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1.
Channel bifurcations can be found in river network systems from high gradient gravel-bed rivers to fine-grained low gradient deltas. In these systems, bifurcations often evolve asymmetrically such that one downstream channel silts up and the other deepens and, in most cases, they eventually avulse. Past analytical and numerical studies showed that symmetric bifurcations are unstable in high and low Shields stress conditions resulting in asymmetric bifurcations and avulsion, while they can be stable in the mid-Shields range, but this range is smaller for larger width-to-depth ratio. Here, using a one-dimensional (1D) numerical model, we show that effects of sediment grain size and of channel slope are much larger than expected for low-gradient systems when a sediment transport relation is used that separates between bedload and suspended load transport. We found that the range of Shields stress conditions with unstable symmetric bifurcations expanded for lower channel slopes and for finer sediment. In high sediment mobility, suspended load increasingly dominates the sediment transport, which increases the sediment transport nonlinearity and lowers the relative influence of the stabilizing transverse bedslope-driven flux. Contrary to previous works, we found another stable symmetric solution in high Shields stress, but this only occurs in the systems with small width-to-depth ratio. This indicates that suspended load-dominated bifurcations of lowland rivers are more likely to develop into highly asymmetric channels than previously thought. This explains the tendency of channel avulsion observed in many systems. 相似文献
2.
《地震工程与结构动力学》2018,47(5):1270-1290
The 2012 Emilia earthquake (in Northern Italy) caused extensive damage to existing prefabricated reinforced concrete structures. These buildings were found being extremely vulnerable because, being designed for vertical loads only, they featured friction‐based connections between structural elements, most commonly between beams and columns. Given the large diffusion of these structures, their seismic retrofit is critical. Various techniques have been proposed in the literature, in most of which friction‐based connections are removed by inserting mechanical connectors that will make beam‐column connections hinged. These approaches lead to a significant increase of the base shear and therefore require strengthening of columns. The paper presents dissipative devices based on carbon‐wrapped steel tubes to be used as an alternative low‐damage solution for the retrofit of beam‐column connections. The first part of the paper presents results of experimental tests on the devices and discusses their dissipative behaviour. The succeeding parts of the paper present numerical analyses on simple structures reinforced with the proposed device. The results of the numerical study show how the introduction of the dissipative devices produces a significant reduction of forces transmitted to the structure, by comparing the seismic response of simple frame structures equipped with dissipative devices with the response of equivalent elastic systems. 相似文献
3.
J. Horák 《Studia Geophysica et Geodaetica》2000,44(1):26-37
The inertial manifold is a positive invariant set which exponentially attracts all the trajectories of a dissipative dynamic system. It was introduced for the purpose of studying the asymptotic behaviour of such systems. The initial infinitely dimensional dynamic system, generated by a partial differential evolution equation, can be projected on to it, in order to obtain the final system of ordinary differential equations (inertial equations). These equations then simulate the initial object. Although the inertial manifold is an object relatively simpler than the attractor (a very complicated set of non-integer dimension may be an attractor) it is more difficult to prove its existence than that of the attractor. The equation of a barotropic fluid on a rotating spherical surface is one of the examples of dissipative dynamic systems with an inertial manifold. This kindles the hope that also the equations of the dynamics of the real atmosphere will have an inertial manifold. The reduction of the sample system to this Lipschitz manifold of finite dimension thus justifies us in analysing the behaviour of the atmosphere on non-linear models of finite dimensions and few parameters, in a finite system of ordinary differential equations. 相似文献
4.
5.
Abstract We study the nonlinear stability of MHD waves propagating in a two-dimensional, compressible, highly magnetized, viscous plasma. These waves are driven by a weak, shear body force which could be imposed by large scale internal fluctuations present in the solar atmosphere. The effects of anisotropic viscosity (leading to a cubic damping) and of the nonlinear coupling of the Alfven and the magnetoacoustic waves are analysed using Galerkin and multiple-scale analysis: the MHD equations are reduced to a set of nonlinear ordinary differential equations which is then suitably truncated to give a model dynamical system, representing the interaction of two complex Galerkin modes. For propagation oblique to the background magnetic field, analytical integration shows that the low-wavenumber mode is physically unstable. For propagation parallel to the background magnetic field the high-wavenumber wave can undergo saddlenode bifurcations, in way that is similar to the van der Pol oscillator; these bifurcations lead to the appearance of a hysteresis cycle. A numerical integration of the dynamical system shows that a sequence of Hopf bifurcations takes place as the Reynolds number is increased, up to the onset of nonperiodic behaviour. It also shows that energy can be transferred from the low- wavenumber to the high-wavenumber mode. 相似文献
6.
River bifurcations are key nodes within braided river systems controlling the flow and sediment partitioning and therefore the dynamics of the river braiding process. Recent research has shown that certain geometrical configurations induce instabilities that lead to downstream mid‐channel bar formation and the formation of bifurcations. However, we currently have a poor understanding of the flow division process within bifurcations and the flow dynamics in the downstream bifurcates, both of which are needed to understand bifurcation stability. This paper presents results of a numerical sensitivity experiment undertaken using computational fluid dynamics (CFD) with the purpose of understanding the flow dynamics of a series of idealized bifurcations. A geometric sensitivity analysis is undertaken for a range of channel slopes (0.005 to 0.03), bifurcation angles (22° to 42°) and a restricted set of inflow conditions based upon simulating flow through meander bends with different curvature on the flow field dynamics through the bifurcation. The results demonstrate that the overall slope of the bifurcation affects the velocity of flow through the bifurcation and when slope asymmetry is introduced, the flow structures in the bifurcation are modified. In terms of bifurcation evolution the most important observation appears to be that once slope asymmetry is greater than 0.2 the flow within the steep bifurcate shows potential instability and the potential for alternate channel bar formation. Bifurcation angle also defines the flow structures within the bifurcation with an increase in bifurcation angle increasing the flow velocity down both bifurcates. However, redistributive effects of secondary circulation caused by upstream curvature can very easily counter the effects of local bifurcation characteristics. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
7.
A study on subsurface airflow plays a vital role in quantifying the effectiveness of natural attenuation of volatile organic compounds (VOCs) or in determining the need of engineering systems (e.g., soil vapor extraction of VOCs). Here, we present a new analytical solution for describing the subsurface airflow induced by barometric pressure and groundwater head fluctuations. The solution improves a previously published semi‐analytical solution into a fully explicit expression and can save much computation efforts when it was used to estimate the soil permeability and porosity, which was demonstrated by a hypothetical example. If the groundwater head and barometric pressure fluctuations have the same frequency and the same order of magnitude for the amplitudes, each or the combination of both fluctuations will generate the air exchange volumes of the same order of magnitude through the ground surface. Particularly, the air exchange volume caused by the combined fluctuations increases with the upper layer's permeability and lower layer's porosity and decreases with the phase difference between these two fluctuations, fluctuation frequency, and the upper layer's thickness. The air exchange volume may decrease quickly to zero essentially when the upper layer's permeability decreases 10‐fold and decrease fourfold to fivefold when the phase difference decreases from π to zero. 相似文献
8.
Maarten G. Kleinhans Robert I. Ferguson Stuart N. Lane Richard J. Hardy 《地球表面变化过程与地形》2013,38(1):47-61
River bifurcations are critical but poorly understood elements of many geomorphological systems. They are integral elements of alluvial fans, braided rivers, fluvial lowland plains, and deltas and control the partitioning of water and sediment through these systems. Bifurcations are commonly unstable but their lifespan varies greatly. In braided rivers bars and channels migrate, split and merge at annual or shorter timescales, thereby creating and abandoning bifurcations. This behaviour has been studied mainly by geomorphologists and fluid dynamicists. Bifurcations also exist during avulsion, the process of a river changing course on a floodplain or in a delta, which may take 102–103 years and has been studied mainly by sedimentologists. This review synthesizes our current understanding of bifurcations and brings together insights from different research communities and different environmental settings. We consider the causes and initiation of bifurcations and avulsion, the physical mechanisms controlling bifurcation and avulsion evolution, mathematical and numerical modelling of these processes, and the possibility of stable bifurcations. We end the review with some open questions. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
9.
An array designed to detect changes of resistivity in the upper 10 km of the crust has been in continuous operation since 1988. Long period (T=300–7200 s) telluric signals are recorded digitally on 5–16 km long dipoles. Telluric transfer functions are computed between field dipoles and two reference dipoles and then decomposed to examine daily fluctuations of those functions. Based on analysis of these daily fluctuations, stabilities of 0.1–0.3% are achieved. No intermediate term resistivity changes are seen associated with any of the 5 Mb>4.0 earthquakes that have occurred since 1989. Short term fluctuations are seen associated with most of these earthquakes, however. Minimum bounds on resistivity changes that could have caused the fluctuations in the telluric responses range from 1.7 to 13.0%. While these are within the ranges seen in other field studies, the statistical significance of these fluctuations is low because they also occur at many times that cannot be correlated to earthquakes. Comparison of the one potential resistivity change associated with a strain change results in a strain sensitivity that is unrealistically high. This possible resistivity change coincides spatially with a temporary variation in seismic travel time in May–June 1989, however. It is likely that many of the fluctuations in the telluric responses are random variations except for the one in 1989, and recording through the next M6 earthquake will be needed in order to detect a statistically significant resistivity change. 相似文献
10.
In this paper the efficiency of various dissipative mechanisms to protect structures from pulse‐type and near‐source ground motions is examined. Physically realizable cycloidal pulses are introduced, and their resemblance to recorded near‐source ground motions is illustrated. The study uncovers the coherent component of some near‐source acceleration records, and the shaking potential of these records is examined. It is found that the response of structures with relatively low isolation periods is substantially affected by the high‐frequency fluctuations that override the long duration pulse. Therefore, the concept of seismic isolation is beneficial even for motions that contain a long duration pulse which generates most of the unusually large recorded displacements and velocities. Dissipation forces of the plastic (friction) type are very efficient in reducing displacement demands although occasionally they are responsible for substantial permanent displacements. It is found that the benefits by hysteretic dissipation are nearly indifferent to the level of the yield displacement of the hysteretic mechanism and that they depend primarily on the level of the plastic (friction) force. The study concludes that a combination of relatively low friction and viscous forces is attractive since base displacements are substantially reduced without appreciably increasing base shears and superstructure accelerations. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
11.
The paper illustrates a probabilistic methodology for assessing the vulnerability of existing reinforced concrete (RC) buildings with limited ductility capacity retrofitted by means of dissipative braces. The aim is to highlight the most important parameters controlling the capacity of these coupled systems and specific aspects concerning the response uncertainties. The proposed methodology is based on the use of local engineering demand parameters for monitoring the seismic response and on the development of component and system fragility curves before and after the retrofit. In the first part of the paper, the methodology is illustrated by highlighting its advantages with respect to the existing approaches. Then, its capability and effectiveness are tested by considering a benchmark two‐dimensional RC frame designed for gravity‐loads only. The frame is retrofitted by introducing elasto‐plastic dissipative braces designed for different levels of base shear capacity. The obtained results show the effectiveness of the methodology in describing the changes in the response and in the failure modalities before and after the retrofit, for different retrofit levels. Moreover, the retrofit effectiveness is evaluated by introducing proper synthetic parameters describing the fragility curves and by stressing the importance of employing local engineering demand parameters (EDPs) rather than global EDPs in the seismic risk evaluation of coupled systems consisting in low‐ductility RC frames and dissipative braces. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
12.
The paper presents and analyzes, from the point of view of smooth dynamic systems theory, a two-layer baroclinic model of
the troposphere in geostrophic approximation. The model describes airflow in β-channel within the tropospheric part of the
main Hadley circulation cell. It enables to obtain, after application of the Galerkin method, a fairly simple low-parametric
dynamic system describing the phenomena of non-linear interactions, bifurcations and blocking in the atmosphere. This enables
to take into consideration such basic factors influencing the atmospheric dynamics like the heat exchange within the surface,
orography, vertical variability of zonal wind and hydrostatic stability.
Impact of zonal thermal variability of the surface and vertical shear of zonal wind in the troposphere on the orographic bifurcation
was investigated and the oscillation character in the dynamic system after Hopf bifurcation of the second kind was analyzed.
Additionally, the model dynamics was investigated in conditions including momentum forcing in the upper and lower parts of
the troposphere and excluding orographic interaction, as well as in the conditions of thermal interaction between the troposphere
and the surface for the vertical shear of zonal wind in both tropospheric layers. Impact of the mean zonal wind in the troposphere
on the properties of model dynamics was assessed.
It was proved that zonally varied surface temperature and layered mean zonal wind in the atmosphere are the parameters that
have basic influence on the model dynamics. They cause numerous bifurcations and strongly influence the periods of oscillations
of the model variables. They are often Hopf bifurcations of the second kind during which tropospheric states fairly distant
from the ones before the bifurcations are generated. This significantly influences the model predictability. 相似文献
13.
The onset of Alfvén intermittent chaos in space plasmas is studied by numerically solving the derivative non-linear Schrödinger equation (DNLS) under the assumption of stationary Alfvén waves. We describe how the Alfvénic fluctuations of the magnetic field can evolve from periodic to chaotic behavior through a sequence of bifurcations as the plasma dissipation is varied. The collision of a chaotic attractor with an unstable periodic orbit leads to the generation of strongly chaotic behavior, in an event known as interior crisis. We also show that in the DNLS equation, chaotic attractors coexist with nonattracting chaotic sets responsible for transient chaotic behaviors. After the interior crisis point, a wide chaotic attractor can be decomposed into two coupled nonattracting chaotic sets, resulting in intermittent chaotic time series. Understanding transient chaos is a key to understand intermittency in space plasmas. 相似文献
14.
Leif M. Burge 《地球表面变化过程与地形》2006,31(10):1211-1226
This study presents the first detailed field‐based analysis of the morphology of bifurcations within anabranching cobble–gravel rivers. Bifurcations divide the flow of water and sediment into downstream anabranches, thereby influencing the characteristics of the anabranches and the longevity of river islands. The history, morphology, bed grain size, and flow vectors at five bifurcations on the Renous River, New Brunswick, Canada, were studied in detail. The angles of bifurcations within five anabranching rivers in the Miramichi basin were investigated. The average bifurcation angle was 47°, within the range of values cited for braided river bifurcations. Bifurcation angle decreased when anabranches were of similar length. Shields stresses in channels upstream of bifurcations were lower than reported values for braided rivers. Stable bifurcations displayed lower Shields stresses than unstable bifurcations, contrary to experimental results from braided river bifurcations. Bifurcations in anabranching rivers are stabilized by vegetation that slows channel migration and helps to maintain a uniform upstream flow field. The morphology of stable bifurcations enhances their stability. A large bar, shaped like a shallow ramp that increases in elevation to floodplain level, forms at stable bifurcations. Floodplains at stable bifurcations accrete upstream at rates between 0·9 and 2·5 m a?1. Bars may also form within the entrance of an anabranch downstream of the bifurcation node. These bars are associated with bifurcation instability, forming after a period of stability or an avulsion. Channel abandonment occurs when a bar completely blocks the entrance to one anabranch. The stability of channels upstream of bifurcations and the location of bars at bifurcations influence bifurcation stability and the maintenance of river anabranching in the long term. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
15.
Assessment of groundwater recharge influenced by floodwater spreading: an integrated approach with limited accessible data 总被引：3，自引：3，他引：0
M. Pakparvar K. Walraevens S.A.M. Cheraghi Gh. Ghahari W. Cornelis D. Gabriels 《水文科学杂志》2017,62(1):147-164
To assess the response of groundwater to artificial recharge through floodwater spreading (FWS) a combination approach of water table fluctuations and water budget was used. In this process, water level data in six observation wells installed inside and around the site of the FWS systems together with the amount of rainfall and volume of floodwater diverted to the system were examined during the period 1993–2012. Specific yield was also determined based on measured soil hydraulic properties for three experimental wells hand drilled within the FWS systems. The observation wells located inside the FWS systems were less susceptible to drought and abstractions than the other wells in the area. The hydrograph of the wells inside the FWS showed a large disparity in rises (0.5–2.05 m) after the two major floods in 2004 and 2005 due to systems closure in 2004. The water budget calculated based on water table fluctuations for 2010/11 showed that the contribution of FWS systems to total recharge in the study area was about 57–61%.
EDITOR D. KoutsoyiannisASSOCIATE EDITOR S. Kanae 相似文献
16.
Water and sediment distribution by river bifurcations is often highly unbalanced. This may result from a variety of factors, such as migration of bars, channel curvature and backwater effects, which promote an uneven partition of flow and sediment fluxes in the downstream branches, which we call ‘forcings’. Bifurcations also display an intrinsic instability mechanism that leads to unbalanced configurations, as occurs in the idealized case of a geometrically symmetric bifurcation, which we call ‘free’, provided the width-to-depth ratio of the incoming flow is large enough. Most frequently, these free and forced mechanisms coexist; however, their controlling roles in bifurcation dynamics have not been investigated so far. In this paper we address this question by proposing a unified free-forced modelling framework for bifurcation morphodynamics. Upstream channel curvature and different slopes of downstream branches (slope advantage) are specifically investigated as forcing effects typically occurring in bifurcations of alluvial channels. The modelling strategy is based on the widely used two-cell model of Bolla Pittaluga et al. (Water Resources Research, 2003, 39 (3), 1–13), here extended to account for the spatially non-uniform fluxes entering the bifurcation node. Results reveal that the relative role of free and forced mechanisms depends on the width-to-depth ratio falling above or below the resonant threshold that controls the stability of free bifurcations: when the main channel is relatively wide and shallow (super-resonant regime) the bifurcation invariably evolves towards unbalanced configurations, whatever the combination of curvature and slope advantage values, which instead control the bifurcation response under sub-resonant conditions. Detection of the resonant aspect ratio as a key threshold also releases the modelling approach from the need for parameter calibration that characterized previous approaches, and allows for interpreting under a unified framework the opposite behaviours shown by gravel-bed and sand-bed bifurcations for increasing Shields parameter values. © 2018 John Wiley & Sons, Ltd. 相似文献
17.
A.D. Heathershaw 《Continental Shelf Research》1985,4(4):485-493
Observations of internal wave current fluctuations at a site on the European continental shelf are described. These have revealed current ‘pulses’ of regular tidal (M2) phase which may be associated with internal tides generated at the shelf-edge. Current ‘pulses’ have been observed with amplitudes of 30 to 40 cm s?1 superimposed on peak spring tidal currents of the order 60 to 70 cm s?1. The measurements have shown that these fluctuations extended throughout the bottom mixed layer to within at least 2 m of the sea bed where they may play an important role in modifying sediment transport rates. 相似文献
18.
The specific features of the generation and intensification of internal gravity wave structures in different atmospheric-ionospheric regions, caused by zonal local nonuniform winds (shear flows), are studied. The model of the medium has been explained and an initial closed system of equations has been obtained in order to study the linear and nonlinear dynamics of internal gravity waves (IGWs) when they interact with the geomagnetic field in a dissipative ionosphere (for the D, E, and F regions). 相似文献
19.
Conceptual design and full‐scale experimentation of cladding panel connection systems of precast buildings 
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下载免费PDF全文 Despite the long series of European research projects that has led to the setting of fully reliable seismic design criteria for precast structures, recent earthquakes have shown that a weak point still exists in the proportioning of the connection systems of cladding wall panels. Following this finding, this paper outlines an organic setting of the design problem of precast concrete structures including cladding–structure interaction and describes three possible solutions, namely, the isostatic, integrated, and dissipative systems. The related fastening arrangements, with the use of existing and innovative connection devices, are also described. This paper comments on the results of the pseudo‐dynamic and cyclic tests performed at ELSA Laboratory of the European Joint Research Centre of Ispra (Italy) on a full‐scale prototype of precast structure. The conception and the experimental performance of the structure with nine different configurations of either vertical or horizontal wall panel claddings are presented. The analysis of the results highlights the effectiveness of the different solutions in a comparative way. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
20.
传统基于地震背景噪声互相关的层析成像技术,主要利用了互相关函数的相位信息,即利用走时来反演地球介质的速度和各向异性特征.最近,开始有研究人员利用互相关函数的振幅信息提取介质的衰减.主要是将二维弹性情形下,互相关函数正比于第一类零阶贝塞尔函数J_0(k_0r)这一结论,通过引入指数衰减系数,直接推广到衰减介质情形,令衰减介质中背景噪声互相关函数类比于J_0(k_0r)·e~(-α(ω)r),以此来反演介质的衰减系数α.然而,在衰减介质中,互相关受源的方位平均的影响,这种简单的推广,可能无法提取可靠的衰减系数.本文基于平面波的叠加模型,研究不同的互相关定义和坐标选择下,衰减介质中两点间互相关函数的理论表达式.结果表明,在平面波叠加模型下,互相关函数的表达形式随着坐标原点的选择,并因而随着源分布的变化而变化,对不同的归一化因子,表达式也不尽相同.利用J_0(k_0r)·e~(-α(ω)r)的形式拟合背景噪声的观测数据得到的衰减比实际值偏小. 相似文献