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.     

Acoustic propagation from a spiral wave front source in an ocean environment

A spiral wave front source generates a pressure field that has a phase that depends linearly on the azimuthal angle at which it is measured. This differs from a point source that has a phase that is constant with direction. The spiral wave front source has been developed for use in navigation; however, very little work has been done to model this source in an ocean environment. To this end, the spiral wave front analogue of the acoustic point source is developed and is shown to be related to the point source through a simple transformation. This makes it possible to transform the point source solution in a particular ocean environment into the solution for a spiral source in the same environment. Applications of this transformation are presented for a spiral source near the ocean surface and seafloor as well as for the more general case of propagation in a horizontally stratified waveguide.     

平板式螺旋相位板的设计与应用

传统的螺旋相位板是一种利用沿方位角方向介质材料高度递增实现对光束相位调控产生涡旋光束的光学器件,由于这种特殊的几何结构特征使其不能通过相位板的叠加而调控出射光束所携带的角量子数.本文基于坐标变换方法将介质材料沿方位角方向折射率不变而高度递增的传统螺旋相位板变换为一种介质材料沿方位角方向高度不变而折射率递增的平板式螺旋相位板.通过理论分析与数值模拟,发现本文所设计的平板式螺旋相位板不仅与传统螺旋相位板一样能够产生高质量的涡旋光束,而且平板式螺旋相位板的高度和涡旋光束携带的角量子数可以根据介质材料的折射率选取而任意调节.为了实际应用的需要,可以通过叠加多层平板式螺旋相位板以获得不同角量子数的涡旋光束.这种平板式螺旋相位板在光传输、光通信等领域具有广阔的潜在应用价值.     

Simulation of anisoplanatism of adaptive optical system in inhomogeneous turbulent atmosphere

The goal of this study was to analyse anisoplanatism of adaptive optics under an inhomogeneous turbulent atmosphere over a pupil of finite size. By means of a numerical model with layers of turbulence software was proposed by which point spread function (PSF), optical transfer function (OTF) as well as system isoplanatic angle can be calculated. Atmospheric turbulence was simulated with the aid of a set of moving random phase screens with arbitrary statistics. Both reference and target are assumed to be the point light sources. To simulate atmospheric turbulence we applied the concept of a number of moving random phase screens with Kolmogorov spectrum. In my investigation I used the model of the Shack-Hartmann wave front sensor and the ideal model of a wave front adaptive mirror that is assumed to reproduce a given number of Zernike polynomials without time delays. The designed software allows calculation of instantaneous and average values of phase correction errors at different angles between a reference beacon and target source. Simulations can be made with a broad range of parameters of an adaptive system and atmospheric turbulence. The system of the model allows changing of the control algorithm of phase correction. Both common phase conjugation and weighted phase conjugation algorithm have been tested. This program is capable of calculating the effects of beam diffraction during propagation in the atmosphere.     

Direct estimation of aberrating delays in pulse-echo imaging systems

Nearfield fluctuations in wave propagation velocity and system timing errors are among the sources of focusing aberrations in pulse-echo imaging systems. For situations in which the source of these errors can be modeled by a stationary phase aberrator placed in front of the transmitter and receiver aperture, appropriate electronic delays might be applied to the signals associated with each array element in order to restore the system to focus. A method is described and evaluated for estimating the set of aberrating delays in a linear array utilizing data from a single two-dimensional scan. The underlying principle is analogous to that of phase closure used for one-way passive interferometry and readily generalizes to two-dimensional arrays. Although the following theory is developed in the context of acoustic imaging, the general approach is applicable to other pulse-echo systems, such as radar.     

Supersonic radiatively cooled rotating flows and jets in the laboratory

The first laboratory astrophysics experiments to produce a radiatively cooled plasma jet with dynamically significant angular momentum are discussed. A new configuration of wire array z pinch, the twisted conical wire array, is used to produce convergent plasma flows each rotating about the central axis. Collision of the flows produces a standing shock and jet that each have supersonic azimuthal velocities. By varying the twist angle of the array, the rotation velocity of the system can be controlled, with jet rotation velocities reaching approximately 18% of the propagation velocity.     

Focusing of helico-conical cosh-Gaussian beam

Helico-conical beam is attractive for its phase front is nonseparable helical and conical phase function, which produces a spiral intensity distribution occurs at the focal plane. In this letter, the focal properties of cosh-Gaussian beam with helico-conical wavefront were investigated to show that helico-conical phase front does not always produce spiral intensity distribution. On increasing decentered parameters in cosh parts of the beams, the spiral focal pattern evolves into multiple intensity peaks with center main strongest peak. Under condition of higher decentered parameters, the focal spiral curve disappears. In addition, the topological charge also affects the focal pattern considerably.     

Experimental observation of tangent-azimuthal non-spiral optical vortex

Optical vortices have attracted much attention recently due to their novel properties and widening applications. And lots of optical vortices can be obtained though most of them turn on spiral pattern on increasing azimuthal angle. In this paper, one kind of non-spiral optical vortex was proposed whose front phase distribution is tangent function of azimuthal angle. And this kind of optical vortices were also observed experimentally by computer-generated hologram method. It was found that when topological charge is smaller than unit one, vortex beam shape changes considerably on increasing topological charge, from hollow pattern to curve shape. When topological charge is bigger than unit and is times of 0.5, vortex beam turns symmetrical polygonal pattern though there is crack between adjacent sides, and the side number is twice of topological charge.     

Connecting Spin and Statistics in Quantum Mechanics

The spin-statistics connection is derived in a simple manner under the postulates that the original and the exchange wave functions are simply added, and that the azimuthal phase angle, which defines the orientation of the spin part of each single-particle spin-component eigenfunction in the plane normal to the spin-quantization axis, is exchanged along with the other parameters. The spin factor (?1)^2s belongs to the exchange wave function when this function is constructed so as to get the spinor ambiguity under control. This is achieved by effecting the exchange of the azimuthal angle by means of rotations and admitting only rotations in one sense. The procedure works in Galilean as well as in Lorentz-invariant quantum mechanics. Relativistic quantum field theory is not required.     

圆弧片状声源向地层中辐射声场的有限差分模拟

提出了一种可用于随钻声波测井的非对称的圆弧片状声源,旨在发展一种能够在钻井过程中精确确定反射体位置的三维反射声波远探测方法。研究了在复杂的井孔条件(钻铤偏心、各向异性和不规则井孔形状)下,该声源向地层中辐射的声场的特征,进一步分析了不同井况条件对三维随钻远探测声波测井的影响。数值模拟结果表明,在圆形井孔、钻铤居中条件下,无论快速地层还是慢速地层,圆弧片状声源均能够向地层中定向辐射声波信号,其辐射指向性图主瓣3 dB角宽窄,旁瓣级低,方位分辨率较高,适用于随钻三维反射成像测井;井壁粗糙程度、地层各向异性等因素对声场特征影响不大,说明该方法适用于粗糙井壁和各向异性地层情况;钻铤偏心和井壁一侧的破坏对辐射声场的影响较大。对本文的模型而言,在破坏深度小于3 cm或者偏心距离小于2 cm时,声场仍有较好的方位特征,该方法仍然适用。而当破坏深度大于6 cm,声场指向性图出现多个幅值较大的角瓣,可能无法利用其确定反射体的方位。本文的研究结果为三维随钻反射声波仪的设计和研发提供了必备的理论基础。      

共轴双涡旋声束形成离轴多涡旋的定位

涡旋声束具有螺旋的相位波前,中轴线上形成声强为零的相位奇点,其所携带的轨道角动量在粒子操控领域有着广阔的应用前景。传统声涡旋只在传播轴线上形成一个拓扑荷可控的涡旋波束,这限制了声涡旋的应用灵活性。基于环形点声源阵列和相位编码技术,利用奇偶声源分别产生共轴双涡旋声束的声场叠加,在传播截面上形成了具有中心涡旋和子涡旋的离轴多涡旋声场;研究了双涡旋拓扑参数对离轴涡旋的个数、位置及拓扑荷的影响,基于声涡旋的径向声压和相位分布,确定了离轴涡旋的离轴半径,并结合声源位置推导子涡旋中心方位角的计算公式,实现离轴涡旋的精确定位。本研究突破了沿轴分布的涡旋声场只能形成单点涡旋势阱的操控局限,为利用离轴多涡旋实现多点粒子捕获提供了理论依据,促进涡旋声场在精确粒子操控和传输方面的高效应用。      

A Three-Dimensional Ray-Tracing Study of R-X Mode Waves during High Geomagnetic Activity

We further present a three-dimensional (3D) ray-tracing study on the propagation characteristic of the superluminous R-X mode waves during high geomagnetic activity following our recent two-dimensional results [J. Geophys. Res. 112(2007)A10214]. We perform numerical calculations for this mode which originates at specific altitude r=2.0R_E in the source cavity along a 70° night geomagnetic field line. We demonstrate that the ray path of the R-X mode is essentially governed by the azimuthal angle of the wave vector k. Ray paths starting with azimuthal angle 180° (or in the meridian plane) can reach the lowest latitude, but stay at relatively higher latitudes with the azimuthal angles other than 180° (or off the meridian plane). The results further supports the previous finding that the R-X mode may be physicallypresent in the radiation belts under appropriate conditions.     

线性黏弹性各向异性介质速度频散和衰减特征研究

地层岩石既非各向同性的,也非完全弹性的,正确地描述波在地层中的传播需要搞清楚岩石的各向异性和黏弹性特征.针对裂缝性地层的裂缝发育方位问题,建立具有任意方位角的方位各向异性黏弹性本构关系.使用均匀平面简谐波分析方法研究其频散关系,得到Christoffel方程,进而获得均匀平面波的复速度、相速度、衰减系数和品质因子的表达式.通过黏弹性方位各向异性页岩和砂岩进行模拟,研究了波场在地层中的传播特征,如相速度、衰减系数和品质因子等随频率、方位和入射角的变化关系 关键词： 黏弹性 各向异性 相速度 衰减系数 品质因子     

Spiral Wave in Small-World Networks of Hodgkin--Huxley Neurons

The effect of small-world connection and noise on the formation and transitionof spiral wave in the networks of Hodgkin-Huxley neurons are investigated in detail. Some interesting results are found in our numerical studies. i) The quiescent neurons are activated to propagate electric signal to others by generating and developing spiral wave from spiral seed in small area. ii) A statistical factor is defined to describe the collective properties and phase transition induced by the topology of networks and noise. iii) Stable rotating spiral wave can be generated and keeps robust when the rewiring probability is below certain threshold, otherwise, spiral wave can not be developed from the spiral seed and spiral wave breakup occurs for a stable rotating spiral wave. iv) Gaussian white noise is introduced on the membrane of neuronsto study the noise-induced phase transition on spiral wave in small-world networks of neurons. It is confirmed that Gaussian white noise plays active role in supporting and developing spiral wave in the networks of neurons, and appearance of smaller factor of synchronization indicates high possibility to induce spiral wave.     

Termination of spiral wave breakup in a Fitzhugh-Nagumo model via short and long duration stimuli

Rotating spiral waves have been observed in a variety of nonlinear biological and physical systems. Spiral waves are found in excitable and oscillatory systems and can be stationary, meander, or even degenerate into multiple unstable rotating waves (a process called "spiral wave breakup"). In the heart, spiral wave breakup is thought to be the underlying mechanism of cardiac fibrillation. The spatiotemporal complexity of multiple unstable spiral waves is difficult to control or terminate. Here, the mechanisms of the termination of spiral wave breakup in response to global stimulation are investigated. A modified Fitzhugh-Nagumo model was used to represent cellular kinetics to study the role of the fast (activation) and slow (recovery) variables. This simplified model allows a theoretical analysis of the termination of spiral wave breakup via both short and long duration pulses. Simulations were carried out in both two-dimensional sheets and in a three-dimensional geometry of the heart ventricles. The short duration pulses affected only the fast variable and acted to reset wave propagation. Monophasic pulses excited tissue ahead of the wave front thus reducing the amount of excitable tissue. Biphasic shocks did the same, but they also acted to generate new wave fronts from the pre-existing wave tails by making some active regions excitable. Thus, if the short duration stimuli were strong enough, they acted to fill in excitable tissue via propagating wave fronts and terminated all activity. The long duration wave forms were selected such that they had a frequency spectrum similar to that of the pseudoelectrocardiograms recorded during fibrillation. These long duration wave forms affected both the recovery and activation variables, and the mechanism of unstable multiple spiral wave termination was different compared to the short duration wave forms. If the long duration stimuli were strong enough, they acted to alter the "state" (i.e., combination of fast and slow variables) of the tissue throughout 1.5 cycles, thus "conditioning" the tissue such that by the end of the stimuli almost no excitable tissue remained. The peak current, total energy, and average power of stimuli required to terminate spiral wave breakup were less for the long duration wave forms compared to the short duration wave forms. In addition, closed loop feedback via stimulation with a wave form that was the difference of the pseudoelectrocardiogram and a strongly periodic chaotic signal was successful at terminating spiral wave breakup. These results suggest that it may be possible to improve cardiac defibrillation efficacy by using long duration wave forms to affect recovery variables in the heart as opposed to the traditional brief duration wave forms that act only on the fast variables. (c) 2002 American Institute of Physics.     

Selection of spiral waves in excitable media with a phase wave at the wave back

Universal relationships between the medium excitability and the angular velocity and the core radius of rigidly rotating spiral waves in excitable media are derived for situations where the wave front is a trigger wave and the wave back is a phase wave. Two universal limits restricting the region of existence of spiral waves in the parameter space are demonstrated. The predictions of the free-boundary approach are in good quantitative agreement with results from numerical reaction-diffusion simulations performed on the Kessler-Levine model.     

相控圆弧阵声源在TI地层井孔中产生的声场

研究了相控圆弧阵声源在各向异性地层井孔中产生的声场的特征,重点分析了该声源对于地层横波各向异性的方位敏感性,旨在探索一种新的地层声学各向异性测量方法。结果显示:相控圆弧阵声源在地层中可激发多种方位阶数的声场,主要成分为单极子波场和偶极子波场;在HTI地层竖直井中,方位角不同的圆弧阵声源激发的波动的速度不同,相控圆弧阵声源对于地层横波速度的方位各向异性的敏感性主要是它所激发的弯曲波的贡献。相比于传统的正交偶极子声波测井而言,采用相控圆弧阵声源可以评价井附近地层的周向非均质性,可以提高井周测量覆盖次数,提高反演结果的可靠性。另外,相控圆弧阵声源兼具备多极子声源的功能,优化的相位控制方式可以实现方位各向异性的最佳测量。      

用运动控制器来终止心律失常

采用Luo-Rudy相Ⅰ心脏模型对通过局部电击使细胞复极化来消除心脏中的螺旋波和时空混沌进行数值模拟.提出利用控制器局部电击螺旋波波头周围的心肌细胞来抑制螺旋波的旋转,使螺旋波漂移出边界,进而控制螺旋波和时空混沌.数值模拟表明:适当选择控制的格点数和膜电位控制阈值,螺旋波和时空混沌都可以被抑制.最少的控制格点数为9个,...     

Focused antenna array in a strongly inhomogeneous medium

A focused acoustic antenna array is considered in a strongly inhomogeneous stationary medium. An opportunity is indicated to determine the coordinates of a number of objects by active location. It is assumed that, in insonifying the objects by a wave with an arbitrary wave front, they scatter spherical waves and are sufficiently separated in distance and angle to be resolved by the same array in a homogeneous medium. The procedure of determining the coordinates of the objects involves a wave front inversion for distinguishing between the signals from different objects. The coordinates are determined by estimating the parameters for each individual object. The parameter estimation procedure is shown to provide a high efficiency of extracting the argument of a complex signal. The results of the numerical modeling and solution of the problem are presented.     

基于单层反射超表面的宽带圆极化高增益天线设计

基于Pancharatnam-Berry相位原理, 设计了一种宽带圆极化反射聚焦超表面并将其应用到提高天线增益中. 首先提出了一种变形十字超表面单元, 在11-16 GHz频带范围内能够实现高效同极化转换, 并基于该单元构建了宽带反射聚焦超表面. 仿真结果表明, 垂直入射的右旋圆极化平面波宽带聚焦效果明显. 然后利用单向阿基米德螺旋天线对超表面进行照射, 其辐射的球面波经超表面反射后得到了近平面波, 有效地提高了天线的增益. 最终对所设计的天线系统进行加工并测试, 结果表明系统的-1 dB增益带宽达到25% (12.5-16 GHz), 在该频带范围内峰值增益均高于19 dBc且轴比小于3 dB. 此外, 在12-15.5 GHz范围内天线口径效率均超过50%.