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981.
The previously derived equation (Agekyan 1974) for the derivative ?f/?n of the field of directions along the normal to a trajectory is approximate, because differentiating along the normal takes the point out of the orbit and changes the third integral of motion. However, on the envelope of the trajectory, i.e., on the contour of an orbit or a fold, ?f/?n undergoes a discontinuity of the second kind. Many authors have used this property to find points of the contours of orbits and folds. Although the integrable equation is approximate, the envelope points are determined accurately. 相似文献
982.
Symmetric tensors are typically encountered during investigations associated with stress and strain analysis and, thus, they are of particular interest to geophysicists and geodesists. Furthermore, symmetric tensors are studied using eigentheory analysis which provides the decomposition of the tensor on its principal components (n independent eigenvalues and the corresponding eigenvectors). In this paper, an analytical expression of the covariance matrix of the eigenvalues and eigenvectors of an n-D symmetric tensor is derived based on the principles of linear algebra and differential calculus. Through numerical tests, the proposed formulation is proven to give realistic uncertainty estimates of the determined eigenparameters. The methodology also reveals the significant impact on uncertainty assessments when the parameter dependencies between principal components are neglected. 相似文献
983.
Spatial variability, mechanisms and propagation of change in an active meandering river 总被引:3,自引:2,他引:3
This paper addresses questions of the spatial pattern of instability and the mechanisms of change in an active meandering river, particularly whether and how change is propagated. More than 20 years of monitoring of a sequence of nearly 100 bends on one dynamic meandering river, combined with historical data and previous analyses of processes of change, provide a unique insight into the link between annual changes produced by erosion and deposition and the longer-term changes in planform. The study reach of the River Dane in NW England exhibits stable and unstable sections adjacent to one another. Rates of movement range up to 3 m a− 1, with maxima occurring in high curvature, free bends. Stable reaches are due to factors of gradient, curvature and bank resistance. Analysis of the large amount of data on occurrence of erosion and deposition in each bend each year reveals no definite association of changes in one bend with another. The detailed evidence of the morphological features in the bends shows that changes do not take place by bars moving progressively through reaches. Case studies of bends upstream of constrained, stable reaches indicate an oscillation of widening and narrowing of the channel, over a period of a few years, producing a net rotation of the bend. These areas are zones of stalling of sediment and change takes place by absorption and lateral movement. Overall, changes tend to be localised and fit the bend theory of meanders, but with low sensitivity reaches pinning the planform for longer periods in certain locations. 相似文献
984.
利用边界积分方程法对土质边坡失稳地球物理模型的视电阻率进行了系统数值计算,研究了土质边坡失稳的电阻率响应特征,揭示了边坡从稳定一失稳演化过程的电阻率变化规律,从而实现了边坡失稳的电阻率响应特征的提取。这些特征信息的获得为利用电法勘探方法探测土质边坡失稳的滑动面或结构面位置提供了依据,同时为实时观测,预测预报土质边坡失稳提供了理论支撑。 相似文献
985.
Impact of predicted climate change on landslide reactivation: case study of Mam Tor, UK 总被引:1,自引:0,他引:1
Global change is expected to result in worldwide increases in temperature and alteration of rainfall patterns. Such changes
have the potential to modify stability of slopes, both natural and constructed. This paper discusses the potential effect
of global climate change on reactivation of landslides through examination of predicted changes in rainfall pattern on the
active landslide at Mam Tor, Derbyshire, UK. This landslide is of Pleistocene origin and is crossed by a road that is now
abandoned. Damaging winter movement is known to occur when precipitation reaches both 1-month triggering and 6-month antecedent
thresholds. Return periods for threshold exceedence is modelled statistically, and the climate change data from the UKCIP
2002 report (Hulme et al. 2002) is applied to this model. For the predicted changes in precipitation, it is shown that the instability threshold could decrease
from 4 to 3.5 years by the 2080s for the medium–high climate change scenario. However, predicted temperature changes could
influence the response of the landslide through increased evapotranspiration leading to a change in the triggering precipitation
thresholds, and this will help counter the impact of changes in precipitation. Analysis of sources of uncertainty in the model
has been used to establish the factors that contribute to the predicted changes in stability. Assessment of these factors
can provide an indication of the potential impact of climate change on landslides in other areas of the UK. 相似文献
986.
Non-Darcy mixed convective flow of water due to external pressure gradient and buoyancy opposed forces are considered in a vertical channel filled with porous medium, which can be either isotropic or anisotropic. The linear theory of stability analysis has been used to numerically investigate the dependence of the transition behavior of the fully developed basic flow on the permeability of the medium. Numerical experiments indicate that mainly two main instability modes appear: Rayleigh–Taylor (R–T) and buoyant instability. For Darcy numbers (Da) ?10−9, R–T instability dominates within the entire Reynolds number (Re) range considered here. It was also found that for the same Re, the fully developed base flow is highly unstable (stable) for porous media with high (low) permeability. Further, it was seen that the disturbance isotherm cells migrate from the channel walls toward the centerline when permeability is reduced. Reducing the permeability by one order of magnitude (corresponding to a decrease of Darcy number from 10−6 to 10−7) increases base flow stability approximately 20-fold. For higher Reynolds numbers, buoyant, mixed and shear instability of the basic flow were found when Da was increased from 10−7 to 10−3. However, for cases in which permeability and porosity behaved as suggested by Carman–Kozeny relation (CKR), buoyant stability was the only mode of instability. Critical values of the Rayleigh (Ra) and Darcy (Da) numbers in the R–T mode of instability were related to each other by the hyperbolic function RaDa = −2.465. 相似文献
987.
988.
Relationship and its instability of ENSO — Chinese variations in droughts and wet spells 总被引:4,自引:0,他引:4
Monthly data of Self-Calibrated Palmer Drought Severity Index (PDSI) from 1951 to 2000 are calculated using historical precipitation and temperature data for Chinese 160 stations. Temporal and spatial patterns of the first empirical orthogonal function (EOF) of the PDSI reveals a fairly linear trend resulting from trends in precipitation and surface temperature, which is similar to the linear PDSI trend during 1951–2000 calculated using all monthly data. The EOF analysis also reveals that the leading mode correlates significantly with ENSO events in time and space. The ENSO EOF shows that during the typical warm phase of ENSO, surface conditions are drier in most regions of China, especially North China, but wetter than normal in the southern regions of Changjiang River, and Northwest China. During the typical cold phase of ENSO, these anomalies reverse sign. From 1951 to 2000, there are large multi-year to decadal variations in droughts and wet spells over China, which are all closely related to strong El Niño events. In other words, when one strong El Niño event happens, there is a possible big variability in droughts and wet spells over China on the multi-year or decadal scale. Studies also suggest that during the last 2–3 decades climate changes over China, especially North China’s drying and northwest China’s wetting, are closely related to the shift in ENSO towards warmer events and global warming since the late 1970s. The instability of the relationship is also studied. It is revealed that there is a good correlation between ENSO and Chinese variations in droughts and wet spells in the 3–8-year band, but the correlation between ENSO and Chinese variations in droughts and wet spells is instable. Studies suggest that there are decadal changes in the correlation: the wavelet coherency between ENSO and Chinese variations in droughts and wet spells is high during 1951–1962 and 1976–1991, but low during 1963–1975 and 1992–2000. 相似文献
989.
在试验研究的基础上对比了无机聚合物硅酸盐和有机聚合物——聚乙烯醇(PVA)、部分水解聚丙烯酰胺(PHPA)、非水解聚丙烯酰胺(PAM)和聚丙烯酸钾防塌能力的大小;由试验得出吸附成膜速度的快慢和保护膜的致密程度是决定钻井液体系防塌能力的关键因素;钻井液体系的防塌能力是无机聚合物和有机物质协同作用的结果;部分水解聚丙烯酰胺钻井液体系的防塌能力较非水解聚丙烯酰胺钻井液体系的防塌能力强;Na-CMC和PHP的加入可提高吸附膜的致密性及体系的防塌能力。 相似文献
990.