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1.
船舶在冰区航行时,船体会受到多种类型激振力的作用而产生不同程度的结构振动。船体振动会对船体结构安全以及船上人员的正常工作产生不利影响。船体结构振动的现场测量可较为准确地分析船体振动的基本特性,确定船体振动与外界荷载因素的对应关系。在2015—2016年中国第32次南极科学考察过程中,对"雪龙"号极地科学考察船的冰激振动开展了现场测量。本研究对比了船体在无冰区和冰区航行过程中的船体振动变化,分析了船体在不同航速、冰厚以及不同破冰航行方式下的振动规律。测量结果表明:船体冰激振动随着航速和冰厚的增加而显著增大;在船体采用冲撞式破冰时横向振动最为明显,而在连续式破冰时,垂向振动最为明显。以上研究可为冰区航行中船体振动的现场监测技术提供有益参考,并有助于分析海冰对船体振动规律的影响。 相似文献
2.
S. D. Chopra 《Geophysical Journal International》1957,7(6):338-346
Summary. We derive the frequency equation and the condition of existence of Stoneley type waves with antisymmetric vibrations which can be propagated along the interfaces between an internal stratum and two adjacent halfspaces of identical elastic properties, all perfectly elastic, homogeneous and isotropic. The ranges of existence of such waves are next obtained by numerical computation and the results are presented both in tabular form and graphically.
For large values of the frequency, the frequency equation for these waves, like the one for waves with symmetric vibrations discussed in an earlier paper*, reduces to the velocity equation of Stoneley waves propagated along the interface between two halfspaces.
For a low-velocity stratum, the results are similar to those for waves with symmetric vibrations. It is found that, for a pair of media for which waves of some frequency can exist, as the frequency of the waves or the thickness of the stratum is decreased, there is a cut-off value of either below which such waves cannot be propagated.
For a high-velocity stratum, the results are in sharp contrast with those for waves with symmetric vibrations. Stoneley waves of all frequencies can exist for some pairs of materials. As the frequency of the waves or the thickness of the stratum is decreased, that is, as the ratio of the wave-length to the thickness of the stratum is increased, the regions of existence expand until a certain limiting region of existence is obtained when this ratio tends to infinity.
Another difference between waves with symmetric and antisymmetric vibrations is that whereas for the former the phase velocity could not be less than the smaller of the two Rayleigh wave velocities, for the latter there is no theoretical lower limit. 相似文献
For large values of the frequency, the frequency equation for these waves, like the one for waves with symmetric vibrations discussed in an earlier paper*, reduces to the velocity equation of Stoneley waves propagated along the interface between two halfspaces.
For a low-velocity stratum, the results are similar to those for waves with symmetric vibrations. It is found that, for a pair of media for which waves of some frequency can exist, as the frequency of the waves or the thickness of the stratum is decreased, there is a cut-off value of either below which such waves cannot be propagated.
For a high-velocity stratum, the results are in sharp contrast with those for waves with symmetric vibrations. Stoneley waves of all frequencies can exist for some pairs of materials. As the frequency of the waves or the thickness of the stratum is decreased, that is, as the ratio of the wave-length to the thickness of the stratum is increased, the regions of existence expand until a certain limiting region of existence is obtained when this ratio tends to infinity.
Another difference between waves with symmetric and antisymmetric vibrations is that whereas for the former the phase velocity could not be less than the smaller of the two Rayleigh wave velocities, for the latter there is no theoretical lower limit. 相似文献
3.
Natural Resources Research - In surface mining, blasting is an indispensable method for fragmenting rock masses. Nevertheless, it can inherently induce many side effects like ground vibrations. At... 相似文献
4.
Tribe Jarryd Koroznikova Larissa Khandelwal Manoj Giri Jason 《Natural Resources Research》2021,30(6):4673-4694
Natural Resources Research - Ground vibrations induced during rock fragmentation by blasting remain a potential source of hazard for the stability of nearby structures. In this paper, to forecast... 相似文献
5.
In this study we apply the correspondence principle for free vibrations of a homogeneous viscoelastic solid derived by Fisher & Leitman to obtain the torsional modes of a homogeneous viscoelastic rod. We also extend the correspondence principle, showing that it may be used to find the frequencies of Love waves in a stratified viscoelastic medium. Finally, we apply the correspondence principle to four viscoelastic materials, the Kelvin-Voigt solid, the Maxwell solid, the standard linear solid, and the Achenbach-Chao solid. We show that in each of these cases some care must be used in applying the correspondence principle because of the presence of multiple solutions. We also examine measures of dissipation of the free vibrations, and we determine the conditions under which the logarithmic decrement may be approximated by the process-independent l/ Q of O'Connell & Budiansky. 相似文献
6.
In this study, in-situ testing results are given, and the analytical relationship of the vibrations' amplitudes inside an embankment by the thawing of the subgrade surface of seasonably deep frozen soils is provided. The peculiarities of the vibration waves' propagation during the springtime thawing of soils compared to those during the summertime period and the correlation of the vibrations with the under-rail basement modulus of elasticity are defined. 相似文献
7.
Summary. The symmetry of the differential system for elastic waves, previously noted for plane geometry, is extended to any linear differential system and, in particular, the elastic-gravitational vibrations in a spherical earth. The result remains valid in a linearly viscoelastic medium. The symmetry allows the inverse of the propagator matrix to be obtained by simply 'transposing' the elements of the propagator. With this result, it is shown how the source excitation using a particular integral can be put in a more instructive form, comparable with the result for the excitation of normal modes. 相似文献
8.
Harold Jeffreys 《Geophysical Journal International》1978,55(2):273-281
Summary. Attempts are made to estimate the distribution of damping with depth, based on the observed damping of P waves, of free vibrations, and of surface waves and partly on a discussion by D. L. Anderson and R. S. Hart. Results are consistent in order of magnitude, but could easily be wrong by a factor of 2. Comments are added on the need for further work. Stress is laid on the damping of the free nutation, which indicates that α in the modified Lomnitz law is about 0.2 on an average. 相似文献
9.
基于黄河中游4个水文站1919~2010年还原的天然径流量、1873~2011年的夏季风强度指数、北大西洋涛动指数（NAO）和西太平洋副高指数等资料,利用Morlet小波方法分析了黄河中游河口镇-龙门区间（简称河龙区间）,龙门-三门峡区间（简称龙三区间）和三门峡-花园口区间（简称三花区间）径流量的周期变化,并探索了夏季风、中纬度西风与西太平洋副高等环流因子对黄河中游3个区间径流量周期性变化的影响。研究发现,黄河中游3个区间径流量的变化对夏季风、中纬度西风和西太平洋副高存在差异性响应,季风对于黄河中游的径流量不仅在时间上具有趋势性的影响,更为重要的是存在周期性的控制作用,在80 a长周期上对于整个黄河中游都有控制作用,但影响黄河中游降水-径流过程的不仅仅是夏季风,相关统计结果表明还受到西风带和西太平洋副高周期的影响。3个区间都存在一个中等尺度(25 a)的周期,这个周期在夏季风上没有得到体现,河龙区间和龙三区间的25 a周期是受西风带影响所致,而三花区间的中尺度25 a周期是西太平洋副高指数的强度和西界的作用所致。 相似文献
10.
11.
To investigate the vibration characteristics of a railway subgrade in different seasons, three field experiments were carried out in
the seasonally frozen Daqing area of China during spring, summer, and winter. The vibration characteristics and attenuation rates
of the subgrade induced by passing trains were investigated, and the influences of the season, train speed, train type, train load, and
number of train compartments are described in this paper. The results show that: (1) near the rail track the vibration in the vertical
direction was more significant than in the lateral and longitudinal directions, and as the distance from the railway track increased,
the acceleration amplitudes and the attenuation rates all decreased in all three directions; (2) the acceleration amplitudes and attenuation
rates decreased in the three different study seasons as the distance from the railway track increased, and the attenuation
rates in the freezing period were the largest; and (3) the acceleration amplitude induced by a freight train was greater than that by a
passenger train, and the subgrade vibration increased with increasing passenger train speeds when the number of train compartments
was similar. These results have great significance for enhanced understanding of the characteristics of train-induced vibration
embankment response in seasonally frozen regions, and provide essential field monitoring data on train-induced vibrations in
order to improve the performance criteria of railroading in seasonally frozen regions. 相似文献
12.
根据中国西北地区东部59个气象台站1965-2014年的逐日降水资料,将降水划分为小雨、中雨和大雨3个等级,分析和比较了该地区夏季不同等级降水降水量、降水日数和降水强度的时间演变特征,并对各等级降水对总降水贡献的时间变化进行了讨论。结果表明：（1）西北地区东部降水量和降水日集中在夏季。降水等级越高,降水量和降水日的集中程度越高,夏季大雨量（日）达到了全年大雨量（日）的72.15%（69.19%）;（2）西北地区东部各等级降水量、降水日主要在1996年存在一个由多转少的突变,均存在较明显的2~3 a的短周期;（3）西北地区东部夏季降水总量与中雨等级以上降水量具有相似的年际和年代际变化,相关系数高达0.8,主要受中雨和大雨量的影响,仅在东南部和甘肃中部趋于增加,在甘肃南部和内蒙古中部趋于减少。总降水日则与小雨日变化一致,主要呈减少趋势,总降水强度的增强则主要源自大雨强度的增大。 相似文献
13.
Takeshi Shirabe 《International journal of geographical information science》2014,28(2):314-325
This paper considers the problem of planning a path in a circumstance where its origin is given, but its destination is not specified and is to be selected from among a set of candidate destinations during a trip. A situation like this may be experienced by a group of people who have different preferred destinations, as well as by an individual who is simply indecisive about where to go. To resolve such an uncertainty, one may stay at the origin until he decides on a destination, or choose to proceed on some path that does not overly deviate from a shortest path, whichever destination is eventually chosen, and make a decision on the way. The latter action is sensible when the risk of traveling longer is outweighed by the benefit of buying more time for a better destination decision. The problem of finding such a time-buying path is formulated and a simple algorithm is developed for its solution. Some extensions and applications are also discussed. 相似文献
14.
15.
In order to understand the underlying physics of distributed seismicity better we have considered a 2-D array of slider blocks connected by springs and interacting via static friction with a surface. There is no driving plate in this model. The time evolution of the system is found from numerical simulations in a cellular automata formulation. Energy is conserved and is the single control parameter. The distribution of energies in the springs is found to obey a modified Maxwell-Boltzmann statistics. It is found that the number-size statistics of clusters of unstable sliding blocks is identical to those in percolation clusters in the site-to-site percolation model. There is a well-defined critical point when unstable blocks become connected across the array. It has been previously suggested that distributed seismicity in a seismic zone is the percolation backbone of a 3-D percolation cluster. The fact that low-level seismicity satisfies the Gutenberg-Richter frequency-magnitude relation and is nearly constant in time also suggests that this background seismicity is similar to thermally induced noise. 相似文献
16.
A detailed analysis is made of the current ideas concerning floods of a special type. We examine short-lasting torrential flash floods, one of the most widespread and hazardous natural phenomena in the world characterized by a high rate of development, and by a short duration. It is established that such floods are in a primitive stage of study, which is testified by the fact that there is no general consensus as to what should be treated as a flash flood. It is pointed out that a special term designating them is also absent in many countries. It is determined that the key formation conditions for flash floods include intense short-lasting cloudbursts, the occurrence of a river basin in mountainous regions and a small drainage area; on this basis, it is suggested that they be termed flash floods. It is shown that such floods are of the most widespread occurrence in the northern hemisphere in regions with a temperate and subtropical climate. We suggest the scheme of natural factors for formation of flash floods and their differences from debris flows and floods of other types. It is determined that the main problems of investigating the formation mechanisms and forecasting the aforementioned floods are associated with the small spatiotemporal scale of these phenomena. 相似文献
17.
Stanislav V. Shipunov 《Geophysical Journal International》1997,130(2):405-410
A correct method for determining the direction of a synfolding remanence is described, and a new test, which in a number of cases can discriminate between a true synfolding remanence and a remanence that is the sum of pre- and post-folding components, is suggested. It is shown that it is possible to reveal the folding evolution of a local object under an assumption that the acquisition of the synfolding remanence was synchronous over this object. 相似文献
18.
《Geophysical Journal International》1964,8(4):389-402
Summary. An attempt is made to describe the transient magnetic change caused by a sudden local change of conductivity in the ionosphere. In a rough illustrative model, electric current is taken to be flowing uniformly in a thin plane rigid sheet of uniform isotropic integrated conductivity, I/ϱO e.m.u. A particular type of perturbing current system is defined, and the type of conductivity anomaly which could produce it is derived. It is shown that the problem is equivalent to that of the free decay of the perturbing current system. For a very small change of conductivity, the current system is found to spread radially at a uniform rate, while decaying. The case of a large conductivity anomaly is analysed by a numerical method. It is concluded that effects at a distance would be similar to those produced by a very small change, and that near the anomaly there would be as well, a local decaying current vortex system. It is found in particular, that at a large distance L cm, the magnetic change would commence immediately and reach an extremum after time of order L × 10−6 s. 相似文献
19.
Binbin Lu Martin Charlton Chris Brunsdon Paul Harris 《International journal of geographical information science》2016,30(2):351-368
In this study, the geographically weighted regression (GWR) model is adapted to benefit from a broad range of distance metrics, where it is demonstrated that a well-chosen distance metric can improve model performance. How to choose or define such a distance metric is key, and in this respect, a ‘Minkowski approach’ is proposed that enables the selection of an optimum distance metric for a given GWR model. This approach is evaluated within a simulation experiment consisting of three scenarios. The results are twofold: (1) a well-chosen distance metric can significantly improve the predictive accuracy of a GWR model; and (2) the approach allows a good approximation of the underlying ‘optimal distance metric’, which is considered useful when the ‘true’ distance metric is unknown. 相似文献
20.
黑河流域气象要素与全球性大气环流特征量的多尺度遥相关分析 总被引：2，自引：1，他引：1
黑河流域生态-水文过程集成研究是我国近年来干旱半干旱地区的研究热点,气候变化条件下流域生态-水文过程的响应机制是其中一个重要科学问题。基于1956-2015 年黑河流域气温、湿度、降水观测数据及多个大气环流特征量监测资料,采用小波分析方法研究了流域气象要素与大气环流特征量的周期性及遥相关。结果表明：（1）流域气温与大气环流特征量的遥相关关系最为明显,湿度、降水次之。（2）气温与Niño3.4 SSTA、SOI 的显著遥相关共同周期为12～16 a,与NAO、AO为9～11 a,与WP遥相关周期为5～9 a,与PDO为8～15 a。（3）湿度与NAO遥相关周期为4～5 a,与POL为3～6 a,与WP为9～15 a,与PNA为2～8 a。（4）降水与Niño3.4 SSTA遥相关周期为9～15 a,与NAO为2～4 a,与AO为3～7 a,与POL为11～19 a,与WP为9～14 a。（5）上述各遥相关周期的地域分布、显著时段与时滞特征各有差异。本文研究有助于加深流域气象要素对环流特征量响应机制的认识,为我国干旱半干旱地区内陆河流域严峻的水-生态问题的解决及水资源管理提供依据。 相似文献