空气冲击波作用于柔性防爆墙的透射和绕射效应分析

为研究爆炸空气冲击波作用于柔性防爆墙后发生的透射和绕射现象及规律,采用数值模拟方法计算了防爆墙在5、10、15和20kg四种药量TNT炸药爆炸冲击波作用下墙后压力流场的变化,分析了墙后发生的透射和绕射现象,比较了压力波形的变化特点,得到了墙后压力场变化分布规律。计算结果表明,柔性墙背后的压力存在两个主要峰值,分别为透射压力峰值和绕射压力峰值。透射压力峰值由柔性墙体变形运动引起,并与墙体变形速率有关;绕射压力峰值由冲击波在墙顶绕射传播引起。透射压力与绕射压力的分布与变化规律不同,需区别对待分析。     

高车流量公路声屏障绕射衰减理论模型与计算

公路上设置的声屏障的长度一般为有限长,如何准确的计算出此种情况下声屏障的绕射衰减,是声屏障声学设计的关键。通过建立有限长声屏障的绕射声衰减的理论模型,进行了这方面的探讨。     

Stokes波在铅垂圆柱上绕射的二阶分析

在势流假设下,利用摄动原理,求解 Stokes 波中铅垂圆柱的二阶绕射势,得到完整的二阶波浪压力和波浪力。根据铅垂圆柱的几何特性,采用两种求解方法。一是直接利用外场解析表达式求解;二是设置控制面,外场用解析表达式,内场用简单 Green 函数方法求解。两种方法得到的结果吻合良好,验证了计算方法的有效性,可为求解任意三维物体的二阶绕射问题提供参考。     

基于Fresnel-Kirchhoff理论的无限大声屏障绕射声衰减

从实际工业噪声控制中大面积有壁障噪声源的噪声控制问题出发,应用Fresnel-Kirchhoff理论和Babinet原理,建立计算无限大声屏障绕射声衰减的理论模型。通过数值求解,分析了声源、接收点位置,声波频率和地面反射等对声屏障绕射声衰减的影响。     

不同顶端结构声屏障绕射声衰减量模拟分析

针对不同顶端结构的声屏障对绕射声衰减量的影响,对四种常见的顶端结构利用国标GB/T17247.2-1998中规定的绕射声衰减量公式进行计算分析,根据0-5 000 Hz频域段内四种结构的绕射量变化曲线选择出最佳构型,结果表明Y形和T形结构绕射声衰减量最多,且几乎相等;然后通过声学软件Virtual Lab对Y形和T形结构进行隔声效果仿真分析,根据声影区内声压级分布情况判断降噪效果,结果显示Y形结构的声压级较T形降低覆盖面积更广,并计算这两种结构的插入损失,结果显示Y形结构较T形障板的降噪效果更好。     

几何绕射理论下楔形障碍物衍射

应用几何绕射理论，研究不同角度和不同频率情况下楔形障碍物衍射衰减规律，并结合90°楔形障碍物现实应用，模拟楔形建筑物下声音传播衰减。得出了楔形障碍物插入损失与楔形角度和声波频率的关系，结果与Maekawa实测值比较并进行精度分析。针对现实场景下楔形建筑物的遮挡，应用空间剖分和声线束追踪生成声音路径，模拟楔形建筑物区域的声音衰减。结果显示声音衰减随楔形角度减小和频率增加而增大，几何绕射理论可应用于大区域建筑物群声传播衰减计算中。     

柱间有相对运动坐底圆柱群的绕射辐射分析

圆柱群是深海海工建筑物的主要承力结构.目前对圆柱群进行的水动力分析大多考虑将圆柱群作为一个整体的情况,即柱群中各柱无相对运动;对于柱间有相对运动圆柱群的研究较少.为此,采用精确代数法研究柱间有相对运动坐底直立圆柱群的绕射-辐射问题.首先开展了柱群中各圆柱做给定单独运动的辐射分析;之后在有入射波浪的情况下,分析了各柱有相对运动圆柱群的绕射-辐射问题;进而求得各圆柱上的水动力、振动幅值等量.作为验证,先将退化情况的结果与已有文献进行了对比.进一步的计算结果表明,在某些波数下,分析中是否考虑柱间相对运动对结果影响较大.     

声屏障吸声作用对绕射降噪量贡献的分析

目前对声屏障的插入损失计算所用的公式，没有体现材料吸声对绕射声衰减的作用，只是反映屏障的隔声作用。探讨屏障的吸声性能对绕射降噪量的贡献，并推导出与吸声系数有关的绕射降噪量的公式。     

爆炸冲击波作用于便携式防爆墙的绕射规律

为研究爆炸冲击波作用于便携式防爆墙的绕射规律,利用LS/DYNA软件,采用欧拉耦合的方法,分析了TNT药量不同爆距相同、TNT药量相同爆距不同以及TNT药量相同墙厚不同的条件下墙体对环流超压的影响规律,且拟合出了不同TNT药量时墙后超压峰值公式,并通过防爆墙墙前、墙后超压值与已知试验值对比,验证了计算模型的正确性.结果表明:环流超压峰值随着TNT药量的增加而增加,随着墙体厚度的增加而减少,且其超压峰值出现在墙后约2倍墙高位置处;当爆距大于2.4 m时,环流超压的峰值先递增、后递减,最大环流超压发生在约2倍墙高位置;当爆距小于2.4 m时,最大环流超压向墙体移动,距离墙体后面1.0 m左右.验证了其绕射规律与已知研究结果的一致性.     

规则波作用下开孔圆盘阵列绕射效应研究

在势流理论和线性波理论的假设下,运用特征函数展开法和贝塞尔函数加法理论,研究了规则波作用下开孔圆盘阵列的绕射效应。阵列中每个圆盘都是刚性、开孔的,固定在静水面以下,其厚度相对于入射波波长可以忽略,且通过其表面的流体运动满足达西定律。根据推导出的速度势,进一步得到圆盘的垂荡力和流场的自由表面高程解析表达式。利用该解析解与已发表的孤立圆盘计算结果进行验证;分析了四盘阵列在各种波浪和结构参数下的绕射现象。研究表明：当圆盘开孔率、吃水深度及入射波波长在一定范围内时,圆盘阵列内部将发生显著的相互作用现象,主要表现为迎浪侧圆盘对背浪侧圆盘的遮蔽效应。在特定的入射波波数下,选择适当的四盘阵列圆盘间距,最大可将背浪侧圆盘垂荡力降低至迎浪侧圆盘的76%。同时还发现,四盘阵列中,迎浪侧圆盘上可能发生波能聚焦现象,引起自由表面高程的急剧增大,但这种波能聚焦现象会随着圆盘开孔率的增大而逐渐减弱。     

Improved diffraction integral for studying the diffracted field of a spherical microlens

By introducing a new kind of Green function, we formulate an improved diffraction integral, which can be used to numerically evaluate the diffracted field of a microlens of plane-convex shape. Analytical expressions for the diffracted field of microlens are derived for the case where the curvature radius of the convex surface is larger than the dimension of the microlens aperture. The validity of the results and the diffracted field of the microlens are illustrated with numerical examples. The focal shifts of the diffracted field are found to depend mainly on the Fresnel number N of the microlens.     

Analysis of diffracted fields with the extended theory of the boundary diffraction wave for impedance surfaces

Uniform diffracted fields from impedance surfaces are investigated by the extended theory of boundary diffraction wave (ETBDW). The new vector potential of the ETBDW is constructed by considering the pseudoimpedance boundary condition. The method is applied to the diffraction problem from an impedance half-plane. It is shown that the total fields from an impedance half-plane reduce to the case of a perfectly electric or magnetic conducting and opaque half-plane for special values of surface impedance. The total and diffracted fields are compared numerically with the exact solution for the impedance half-plane and modified theory of physical optics (MTPO) solution for an impedance wedge. The numerical results show that the field expressions are in very good agreement with the exact and MTPO solutions.     

Algorithm study of Collins formula and inverse Collins formula

In the study of a diffraction field of a light wave passing through a symmetrical paraxial optical system, the Collins formula and its inverse are convenient for calculation. The algorithm study of the Collins formula demonstrates that both a single fast Fourier transform algorithm and a double fast Fourier transform algorithm can be used in diffraction calculation. But, whichever algorithm is adopted, only by meeting some specific conditions can the amplitude and phase distributions of a diffracted wave be calculated correctly. Based on the Nyquist sampling theorem, the indispensable conditions to calculate a diffraction field accurately are presented.     

Computer Reconstruction of a Random Rough Surface in the Presence of Higher Diffraction Orders

Abstract

The profile of a random rough surface (RRS), whose mean roughness R_a is greater than the light wavelength, is visualized by computer processing. The surface is presented as a sum of sinusoidal gratings. The light diffracted from this surface field is registered by a photodiode array. The second and third diffraction orders from each grating are taken into account in computer processing of the diffracted field and the mixing field–the field obtained at the mixing of the reference and the diffraction fields. The criterion for taking into account higher diffraction orders is the asymmetry of the diffraction pattern to the left and to the right relative to the central peak (the field of zero diffraction orders obtained from each grating) The number of the diffraction orders higher than the first is defined from the average intensity distribution between the central peak and the diffraction orders to the left and to the right at arbitrary light wavelength. The surface profile is reconstructed by a computer program and the mean roughness R_a is calculated. The obtained value of R_a is in satisfactory agreement with that measured by the contact pin method.     

The theory of the boundary diffraction wave for wedge diffraction

A new potential function, line integration which gives the edge diffracted fields, is constructed for wedge diffraction by using the method of modified theory of physical optics. The surface integrals are transformed into line integrals by the technique of asymptotic reduction. As an application of the novel potential function, the diffracted field is obtained for the geometry of a wedge for arbitrary incidence of plane waves.     

Diffraction of shear waves by a rigid strip in a medium of monoclinic type

Summary The paper discusses the two dimensional problem of diffraction of shear waves by a rigid strip in an infinite medium of monoclinic type. This problem is reduced to a system of dual integral equations of which the solution provides the diffracted field. The method of steepest descent has been used in the determination of the diffracted fields at a large distance from the strip. Diffraction pattern for displacement and stress field have been computed and the effect of anisotropy is distinctly marked.With 6 Figures     

提高背景噪声下结构辐射声强测量精度的研究

背景噪声存在会引起薄壁结构表面辐射声强和声功率的测量误差。将探头屏蔽起来是降低这一误差的方法之一。本文分别对两种声屏蔽罩和局部声屏蔽空间进行了研究。结果表明,屏蔽罩虽然使用方便,但对薄壁结构表面声辐射这类抗性很强的声场.它的放入将破坏局部声场,引起测量的偏度误差,不宜采用。就本身声学特性来说.吸声材料构成实心锥的声屏蔽罩明显优于空心锥形的.在被测声场抗性不强的情况下可以采用。本文提出的局部声屏蔽空间解决了声屏罩引起的问题,当背景噪声的总声压级比信号的总声压级高出10dB时,总声强的测量误差低于1dB,声强细谱的误差小于2dB,有效地抑制了背景噪声引起的声强测量误差。     

Aberrations of diffracted wave fields. I. Optical imaging

The concept of aberrations of diffracted waves is revisited by using the Rayleigh-Sommerfeld theory of diffraction, and it is pointed out that these aberrations are in a class by themselves; they are only deceptively similar to the aberrations of a rotationally symmetric imaging system. Although an exact Fourier-transform expression can be written for the diffracted wave field, its numerical calculation is cumbersome because of the dependence of the aberrations of the diffracted wave on the observation point. This is true regardless of whether the diffracted wave field is observed on a plane or a hemisphere. It is shown that the Fresnel and Fraunhofer approximations, which neglect these aberrations and thereby simplify the calculations, are valid in imaging applications.