A Laboratory Study of Strong Modulation of Radar Signals due to Long Waves on the Water Surface Covered with a Surfactant Film

Modulation of radar backscatter due to long wind waves in the presence of surfactant films is studied for the first time under wave-tank conditions. It is found that the modulation coefficient can increase significantly with increasing surfactant concentration. We show that the existing theoretical models of modulation of free surface waves underestimate the observed modulation levels. It is demonstrated that the experimental radar Doppler shifts cannot be explained by only the scattering due to free gravity-capillary waves (GCW) and are also determined by coupled waves, which are nonlinear harmonics of the longer wind wave components. Estimates of the relative intensity of coupled waves in the wind-wave spectrum at the Bragg wavelength are presented. It is shown that the contribution of coupled waves is considerable and increases with increasing surfactant concentration, thus confirming the hypothesis for a cascade mechanism of strong backscatter modulation in the long-wave field.     

Compton散射下强激光等离子体空气冲击波波前的传输特性

 根据球对称激光等离子体空气冲击波在自由空间中传输的约束条件,对多光子非线性Compton散射的强激光等离子体空气冲击波波前的传输特性进行了研究,给出了散射下空气冲击波波前的运动方程,并进行了数值模拟。结果表明：该冲击波的衰减过程不仅与爆炸源和爆炸过程的特性、释放总能量、空气的弹性有关,而且还与散射有关,散射效应使冲击波的初始半径增大,衰减过程加快,能量转移率提高;数值模拟与实验结果符合得很好。     

Incredible negative values of effective electromechanical coupling coefficient for surface acoustic waves in piezoelectrics

The extraordinary case of increase in velocity of surface acoustic waves (SAW) caused by electrical shorting of the surface of the superstrong piezoelectric crystal potassium niobate, KNbO3, is numerically found. The explanation of this effect is based on considering SAWs as coupled Rayleigh and Bleustein-Gulyaev modes. A general procedure of approximate decoupling of the modes is suggested for piezoelectric crystals of arbitrary anisotropy. The effect under study takes place when the phase velocity of uncoupled sagittally polarized Rayleigh waves is intermediate between the phase velocities of uncoupled shear-horizontal Bleustein Gulyaev waves at the free and metallized surfaces. In this case, the metallization of the surface by an infinitely thin layer may cause a crossover of the velocity curves of the uncoupled waves. The presence of the mode coupling results in splitting of the curves with transition from one uncoupled branch to the other. This transition is responsible for the increase in SAW velocity, which appears to be greater than its common decrease produced by electrical shorting of the substrate surface.     

无缝钢轨中温度对应力非线性超声检测的影响

研究了温度与应力共同作用下的非线性超声响应规律。构建了纵波在无缝钢轨内传播的数学模型,将传统原子间势能引入非线性波动方程,得到了无缝钢轨在自由状态及受约束状态两种情况下超声非线性系数随温度变化的函数关系。在理论分析的基础上,进行了无缝钢轨受约束时内部温度应力变化的实物模拟实验,获得的非线性系数变化趋势与理论预期相一致。研究结果表明:温度和应力都对固体介质非线性影响明显,介质非线性系数随着温度的升高而增大,随着内部压应力的增加而减小。因此,当利用非线性系数检测钢轨纵向应力时,需要对温度的影响进行补偿。      

On the long-range detection and study of undamped directed temperature waves generated during the interaction between a cavitating water jet and targets

The features of the propagation of undamped thermal (temperature) waves in air are investigated. The presence of these waves is a consequence of solution of the heat equation taking into account the relaxation of local thermal perturbation. It is shown that such waves can exist only in media with a finite (nonzero) time of local thermal relaxation, and their frequencies are determined by this time. The time of relaxation in air depends on the gas composition, its temperature and increases with a decrease in pressure. Under normal conditions, the minimum frequency of undamped waves in air corresponds to 70–80 MHz. One of the methods for exciting these waves is associated with pulsed heating of the surface of a medium bordering air. Pulsed heating on account of the application of shock waves generated during water jet cavitation is used. It is shown for the first time that these waves with frequencies in the range of 70–500 MHz can propagate in air without damping over a distance of up to 2 m.     

On the origin of the inverted-barometer effect at subinertial frequencies

Summary An analytical model, simulating the frictionless response of the sea contained in a rotating, rectangular channel of arbitrary width to air pressure waves travelling at varying directions, is developed. Since planetary atmospheric waves are of primary interest as forcing agents, a solution is found for subinertial frequencies. For an atmospheric wave travelling along a channel whose width is close to the Rossby deformation radius, the model predicts sea levels and currents organized in two coastal waves and a geostrophic current system prevailing in mid-basin. The right-hand coastal wave is more pronounced than the left-hand wave. The structure is coupled to the atmospheric wave, and is resonantly driven when the phase velocity of the forcing wave approaches the Kelvin wave velocity. Along the coasts a quasi-static adjustment occurs under off-resonant conditions. When the atmospheric wave is moving across the channel at a sharp angle, the response of the sea is enhanced for the apparent along-channel velocities below those of free shallow-water waves, due to reflections at channel boundaries. For the atmospheric wave that travels at right angle across the channel, the resonance is not possible, and the sea level undershoots a simple inverted-barometer response. Both travelling and standing waves appear in the channel. In the narrow-channel limit only a standing wave remains, with a nodal line in the middle of the channel. In the central part of the channel the currents are almost geostrophic at very low frequencies. The model is used to interpret some aspects of the response of the Mediterranean Sea to planetary-scale atmospheric forcing. In particular, it is shown that resonant transfer of energy from the atmosphere to the sea is most unlikely, since planetary atmospheric waves are rather slow and they travel along the main axis of the Mediterranean basin.     

On Mechanism of Influence of Low Intense Millimeter Waves on Air Content in Water

The two-stage mechanism is offered, on which the millimeter waves rises concentration of free air in water. At first, millimeter waves localize heating in a volume of the sample, creating supersaturated concentration of the dissolved air in a fluid. Then superfluous air diffuses in embryonic microbubbles present in a fluid. The experimental data confirm the mechanism suggested.     

Solitary waves in a long structure of charged particles confined in the linear Paul trap

This paper demonstrates the possibility of generation of density waves in the form of individual humps in a long structure of the one-component Coulomb system of dust particles confined in the linear Paul traps in air under normal conditions Our numerical simulations by particle-in-cell method support this possibility. Physical possibility of analogous the hump density waves (caustics) is discussed by V.I. Arnold in his book ‘Catastrophe Theory’ and is caused by the nonuniform velocity distribution of dust particles.     

平面冲击下铜的拉伸应变率相关特性研究

本文对冲击加载下高纯无氧铜的拉伸应变率相关特性进行了实验研究.实验中利用磁测速系统测试撞击前飞片速度,利用光纤位移仪——多普勒探针系统测试样品自由面粒子速度剖面.对自由面速度剖面的特征参量进行计算分析,结果表明:铜样品的层裂强度随着拉伸应变率的增加而增加,对比发现层裂强度不仅受加载条件的影响,同时受到材料本身微细观结构影响;同时随着拉伸应变率的增加,自由面速度的回跳斜率呈现出先缓慢增加后迅速增加的临界特性;最后,通过层裂样品中波系相互作用,给出了自由面速度回跳过程中的振荡特征随着拉伸应变率增加而逐渐消失的物理过程.     

Elastic properties of finite three-dimensional solid phononic-crystal slabs

We study theoretically, by means of layer-multiple scattering techniques, the propagation of elastic waves through finite slabs of phononic crystals consisting of metallic spheres in polyester matrix, embedded in air. We focus on the study of modes localized on the surfaces of the structure, investigating the physical parameters which influence and determine their appearance. Our results reveal the existence of absolute phononic frequency gaps in these finite structures, and point out the possibility, under an appropriate choice of the parameters, of tunable regions of frequency free of propagating and/or surface-localized modes. This could be very useful in the design of devices related to frequency filtering, waveguiding, etc.     

Pulsed TV holography and tomography for the study of transient waves in air

Transient waves in air are recorded and reconstructed using pulsed TV holography and computerized tomography (CT). Experiments are performed with an electrical discharge between two electrodes as the acoustic wave source. The free space wave-fronts and pressure fields are reconstructed. Waves reflected and diffracted by different obstacles are also recorded and reconstructed in three dimensions. Speckle averaging and image processing techniques are used to get the high quality projection fields needed for CT reconstruction.     

Generation and amplification of electromagnetic waves by an annular electron beam in a radial electric field in free space

Mechanisms for the generation and amplification of electromagnetic waves by a thin-walled annular beam of electrons rotating in a radial electric field in free space are studied theoretically. It is shown that electromagnetic waves can be generated and amplified under the Cherenkov resonance conditions. The frequencies and growth rates of the generated waves are determined, and the propagation characteristics and amplification coefficients of the amplified waves are found.     

Numerical simulations of shock-wave interaction with a boundary layer in the plane supersonic flows with jet injection

A supersonic air flow in a plane channel with a transverse turbulent jet of hydrogen injected through a slot on the bottom wall is simulated. The algorithm for solving the Favre-averaged Navier-Stokes equations for the flow of a perfect multispecies gas on the basis of the WENO scheme is proposed. The main attention is paid to the interaction of the shock-wave structure with the boundary layers on the upper and lower duct walls under the conditions of an internal turbulent flow. Namely, a detailed study of the structure of the flow is done, and separation and mixing depending on the jet slot width are investigated. It is found that in addition to well-known shock-wave structures produced by the interaction of the free stream with the transverse jet and the bow shock interaction with the boundary layers near the walls, an additional system of shock waves and the flow separation arise on the bottom wall downstream at some distance from the jet. The comparison with the experimental data is performed.     

Anomalous transparency of water-air interface for low-frequency sound

Sound transmission through a water-air interface is normally weak because of a strong mass density contrast. We show that the transparency of the interface increases dramatically at low frequencies. Almost all acoustic energy emitted by a sufficiently shallow monopole source under water is predicted to be radiated into air. Increased transparency at lower frequencies is due to the increasing role of inhomogeneous waves. For sources symmetric with respect to a horizontal plane, transparency is further increased by a destructive interference of direct and surface-reflected waves under water. The phenomenon of anomalous transparency has significant geophysical and biological implications.     

Bloch定理与散射平面波的干涉

从波动学的角度分析空晶格模型中的自由电子在周期性晶体势微扰作用下转变为晶体中Bloch电子的过程,揭示出了这一过程的物理实质,结果表明,在晶体势微扰作用下,代表空晶格模型中自由电子定态波函数的行进平面波在晶体内各点处产生了各种许可波矢的散射平面波,Bloch定理是晶体微观结构的平移对称性（即周期性）使得这些散射平面波产生干涉的结果。     

Physical investigation of acoustic waves induced by the oscillation and collapse of the single bubble

The objective of this paper is to apply both experimental and numerical methods to investigate acoustic waves induced by the oscillation and collapse of a single bubble. In the experiments, the schlieren technique is used to capture the temporal evolution of the bubble shapes, and the corresponding acoustic waves. The results are presented for the single bubble generated by a low-voltage bubble generator in the free field of water. During the numerical simulations, a three-dimensional (3D) weakly compressible model is introduced to investigate the single bubble dynamics, including the generation and propagation of acoustic waves. The results show that (1) Compression wave, rarefaction wave and shock wave are generated during expansion stage, collapse stage and rebound stage of the bubble respectively. (2) Compression waves are induced by the rapid expansion of the bubble and eventually steepen into one shock wave propagating outward in the liquid, then another strong shock wave is emitted at the final collapse stage. The velocity and pressure of the liquid field increases after the shock wave. (3) Rarefaction waves are generated during the collapse stage due to the contraction of the bubble. The rarefaction wave reduces the liquid pressure and its spatial distribution is dispersive. The pressure of these acoustic waves and their effect on the liquid velocity attenuate with the increase of propagation distance.     

Transmission of low-frequency sound through the water-to-air interface

L.M. Brekhovskikh revealed and studied the important role played by inhomogeneous waves emitted by a point source when they pass through an interface with a medium in which the velocity of sound is lower, for example, from water to air. This paper studies the energy characteristics of sound emitted into air by an underwater point source. The energy transfer due to inhomogeneous waves is shown to cause the phenomenon of anomalous transparency of the interface for low-frequency sound. The anomalous transparency manifests itself in that the energy flux through the interface increases with decreasing frequency of sound and, at sufficiently low frequencies, almost all of the acoustic energy produced by the underwater source is emitted into air. Conversely, at high frequencies, when the contribution of the inhomogeneous waves becomes negligible, the water-to-air interface is similar to a perfectly reflecting surface and almost all of the acoustic energy produced by the source is emitted into water. The anomalous transparency phenomenon changes the conventional opinion on the possibility of acoustic coupling between points in water and air and on the role played by physical processes evolving in the water column in generating atmospheric acoustic noise.     

用非饱和三相孔弹模型研究浅层土壤中地震波的传播特性

研究浅层土壤中声波耦合的地震波的传播特性, 用于声波探雷技术的机理分析. 根据浅层土壤具有孔隙度和可压缩性的特点, 利用非饱和三相孔隙介质中的地震波模型, 研究了土壤孔隙度、含水饱和度等参数对地震波传播特性的影响. 计算结果显示: 在给定的参数条件下, 地震波的传播速度和衰减系数均随频率的增加而增加; 纵波的传播速度随孔隙度的增加而减小, 横波的传播速度随孔隙度的增加而增加; 地震波的传播特性随含水饱和度的增加变化比较复杂. 通过对计算结果与已发表实验结果的比较分析, 讨论了解析方法的可行性, 为声-地震耦合机理及其在声波探雷研究中的应用提供了一定的理论基础. 关键词： 声-地震耦合 地震波 孔隙度 声波探雷     

Softening of ionic crystals as a result of a change in the spin states of structural defects under paramagnetic resonance conditions

The purpose of this work was to investigate the plastic properties of Ca-or Eu-doped NaCl and KCl single crystals in crossed constant and microwave magnetic waves under paramagnetic resonance conditions. It was found that when the photon energy of the microwave field equals the Zeeman splitting of the electronic spin sublevels, resonance softening of the crystals, manifested as an increase in the free path of individual dislocations and the macroplastic flow velocity as well as a decrease of the microhardness of the crystals, is observed. It was established that metastable Ca-and Eu-impurity complexes, which are also sensitive to the constant magnetic field in the absence of the microwave field, as well as complexes formed by dislocations and point defects are responsible for resonance softening.     

Remote ultrasonic defectoscopy of sound radiating objects through the air

A method of obtaining ultrasonic images of objects radiating ultrasonic acoustic waves in the air is proposed. Object images are retrieved by spatially matched filtering of the amplitude-phase distribution of the ultrasonic field measured in a planar rectangular region at a distance from the radiating object under investigation. The field is measured at different frequencies in a wide band, which allows one to reduce the level of noise and artifacts in the retrieved images by processing in a wide frequency band.