基于信道传播理论的多基地远程海底混响研究

针对浅海多基地远程海底混响, 以声传播的线性信道近似理论为基础, 通过确定海底界面的不均匀起伏特性, 对入射声波的远距离异地混响脉冲响应函数进行求解. 对非水平分层介质的浅海波导, 利用绝热简正波求解信道的传输函数, 得出了小角度斜坡海底的远程海底混响强度, 以下坡海底为例, 分析对比了不同海底倾角斜坡海底对远程异地混响强度及其衰减规律的影响. 理论仿真表明, 波导的传输效应对多基地远程海底混响强度的影响不可忽略.     

海底表层声场垂直衰减分析

声波在海底中的传播衰减规律是海洋声学中值得研究的课题。根据简正波理论推导了高声速海底表层声场垂直衰减系数的解析表达式,并给出了其理论上限。数值仿真与实验研究表明:对液态海底模型,海底表层声场垂直衰减系数与声波频率基本呈正比;对于特定频率,海水-海底声速比是影响垂直衰减系数的最主要因素,比值越小垂直衰减系数越大;对于斜率较大的楔形海底地形,且声源在深水区、接收点在浅水区的情形,垂直衰减系数会大大小于其理论上限。      

圆柱面上再附边界层流动的级数分析

按普朗特边界展理论,将圆柱面上再附边界层流动表示成级数的形式,并根据实验所测圆柱表面上的压强分布导出再附边界层外边界上的流速分布,取有限项级数解的形式解出了圆柱表面上再附边界层内的流速分布及流动再分离点。     

关于非牛顿型流体边界层的研究

一、引言 近年来,随着石油工业、纺织工业以及化学工业的发展,非牛顿型流体力学引起了学术界的广泛的注意。关于此类流体的边界层理论目前亦正在形成中。 本文即系从B.B.流体边界层一般积分关系式的概念出发,导出适合于此类流体边界层的动量方程,并借此寻求流体沿平板运动时滞流边界层发展的定量关系式。 二、非牛顿型流体边界层的动量方程     

多孔表面推迟高超声速边界层转捩的机理

利用线性稳定性理论分析（LST）结合直接数值模拟（DNS）研究高超声速多孔表面边界层流动的失稳特征,分析多孔表面推迟高超声速边界层转捩的机理.在Ma=6,Re=2.0×10⁴（参考长度为入口处边界层位移厚度）条件下获得平板边界层及不同孔隙排列情形下平板边界层的典型流动特征,并采用LST方法分析光滑平板及多孔平板扰动的增长率及累计放大率.研究表明三维顺排及错排多孔表面都可以抑制第二模扰动的发展,推迟高超声速边界层转捩,但顺排多孔表面推迟高超声速边界层转捩能力更强.     

叶型表面紊流边界层的差分计算方法

为了计算叶型表面的层流和紊流边界层,我们对Cebeci-smith可压紊流边界层的差分计算方法,进行了简化,编出了计算机程序.计算结果与现有理论解比较,误差很小.为了适应叶栅中强逆压梯度的流动,我们又采用剪应力函数为主要变量,对紊流边界层方程进行求解.试算表明后一种方法,对求解强逆压梯度的边界层相当稳定,可以一直算到开始出现反流的脱离点.后一种方法的计算结果与理论解相比误差比前一种方法略大,但对于工程应用已足够准确.可以在工程计算中应用.     

三维边界层内诱导横流失稳模态的感受性机理

边界层感受性问题是层流向湍流转捩的初始阶段,在转捩过程中起关键性作用,尤其是三维边界层流动.因此,研究三维边界层感受性问题对进一步理解层流向湍流转捩机理以及湍流成因具有重要的理论意义.采用数值方法研究自由来流湍流与三维壁面局部粗糙相互作用下三维边界层的感受性问题,确定是否能在三维边界层内寻找一种新的横流失稳模态;确定在何种条件下三维边界层内能诱导出定常、非定常的横流失稳模态;探索自由来流湍流的强度、展向波数和法向波数以及三维壁面局部粗糙的大小和结构类型等因素在自由来流湍流与三维壁面局部粗糙作用下三维边界层内被激发出的感受性过程中有何影响,并确定何种横流失稳模态在三维边界层感受性过程中占据何种地位.对自由来流湍流与三维壁面局部粗糙作用激发三维边界层内感受性问题的深入研究,将有助于完善流动稳定性与湍流理论,为层流向湍流转捩过程的预测与控制提供合理的理论依据.     

固体推进剂侵蚀燃烧计算显函分析

一、前言 侵蚀燃烧作为一个对固体火箭发动机设计水平密切相关的重要课题,国内外经30年的努力,至今还没能完全掌握它的细节。代表性的理论是L-R公式和K.K.Kuo的化学反应湍流边界层理论等。为更清楚地了解侵蚀燃烧规律性,本文对燃烧表面附近边界层的气流状态,根据已有理论定量作了分析,并由宏观传热理论导出计算侵蚀燃烧速度的理论显式。     

回转体边界层转捩区声辐射的预报方法研究

基于边界层理论和转捩区声辐射理论,利用Liepmann理论Krane偶极子声源改进模型,在研究回转体表面的压力梯度和声衍射损失对转捩区声辐射影响的基础上,改进了回转体边界层转捩区声辐射的预报方法。结果表明:与零压力梯度相比,逆压梯度下的转捩区将导致更高的噪声辐射,利用改进方法预报的噪声级与实验值的一致性较好。     

浅海海底反射系数幅值参数的反演

理论分析了一种通过混响强度衰减特性获取海底反射系数的幅值参数的方法.将海底反射系数的幅值参数和相位参数引入到全波动混响模型中,为海底反射系数的反演提供理论基础。理论分析和数值仿真表明,在小掠射角条件下,利用混响强度衰减特性反演海底反射系数幅值参数的可行性和准确性。该反演方法只需要输入4个变量:本地混响强度的衰减特性,反射系数的相位参数,海深以及海深处的声速,同时要求混响数据具有一定的混响噪声比(大于6 dB)才能够使反演结果准确可信。根据本地静态混响实验数据成功反演得到海底反射系数的幅值参数.      

Characterization of mechanical and geometrical properties of a tube with axial and circumferential guided waves

Guided waves propagating in cylindrical tubes are frequently applied for the characterization of material or geometrical properties of tubes. In a tube, guided waves can propagate in the axial direction and called axial guided waves, or in the circumferential direction called circumferential guided waves. Dispersion spectra for the axial and circumferential guided waves share some common behaviors and however exhibit some particular behaviors of their own. This study provides an investigation with theoretical modeling, experimental measurements, and a simplex-based inversion procedure to explore the similarity and difference between the axial guided waves and circumferential guided waves, aiming at providing useful information while axial and circumferential guided waves are applied in the area of material characterization. The sensitivity to the radius curvature for the circumferential guided waves dispersion spectra is a major point that makes circumferential guided waves different from axial guided waves. For the purpose of material characterization, both axial and circumferential guided waves are able to extract an elastic moduli and wall-thickness information from the dispersion spectra, however, radius information can only be extracted from the circumferential guided waves spectra.     

Power flow in coupled bending and longitudinal waves in beams

In complex structures, curvature and impedance discontinuities (e.g., junctions) couple bending and longitudinal waves. Propagation losses for longitudinal waves are often much less than losses for bending waves, and damping treatments often less effective on longitudinal waves. When the dissipation in longitudinal waves is less than that on bending waves, longitudinal waves can provide an efficient means of power flow between bending waves generated at one location and bending waves that are a source of acoustic radiation at another location. In order to design and locate effective treatments, knowledge of the power flow in longitudinal as well as bending waves is required. The measurement of power in both bending and longitudinal waves when both waves are present is demonstrated. Measurements conducted on a straight beam and a T-beam are compared to predictions obtained using finite element methods. The effect of coupling between waves at the junction in a T-beam is illustrated using results from measurements of power flow.     

Weak compressible magnetohydrodynamic turbulence in the solar corona

This Letter presents a calculation of the power spectra of weakly turbulent Alfvén waves and fast magnetosonic waves ("fast waves") in low- plasmas. It is shown that three-wave interactions transfer energy to high-frequency fast waves and, to a lesser extent, high-frequency Alfvén waves. High-frequency waves produced by MHD turbulence are a promising explanation for the anisotropic heating of minor ions in the solar corona.     

Van der waals waves in free-surface liquids

It is shown that, along with gravity waves, surface and internal waves caused by van der Waals forces may exist in a liquid with a horizontal free surface. A dispersion relation is found by using the stepwise approximation for the coefficients of a wave equation derived for these waves. The surface waves are similar to surface gravity waves in dispersion and amplitude distribution but differ in frequency by several orders of magnitude. Another sequence of roots in the spectrum corresponds to internal van der Waals waves that have an upper frequency bound and the dispersion law typical of a multimode waveguide.     

Tunneling of evanescent waves into propagating waves

The tunneling of evanescent waves into propagating waves is related to the convolution of the high spatial frequencies of the source with those of the detectors. Such an approach is demonstrated by treating the evanescent waves which are diffracted from very narrow apertures in a plane screen (with dimensions much smaller than the wavelength) and are converted to propagating waves by tip detectors. The mechanism responsible for the conversion of evanescent waves into propagating waves is explained and a general formula for the conversion of evanescent waves into propagating waves is derived. PACS 42.25.Fx; 42.30.Kq; 42.25.Bs     

A theoretical study of ultrasonic wave transmission through a fluid-solid interface

This article develops a model for the study of the transient ultrasonic waves radiated by a transducer in a liquid and transmitted into a solid through a plane interface. The method is an extension to the transient case, of the angular spectrum method previously developed for the monochromatic case. It is based on the decomposition of the ultrasonic field, in impulse plane waves. The radiated waveform is calculated at any point in the field by a simple summation of these impulse plane waves, where the propagation delay and the refraction have been taken into account. These plane waves are, first of all, delayed by an amount of time corresponding to the travel time up to the considered field point. The transmission through the plane interface is taken into account by using Snell refraction laws and transmission coefficients. In the obtained results all the waves previously described by other authors are highlighted: direct wave, edge waves, head waves as well as subsurface waves with a clear resolution between compression and shear waves.     

Gravitational Waves and Extra Dimensions: A Short Review *

We give a brief review on the recent development of gravitational waves in extra-dimensional theories of gravity. Studying extra-dimensional theories with gravitational waves provides a new way to constrain extra dimensions. After a flash look at the history of gravitational waves and a brief introduction to several major extra-dimensional theories, we focus on the sources and spectra of gravitational waves in extra-dimensional theories. It is shown that one can impose limits on the size of extra dimensions and the curvature of the universe by researching the propagations of gravitational waves and the corresponding electromagnetic waves. Since gravitational waves can propagate throughout the bulk, how the amplitude of gravitational waves decreases determines the number of extra dimensions for some models. In addition, we also briefly present some other characteristics of gravitational waves in extra-dimensional theories.     

在金属与光折变晶体界面形成的表面波研究

应用质点振荡模型和数值模拟方法研究了在金属与光折变晶体界面形成表面波的条件及其能量变化. 结果表明: 传播常数的正负影响表面波的类型及波能量分布, 当传播常数取负值时在界面处形成非局域表面波, 取正值时在界面处形成振荡表面波和局域表面波, 局域表面波的能量随传播常数的变大而单调递增. 在一给定的物理系统中, 可通过调节决定非线性效应强度的可变参量控制不同阶数局域表面波模及其传播波形.     

Nonlinear effects in trapped modes of standing waves on the surface of shallow water

It was shown that traveling waves may coexist with standing waves in a planar infinitely long channel filled by ideal liquid with a free surface. The standing waves are localized near a dynamic inclusion—a massive die on an elastic base. The amplitude of the traveling waves may be turned to zero by appropriately selecting the vibration frequency of the die. The standing waves arise because the vibration eigenfrequencies have a mixed spectrum; that is, the discrete and continuous spectra superpose. Nonlinear effects were observed for the first time when standing waves form in shallow water. In particular, a relationship between the die weight necessary to excite trapped modes, die dimensions, and vibration frequency was derived. It was shown that the nonlinear effects cause double-frequency traveling waves with amplitudes of the next order of smallness. These traveling waves vanish if the die geometry is properly chosen, as for the waves of the zeroth order.