions are also given for the case of prescribed source sound propagation in a laminar boundary layer as an illustration of the effects of mean convection. Copyright © 1999 John Wiley & Sons, Ltd.     

Non-homogeneous radiation and outflow boundary conditions simulating incoming acoustic and vorticity waves for exterior computational aeroacoustics problems

A set of non-homogeneous radiation and outflow boundary conditions which automatically generate prescribed incoming acoustic or vorticity waves and, at the same time, are almost transparent to outgoing sound waves produced internally in a finite computation domain is proposed. This type of boundary condition is needed for the numerical solution of many exterior aeroacoustics problems. In computational aeroacoustics, the computation scheme must be as non-dispersive and non-dissipative as possible. It must also support waves with wave speeds which are nearly the same as those of the original linearized Euler equations. To meet these requirements, a high-order/large-stencil scheme is often necessary. The proposed non-homogeneous radiation and outflow boundary conditions are designed primarily for use in conjunction with such high-order/large-stencil finite difference schemes. © 1998 John Wiley & Sons, Ltd.     

一类新的差分格式优化目标函数

对差分格式进行优化处理可以提高其谱精度。与高精度(指Taylor展开精度)格式相比,优化格式放大因子的误差随波数的变化不是单调的,而是必然会出现极值点,这样就存在临界距离R_cr,在此距离内优化格式描述的数值波的积累误差小于高精度格式,而超出此距离后优化格式的误差反而大,对于非定常流及气动声学计算来说,控制差分格式的临界距离是必要的。一般的优化目标函数以每个时间推进步的误差为基础(即放大因子法),R_cr不能在优化过程中确定。对此进行分析,指出积累误差的重要性并提出以此为基础的新的优化目标函数,这样在对格式进行优化时可以直接指定临界距离,从而为控制谱精度提供方便。     

An investigation into the noise generation of CPAP devices: Experimental and numerical study

Continuous positive airway pressure (CPAP) devices are popularly used for obstructive sleep apnoea (OSA) treatment. However, the noise level emission from these devices has been identified as a potential factor for patient’s discomfort and rejection. There is a need to obtain information on the noise characteristics and source locations in order to tackle the most serious noise source within these devices. A typical CPAP device was used for the investigation and its sound characteristics and sound power levels were determined. The noise generated from a centrifugal fan was also independently investigated to address its contribution to the overall noise of the device. Frequency analysis suggested that the noise generated from both the CPAP device and the fan is broadband in nature with discrete peaks containing rotational and non-rotational components. The broadband components were then studied in detail using numerical simulation approach. Computational aeroacoustics (CAA) method with hybrid approach was used to a three-dimensional (3-D) CPAP fluid model to predict the aerodynamic and aeroacoustics behaviours of the device. This showed a complicated flow structure involving flow separation, rotation, and vortices in several locations which resulted in high level of flow turbulence inside the device. The turbulent components were used to estimate the broadband noise level at source using the broadband noise source (BNS) models. It shows the most critical location is at the fan region and at the fan inlet.     

Mathematically consistent boundary conditions and turbulence matching at block interfaces for computational aeroacoustics

The method of characteristics is used to implement the various boundary conditions (e.g. wall and interface) in a high-order computational aeroacoustic (CAA) code developed by the first author. Most characteristic methods do not satisfy Pfaff's condition (which needs to be satisfied for any mathematical relation to be valid). A mathematically consistent and valid method is used in this work to derive the characteristic boundary conditions. Also, a robust and efficient approach for the matching of turbulence quantities at multi-block interfaces is proposed. Various numerical simulation cases were run to validate the concepts. The computed results show that the proposed method is accurate, robust and is in excellent agreement with experimental data. The results also indicate that the matching of turbulence quantities is essential for accurate turbulent flow calculations.     

带制退器的膛口射流噪声数值模拟与实验研究

为了研究膛口装置对膛口噪声气动特性的影响,对带膛口制退器的某小口径武器的膛口射流噪声进行了数值模拟和实验研究。采用计算流体力学CFD (computational fluid dynamics)-计算气动声学CAA (computational aeroacoustics)耦合算法对膛口噪声进行数值模拟,即对膛口流场进行瞬态CFD模拟,获取流场数据,然后利用所得到的结果采用声学方程模拟声源信息求解声场。基于数值模拟结果,分析了膛口流场变化及噪声的指向性分布,并与实验结果进行了对比。研究表明：膛口制退器的安装改变了膛口流场结构,影响了膛口射流噪声的指向性分布。计算结果与实验结果的误差小于9%,验证了该计算方法的可行性。研究结果可为膛口射流噪声的预测及膛口制退器的设计提供一定的参考。

     

射流对空腔噪声抑制效果研究

高速空腔流动的流场结构非常复杂,在一定条件下存在严重的压力、速度等脉动,诱发强烈的噪声,声压级可达到170dB,对腔内的导弹及其自身结构安全构成很大的威胁。应用基于两方程剪切应力(SST)的尺度自适应(SAS)分离流模型的CFD技术和气动声学频域理论(FW-H积分方程),模拟了射流对二维M219空腔(长深比L/D=5)内气动噪声变化情况,研究了空腔流动特性、流场结构及发声机理。在此基础上,首先对比分析了不同射流位置对空腔噪声的抑制情况,以此确定一个工程上可实现的射流位置;然后,对比分析了不同射流状态(不同的射流流量、温度)对空腔噪声的抑制情况。由此可知,跨音速(Ma=0.85)条件下,采用不同的射流状态对空腔噪声具有不同的抑制效果,其中随着射流量及射流温度的增大,噪声抑制效果更加明显。