Fluid-Filled Passive Sonar Calibration Spheres: Design, Modeling, and Measurement

Hollow spheres have long been used as simple underwater targets for testing acoustic projector systems. While spheres offer a mathematically simple shape with a resolvable scattering strength, their usage as a passive target has been less successful due to the complicated manner in which a hollow sphere scatters energy from its exterior and interior as a function of frequency and temperature. Furthermore, a sphere's aspect independent scattering requires a surface area that is physically much greater than a wavelength which in turn requires mechanical support systems that are also large, often with target strengths that rival that of the test target itself. This paper discusses the development of several thin-walled spheres, ranging in diameters from 0.1524 to 0.4953 m, filled with a high-density fluid, to be used collectively as calibrated underwater sonar targets in the 5–50-kHz frequency range and an additional 0.4953-m diameter sphere tested over the range of 5–120 kHz. The combination of the spherical shape and focusing effects of the fluid enhances the acoustic scattering strength of the sphere and produces a significantly greater backscattered response than a rigid sphere. A simple theoretical model is presented to compare several fill fluid possibilities and is then used to compare the chosen fluid, fluorolube, against measured data for each sphere.      

海底底质声学原位测量电路控制系统研究

介绍了一种液压驱动贯入式海底沉积声学原位测量系统的电路控制单元的研究实现过程,以及该控制单元在南海北部海底沉积声学调查中的应用。该电路控制单元以Cortex-A8处理器为核心,集成大容量FLASH存储器,与单片机接口控制板进行串口通讯,实现对声学发射采集单元和机械液压贯入单元的可视化控制和监测。基于该电路控制单元,海底底质声学原位测量系统兼具自容式和在线式两种工作模式,可自容记录或实时采集声学原位测量单元在海底的工作状态数据、海底沉积物声速和声衰减系数等声学特性数据。该声学原位测量系统的实验室联调及南海海试结果表明,使用该电路控制单元对海底底质声学测量过程的监测与控制是有效的,对精确获取海底底质的原位声学特性有重要作用,可以促进海底底质声学原位测量系统的产品化。     

Trajectory model for vertical sphere water-entry in presence of deep-seal cavity

This study presents a piecewise model for determining the vertical distance and velocity evolution with time for a sphere impacting a water surface and submerging to depths beyond deep-seal cavity pinch-off. Experimental data taken with a high-speed camera are presented for varying sphere mass ratios and impact velocities. The semi-empirical model incorporates results from previously published research and is shown to be in good agreement with experiments for heavier spheres but deviates when the sphere is only slightly denser than water. Two causes for the deviation are presented which relate to the dynamics of the cavity pinch-off event and the inception of a trailing vortex ring after the trailing cavity sloughs from the sphere. A model for predicting cavity pinch-off time and sphere position and velocity at the moment of cavity pinch-off is shown to agree well with experimental results for varying sphere mass ratios and impact velocities. The key experimental values are provided for comparison with current and future modeling efforts.     

Extended-aperture underwater acoustic multisource azimuth/elevationdirection-finding using uniformly but sparsely spaced vector hydrophones

Aperture extension is achieved in this novel ESPRIT-based two-dimensional angle estimation scheme using a uniform rectangular array of vector hydrophones spaced much farther apart than a half-wavelength. A vector hydrophone comprises two or three spatially co-located, orthogonally oriented identical velocity hydrophones (each of which measures one Cartesian component of the underwater acoustical particle velocity vector-field) plus an optional pressure hydrophone. Each incident source's directions-of-arrival are determined from the source's acoustical particle velocity components, which are extracted by decoupling the data covariance matrix's signal-subspace eigenvectors using the lower dimensional eigenvectors obtainable by ESPRIT. These direction-cosine estimates are unambiguous but have high variance; they are used as coarse references to disambiguate the cyclic phase ambiguities in ESPRIT's eigenvalues when the intervector-hydrophone spacing exceeds a half-wavelength. In one simulation scenario, the estimation standard deviation decreases with increasing intervector-hydrophone spacing up to 12 wavelengths, effecting a 97% reduction in the estimation standard deviation relative to the half-wavelength case. This proposed scheme and the attendant vector-hydrophone array outperform a uniform half-wavelength spaced pressure-hydrophone array with the same aperture and slightly greater number of component hydrophones by an order of magnitude in estimation standard deviation. Other simulations demonstrate how this proposed method improves underwater acoustic communications link performance. The virtual array interpolation technique would allow this proposed algorithm to be used with irregular array geometries     

Resonances of elastic scatterers in fluid half-spaces

A methodology is presented for the spectral analysis of the echo returns from elastic spheres submerged in a fluid half-space. The influence of a target depth on its resonances as manifested in the scattered field is assessed. Results are expressed in terms of Debye potentials in the fluid and in the solid. The unknown coefficients arising in these potentials are determined by the application of appropriate boundary conditions at the surface of the elastic sphere and at the free boundary of the fluid. These boundary conditions are satisfied exactly through the use of some known transformations of the basic wave functions. As the depth increases, our results degenerate to earlier results for spheres in unbounded media. In spite of the spherical symmetry of the scatterer, the field is found to be azimuthally dependent because of the influence of the plane free-surface of the fluid. The saddle-point method is used in the asymptotic evaluation of certain coefficients which exhibit the influence of depth on the complex eigenfrequencies of the scatterer. We demonstrate that as the sphere's depth increases, the influence of the boundary decreases. The rate of decrease increases as the azimuthal wavenumbermincreases. Thus the influence of the boundary is greatest for eitherm = 0, or at shallow depths, or both.     

再论采用双重投影法的椭球面日晷投影

为减小椭球面日晷投影的变形,讨论了椭球面日晷投影的特性,分析了3种球面下双重投影法的变形大小,结果表明采用高斯等角球面,投影长度变形最小,适合于各种比例尺的地图海图制图,可以使船只沿着拟大地线方向航行,迅速到达目的地。     

Detection of buried targets using a synthetic aperture sonar

This paper presents observations of buried target detections made using a 20-kHz synthetic aperture sonar. At grazing angles below the critical angle, surprisingly high signal-to-noise detections were made of cylindrical targets buried at depths of 15 and 50 cm. During a separate set of measurements, buried spheres were clearly seen at steep grazing angles, but were generally not seen below the critical angle. Since scattering from wave-generated sand ripples may contribute to detections at grazing angles below critical angle, the information available on the ripple fields is discussed and used in acoustic backscatter simulations for the buried spheres. Lack of information on the ripple height precludes a definitive explanation for the absence of buried sphere detections at subcritical grazing angles.     

AN ANALYSIS OF ACOUSTICAL SCATTERING FROM ELASTIC OBSTACLES BY FINITE AND BOUNDARY ELEMENT COMBINATION APPROACH

A method combining boundary and finite element approaches as well as modal analysis to solve the acoustical scattering from elastic obstacle in infinite fluid is described. An elastic cylinder with finite length immersed in water is chosen as an example for numerical demonstration. The computed frequency response and direction pattern of the backscattering target strength of the cylinder are conformable to those measured.     

Motions of a spherical submarine in waves

The free motions in waves of submerged vehicles with a spherical hull from but different metacentric heights are sought. The problem is analysed by considering the submerged vehicle as a neutrally buoyant sphere. The solutions to two independent problems, namely the radiation problem and the diffraction problem, are required. Nondimensional parameters known as the added mass, damping and diffraction coefficients for neutrally buoyant spheres are derived and computed values of these coefficients are presented in tabulated form. The responses of surge, heave and pitch are explicitly expressed by these coefficients and the metacentric height of the submarine. A spherical submarine is practically motionless relative to the particle movement of waves except at the vicinity of reasonant frequency, which is governed by the value of metacentric height.     

Temporal fluctuations in the acoustic scattering frombottom-deployed objects and localized biological treatments

A field experiment was carried out to examine the time variation of scattering from man-made objects placed near the water-sediment interface and within the sediment. The objects (spheres) were monitored for a period of about two months using a sonar system capable of measuring scattering levels, bottom bathymetry, and correlation of scattering over time. In addition, divers performed focalized biological treatments that were also monitored over extended periods. The results of these monitoring activities are presented and related to previous studies that used the same data sets for other purposes. One notable result is that the buried sphere becomes undetectable (by scattering level alone) within two days of deployment. The rapid changes in the first few days after the buried sphere is introduced are quantified relative to the rate of changes for undisturbed regions of the sediment     

岩样孔隙汞渗入量超声法精密测定

采用声学方法精密测定汞被高压强压入储油岩样的渗入体积，对岩芯孔隙结构测定中的汞体积计量方法作了重要改进。本方法以无水酒精作为计量工作介质，在其中发射声波，通过对汞界面反射声波的识别，进行非接触的声学测量，从根本上解决了当前国内外采用的电阻法，电容法无法避免的由于汞腐蚀或油污染而要求经常进行与有毒汞接触的清洗，并且确保了计量系统工作性能的长期稳定与准确可靠。在０．０１ＭＰａ至５０ＭＰａ压强范围内连续计量，量程０～１０ｍｌ，准确度０．０１ｍｌ。     

An alternative structural system for an offshore airport

An alternative floating system for an offshore airport and other reclamation works is presented. It consists of precast boxes filled with FRP hollow spheres which sit on pile system. A top slab system consisting of precast and cast in situ concrete slabs will make the whole system monolithic. The light weight but strong spheres will enable the system to make use the uplift force from water. The precast boxes and slab system will transfer the difference between the gravity and uplift forces, as well as any lateral forces to the pile system. Finally, a proposed method of any lateral forces to the pile system. construction is also presented.     

声脉冲在液态平面层系上反射的数值计算

对入射平面脉冲声波,由其频谱和层系的反射系数可通过数值计算得到其在液态平面层系上的反射波。这对室内与海上声学实验及某些类型的水声仪器设备的研制都是一个有用的辅助手段。本文举出了一些计算的例子。     

An integrated geochemical and hydrodynamic model for tidal coastal environments

In this paper, the design, calibration and application of an integrated geochemical–hydrodynamic model are described. The model comprises three parts: A hydrodynamic submodel that was adopted from a depth-averaged, semi-implicit hydrodynamic model, a geochemical submodel based on equilibrium partitioning of chemicals between aqueous and particulate phases, and a particle dynamic submodel that simulates resuspension, transport and settling of suspended particulate matter (SPM). The integrated model was implemented in San Diego Bay (SDB), a heavily urbanized, semi-closed mesotidal embayment. A series of model calibrations were carried out based on observations on salinity, polychlorinated biphenyls (PCBs) and SPM. Salinity calibrations indicated that only 15% of precipitation in the drainage area of SDB could reach the bay, presumably due to the dams on the tributary rivers. Steady-state calibrations of PCBs based on fixed concentrations at known ‘hot spots’ have reproduced observed PCB concentrations in both the dissolved and particulate phases. SPM calibrations showed that shipping-induced resuspension produce more SPM than natural processes. Based on the calibrated model, the annual transport of PCBs out of SDB was estimated to be 3.85 kg (3.5 kg and 0.35 kg in the dissolved and particulate phases, respectively), much higher than the previous estimates based on steady-state assumptions. It was also found out that only a small portion of the fine sediment exported from SDB was derived from riverine input. This model can be applied to the studies of the transport and fate of other chemical species. It can be transplanted to other coastal areas as well. The integrated model represents a novel framework in which geochemical processes in coastal environments can be investigated on a truly dynamic basis.     

氮化硅陶瓷空心浮力球在内爆临界状态下的失效分析

氮化硅陶瓷具有密度小强度高的优点,在深潜浮力设备领域具有广阔发展前景。然而陶瓷材料固有的脆性以及万米深海严苛的工作环境,使得氮化硅陶瓷空心浮力球在深水环境下面临着内爆的风险。为探究氮化硅陶瓷空心浮力球在内爆临界状态下的失效问题,设计并进行了三点弯曲试验,确定了氮化硅陶瓷材料的力学性能参数,包括弹性模量和破坏应力;随后对氮化硅陶瓷空心浮力球内爆试验的过程及结果进行了回顾总结;基于标准球壳模型与浮力球实物的CT扫描几何模型,对浮力球在内爆临界状态的失效过程进行了有限元仿真分析。结果表明：氮化硅陶瓷空心浮力球的性能存在一定不稳定性;结构内表面局部过大的拉应力是造成失效的主要原因,对内表面进行预加压处理或能提高其抗内爆性能。     

Using directional data method for analysis on typhoon movement and its probability distribution features

In many scientific fields such as meteorology, biology, geography, oceanography and astronomy, the angle value is very important, for example, wind directions, and the directions on which animals and birds migrate. This kind of data is called as directional data. In some problems, it is not expressed as "angle" value, such as month by month mean temperature, however, it can be changed into a directional data through some transformations. The statistical method of directional data means that the methods of mathematical statistics are used in the treatment of directional data. In this way, data are considered as the points on two-dimensional cycle or higher dimensional sphere. Therefore, this method can also be called statistics on sphere. Because of its own characteristical feature, the statistics of directional data much differ from common statistics. Up to now, research work in this field has just started in China. In this paper, a statistical study on typhoon moving directions after their formation ov     

Control synthesis and adaptation for an underactuated autonomousunderwater vehicle

The motion of an autonomous underwater vehicle (AUV) is controllable even with reduced control authority such as in the event of an actuator failure. In this paper we describe a technique for synthesizing controls for underactuated AUV's and show how to use this technique to provide adaptation to changes in control authority. Our framework is a motion control system architecture which includes both feed-forward control as well as feedback control. We confine ourselves to kinematic models and exploit model nonlinearities to synthesize controls. Our results are illustrated for two examples, the first a yaw maneuver of an AUV using only roll and pitch actuation, and the second a “parking maneuver” for an AUV. Experimental results for the yaw maneuver example are described     

Description of a secure reliable acoustic system for use in offshore oil Blowout Preventer (BOP) or wellhead control

The search for oil in deeper water depths has generated the need for secure, reliable, low error acoustic control systems in the presence of a highly complex man-made noise-like environment. This paper discusses an acoustical command, control, and telemetering system developed for emergency subsea Blowout Preventer (BOP) operation that has been structured around a unique coding design. The system described has been successfully demonstrated in this acoustic environment at a deployed water depth in excess of 6400 feet (sim 1950m).     

Comparison of oscillatory and stationary flow through porous media

Experiments were conducted in an oscillatory water tunnel to investigate what effects temporal inertia has on the resistance of a granular medium. The flow law governing stationary porous media flow is reasonably well accepted and understood but the effects of unsteady flow have commonly been neglected. The present research was designed to assess the magnitude of the acceleration effects on media of uniformly packed spheres of equal diameter and on one sample of randomly placed stone.Oscillatory flow tests were made in a large oscillatory flume tunnel with periods varying from 3 to 12 seconds. The influence of properties of the medium (grain size and porosity) were tested by using spheres with two different diameters and packing each size sphere in different geometric arrangements. Tests made on a stone sample provided a qualitative assessment of the effects of more random material properties.For the experiments described in this paper, the Forchheimer unsteady-stationary flow law described the oscillatory measurements well when velocities and energy losses were maximum. Empirical coefficients determined from steady-stationary flow were generally found to apply to the unsteady flow, however some evidence of dependency on the period of oscillation was noted.     

海岸带空间要素分析查询系统构建研究——以山东莱州湾为例

海岸带开发与保护是我国保持海岸带可持续发展所急需解决的问题。以莱州湾为示范区 ,利用 GIS技术建立海岸带空间要素分析查询系统 ,旨在为实施海岸带的综合管理构建一个高技术支撑的信息分析系统