考虑界面状况时柱状弹性固体的声波散射

导出了求解母材料固体中镶嵌的柱状弹性固体(两固体间存在界面薄层)声波散射系数的一般表达式.根据Flax的共振散射理论,对背向散射谱中的共振模式进行了识别.利用模拟界面薄层的弹簧模型,考察了界面层切向劲度常数K_T对共振模式形态的影响 关键词： 声波散射 共振模式 界面     

固体中多孔媒质球的声共振散射特性

利用Flax等提出的声共振散射模型,以铁中多孔石墨为例数值上研究了平面声纵波入射时固体中多孔媒质球体背向散射纵波和散射横波的频谱以及不同阶分波的共振散射谱,并由此研究了铁中多孔球形石墨不同模式的共振频率随石墨孔隙率的变化。结果表明,球形石墨的背向散射谱及各阶分波中的共振频率均随孔隙率的增加向低频移动,其形态也发生变化。因此,如果用实验测定球墨铸铁样品的超声背向散射谱或各阶分波中共振频率,有可能确定多孔石墨的孔隙率。     

Mie理论在生物组织散射特性分析中的应用

根据等效颗粒散射模型，运用经典的Mie理论，对生物组织的散射相函数、各向异性因子及散射系数进行了数值计算.计算结果表明：可见光照射生物组织时，各向异性因子、散射系数随等效颗粒直径增大而增大；等效颗粒直径较小时，各向异性因子、散射系数随入射光波长增大而单调减小；随着等效颗粒直径增大，各向异性因子、散射系数随入射光波长变化不再具有单调性.上述计算结果可合理解释公布的实验结果. 关键词： 生物组织 散射 Mie理论 等效颗粒     

粘弹包膜气泡的声散射特性

在线性条件下研究了单个粘弹包膜气泡的声散射特性,并用WT散射法求解了造影剂的多气泡散射特性。首先建立了超声造影剂中单个粘弹包膜气泡的声散射物理模型,结合包膜内外声波方程和边界条件解得散射矩阵,并得到散射系数。通过数值计算用简化散射截面积曲线来表征单个气泡的声散射特性,用声色散曲线和衰减系数曲线来表征气泡的集总散射特性。研究结果表明:增加包膜的弹性参数μe的值,不仅使得气泡共振频率增高,还使气泡散射共振峰值相应加大;增加包膜的粘性参数μv的值,将使得散射的共振峰值降低;气泡的另两个参数λe和λv对散射特性几乎没有影响。     

纤维复合媒质中的超声衰减

为了精确估计纤维复合煤质中散射所引起的超声衰减，必须计及纤维之间的多次散射现象本文提出了纤维复合煤质中任一点有效入射声场的计算方法当超声纵波传播方向与纤维轴线相垂直时，散射横波并不影响有效入射声场；导出了考虑多次散射后超声衰减系数的表达式；数值计算了玻璃／金属铝复合媒质中纵波衰减系数随频率的变化关系 关键词：     

Mie散射物理量的数值计算

介绍Ｍｉｅ散射物理量，如振幅函数，强度函数，偏振度，散射系数，消光系数和吸收系数等的数值计算法，给出这一算法的计算公式和运行时间，并与其它Ｍｉｅ散射物理量计算算法进行比较。     

低能π介子—核子散射

本文应用向前方向附近的定角色散关系推出了π介子-核子散射的s波和p波振幅的积分方程。方程中包含湮没过程的贡献,其中主要贡献来自二π介子中间态,为了检验ππ作用在π介子-核子散射过程中的效应,将散射相移观察值代入方程中作了数值计算。利用了核子电磁结构形式因子的实验知識,取共振能量平方t_r=20,得到了I=J=1ππ散射共振宽度参数r≌0.1。此结果可同时符合三个低角动量分波振幅的实验材料,但共振宽度要比观察值小。     

球形粒子在聚焦拉盖尔-高斯光束中的散射特性研究

研究了球形粒子在聚焦拉盖尔-高斯光束中的散射特性. 根据广义Mie理论, 推导出球形粒子在聚焦拉盖尔-高斯光束中散射系数的解析公式. 针对光束的电场分布及粒子散射强度进行了数值仿真, 讨论了散射强度随散射角、散射球粒子半径和拓扑荷的变化特性, 并通过散射系数解释了散射强度分布的振荡现象. 结果表明, 在聚焦拉盖尔-高斯光束照射下, 球形粒子的后向散射强度随着粒子半径的增大而逐渐增大; 后向散射强度开始增大时对应的粒子半径与拓扑荷有关. 通过与高斯光束的对比, 可以看出球形粒子在聚焦拉盖尔-高斯光束中散射特性的差异, 使其在粒径测量、光通信和大气后向散射探测等方面具有潜在应用价值.     

具有二维fBm特征的分层介质粗糙面电磁散射的特性研究

运用微扰法研究了平面波入射分层介质粗糙面的电磁散射,推出了不同极化状态下的双站散射系数公式.采用二维fBm分形粗糙面来模拟实际的分层介质粗糙面,结合二维fBm分形粗糙面的功率谱导出了平面波入射二维fBm分形分层介质粗糙面的散射系数计算公式.通过数值计算得到了HH极化下双站散射系数随散射角的变化曲线,讨论了分维、底层介质介电常数、中间介质介电常数和厚度及入射波频率对双站散射系数的影响,得到了二维fBm分形分层介质粗糙面散射系数的分维特征、基本特征、分区特征和随频率变化的特征. 关键词： 电磁散射 二维fBm分形粗糙面 分层介质 微扰法     

海面冰层对声波的反射和散射特性

北极海面冰层复杂多变,其对声波的反射和散射严重影响冰下水声信道的传输特性,建立海面冰层的声波反射和散射模型对冰下水声通信研究具有重要意义.假设海面冰层为多层固体弹性介质且冰-水界面粗糙,满足微扰边界条件,导出声波从海水介质入射到海面冰层时相干反射系数满足的线性方程组.对相干反射系数随声波频率、掠射角、冰层厚度的变化进行数值分析.进一步引入根据散射声场功率谱密度计算散射系数的方法,改变掠射角,对冰层厚度、散射掠角对散射系数的影响进行研究.     

Matching of asymptotic models in scattering of a plane acoustic wave by an elastic cylindrical shell

Scattering of a plane harmonic acoustic wave by an elastic cylindrical shell is considered. The procedure based on matching of asymptotic models is developed. The long-wave models (the Kirchhoff-Love theory (or its refinement) and the long-wave high-frequency approximation) are exploited in the vicinities of zero frequency and thickness resonance frequencies, whilst the model of a fluid-loaded flat elastic layer is utilized outside the aforementioned vicinities. Comparison with the exact solution demonstrates that the proposed approach is highly efficient for total synthesis of the scattered pressure as well as for uniform approximation of the resonant curves associated with partial modes.     

Circumferential resonance modes of solid elastic cylinders excited by obliquely incident acoustic waves

When an immersed solid elastic cylinder is insonified by an obliquely incident plane acoustic wave, some of the resonance modes of the cylinder are excited. These modes are directly related to the incidence angle of the insonifying wave. In this paper, the circumferential resonance modes of such immersed elastic cylinders are studied over a large range of incidence angles and frequencies and physical explanations are presented for singular features of the frequency-incidence angle plots. These features include the pairing of one axially guided mode with each transverse whispering gallery mode, the appearance of an anomalous pseudo-Rayleigh in the cylinder at incidence angles greater than the Rayleigh angle, and distortional effects of the longitudinal whispering gallery modes on the entire resonance spectrum of the cylinder. The physical explanations are derived from Resonance Scattering Theory (RST), which is employed to determine the interior displacement field of the cylinder and its dependence on insonification angle.     

Local vibration of an elastic plate and zero-group velocity Lamb modes

Elastic plates or cylinders can support guided modes with zero group velocity (ZGV) at a nonzero value of the wave number. Using laser-based ultrasonic techniques, we experimentally investigate some fascinating properties of these ZGV modes: resonance and ringing effects, backward wave propagation, interference between backward and forward modes. Then, the conditions required for the existence of ZGV Lamb modes in isotropic plates are discussed. It is shown that these modes appear in a range of Poisson's ratio about the value for which the cutoff frequency curves of modes belonging to the same family intercept, i.e., for a bulk wave velocity ratio equal to a rational number. An interpretation of this phenomenon in terms of a strong repulsion between a pair of modes having a different parity in the vicinity of the cutoff frequencies is given. Experiments performed with materials of various Poisson's ratio demonstrate that the resonance spectrum of an unloaded elastic plate, locally excited by a laser pulse, is dominated by the ZGV Lamb modes.     

用于储罐底板缺陷检测的超声兰姆波模式研究

从超声兰姆波的声场方程出发,得到板材表面离面位移(法向位移)为零的条件,即当超声兰姆波的相速度等于板材介质的纵波声速时,在板材表面的离面位移为零。在给定适当频厚积的条件下,分别数值模拟了仅有切向位移而无离面位移的A1、S1、A2和S2兰姆波模式在有液体负载的单层钢板中的传播情形。结果表明:离面位移为零的S2模式频散较小且对板中缺陷更为敏感。     

Elastic waves in noncohesive frictionless granular crystals

An ordered structure of noncohesive spherical beads constitutes a phononic crystal. This type of media combines the properties of wave propagation in phononic crystals (dispersion due to the geometrical periodicity) with the properties of wave propagation in granular media (nonlinearities, rotational degree of freedom) and gives the opportunity to have interesting features as tunable frequency band gaps for example.In this work, the acoustic bulk modes of a hexagonal close packed (hcp) structure of beads, considered as rigid masses connected by springs, are theoretically evaluated and their associated resonance frequencies are compared to experimental results. When friction is neglected, the elastic interaction between the beads are reduced to a normal spring interaction given by the Hertz theory. According to this theory, the rigidity of the contact depends on its static loading. The theory predicts the existence of elastic transverse and longitudinal acoustical-type modes and transverse and longitudinal optical-type modes.The acoustic transfer function of a hcp crystal slab built with stainless steel beads is measured and its resonance frequencies are compared to the theoretical predictions. Despite some differences between theory and experiments, which could come for instance from the disordered character of the contact loads, the developed theory and the experimental results show relatively good agreement.     

纵模对光子晶体中类狄拉克点传输特性的影响

本文通过数值仿真分析了无序正方晶格光子晶体中类狄拉克点的光子传输特性. 结构中的无序是通过随机移动氧化铝介质柱的位置来实现. 研究发现, 由于纵模被激发出来, 在类狄拉克点及其附近无序对结构透射率的影响是不同的. 在类狄拉克点, 由于纵模的干扰, 透射率随着无序的增加而减小, 与通带的行为类似. 在不受纵模干扰的类狄拉克点附近, 透射率几乎不受无序的影响, 这主要是由于结构可以等效为近零折射率材料, 等效的波长非常大. 本文的研究结果有助于人们进一步理解光学纵模和零折射率材料.     

Stabilization mechanisms of longitudinal pulsations in rotating detonation combustors

This work investigates the stabilization mechanisms of two types of longitudinal pulsations in rotating detonation combustors. The first type is linked to operating modes with two counter-rotating waves in combustors with open outlets and appears as a minor peak in the pressure spectrum. The second type is observed as pulsed operation of the combustor when the outlet is restricted. Different combustor lengths are studied and the susceptibility to these longitudinal pulsations is investigated. Pressure measurements along the length of the combustor and around the perimeter are used to identify the operating mode and to describe the propagation and stabilization mechanisms of the two longitudinal modes. The results show that both modes are linked to the longitudinal acoustic resonance of the combustor. The length-to-perimeter ratio and the mass flux are identified as the driving parameters for the existence of these longitudinal modes. The first mode is shown to be an acoustic resonance supported by the intersections of counter-rotating waves. The second mode is controlled by the reflection of an explosion induced shock wave propagating through a high velocity bulk flow.     

套管中模式波的响应特征*

在固井质量评价中主要利用套管中模式波的幅度或衰减变化反映水泥的胶结状况,不同测井仪器在套管中激发的模式波的类型不同,研究套管中各模式波的传播特征和影响因素可充分挖掘测量数据的潜在应用价值。CBL和SBT等测井仪器在套管中主要激发拉伸波,类同于平板中的零阶对称Lamb波,水泥环封隔测井仪器的斜入射模式在套管中主要激发套管弯曲波,类同于平板中的零阶反对称Lamb波,垂直入射模式激发套管共振波,类同于平板中的高阶对称Lamb波。该文重点分析了这些模式波的衰减特征及其对微环的响应,套管弯曲波在套后耦合轻质水泥时对微环不敏感,但在套后耦合常规水泥或重水泥时,其衰减明显高于胶结良好的状况;拉伸波对微环的存在最为敏感;套管共振波对微环不敏感。     

Experimental study of coupled vibration in a finite periodic dual-layered structure with transverse connection

The analytical equations of the transfer matrix method are further derived for the multi-coupled vibration of flexural and longitudinal waves in a periodic dual-layered beam structure with connection branches, with full consideration given to the flexural and longitudinal motions that are tri-coupled at each connection. Measurements of mobilities at the junctions on the uni-layered beam and the cross-layered beam are made. The numerical results agree well with the experimental results at all frequencies from 10 to 2000 Hz, which verifies the theoretical methodology for the multi-coupled vibration in a finite dual-layered beam. The cross-layer energy transmission is calculated, which reveals that the transmitted longitudinal energy is enhanced not only at the longitudinal resonant modes but also at the flexural resonant modes of the connection branches due to the structural wave coupling. The flexural energy is excited by wave coupling and becomes stronger at the longitudinal resonant modes and the flexural resonant modes of the connection branches. The cross-layer vibration motions from coupled waves in the branches can be effectively controlled by the attached cantilevers with mass at the resonance modes. This method can be used to control the structure-borne sound transmission in multi-layer beam structures.