Numerical simulation of Bragg scattering of sound by surface roughness for different values of the Rayleigh parameter

Numerical simulation methods are described for the spectral characteristics of an acoustic signal scattered by multiscale surface waves. The methods include the algorithms for calculating the scattered field by the Kirchhoff method and with the use of an integral equation, as well as the algorithms of surface waves generation with allowance for nonlinear hydrodynamic effects. The paper focuses on studying the spectrum of Bragg scattering caused by surface waves whose frequency exceeds the fundamental low-frequency component of the surface waves by several octaves. The spectrum broadening of the backscattered signal is estimated. The possibility of extending the range of applicability of the computing method developed under small perturbation conditions to cases characterized by a Rayleigh parameter of ≥1 is estimated.     

Effects of wind waves on horizontal array performance in shallow-water conditions

We analyze the influence of statistical effects of the propagation of an acoustic signal excited by a tone source in a shallow-water channel with a rough sea surface on the efficiency of a horizontal phased array. As the array characteristics, we consider the angular function of the array response for a given direction to the source and the coefficient of amplification of the signal-to-noise ratio (array gain). Numerical simulation was conducted in to the winter hydrological conditions of the Barents Sea in a wide range of parameters determining the spatial signal coherence. The results show the main physical effects of the influence of wind waves on the array characteristics and make it possible to quantitatively predict the efficiency of a large horizontal array in realistic shallow-water channels.     

Time reversed wave propagation experiments in chaotic micro-structured cavities

The elastic wave propagation in strongly scattering solid-state cavity consisting of a thin micro-patterned silicon wafer is studied experimentally. The chaotic behavior is induced by the irregular boundary of the cavity and/or by fabricating patterns of small holes in the wafer by laser machining. The pattern and hole size are designed with length scales matching the wavelength 相似文献   

Efficiency of excitation of space-charge waves in a thin-film semiconductor structure with a single strip Schottky barrier

Results of a theoretical analysis of the influence of various factors on the efficiency of excitation of space-charge waves in thin-film semiconductor structures by a single strip Schottky barrier are presented. It is shown that to increase the efficiency of conversion of a microwave signal into space-charge waves, it is advisable to use a Schottky barrier with a small contact potential with optimized values of the width of the Schottky barrier and the electron concentration in the film. Zh. Tekh. Fiz. 69, 128–130 (January 1999)     

考虑信号灯影响的交通流模型与数值模拟

基于信号灯交通系统中跟车行为的特点,改进了现有跟车方程,提出一个考虑信号影响的交通流模型.数值计算结果表明,该模型可以很好地再现车流的聚集与消散、停车波和启动波的演化规律及传播特性,证明模型是合理有效的. 关键词： 交通流模型 信号灯 停车波 启动波     

电磁波在非均匀碰撞等离子体中的透射增强效应

为研究碰撞等离子体对电磁波传输性质的影响,基于电磁波在介质中的传输特性,将等离子体作为一种特殊的介质,针对一定实验条件下的高功率微波（HPM）大气等离子体与一定范围电磁波的透射特性开展了实验、理论及仿真研究。研究发现:S波段HPM在50 Pa真空下形成的等离子体对不同频率的电磁波透射特性具有较大影响,且在一定频率范围内有规律地出现电磁波透射信号增强效应现象;获取了一系列不同频率连续电磁波穿过HPM等离子体区域的透射波形,并对波形进行了归一化处理,在32.4 GHz下,连续电磁波穿过有无等离子体区域的透射系数约有2倍的差异。建立了仿真模型,获得31.5～32.5 GHz范围内透射系数分布曲线图,穿过等离子体的电磁波出现透射增强效应,且在某些频点上出现了约1.9倍的透射增强。该研究成果为HPM大气等离子体在隐身、应急通讯、黑障通讯等方面的应用提供了重要的技术支撑。     

2~4μm可调谐OPO参量允差的数值分析

 计算了小信号近似下，KTP晶体中三波互作用允许角，分析了允许角随输出波长的变化规律。研究了Nd:YAG 1 064nm泵浦的KTP-OPO在x — z调谐面上的相位匹配，输出波长不同时失配角的变化对相位失配系数的影响，晶体长度对允许角的影响，以及在一定的调谐精度范围内晶体长度对转换效率的影响。所得结果对红外可调谐参量振荡器的技术设计提供了理论依据。     

Absorption of finite amplitude focused ultrasound

Predictions of the absorption of focused finite amplitude waves based on weak shock theory have been tested experimentally. The characteristics of this absorption are qualitatively different from those associated with small signal losses. Under appropriate conditions, the absorption of finite amplitude ultrasound is determined largely by source amplitude, field geometry, and the nonlinear properties of the medium and is only weakly dependent upon the small signal absorption coefficient of the material. These effects are seen most dramatically in sharply focused sound fields. To emphasize nonlinear absorption in an experimental test of these predictions, measurements of heating were made in agar which has a very small linear absorption coefficient. Under appropriate conditions, nonlinear losses can make the effective absorption coefficient of this poorly absorbing material somewhat greater than the soft tissues of the body.     

基于群速度失配的超声兰姆波二次谐波的时域测量方法

考虑到发射和接收换能器对超声兰姆波时域二次谐波信号所带来的不可避免的影响,提出一种基于基频与二倍频兰姆波群速度失配的超声兰姆波二次谐波的时域测量方法。当基频与二倍频超声兰姆波的相速度匹配而群速度失配时,在超声兰姆波传播过程中所发生的二次谐波信号,在时域上可与源于斜劈换能器的二次谐波信号相分离。采用仅源自于基频兰姆波的时域二次谐波的积分振幅,定量描述兰姆波二次谐波的发生效率。以铝板中传播的兰姆波为例,给出了时域二次谐波的具体测量过程。本文提出的测量方法放宽了超声兰姆波二次谐波的测量条件,且扣除了换能器对二次谐波信号所带来的影响,所测得的二次谐波信号完全来自于基频兰姆波时域信号的二次谐波发生效应。      

声表面波在圆弧处反射及透射的数值研究*

为了研究直达表面波在圆弧过渡面处传播性能的变化,采用有限元方法模拟了热弹机制下,线性脉冲激光辐照金属铝块表面时激发的表面波在近表面传播过程中,在不同曲率半径的圆弧处发生的反射及透射现象,建立了圆弧半径与反射表面波以及透射表面波时域信号特征之间的联系。计算结果表明:曲率半径与表面波中心波长的数值关系对表面波在圆弧处的传播有显著的影响;同时证明了根据透射表面波信号的到达时间可以反演圆弧半径的大小,为之后利用表面波信号定量检测材料表面圆弧凹痕的深度提供了理论依据。     

板状结构Lamb波频散补偿与聚焦接收成像检测

对板状结构中Lamb波的频散和多模现象进行了理论分析,采用有限元仿真方法获取了Lamb波与缺陷作用后的回波信号,对信号进行聚焦接收处理,即在一定距离范围内利用导波频散特性对各个模式信号进行频散补偿,提取补偿后信号的幅值,结果表明当所得信号幅值最大时,所对应的补偿距离等于缺陷与换能器之间的实际距离.利用这一结论,提出了以...     

Noncontact measurement of the thermal diffusivity of IR semi-transparent and semiconducting n-CdMgSe mixed crystals by means of the photothermal radiometry

In this paper we present results of noncontact measurements of the thermal diffusivity of infrared semi-transparent n-CdMgSe mixed semiconductor crystals by means of the photothermal radiometry (PTR) in a transmission configuration. In order to overcome an influence of the infrared semi-transparency and plasma waves on the PTR signal from n-CdMgSe mixed crystals the samples were covered by thin aluminum foils on both sides. The thermal diffusivities of n-CdMgSe mixed crystals were estimated from PTR phase frequency characteristics using a well-known formula. It was found that the obtained results are underestimated in comparison to thermal diffusivities estimated from the PPE (photopyro-electric) measurements. A three layer model of a PTR signal was applied in order to estimate an error in determination of the thermal diffusivity of a sample caused by aluminum foils.     

Experimental study of propagation of instability waves in a submerged jet under transverse acoustic excitation

An experimental study was conducted on the specific features of instability wave propagation in the mixing layer of a turbulent jet when the jet is excited by an external acoustic wave. We used the technique of conditional phase averaging of data obtained by particle image velocimetry using the reference signal of a microphone placed near the jet. The influence of the excitation frequency on the characteristics of large-scale structures in the mixing layer was investigated. It is shown that the propagation patterns of the instability waves agree well with previously obtained data on the localization of acoustic sources in turbulent jets.     

Interaction of microwave signals in a resonance transmission line based on magnetostatic waves

Interaction between two (small and large) regular microwave signals that simultaneously pass through a resonance transmission line based on backward volume magnetostatic waves is studied experimentally in different frequency ranges. It is found that the amplitude-frequency response of the small signal near the frequency of the large one depends on the frequency of the latter; namely, the amplitude of the small signal is affected by spin-wave packets parametrically excited by magnetostatic and electromagnetic pumps.     

Neuromorphic Calculations Using Lateral Arrays of Magnetic Microstructures with Broken Translational Symmetry

A possibility to control the characteristics of spin waves in a lateral array of magnetic microstructures with broken translational symmetry is demonstrated. The regimes of spatial and frequency selection of the spinwave signal are studied by Brillouin light scattering and by numerical simulations. The micromagnetic simulation is used to study the effect of geometric parameters on the characteristics of dipole-coupled spin waves. The specific features of the coupling between the transverse modes propagating in the system with broken translational symmetry are revealed. The results can be applied to develop multiplexers, power dividers, couplers, and the ultrahigh frequency signal processing circuits using the neuromorphic principles, which are based on the lateral arrays of magnetic microstructures.     

Spatial–frequency selection of magnetostatic waves in a two-dimensional magnonic crystal lattice

Features of the propagation of magnetostatic waves in a tangentially magnetized magnonic crystal structure based on iron–yttrium garnet with a two-dimensional array of grooves on the surface are studied. Numerical simulation is performed by the finite element method and the dispersion characteristics and the spatial distribution of fields of eigenmodes of surface magnetostatic waves propagating in this structure are calculated. The characteristics of waves in the magnonic crystal structure are experimentally studied by means of Brillouin scattering. It is shown that the formation of waveguide channels is possible when the frequency of the input signal is close to the frequency of the band gap of the structure.     

磁化分层等离子体中电磁波传播特性研究

采用混合矩阵法,分析了磁化分层等离子鞘套对斜入射电磁波传播特性的影响,分别计算了不同入射角以及外加磁场下电磁波透射系数和极化特性的变化。以GPS导航信号右旋圆极化波(RCP)为例,研究了磁场、电子密度对电磁波右旋圆极化特性的影响。结果表明,外加磁场能够使右旋圆极化波在等离子体中的阻带向高频方向移动,此外,外加磁场能在一定程度上改善斜入射是圆极化波的极化特性,有利于GPS信号接收。     

Impact of the spatial coherence on self-interference digital holography

Owing to the unique feature that the signal and reference waves of self-interference digital holography (SIDH) contain the same spatial information from the same point of object, compared with conventional digital holography, the SIDH has the special spatial coherence properties. We present a statistical optics approach to analyzing the formation of cross-correlation image in SIDH. Our study reveals that the spatial coherence of illumination light can greatly influence the imaging characteristics of SIDH, and the impact extent of the spatial coherence depends substantially on the recording distance of hologram. The theoretical conclusions are supported well by numerical simulation and optical experiments.     

小样本字典学习的兰姆波模态识别方法*

针对兰姆波多模态识别问题,提出了基于小样本字典学习的模态识别方法。将多层复合板的频散特性看作一个线性时不变系统,首先,根据频散知识模拟各个模态传播特定距离后的信号,提取走时和能量特征创建字典;其次,获取待测信号的走时特征,通过查询字典来识别兰姆波模态;最后,根据能量参数估计结果,实现待测信号中各模态信号的分离和重构。通过对三层粘接的AAA板(铝板-亚克力板-铝板,每层厚度为2mm)中传播距离为0.3m、0.5m的直达波和反射波的实验验证,结果显示该方法对A0、S0模态的有效识别和各个波包信号的准确重建。     

Primordial gravity waves and weak lensing

Inflation produces a primordial spectrum of gravity waves in addition to the density perturbations which seed structure formation. We compute the signature of these gravity waves in the large scale shear field. The shear can be divided into a gradient mode (G or E) and a curl mode (C or B). The latter is produced only by gravity waves, so the observations of a nonzero curl mode could be seen as evidence for inflation. We find that the expected signal from inflation is small, peaking on the largest scales at l(l+1)C(l)/2pi<10(-11) at l=2 and falling rapidly thereafter. Even for an all-sky deep survey, this signal would be below noise at all multipoles.