共查询到20条相似文献，搜索用时 31 毫秒
1.
本文研究了中纬电离层Rayleigh-Taylor不稳定性和漂移不稳定性的基本性质.如果不存在中性风和背景电场,中纬Rayleigh-Taylor不稳定性的增长率比赤道相应条件下的小.中性风和电场对中纬Rayleigh-Taylor不稳定性有重要影响.当中性风速度达到10m/s时,其作用就超过了重力.漂移不稳定性也受到重力、中性风和电场的影响,离子-中性粒子碰撞能降低漂移不稳定性的增长率.本文的分析表明,在赤道电离层容易产生大尺度的Rayleigh-Taylor不稳定性,在中纬电离层易产生小尺度的漂移不稳定性. 相似文献
2.
Abstract The linear, normal mode instability of barotropic circular vortices with zero circulation is examined in the f-plane quasigeostrophic equations. Equivalents of Rayleigh's and Fjortoft's criteria and the semicircle theorem for parallel shear flow are given, and the energy equation shows the instability to be barotropic. A new result is that the fastest growing perturbation is often an internal instability, having a finite vertical scale, but may also be an external instability, having no vertical structure. For parallel shear flow the fastest growing perturbation is always an external instability; this is Squire's theorem. Whether the fastest growing perturbation is internal or external depends upon the profile: for mean flow streamfunction profiles which monotonically decrease with radius, the instability is internal for less steep profiles with a broad velocity extremum and external for steep profiles with a narrow velocity extremum. Finite amplitude, numerical model calculations show that this linear instability analysis is not valid very far into the finite amplitude range, and that a barotropic vortex, whose fastest growing perturbation is internal, is vertically fragmented by the instability. 相似文献
3.
1 INTRODUCTION AND REVIEW Numerous investigations related to vertical velocity and suspension concentration distributions have been undertaken to the steady, uniform, open channel turbulent flow. The problems of sediment mixed water flows are of direct interest to the practical situations in the field of river sedimentation, coastalsediment transport and in the field of two-phase flow in particular. In hydraulic open channel flow, the vertical velocity profile is usually described by… 相似文献
4.
伸缩缝刚度对大跨度悬索桥动力特性的影响 总被引：1，自引：1，他引：0
伸缩缝作为大跨度桥梁与引桥之间的重要连接构件,其抗推刚度及可能存在的变异性对主桥及引桥动力特性的影响不可忽略。本文建立了大跨度悬索桥及引桥的有限元模型,采用弹簧单元模拟加劲梁与引桥箱梁之间的伸缩缝,分析伸缩缝刚度对悬索桥及引桥自振特性及其地震响应的影响规律。分析结果表明:伸缩缝刚度对加劲梁的横弯振型、竖弯与纵飘耦合振型的频率有明显的影响;伸缩缝刚度的变化会导致加劲梁与引桥的振型相互耦合,同时这些振型的频率发生相应的突变,当伸缩缝刚度较大时,加劲梁两个竖弯与纵飘的耦合振型解耦成为独立的竖弯和纵飘振型;当引桥与悬索桥加劲梁的纵飘振型发生耦合时,在纵向和竖向地震作用下的悬索桥及引桥的地震响应达到最小。伸缩缝刚度对悬索桥动力特性影响的分析可为悬索桥的模态参数确认、损伤识别、抗震性能分析提供有价值的借鉴。 相似文献
5.
本文讨论了旋转型La coste地震计的原理和各种悬挂方式及其周期表达式。指出消除“拐腿”效应和振幅效应的最好办法是采用零长弹簧（l=0）并使绞链簧片的弹率c1=-M′（0）否则,地震计将表现为正“拐腿”效应、正振幅效应或反效应,它们严重地阻碍着固有周期的增长。绞链簧片弹率c1固定后,地震计只能工作在对应的周期。列举了满足La coste悬挂条件时各种因素变化对周期和零点的影响。 相似文献
6.
The vibration control of megaframes with suspension systems is developed in this paper. The interaction between the megaframe and the suspension systems is first analysed, then asynchronous driving principle is proposed for vibration control of such structures. A numerical example is presented to show the application of asynchronous driving principle in the design of vibration control. The response of the megaframe with suspension systems under evolutionary random excitation indicates the feasibility and effectiveness of the proposed method. The vibration control method, though studied in a special case of the megaframe with suspension systems, is also applicable to the vibration control of combined structures with large secondary‐to‐primary mass ratio. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
7.
The classical continuum model for the linear vertical vibrations of a suspension bridge (Bleich et al., 1950 [1]) is re-examined. The primary objective of the study is to extend the definitive analytical and numerical results of Irvine and Caughey (1974) [2], Irvine and Griffin (1976) [3] and Irvine (1980, 1981) [4], [5] for the natural frequencies, mode shapes, and modal participation factors for an extensible suspension cable, which depend on one dimensionless parameter related to the elasticity of the cable, to the case of a stiffened suspension bridge in which the response depends also on a second dimensionless parameter related to the stiffness of the girder. The continuum suspension bridge model is also used to understand the pattern of variation of mode shapes as a function of cable elasticity and girder stiffness, which has been shown by West et al. (1984) [6] to be considerably more complex than that for a suspension cable. Finally, the threshold amplitudes of free vibrations that would result in the incipient slackening of the hangers are determined. 相似文献
8.
Field swelling instability is one of the most important instabilities in plasmas in which electrons are hotter than ions. In contrast to diamagnetic (mirror) instability belonging to the branch of slow magnetosonic waves, field swelling instability corresponds to the branch of fast magnetosonic waves. The theory of this instability was developed by (Basu and Coppi, 1982, 1984) in the early 1980s in the idealized zero ion temperature limit in the context of the magnetohydrodynamic model. To make the model more realistic, it is necessary to complete it with finite ion temperature effects. In addition, we consider the field swelling theory in the context of a more instructive quasihydrodynamic approach which showed a good performance on examination of the mirror instability. Here, it appears to be necessary to use only the transverse plasma pressure balance condition and Liouville theorem for calculating the transverse pressure variation. This consideration is simpler and clearer and makes it possible to understand in more detail the physical nature of the instability and to prepare the necessary basis for interpreting observational data. 相似文献
9.
Determination of the suspension competence of debris flows based on particle size analysis 总被引：1，自引：0，他引：1
The determination of the critical particle size between solid and fluid phases, i.e., the suspension competence, is fundamental for debris flow. A method for determining suspension competence based on particle size analysis is presented in this paper. Suspension competence of static experimental water-debris mixtures prepared with the sediment of Jiangjia Gully is -0.025 mm if the bulk density is less than 1,800 kg m-3 and it increases with bulk density of more concentrated mixtures. Suspension competence of natural debris flows in Jiangjia Gully increases exponentially with the bulk density. These two data sets are compared in order to understand the suspension mechanism. It is concluded that turbulence may play a leading role in particle suspension in non-viscous and sub-viscous debris flows, while in viscous debris flows both matrix strength and excess pore water pressure play important roles. 相似文献
10.
We examine the three-dimensional, nonlinear evolution of columnar vortices in a rotating environment. As the initial vorticity distribution, a wavetrain of finite amplitude Kelvin-Helmholtz vortices in shear is employed. Through direct numerical simulation of the Navier-Stokes equations we seek to better understand the process of maturation of the various three-dimensional modes of instability to which such vortical flows are subject, especially those which exist as a consequence of the action of the Coriolis force. In the absence of rotational influence, we thereby demonstrate that the nonlinear evolution of columnar vortices is most strongly controlled by one or the other of two mechanisms. One mechanism of instability is identifiable as a so-called elliptical instability, which promotes the initial bending of vortex tubes in a sinusoidal fashion, while the other is a hyperbolic mode, which is responsible for the development of streamwise vortex streaks in the "braids" between adjacent vortex cores. In the rotating case, anticyclonic vortices are strongly destabilized by weak background rotation, while rapid rotation stabilizes both the cyclones and anticyclones. The strong anticyclones are subject to two distinct forms of instability, namely a Coriolis force modified elliptical instability and an inertial (centrifugal) instability. The former instability is very similar to the nonrotating form of the elliptical instability as it promotes bending of vortex tubes, while the latter instability grows on the edge of the vortex core and generates streaks of vorticity, which surround the vortex core itself. These results of direct numerical simulation fully verify the results of previous linear stability analyses. Taken together, they provide a simple explanation for the broken symmetry that is often observed to be characteristic of the von Karman vortex streets that develop in the atmospheric lee of oceanic islands. 相似文献
11.
Waves with a large incidence angle in deep water can drive a morphodynamic instability on a sandy coast whereby shoreline sand waves, cuspate forelands, and spits can emerge. This instability is related to bathymetric perturbations extending offshore in the shoaling zone. Here, we explore a different mechanism where the large incidence angle is supposed to occur at breaking and the bathymetric perturbations occur only in the surf zone. For wave incidence angles at breaking above ≈?45°, the one-line approximation of coastal dynamics predicts an unstable shoreline. This instability (EHAWI) is scale-free and the growth rate increases without bound for decreasing wavelength. Here we use a 2DH morphodynamic model resolving surf zone instabilities to investigate whether EHAWI could approximate a real instability in nature with a characteristic length scale. Assuming very idealized conditions on the bathymetric profile and sediment transport, we find a 2DH instability mode consisting of shore-oblique up-current bars coupled to a meandering of the longshore current. This mode grows for high-angle waves, above about 30° (offshore) and the maximum growth rate occurs for the angle maximizing the angle at breaking, about 70° (offshore). The dominant wavelength is of the order of the surf zone width. Interestingly, for long sand waves, the growth rate never becomes negative and it matches very well the anti-diffusive behavior of EHAWI. This distinguishes the present instability mode from other modes found in previous studies for other bathymetric and sediment transport conditions. Thus, we conclude that EHAWI approximates a real morphodynamic instability only for quite particular conditions. In such case, a characteristic length scale of the instability emerges thanks to surf zone processes that damp short wavelengths. 相似文献
12.
The geological condition may vary largely from one support to another for a long span bridge. The effect of geological variability and spatial variation produced by propagation and coherence loss of seismic ground motion on the response of long span suspension bridges is investigated in this article. The case of Jiangyin Yangtse River Bridge, a suspension bridge with a main span of 1385 m, is studied in detail. Numerical results show that the geological difference at the supports has a significant effect on the seismic response of long span suspension bridges and, that it is unacceptable to neglect the difference. The effect of the propagation of seismic ground motion on the response of long span suspension bridges is far more important than that of the coherence loss. The response of bridges varies greatly with the horizontal apparent velocity in a very complex way, and there is a critical horizontal velocity for a given response quantity. 相似文献
13.
Grain-size distributions of suspended load over a sand-gravel bed at two different flow velocities were studied in a laboratory flume.The experiments had been performed to study the influence of flow velocity and suspension height on grain-size distribution in suspension over a sand-gravel bed.The experimental findings show that with an increase of flow velocity,the grain-size distribution of suspended load changed from a skewed form to a bimodal one at higher suspension heights.This study focuses on the determination of the parameter β_n which is the ratio of the sediment diffusion coefficient to the momentum diffusion coefficient of n th grain-size.A new relationship has been proposed involving β_n,the normalizing settling velocity of sediment particles and suspension height,which is applicable for widest range of normalizing settling velocity available in literature so far.A similar parameter β for calculating total suspension concentration is also developed.The classical Rouse equation is modified with β_n and β and used to compute grain-size distribution and total concentration in suspension,respectively.The computed values have shown good agreement with the measured values of experimental data. 相似文献
14.
Geometrical textures of faults，evolution of physical field and instability characteristics 总被引：9，自引：0，他引：9
Ｇｅｏｍｅｔｒｉｃａｌｔｅｘｔｕｒｅｓｏｆｆａｕｌｔｓ，ｅｖｏｌｕｔｉｏｎｏｆｐｈｙｓｉｃａｌｆｉｅｌｄａｎｄｉｎｓｔａｂｉｌｉｔｙｃｈａｒａｃｔｅｒｉｓｔｉｃｓＪＩＮＭＡ（马瑾），ＳＨＥＮＧ－ＬＩＭＡ（马胜利），ＬＩ－ＱＩＡＮＧＬＩＵ（刘力强），Ｚ... 相似文献
15.
Richard Blender 《地球物理与天体物理流体动力学》2019,113(5-6):594-601
ABSTRACTThe instability of ideal non-divergent zonal flows on the sphere is determined numerically by the instability criterion of Arnold (Ann. Inst. Fourier 1966, 16, 319) for the sectional curvature. Zonal flows are unstable for all perturbations besides for a small set which are in approximate resonance. The planetary rotation is stable and the presence of rotation reduces the instability of perturbations. 相似文献
16.
This study utilises the equivalent granular state parameter, ψ⁎, as a key parameter for studying static and cyclic instability and their linkage. ψ⁎ can be considered as a generalisation of the state parameter as first proposed by Been and Jefferies so that the influence of fines content in addition to stress and density state can be captured. Test results presented in this study conclusively showed that ψ⁎ at the start of undrained shearing and ηIS, the stress ratio at onset of static instability, can be described by a single relationship irrespective of fines content for both compression and extension shearing. This single relationship is referred as instability curve. However, the instability curve in extension shearing is different from that of compression. In this paper, the capacity of the instability curve in predicting triggering of cyclic instability was evaluated experimentally. An extensive series of undrained one-way (compression) and non-symmetric two-way cyclic triaxial tests, in addition to monotonic triaxial tests in both compression and extension were conducted for this evaluation. Furthermore, a published database for Hokksund sand with fines was also used. Test results show that cyclic instability was triggered shortly after the cyclic effective stress path crossed the estimated ηIS-zone(s) as obtained from instability curve(s) irrespective of whether instability occurs in the compression or extension side. 相似文献
17.
The magnetopause separates the geomagnetic field from the interplanetary plasma and performs finite motions under the action
of the solar wind pressure variable in time. Accelerations originating in this case result in that the necessary condition
for the development of the Rayleigh-Taylor instability is formed at quite a definite motion phase. We can anticipate that
the instability will develop during compression of the magnetosphere. It should be taken into account that the magnetopause
is a potential tangential discontinuity. On the one hand, a plasma flow along the magnetopause results in a decrease in the
Rayleigh-Taylor instability threshold. On the other hand, the Kelvin-Helmholtz instability threshold, typical of the tangential
discontinuity, also decreases during the magnetosphere compression phase. Thus, if we speak about the magnetopause, it is
natural and necessary to jointly consider both types of instability. Main information on the combined Rayleigh-Taylor-Kelvin-Helmholtz
instability is presented, the dispersion equation determining the evolution of small perturbations is considered, and the
possible geophysical applications to the theory (e.g., penetration of the solar plasma into the magnetosphere, excitation
of global Pc5 oscillations) are indicated. 相似文献
18.
本文用二维MHD数值模拟研究了地球磁层顶同时存在速度剪切和磁场剪切时Kelvin-Helmholtz不稳定性（K-H）和撕裂模不稳定性（TM）的耦合过程。在雷诺数和磁雷诺数确定时,Alfvèn马赫数值（MA）对耦合特性起决定性作用。在本文选取的参数条件下,若MA<0.4,自发TM占主导地位;当0.4≤MA<1.4时,TM受到K-H明显调制;如果MA≥1.4,K-H引起的涡旋运动起控制作用,导致一种新的不稳定性产生。该不稳定性称作涡旋撕裂模不稳定性。其饱和后长时间渐近状态由一个大尺度的流体涡旋和同心磁岛组成。在地球磁层顶通量传输事件（FTEs）中它可能起着重要作用。 相似文献
19.
O. M. Podvigina 《Izvestiya Physics of the Solid Earth》2011,47(5):440-445
The onset of Boussinesq convection in a horizontal layer of an electrically conducting incompressible fluid is considered.
The layer rotating about a vertical axis is heated from below; a vertical magnetic field is imposed. Rigid electrically insulating
boundaries are assumed. The loss of stability of the trivial steady state, which occurs as the Rayleigh numbers increase,
can be accompanied by the development of a monotonic or an oscillatory instability, depending on the parameter values of the
problem at hand (the Taylor number, the Chandrasekhar number, the kinematic and the magnetic Prandtl numbers). When the instability
is monotonic, the emerging convective rolls themselves are also unstable if the Taylor number is sufficiently large (the so-called
Küppers-Lortz instability takes place). In the present work it is studied how the critical value of the Rayleigh number, the
type of the trivial steady state instability, and the critical value of the Taylor number for the Küppers-Lortz instability
depend on the kinematic and the magnetic Prandtl numbers. We consider the values of the Prandtl number not exceeding 1, which
is typical for the outer core of the Earth. 相似文献
20.
The plasma instability process during internal gravity wave propagation through the ionospheric E region is considered. The growth rate of the instability has been found and it has been shown that it depends on perturbation wavelength, gravity wave parameters and direction of propagation. The conditions for the instability are favorable when the vorticity of the associated neutral motion becomes antiparallel to the geomagnetic field. In the proposed instability mechanism plasma irregularities could seed the large-scale sporadic E layer structuring because they are generated in situ as a part of the same neutral wind structure that serves to initiate the formation of the layer. 相似文献