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本文提出一种用高频正入射声脉冲估测海底表层沉积物衰减系数的方法。在一定的条件下,正入射声脉冲海底回波的包络含有指数衰减因子exp(-βct)。如果声波频率足够高,回波包络的形状主要取决于声波在沉积物中传播的指数衰减。因此,从回波包络可以估算沉积物的衰减系数。本文在细密分层模型的基础上推导出回波包络,这可以看作是对高频声波在沉积物中体积散射机理的一种解释。文中给出了海上实验的测量结果。 相似文献
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一、前言实际的水位变化除包括天文潮变化外,还含有非天文因素影响而产生的非潮汐性质的水位变化。用一日24小时逐时水位平均求得的日平均水位,基本上滤去了日周期以下的高频分潮,而非潮汐性质的水位变化仍包括在日平均水位变化之中。不过这儿要指出,日平均水位仍包含日和半日以及更高频的潮汐成份,这必然形成Nyquist折叠现象,会使谱分析中形成的极值出现虚像。但是在研究大尺度的低频波动还常采用日平均水位 相似文献
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由于预载下土体固结,海底浅基础的承载力会随作业时间的增加而改变,其时变效应评估困难。基于修正剑桥模型,采用水土耦合有限元方法研究了预载作用下浅基础在正常固结黏土海床中承载力破坏包络面的时变规律。在验证数值模型准确性后,通过位移探针测试获取复合加载模式下浅基础的破坏包络面,揭示了预载和固结程度对基础承载力和破坏包络面的影响,给出了预载作用下浅基础承载力包络面计算方法。结果表明:随着预载比增加,固结单轴承载力呈现线性增长,固结承载力增幅在水平向最大;部分固结承载力相对增幅与预载比无关,而随固结度变化;破坏包络面形状由预载比控制,而包络面大小由预载比和固结度共同控制。研究结果可为海洋浅基础的时变承载力评估提供参考依据。 相似文献
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南极大气的成份已经受到人类活动的影响。主要由氟氯碳(chlorofluoro-carbons)引起的南极“臭氧洞”(见图9)是最显著、最富戏剧性的全球变化之一。南极在全球大气成份与全球变化中起着很独特的作用,如臭氧洞在极地地区尤为明显。 相似文献
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浙江苍南电厂冷却水温排放的数值模拟 总被引:1,自引:0,他引:1
电厂排放的冷却水水量大且热量高,其排放的热水将引起周围水域水温的升高,导致不同程度的热污染.针对浙江苍南电厂附近海域水流和污染物的对流扩散特点,采用MIKE21软件对电厂温排水进行数值模拟,分析不同潮型、不同流量及不同季节下的最大温升包络面积和取水口的温升变化.研究结果表明,电厂冷却水温排放的最大温升包络面积范围与流速和水深的关系密切.从本工程的情况来看,在高温升(4 ℃)区以保证率为97%的低水位潮型下的最大温升包络面积为最大,达0.69 km2,大潮、中潮和小潮相应的面积分别为0.63、0.66和0.48 km2;在低温升(0.5 ℃)区以小潮相应的面积为最大,达14.96 km2,中潮、大潮和保证率为97%的低水位潮型相应的面积分别为13.48、10.30和6.90 km2.温升包络面积与电厂冷却水的排放量不为线性关系.冬季电厂冷却水排放的温升包络面积大于夏季,冬季4 ℃温升包络面积是夏季的3倍,其它各级温升包络面积为夏季的1.1~1.4倍.由于取排水口被挡沙防浪堤隔开,故取水口区域的温升较小.研究成果可为电厂的建设提供依据. 相似文献
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以子波变换分析高频组成波周期信息并对超射现象进行动力学研究。高频波相位相互耦合,其能量变化以主频波周期为特征时间尺度。主频波对高频波有调制作用并可导致高频波的混沌运动。高频波在混沌海中破碎机会较多,始终不能积累较高的能量,其能量低于其作为主频波时具有的能量。 相似文献
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利用涌浪影响下短时段内的冲流带滩面高频高程数据和碎波带波流资料,在奇异谱分析(SSA)的基础上,对比研究了不同形态滩面的冲淤变化趋势、趋势分布形状、冲淤变化周期和冲淤变化强度,以及同一条剖面不同桩点间各因素间的变化关系;用交叉谱方法探索了每分钟滩面高频冲淤变化与碎波带长重力波间的作用关系。分析结果表明,滩角韵律地形引起的冲流分流作用促进了滩脊向滩谷的泥沙转运,冲流带滩面存在明显的长重力波频段的周期性冲淤振动,滩面冲淤振动强度由滩面下部向上部递减,碎波带长重力波对滩面高频冲淤变化起重要作用。 相似文献
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基于HYDROLIGHT光学模拟软件模拟水表面以下辐照度比R(0~-)和参数f值,着重分析多次散射作用对f值的贡献率以及太阳天顶角、风速和b_b/(a+b_b)对贡献率的影响,结果表明:多次散射作用对参数f的贡献率受到太阳天顶角、风速和b_b/(a+b_b)的共同影响,使得多次散射作用具有一定的时空差异;贡献率P_(ms)在太阳天顶角从0(°)~85(°)的范围内,随太阳天顶角的增加呈现波动状态,但这种波动随b_b/(a+b_b)值的增加而减弱,P_(ms)在0m/s到4m/s的范围内存在较大的波动,而当风速增加到4m/s时,几乎不随风速的变化而变化;多次散射作用对参数f的贡献率最大值可以达到24.41%,因此在内陆湖泊高散射特性水体中必须考虑多次散射作用对R(0~-)的影响. 相似文献
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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. 相似文献
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This study gives a new approximate analytic solution for water wave scattering by a submerged horizontal porous disk in the context of the linear potential theory. The solution is based on the domain decomposition method. The velocity potentials are determined by two different approaches. One approach is to adopt decompositions for velocity potentials, and the other is to expand the vertical derivative of the velocity potential on the porous disk along the radial direction. Hence the velocity potentials are determined by the matched eigenfunction expansions. Differing from previous solutions with respect to the porous disk, the present solution needs no complex dispersion relations. Thus the new solution is easier for numerical implementation. According to numerical examples, the convergence of the present solution is satisfactory. In addition, the present predictions of the wave surface elevation and the vertical wave force on the disk agree very well with previous results by different approaches. The present solution can also be extended to other structures involving disks, such as a fish cage, a porous disk with finite thickness, and a submerged elastic disk. 相似文献
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Based on the dynamic theories of water waves and Mindlin plates,the analytic solution of interaction between surface waves and two-dimensional floating elastic plates with edge-restraint is constructed by use of the Wiener-Hopf technique.Firstly,without regard for elastic edge restraint,the wave-induced responses of elastic floating plate analyzed by the present method are in good agreement with the results from literature and experimental results.Therefore,it can be shown that the present method is valid.Secondly,three end-restraint cases(i.e.,the left-end elastic restraints,the both-end elastic restraints,and the right-end elastic restraints)are proposed to reduce the vibration of floating plates,in which the spring is used to connect the sea bottom and the floating plate's left(or right)edge.The relations between the spring stiffness and the parameters of wave-induced responses of floating plates are discussed.Moreover,the effective method to reduce the vibration of floating elastic plates can be obtained through comparison. 相似文献
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Lynch J.F. Colosi J.A. Gawarkiewicz G.G. Duda T.F. Pierce A.D. Badiey M. Katsnelson B.G. Miller J.E. Siegmann W. Ching-Sang Chiu Newhall A. 《Oceanic Engineering, IEEE Journal of》2006,31(1):33-48
Results and recommendations for evaluating the effects of fine-scale oceanographic scattering and three-dimensional (3-D) acoustic propagation variability on the Effects of Sound on the Marine Environment (ESME) acoustic exposure model are presented. Pertinent acoustic scattering theory is briefly reviewed and ocean sound-speed fluctuation models are discussed. Particular attention is given to the nonlinear and linear components of the ocean internal wave field as a source of sound-speed inhomogeneities. Sound scattering through the mainly isotropic linear internal wave field is presented and new results relating to acoustic scattering by the nonlinear internal wave field in both along and across internal wave wavefront orientations are examined. In many cases, there are noteworthy fine-scale induced intensity biases and fluctuations of order 5-20 dB. 相似文献
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Brillouin light scattering technique can be successfully used to determine the whole set of elastic and piezoelectric constants of a ZnO single crystal irradiated by different laser energy densities, into a micron range (radiation layer thickness). It is found that the scattering intensity, the linewidth and the Brillouin scattering shift of acoustic phonons are all strongly dependent on laser energy density. Based on the sound propagation equations and these results, the directional dependences of the compressional and shear moduli of the irradiated ZnO sample in the (001) plane are investigated. It is found that under an appropriate laser condition, 248 nm KrF excimer laser irradiation can significantly improve the surface quality and increase the elastic properties of ZnO single crystal. This procedure has potential applications in the fabrication of ZnO-based surface acoustic wave and optic-electronic devices. 相似文献
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The authors study the scattering of monochromatic plane acoustic waves incident at 90° angles relative to the axis of symmetry (i.e. broadside or beam aspect) on solid elastic spheroids. In this analysis, the aspect ratios of the spheroids vary in the range 2⩽L / D ⩽5 in steps of one. The nondimensional frequency kL /2 is kept within the band 2⩽kL /2⩽24. A numerical solution based on a modification of the T -matrix method is generated. The authors generate predictions for the backscattered echoes and graphically display their frequency dependence in order to study the resonance features present within them. In this three-dimensional study, the authors identify the (leaky) Rayleigh-type resonances consistent with those present in infinite cylinders 相似文献
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We present a study of scattering from objects in a free unbounded ocean and for objects near an interface and in a waveguide. Major emphasis is on results and discussion of theoretical development is limited to the essentials. An examination of backscattered echoes is presented for elastic targets. Angular distributions are examined for all target types, as well as environments, with the exception of objects in a waveguide. In a waveguide, beamforming techniques are most appropriate for detection of objects. We also examine rigid, soft, and elastic objects that are either spheroidal or cylindrical. Aspect ratios studied range from 3:1 to 24:1, andkL/2 ranges (k is wavenumber andL is the length of the object) are from 3 to 120. 相似文献
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In this work, sloshing flows were successfully simulated by using a coupled numerical scheme between smoothed particle hydrodynamics (SPH) and smoothed point interpolation method (S-PIM) (SPH-SPIM coupled method). SPH is a Lagrangian particle method to solve flow fields while S-PIM is developed to deal with the structure dynamics. A coupling scheme is proposed, the key of which is that the fluid and solid fields are not necessary to be discretized by the same resolution. The stability, accuracy, convergence and conservation of the SPH-SPIM coupled method were validated by the case of hydrostatic water column on an elastic plate. Then, a wave impact problem was simulated to verify that the present SPH method worked well for sloshing flows. Finally, two sloshing problems with an elastic baffle were simulated, which validated the accuracy and stability of the method in predicting the fluid-structure interaction (FSI) features during the process of sloshing. It has been found that both the shape of the free surface and the large deformation of the elastic baffle can be well captured by the present method, which shows the potential of the present method to be a good candidate for simulating sloshing problems. 相似文献