三维FW-H方程与CAA数值模拟匹配技术研究

本文研究了三维FW-H方程与CAA数值模拟匹配技术。首先验证了适于亚音任意运动声源的Farassat时域公式和适于亚音匀速直线运动声源的Lockard频域公式,采用均匀流中单极子源声辐射问题对两类公式进行了校核。进一步,采用FW-H／CAA匹配技术对风扇／压气机前传声进行了预测。近声场基于轴对称三维CAA方法获得,远声场则基于近声场数据采用三维可穿透FW-H方程时域公式进行预测,并校核算例着重分析了不同积分面对远场声指向性的影响。本文研究证实了FW-H／CAA数值模拟匹配技术的可行性和解决工程实际问题的潜力。     

圆柱及切割圆柱低Mach数绕流气动声辐射

以柱体绕流声辐射为研究对象,探讨低Mach数流动气动声定量预测方法。首先,理论分析Lighthill声类比理论与基于流体动力学压力对时间求导的扰动分离模型之间内在联系。其次,利用计算流体动力学对圆柱及其尾部切割后的亚临界流态绕流进行二维数值求解,分析两种湍流模型的流场计算效果。最后,分别使用声类比理论FW-H方程及扰动分离模型进行柱体绕流气动声辐射定量预测,分析声指向性及频谱特性。数值结果表明声类比理论与扰动分离模型预测的声辐射频谱相似,但前者的幅值略高;圆柱尾部切割使尾流脉动增强,导致声辐射变大。     

辐射扩散方程的自适应坐标变换方法

借鉴自适应坐标变换流体力学方法和其他一些有益工作，研究辐射扩散方程的自适应坐标变换方法，并考虑辐射扩散和流体力学耦合的辐射流体力学问题。对辐射扩散方程和一般二维对流扩散方程，从积分和微分形式出发，导出自适应坐标变换下的等价形式，对数值离散格式进行初步讨论。对二维三温辐射扩散方程，采用时空有限体积方法，进行数值离散；利用局部函数近似离散扩散算子。利用合理的进程分裂，简化耦合问题的计算。     

Amiet剪切层理论的角度折射验证研究

在开口射流风洞中进行气动噪声的定位测量试验时,射流边界的剪切层会对声传播产生折射影响。在处理过程中需要对传声器测量的信号进行剪切层修正,修正结果将直接影响到波束形成对气动噪声源定位的准确性。本文针对经典的Amiet剪切层修正理论,综合计算气动声学(Computational aeroacoustics,CAA)数值计算与试验测试,对该理论进行了验证,研究表明CAA计算与Amiet理论吻合,Amiet理论能够有效修正剪切层对声波的角度折射影响。     

基于非结构网格CE/SE算法圆柱绕流气动声场模拟

本文采用时空守恒(CE/SE)格式求解Navier-Stoks方程与Euler方程,数值模拟了绕圆柱流动引发的气动噪声源及传播问题.对近声场与远声场采用了不同的控制方程和边界条件,分别得到了流场与声场解.将其与采用大涡模拟和FW-H方程得到的结果进行了对比,表明本文的数值方法能够很好地反映流场与声场的形态,能较好地模拟声场指向性及流场中的拟声现象,相比LES/FW-H方法能更精确地反映流场与声场的相互关系.     

连续谱Faddeev积分方程的新解法

在动量空间中具有定域势的Faddeev方程是二维积分方程,在破裂过程和三体散射一类的连续谱情况下,方程的积分核是奇异的。本文根据奇异积分方程一般理论提出一种求解二维方程的数值方法。实践证明数值解是收敛的,全运动学微分截面的计算值与实验数据十分符合。     

子波变换在声辐射和声散射数值解中的应用

通过将边界变量用于波展开,获得了求解二维及三维轴对称声辐射和声散射的边界积分方程的子波谱方法,既可求解Ｄｉｒｉｃｈｌｅｔ、Ｎｅｕｍａｎｎ问题,也可求解混合过值问题;它能处理任意边界条件的油对称体．用三维元方法解决了三维轴对称问题边界于波谱方法的奇异积分。给出了二维问题奇异积分的近似积分公式。给出了子波谱方法的系数计算方法,它与传统的边界元系数计算方法相似,易于计算机程序实现,能处理复杂的边界几何形状。该方法的优点是可以获得稀疏的系数矩阵。算例表明：该方法收敛较快,精度高。     

数值模拟输运问题的一种破开算子方法

本文通过破开算子方法,把二维输运问题的控制方程破开为对流问题和扩散问题。在任意四边形网格的离散下,用特征线法解对流问题,并采用伽辽金加权余量法,从而有效地减少插值所引起的数值阻尼,提高计算精度。用有限单元法和迭代计算格式解扩散问题。由于采用了辛普生积分公式,在每个时间步长都不需要求逆矩律,节省了计算时间。算例表明,本文数值模拟结果与精确的理论解吻合较好。     

有厚度平板尾缘可压缩剪切层中的涡结构数值模拟

采用一个新型Fu Ma高精度UCD5 SCD6紧致差分算法,通过直接求解二维Navier Stokes方程,成功实现了有厚度平板尾缘可压缩剪切层中涡结构的数值模拟,并考查了平板厚度对其的影响.计算对流马赫数M_c=0.3,平板厚度分别为1,2,3,4个参考长度.结果表明,增加平板厚度可促使平板尾缘可压缩剪切层中的涡提前卷起,有利于两股气流混合.     

非均匀随机散射介质层的多次散射和热辐射的分层迭代解

为纳入矢量辐射传输方程的多次散射，将非均匀平行分层分布的随机散射介质划分成多个薄 层，利用各薄层的低阶Mueller矩阵解推导出整层的高阶散射迭代解.用这一方法计算了具有 占空比、粒子大小和温度廓线非均匀分布的平行分层随机密集球形粒子积雪层的极化热辐射 .数值结果与一层均匀随机散射层的离散坐标-特征值方法作了比较，讨论了多次散射和热辐 射与散射层各特征参数的关系，与积雪层微波遥感数据进行了对比. 关键词： VRT方程 平行分层 高阶散射解 迭代解     

空腔流动的大涡模拟及气动噪声控制

大涡模拟(LES)和三维的Ffowcs Williams-Hawkings声学比拟方法相结合,研究空腔过流的一种噪声控制措施.空腔的底板/后墙使用多孔壁板,因此流体可以穿透空腔壁面,多孔效果使用Darcy压力-速度关系模拟.声源流场由LES计算,声辐射和远声场由声学比拟获得.结果表明,这种措施有效地减弱了空腔内的压力脉动和远场声辐射,低频脉动Rossiter模数对应的波动幅值被有效抑制,声源中偶极子占优项大幅度减小,从而抑制了声辐射.     

偶极横波远探测典型地质体解释模型研究及应用

偶极横波远探测技术是近年来发展起来的一门重要的地球物理测井技术,它把常规测井技术的测量范围从井周1 m左右提高到几十米,在井旁裂缝和孔洞等地质体的探测中取得了较好的应用效果。本文根据偶极横波辐射声场和接收声场的互易性,提出了一种快速计算井旁地质体反射声场的解析方法——远场渐近解,实现井旁孔洞、裂缝等典型地质体反射声场的快速计算,分析了不同地质体的远探测成像特征,在此基础之上,与已有实际井资料结合建立了偶极横波远探测的解释模型,并将其应用于海上实际井旁地质体评价。应用结果表明,偶极横波远探测解释模型可用于海上缝洞型地层识别、井旁断层或断裂带判断等,效果明显。     

Suppression effect of jet flow on pulsating pressure of cavity using scale-adaptive simulation model

The suppression of the aerodynamic noise in the cavity has a great significance to solve relevant puzzles of weapon bays.Acoustic field of the standard cavity model is simulated by using the computational fluid dynamics technology based on scale-adaptive simulation(SAS)model.The results obtained by the proposed method in this paper show reasonable agreement with experiments.On the basis of this,effect of different jet flow rates on the time-averaged variables,turbulent kinetic energy,root mean square(RMS)of sound pressure,sound sources distribution and the pulsating pressure distribution in the cavity is studied.The analysis shows that the jet flow has great influence on the cavity flow field and the distribution of pulsating pressure RMS by changing the morphology of the shear layer.The most obvious of these measures is spout4 configuration,the influence mainly in the form of reducing the pulsating pressure of the whole cavity and changing the sound pressure level in the far field.The results show that different jet flow rates have different control effects on pulsating pressure in the cavity and sound pressure level in the far field.Furthermore,the jet flow rates and the suppression effect on the pulsating pressure have no linear relation.     

变声速弹性沉积层下压缩波与剪切波的耦合影响

针对压缩波声速n²线性分布、剪切波声速平方线性分布的变参数弹性海底沉积层中声场建模问题,建立了关于压缩波与剪切波本征函数的偏微分耦合方程组,基于微扰法,通过逐次微分给出了本征函数的近似解析解.理论分析表明,只有剪切波声速平方的梯度不为零的情况下,沉积层中的压缩波才会与剪切波产生耦合.压缩波与剪切波耦合的影响会带来本征值的变化,其对近场声传播的影响较小,但远距离声传播损失的预报在考虑耦合后与实测数据符合得更好.此外,耦合还会导致沉积层中本征函数及其导数的变化,使得耦合与非耦合情况下沉积层中质点位移场不同.通过与商用声场软件COMSOL计算结果的对比分析检验了本文方法的正确性,且本文方法计算时间远小于COMSOL软件运行时间.     

基于平面声源进行结构声辐射有源控制的实验研究

采用分布式平面声源作为次级声源,对振动钢板的声辐射进行了抵消实验,验证了以往研究中的一系列关键理论。实验研究结果表明:一个平面声源可以控制钢板奇-奇模态的声辐射,两个平面源可以控制结构偶-奇或奇-偶模态的声辐射,同时也可以控制结构奇-奇模态的声辐射;平面声源的面积和布放位置对降噪效果有重要影响,采用单个平面声源控制时,平面声源面积越大,控制效果越好;基于近场声压的误差传感策略是有效可行的,实际中,将近场测量面的声功率作为有源控制的目标函数与总声功率作为目标函数是一致的;控制后远场声压和声强都得到有效降低,部分区域的声能向声源流动,近场声压及声强分布也发生显著变化。     

Diffraction-biased shear wave fields generated with longitudinal magnetic resonance elastography drivers

Magnetic resonance elastography (MRE) is a technique for quantifying the acoustic response of biological tissues to propagating waves applied at low frequencies in order to evaluate mechanical properties. Application-specific MRE drivers are typically required to effectively deliver shear waves within the tissue of interest. Surface MRE drivers with transversely oriented vibrations have often been used to directly generate shear waves. These drivers may have disadvantages in certain applications, such as poor penetration depth and inflexible orientation. Therefore, surface MRE drivers with longitudinally oriented vibrations are used in some situations. The purpose of this work was to investigate and optimize a longitudinal driver system for MRE applications. A cone-like hemispherical distribution of shear waves being generated by these drivers and the wave propagation being governed by diffraction in the near field are shown. Using MRE visualization of the vector displacement field, we studied the properties of the shear wave field created by longitudinal MRE drivers of various sizes to identify optimum shear wave imaging planes. The results offer insights and improvements in both experimental design and imaging plane selection for 2-D MRE data acquisition.     

The acoustic emission of a distributed mode loudspeaker near a porous layer

Experimental and theoretical modeling of the vibro-acoustic performance of a distributed mode loudspeaker (DML) suggest that their acoustic emission can be significantly affected by the presence of a porous layer. The amplitude of the surface velocity of the panel and the acoustic pressure on the porous surface are reduced largely in the vicinity of structural resonances due to the additional radiation damping and visco-thermal absorption phenomenon in the porous layer. The experimental results suggest that a porous layer between a rigid base and a DML panel can considerably alter its acoustic emission in the near field and in the far field. This is illustrated by a reduction in the level of fluctuations in the emitted acoustic pressure spectra. These fluctuations are normally associated with the interference between the sound emitted by the front surface of the speaker and that emitted from the back. Another contribution comes from the pronounced structural resonances in the surface velocity spectrum. The results of this work suggest that the acoustic boundary conditions near a DML can be modified by the porous layer so that a desired acoustic output can be attained.     

Duct noise radiation through a jet flow

The radiation of internal (or core) noise for aircraft turbojet or turbofan engines is studied analytically. The geometry of a typical engine is simplified for analytical considerations to a hemispherical shell with a jet flow and internal sound emanating through a circular hole on the axis. A linearized theory is used to derive a flow modified spherical wave equation. A forced separation technique is used to produce a modified Legendre equation describing the angular variation of the acoustic radiation field. Then a numerical technique is described for obtaining a general field solution by progressively imposing continuity of pressure across hemispherical shells as the solution is marched from near field to the far field. In a companion paper, numerical results are presented and compared with experimental results from a test configuration identical to that described by the theory.