随机起伏界面声散射的实验与理论研究

进行了二维粗糙海面声波散射特性的水池实验,测量了不同入射角、散射角以及方位角条件下所对应的散射强度。实验通过不同位置风扇对水面的吹拂获得粗糙水面,分别对水平面上互相垂直的两个方向上的水面波高变化进行了测量,利用周期图法估计出这两个方向上的空间功率谱,验证了实验中的粗糙水面是各向同性的。利用改进的空间域处理技术去除了总声波信号中的直达波和其他固定位置散射体的散射信号,获得了粗糙水面的声波散射信号。利用小斜率近似方法计算了二维粗糙海面的声散射特性。实验与模拟计算结果比较,证实了计算二维粗糙海面声波散射特性的小斜率近似方法的有效性与准确性,相互印证了实验与理论。     

大粗糙度起伏下一维海面声散射建模与分析

针对起伏海面高频声散射计算问题,提出了一种改进的基尔霍夫(Kirchhoff)近似方法。该方法考虑了海面的阴影区和亮区之间的多次散射声场,可对小入射角下大粗糙度起伏海面的散射声场进行计算。以一维余弦和高斯谱海面散射声场的有限元计算结果为标准解验证了所提方法的准确性和适用性。利用该方法计算了一维余弦海面的散射强度,分析了不同入射声波频率和角度下海面散射强度的分布情况,并解释了Bragg散射的产生机理,同时讨论了不同海面均方根高度和相关长度情况下高斯谱海面散射强度的变化规律。     

海面散射问题的各种小波高解之间的联系

本文谨慎地导出了点源海面声散射问题的若干种近似解法,这里感兴趣的是小波高的情况即海面的均方根波高与声波长和海面相关半径相比要小,近似解可分为二类:利用Kirch-hoff近似得到的解和由微扰展开得到的解.证明了在小波高的条件下微扰解优于任何由Kir-chhoff近似得到的解,前者对任意的声波长与海面波长之比都是有效的.在小波高的情况下,当声波长与海面波长之比很小时可以证明所有的近似解是一致的.本文指出了在使用Kirch-hoff近似时可能和已经出现的一些错误.     

双基地海底散射建模与仿真分析

海底的声散射是产生浅海混响的主要原因之一。采用小斜率近似方法研究了粗糙液-液分界面的海底散射问题,通过一个统一的方法实现微扰近似和Kirchhoff近似的无缝衔接且不受表面高度均方根条件限制。类比APL-UW模型,利用最低阶小斜率近似求解粗糙海底界面散射截面,并结合Jackson等人的海底沉积层体积散射截面表达式,建立了一个计算双基地海底散射强度的＂小斜率-体积＂模型。针对几种典型的海底类型进行了数值仿真与性能分析,并将数值预报结果与APL-UW模型进行了对比。结果表明,小斜率-体积模型在整个角度范围内较为准确,并且与APL-UW模型的预报结果吻合程度较高,两者散射强度差值小于2 dB。     

海面波浪起伏对声传播的影响

位于南海某海域同一传播路径上,在2004年和2005年分别进行的两次声传播实验,两次实验中的设备、过程和方法都相同,水文变化也不大,但实验得到的传播损失相差却很大。通过对实验数据的分析表明,两次实验的传播损失差异是由于海面波浪起伏引起的。利用第一次实验数据,考虑海面波浪起伏后反演得到的海底吸收系数可以较好地解释另一次的声传播损失。因此,可以以为浅海海面波浪起伏对声传播的影响很大,在进行水下声传播规律的研究或地声参数反演研究时,应当充分考虑海面波浪起伏的影响。     

浅海表面声道中的传播损失

浅海声传播问题,由于边界的多次反射,使声场的空间结构复杂化。为此采用了空间平滑化的方法计算平均声场,使问题大大简化。同时为了使问题进一步简化,在计算浅海声场时常假设海表面为平坦表面,反射系数的模为1。然而在实际情况下海表面不是平面,海面波浪的起伏大小直接影响海面的反射系数,不能简单地当作1,特别是在浅海表面声道情况下,海面波浪是影响声场的一个十分重要的因素。本文同时考虑了海面和海底的影响,利用平滑平均声场的计速公式[1]对浅海表面声道情况进行数值模拟计算,　与实验结果进行了对照,得到了一些有意义的结论。 实验是在…     

水中目标强度的一致性时域有限差分计算

本文应用一致性时域有限差分法（FDTD）和近场-远场变换计算散射声目标的反射声压强度，找出反射声压级与目标声中心到换能器间距离的关系。数值计算中考虑了自由场和有海面及海面波浪影响时的不同情况。根据声源和目标的几何特征对入射波和散射波分别作了球面和柱面扩散修正，并将计算结果与水池实验结果进行了比较。本文的讨论可对水下目标强度测量方法提供参考。     

浅海起伏海底环境下的声传播

起伏海底对浅海声传播有着极大影响。文章利用一次东海夏季水文数据,结合蒙特卡洛方法统计分析了浅海长距离起伏海底环境下的声场特性,并利用射线模型分析了海底起伏对声场不确定性影响的机理。结果表明,高频声波对海底起伏的变化更为敏感,传播损失的概率分布更为分散。负跃层环境下,当声源位于跃层下方时,起伏海底对位于跃层下方接收点的声传播影响更为明显。等温层环境下,起伏海底对声传播的影响相比于跃层环境影响更小。起伏海底的倾斜度越大,声传播损失越大。对于相同倾斜度的起伏海底,相对起伏的大小,即起伏海底高度与海深的比值,比起伏周期的变化对声传播的影响更大。     

声波由粘弹性介质向流体介质入射情况下的声反射分析

在研究含腔体的结构的声学特性时,会遇到声波从粘弹性介质向流体介质入射的情况。主要研究了声波从粘弹性介质向流体介质斜入射情况下的声反射特性,分析表明：反射声波中有横波和纵波同时存在,纵波声压反射系数随入射角度的增大逐渐增大,横波的声压反射系数随入射角度的增加先增大后减小,在垂直入射时只有纵波,而没有横波存在。这些结论为进一步研究带空腔结构的声学特性打下了基础。     

充水透声结构声目标强度预报方法研究

充水透声结构是水下航行体重要的结构类型之一,主动声呐作用于目标时,声波可以透射充水透声结构外表面进入结构内部,在充水透声壳体结构内表面产生内部散射,从而影响目标的目标强度。文章提出了一种适用于透声结构目标强度预报的考虑透声的修正板块元方法,此方法假设散射声场近似为内表面板块散射声场和与之对应的外表面板块散射声场叠加,基于Z-Buffer算法确定充水透声结构的外、内表面板块之间的透射遮挡对,再考虑外表面板块的反射和透射系数、内表面的反射系数,采用非刚性表面板块元方法对两部分声场进行分别计算,相干叠加后预报总目标强度。此方法考虑了充水透射结构内表面的一次散射,物理概念清晰,提高了板块元方法预报精度。通过双层平板算例验证方法的正确性,并将此方法应用于BeTSSi Ⅱ潜艇模型中的典型透声结构——围壳部分的目标强度仿真计算,最后通过围壳缩比模型目标强度水池试验,验证了提出的方法预报精度。     

Fourth- and higher-order small-perturbation solution for scattering from dielectric rough surfaces

A recursive solution of the small-perturbation method for rough surface scattering is presented. These results permit fourth- and higher-order corrections to rough surface scattering coefficients to be determined in a form that explicitly separates surface and electromagnetic properties. Sample results are presented for the fourth-order correction to the specular reflection coefficient of a rough surface and the sixth-order correction to incoherent scattering cross sections.     

Polarization of specular reflection and near-specular scattering by a rough surface

Polarization of specular reflection and near-specular scattering (NSS) by a randomly rough surface is investigated by the use of a Mueller matrix formulation. The collective effect by a rough surface on the average specular field results in reflectance loss and polarization, which can be explained by an effective medium theory. Effects of random NSS can be represented by a scattering matrix that is partially coherent and polarized. The incoherent and unpolarized part of scattering causes depolarization, and the coherent and polarized parts of scattering change the apparent polarization properties of specular reflection. Results of a simulation and least-squares fit of ellipsometric data to the models including the NSS effect, for a black anodized aluminum sample, are presented. Simultaneous least-squares fits for both ellipsometric data and reflectance data at multiple angles of incidence at three different wavelengths gave approximately the same rms roughness, which agrees with the profilometric values reported previously.     

Diffuse and specular reflectance from rough surfaces

We present a reflection model for isotropic rough surfaces that have both specular and diffuse components. The surface is assumed to have a normal distribution of heights. Parameters of the model are the surface roughness given by the rms slope, the albedo, and the balance between diffuse and specular reflection. The effect of roughness on diffuse reflection is taken into account, instead of our modeling this component as a constant Lambertian term. The model includes geometrical effects such as masking and shadowing. The model is compared with experimental data obtained from goniophotometric measurements on samples of tiles and bricks. The model fits well to samples with very different reflection properties. Measurements of the sample profiles performed with a laser profilometer to determine the rms slope show that the assumed surface model is realistic. The model could therefore be used in machine vision and computer graphics to approximate reflection characteristics of surfaces. It could also be used to predict the texture of surfaces as a function of illumination and viewing angles.     

Enhanced specular peaks in diffuse light scattering from weakly rough metal surfaces

We employ Monte Carlo techniques based on the reduced Rayleigh equations to study an enhanced specular peak that appears in the light scattered from weakly rough metal surfaces. This peak is not associated with the specular reflection but instead appears, with finite angular width, at the specular angle of the mean diffusely scattered intensity. As is the case with backscattering enhancement, the specular peak arises from the interference of contributions of multiple-scattering processes related to surface plasmon polariton excitation. We demonstrate that the specular peak is seen clearly for surface roughness that has a conventional Gaussian power spectrum. Further, we show that the peak appears more distinctly for roughness whose power spectrum has a new rectangular form, which is proposed here with the intent of better isolation of the scattering processes essential to the specular peak. Finally, for a pair of rough surfaces that have appropriately correlated surface roughness, it is found that the cross correlation of scattered amplitudes presents a well-isolated specular peak, which directly demonstrates the constructive interference that produces the effect.     

Unpolarized infrared emissivity with shadow from anisotropic rough sea surfaces with non-Gaussian statistics

The emissivity of two-dimensional anisotropic rough sea surfaces with non-Gaussian statistics is investigated. The emissivity derivation is of importance for retrieval of the sea-surface temperature or equivalent temperature of a rough sea surface by infrared thermal imaging. The well-known Cox-Munk slope probability-density function, considered non-Gaussian, is used for the emissivity derivation, in which the skewness and the kurtosis (related to the third- and fourth-order statistics, respectively) are included. The shadowing effect, which is significant for grazing angles, is also taken into account. The geometric optics approximation is assumed to be valid, which means that the rough surface is modeled as a collection of facets reflecting locally the light in the specular direction. In addition, multiple reflections are ignored. Numerical results of the emissivity are presented for Gaussian and non-Gaussian statistics, for moderate wind speeds, for near-infrared wavelengths, for emission angles ranging from 0 degrees (nadir) to 90 degrees (horizon), and according to the wind direction. In addition, the emissivity is compared with both measurements and a Monte Carlo ray-tracing method.     

Unpolarized emissivity with shadow and multiple reflections from random rough surfaces with the geometric optics approximation: application to Gaussian sea surfaces in the infrared band

The emissivity from a stationary random rough surface is derived by taking into account the multiple reflections and the shadowing effect. The model is applied to the ocean surface. The geometric optics approximation is assumed to be valid, which means that the rough surface is modeled as a collection of facets reflecting locally the light in the specular direction. In particular, the emissivity with zero, single, and double reflections are analytically calculated, and each contribution is studied numerically by considering a 1D sea surface observed in the near infrared band. The model is also compared with results computed from a Monte Carlo ray-tracing method.     

风浪谱模型在高度计风速反演中的应用

依据粗糙海面电磁波镜面散射理论,将根据全波数范围内的风浪谱模型计算的标准化雷达后向散射截面与T/P卫星高度计测得的雷达后向散射截面(Ku波段)进行比较,实现了对高度计风速的反演.研究表明,在所选浮标附近海域,波龄因子对高度计风速反演存在较大影响.与目前高度计业务化算法相比,考虑波龄因子影响后,根据谱模型反演获得的风速与中国近海浮标风速之间均方根误差和平均偏差更小.由于波龄因子可以根据高度计测得有效波高以及业务化算法获得的风速得到,因此根据风浪谱模型反演获得的风速具有广泛的适用性.     

Angular spectral approach to reflection of focused beams with oblique incidence in spherical-planar-pair lenses

The characteristics of the output signal from spherical-planar-pair (SPP) lenses in the ultrasonic microspectrometer (UMSM) are described. The angular spectral approach is used to obtain the mathematical formula of the output signal. Although isotropic specimens are assumed, anisotropic materials can be treated as well with a small modification. A 2-D approximation is introduced to analyze the specular reflection, the Rayleigh critical angular phenomenon, and V (z) of the specular reflection. Numerical calculations are also performed for specimens with copper substrate, steel substrate, and layered structures, by numerically integrating the 2-D formulas of the output signal. It is found that a ray optical treatment is a good approximation of the wave propagation in the SPP lenses. The incident angular dependence of a reflection coefficient can be estimated by tilting the sensor unit.