Low-frequency horizontal acoustic refraction caused by internal wave solitons in a shallow sea

The effect of internal wave solitons on the sound field generated by a point source in a shallow sea is considered. In the framework of the theory of “horizontal rays and vertical modes,” the sound field pattern governed by the aforementioned hydrodynamic effect is investigated. It is shown that solitons can induce time-periodic focusing and defocusing of horizontal rays propagating at shallow angles to the internal wave front. This may result in the formation of “dynamical” horizontal sound channels, which, in its turn, results in considerable temporal fluctuations of the field along the acoustic track oriented along the internal wave front. For the sound field calculations, an approach is developed on the basis of the parabolic approximation in the horizontal plane and the mode representation in the vertical direction. The results obtained can be used for remote monitoring of internal wave packets in a shallow sea.     

Generation of custom modes in a Nd:YAG laser with a semipassive bimorph adaptive mirror

Custom modes at a wavelength of 1064 nm were generated with a deformable mirror. The required surface deformations of the adaptive mirror were calculated with the Collins integral written in a matrix formalism. The appropriate size and shape of the actuators as well as the needed stroke were determined to ensure that the surface of the controllable mirror matches the phase front of the custom modes. A semipassive bimorph adaptive mirror with five concentric ring-shaped actuators and one defocus actuator was manufactured and characterised. The surface deformation was modelled with the response functions of the adaptive mirror in terms of an expansion with Zernike polynomials. In the experiments the Nd:YAG laser crystal was quasi-CW pumped to avoid thermally induced distortions of the phase front. The adaptive mirror allows to switch between a super-Gaussian mode, a doughnut mode, a Hermite-Gaussian fundamental beam, multi-mode operation or no oscillation in real time during laser operation. PACS 42.60.Jf; 42.60.Da; 42.60.By     

准直高功率激光束斜程大气传输中的非线性热畸变效应

本文根据波动光学理论,利用实际大气参数对准直高功率激光在斜程上行大气传输中的非线性热畸变效应进行了数值分析,结果表明,在上行大气传输中,尽管大气气体吸收系数较小,从而产生的热畸变效应也较弱,但它将导致激光光束位相畸变和发散角的显著增大,因此对远距离传输的光束将产生非常严重的不利影响。     

Influence of the benthic front on the mode structure of acoustic field in the ocean

The characteristic features of the mode structure of low-frequency sound field in the ocean are investigated for the ocean regions containing the benthic front, which represents the boundary between the Antarctic bottom water and the deep water. The hydrographic characteristics of the benthic front are described with the use of the data of the international WOCE experiment. On the basis of mathematical simulation, the changes introduced by the benthic front into the phase and group velocities and the vertical structure of modes are estimated for a frequency range of 5–20 Hz. Possible ways of reconstructing the hydrographic characteristics of the benthic front by the mode tomography methods are considered.     

浅海内波影响下的波导不变量变化特性分析

针对浅海内波引起波导不变量变化的问题,利用声场波导不变量的概率分布并结合声场简正波的理论,研究了内波活动下波导不变量的时变性,给出了波导不变量变化的机理和规律.具体结论是,在负跃层波导中,声场的波导不变量的最大概率取值具有明显的频变特性.内波环境下,当声传播方向与内波波阵面平行时,接收声场简正波的幅度变化不大,但是简正波的相慢度差和群慢度差的变化却能引起波导不变量最大概率取值的变化;而当声传播方向与内波波阵面垂直时,内波引起的简正波耦合同样会导致波导不变量的最大概率取值的明显变化.     

Phase transformation of quasicrystals in aluminium-transition metal alloys

Quasicrystals in Al–Mn, Al–Cu–TM (TM = Fe, Cr, Mn and Ru) and Al–Cu–Fe–Cr alloys can undergo two different modes of phase transformation. Discontinuous transformations of quasicrystals are characterized by the existence of a definite reaction front separating the quasicrystalline phase from the resulting crystalline one; the kinetics are controlled by the migration of the reaction front. Continuous transitions, on the other hand, proceed by structural evolution such as modulation or chemical ordering inside the quasicrystalline phase without creating any high-energy interfaces. Both types of transformations are thermally activated and need atomic diffusion.     

Formation,propagation, and focusing of acoustic beams in oceanic waveguides. The quasi-optics approximation

Using the WKB approximation and assuming that the spatial spectrum of modes excited by a vertical array is sufficiently narrow, a quasi-optical theory is developed to describe the fundamental regularities manifesting themselves in the formation, propagation, and focusing of multimode acoustic beams in oceanic waveguides. Functional dependences on the parameters of both the transmitting array and the oceanic waveguide are obtained for horizontal distances at which ordinary beams form refraction focusing zones. Conditions ensuring the formation of a beam with the minimum wave front divergence are formulated for the distribution of the source excitation factor over the array aperture.     

Zone folding induced tunable topological interface states in one-dimensional phononic crystal plates

In previous literature, the realization of topological interface state in one-dimensional periodic system is strongly relied on the tedious parameter adjustment to search for the Dirac cone. In this paper, based on a strategy of zone folding, multiple topological interface modes for the shear horizontal guided waves in one dimensional phononic crystal plate are investigated by using finite element method and eigenmode matching theory, in which the Dirac points are formed by simply making the unit cell double. Significantly, by simply contracting or expanding the stubs can bring the topological phase transition. Furthermore, the topological phase transition is further achieved by varying the height of the stubs. The proposed designs will be more convenient to be applied in real engineering.     

利用水平阵模态域波束形成判别声源深度*

针对近水面声源和水下声源的深度判别问题,根据近水面声源难以激发低阶模态的物理现象,研究利用声源波数谱结构和波数位置的不同来分辨近水面声源和水下声源。通过采用MVDR的谱估计方法进行模态域波束形成,补偿水平阵各阵元之间各号简正波的相位差,获得主瓣窄、旁瓣低的声源信号波数谱。波数谱的波数位置与频率呈近似线性关系,水中声速剖面、海底参数、海深都会影响波数谱的具体结构和位置。此外,声源信号的到达角估计误差同样也会影响波数谱主瓣的位置估计。数值仿真结果表明,在浅海负跃层声速剖面条件下,可利用水平阵模态域波束形成判别声源深度,区分近水面声源和水下声源。     

Instability in Two-Sided Thermocapillary-Buoyancy Convection with Interfacial Phase Change

A new model of two-phase thermocapillary-buoyancy convection with phase change at gas-liquid interface in an enclosed cavity subjected to a horizontal temperature gradient is proposed,rather than the previous onesided model without phase change.We study the onset of multicellular convection and two modes of convective instability,and find four different flow regimes.Their transition map is compared with the non-phase-change condition.Our numerical results show the stabilizing effect of interfacial phase change on the thermocapillarybuoyancy convection.     

Boundary effects in the diffraction of light by volume gratings: II. Phase transmission gratings

The two-wave approximation of the method of characteristics modes is used to analyze theoretically the diffraction of light by volume phase transmission gratings for arbitrary proportions between the average refractive index of the grating and the refractive indices of the ambient media. Simple expressions are derived for calculating the amplitudes of the diffracted waves in the case where the angle of incidence of the primary wave coincides with the Bragg angle. The roles of the front and back boundaries of the grating are discussed separately, and a new method for separating the contribution of the boundary diffraction, which is due to the periodic inhomogeneity of the grating boundaries, is proposed. The experimental possibility of selecting partial Bloch waves in the measurements of the diffraction efficiency of gratings that have high reflection coefficients of both boundaries is demonstrated.     

单杠结构差分离散系统的振动反问题

研究组合结构中的杆-梁-杆组合单支结构,或称单杠结构的模态反问题,从数学上证明单杠结构差分离散系统固有振动的刚度矩阵的符号振荡性,并导出其频率和振型的一些定性性质.在此基础上,探讨由两组位移或应变振型及相应频率构造物理参数的条件和方法,给出两个计算实例.讨论结构参数关于其几何对称轴具有对称性的条件下的模态反问题,并给出计算实例.计算结果表明,上述反问题提法合理,解法正确.     

Phase compensation of laser beam modes

A technique is described for improving the propagation characteristics of high order laser beam modes. A thin phase plate inserted in the beam forces all portions of the wave front to oscillate in phase and yields a substantial improvement in the far field intensity distribution. Depending on the mode order, the peak intensity at a distant target may be readily increased by more than a factor of ten.     

Pekeris波导中简正波的复声强及其应用

在Pekeris波导模型下,关注了简正波的矢量场,讨论了简正波水平复声强和垂直复声强的表述,并分析了其特征.单阶简正波在水平方向是行波,相应的水平复声强仅为有功的;在垂直方向为驻波,相应的垂直复声强仅为无功的.而多阶简正波相互干涉,因此总声场的复声强既有有功分量,也有无功分量,其中只有有功分量参与声能的输运,但无功分量是反映声场信息的重要组成部分.通过对垂直(交互)复声强无功分量和水平交互复声强有功分量的数值分析,对于甚低频率的点源声场,发现当声源深度变化时,上述声场分量的正负号呈有规变化,当接收传感器置 关键词： 目标深度分类 复声强 矢量场 Pekeris波导     

A spiral wave front beacon for underwater navigation: transducer prototypes and testing

Transducers for acoustic beacons which can produce outgoing signals with wave fronts whose horizontal cross sections are circular or spiral are studied experimentally. A remote hydrophone is used to determine its aspect relative to the transducers by comparing the phase of the circular signal to the phase of the spiral signal. The transducers for a "physical-spiral" beacon are made by forming a strip of 1-3 piezocomposite transducer material around either a circular or spiral backing. A "phased-spiral" beacon is made from an array of transducer elements which can be driven either in phase or staggered out of phase so as to produce signals with either a circular or spiral wave front. Measurements are made to study outgoing signals and their usefulness in determining aspect angle. Vertical beam width is also examined and phase corrections applied when the hydrophone is out of the horizontal plane of the beacon. While numerical simulations indicate that the discontinuity in the physical-spiral beacon introduces errors into the measured phase, damping observed at the ends of the piezocomposite material is a more significant source of error. This damping is also reflected in laser Doppler vibrometer measurements of the transducer's surface velocity.     

Method for the distinguish of normal modes in shallow water under near field

This paper proposes a method to distinguish the normal modes in shallow water under near field.The method,which is based on frequency-wavenumber(F-K) transform,employs the 2D matrix comprising near field horizontal multi-channel signals.These signals are transformed from the time-space domain to the frequency-wavenumber domain,and the normal modes in the signals are distinguished by extracting the wavenumbers component at each frequency for each order in the frequency-wavenumber domain.The practical reliability and validity of this method are verified by performing numerical simulation and a tank experiment,and the results showed that the normal modes in the Pekeris waveguide below 200 Hz could be obtained reliably at 500 m by using this method.     

Propagation of beluga echolocation signals

The propagation characteristics of high-frequency echolocation signals (peak energies above 100 kHz) of the beluga (Delphinapterus leucas) were measured while the animal performed a target detection task. The whale was trained to station on a bite plate so that its transmission beam could be measured in the vertical and horizontal planes using hydrophone arrays. The transitional region between the acoustic near- and farfields was also located using an array of hydrophones that extended directly in front of the animal in the horizontal plane. Three distinct modes of signals were observed. Mode 1 signals had click intervals greater than the time required for the signals to travel to the target and back (two-way transit time). Mode 2 signals had click intervals shorter than the two-way transit time, and mode 3 signals had high repetition rates with an average click interval of 1.7 ms, approximately 2% of the two-way transit time. The average click intervals for the modes 1 and 2 signals were 193 and 44 ms, respectively. The vertical and horizontal beam patterns of the mode 1 signals had similar 3-dB beamwidths of approximately 6.5 degrees. The major axis of the vertical beam was directed approximately 5 degrees above the plane defined by the animal's teeth. The near- to farfield transition region was approximately 0.64-0.75 m from the tip of the animal's mouth.     

The efficiency of the reversed wavefront field with a phase shift focusing in random media

The effectiveness of reversed wave focusing in a random atmosphere was studied by using the phase approximation of geometrical optics as well as in an oceanic waveguide using the method of normal modes and horizontal rays. Calculations were made for the turbulence fluctuation obeying a structural function with a 2/3 power law. To provide focusing of the field radiated by a point-like source into a given point, a correction phase factor, determined for a homogeneous medium, was introduced. We estimated that the size of a region near the location of the source wherein the intensity of a field, focused in accordance with the free-space algorithms, decreased due to random inhomogeneities to no less than half of its original size. The minimal number of reference sources necessary for effective focusing at any point in the region under consideration was estimated.     

Comparison of hybrid three-dimensional modeling with measurements on the continental shelf

During the CALOPS 2007 experiment, off the coast of Fort Lauderdale, Florida, three-dimensional (3D) multipath was observed using a bottom mounted horizontal line array during source tows along the 200 m isobath [Kevin D. Heaney and James J. Murray, J. Acoust. Soc. Am. 125(4), 1394-1402 (2008)]. In this paper a hybrid modeling approach is presented to model the 3D sound on the Florida shelf, nearly shaped like the canonical wedge. The hybrid approach combines vertical acoustic normal modes with the parabolic equation solution (in range/cross-range). The approach is shown to satisfy the 3D Cartesian-coordinate wave equation in the limit of adiabatic mode propagation. In the adiabatic mode parabolic equation (AMPE) approach modal phase speeds vs position are used as the input to the parabolic equation computation with dimensions of easting (km) and northing (km). Vertical adiabatic modes and horizontal rays are also computed to illustrate the 3D multipath arrival. The AMPE field is computed for all the modes for each element of the horizontal array. Beamforming vs source range is then conducted and excellent agreement with data is achieved.