首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   8篇
  免费   0篇
  国内免费   1篇
测绘学   1篇
大气科学   4篇
地球物理   1篇
地质学   1篇
天文学   1篇
自然地理   1篇
  2020年   1篇
  2019年   1篇
  2018年   1篇
  2013年   1篇
  2011年   1篇
  2003年   1篇
  1999年   1篇
  1997年   1篇
  1985年   1篇
排序方式: 共有9条查询结果,搜索用时 18 毫秒
1
1.
We present a new measure for the rotation of Lagrangian trajectories in turbulence that simplifies and generalises that suggested by Wilson and Flesch ( Boundary-Layer Meteorol. 84, 411–426). The new measure is the cross product of the velocity and acceleration and is directly related to the area, rather than the angle, swept out by the velocity vector. It makes it possible to derive a simple but exact kinematic expression for the mean rotation of the velocity vector and to partition this expression into terms that are closed in terms of Eulerian velocity moments up to second order and unclosed terms. The unclosed terms arise from the interaction of the fluctuating part of the velocity and the rate of change of the fluctuating velocity.We examine the mean rotation of a class of Lagrangian stochastic models that are quadratic in velocity for Gaussian inhomogeneous turbulence. For some of these models, including that of Thomson ( J. Fluid Mech. 180, 113–153), the unclosed part of the mean rotation vanishes identically, while for other models it is non-zero. Thus the mean rotation criterion clearly separates the class of models into two sets, but still does not provide a criterion for choosing a single model.We also show that models for which = 0 are independent of whether the model is derived on the assumption that total Lagrangian velocity is Markovian or whether the fluctuating part is Markovian.  相似文献   
2.
Tidal bores may appear in some estuaries when the tides quickly reach a high level. This phenomenon is rare but has a strong impact during its short duration: i.e. the river bed is significantly eroded and sediments are then transported. In this paper, the trajectories of suspended particles induced by this flow are numerically studied. Four undular bores with Froude numbers between 1.1 and 1.2 are studied. Despite similar Froude numbers, various initial flow conditions were selected to produce or not an inversion of the flow direction during the bore passage. The particle trajectories associated with each distinct flow configuration are presented and analyzed. These trajectories, estimated by solving the Maxey-Riley equation, appear to be very different even though the Froude numbers of flows are similar. These observations are important because the Froude number is often used to characterize a tidal bore as it describes well the free surface, however, it cannot describe the sediment transport. Finally, Chen's model of wave-current interactions is adapted to fit the cases studied and is applied to the four bores simulated. The results highlight that this latter model can reproduce the observed trajectories and dissociate their different components. From this model, it is shown that the inertial and Basset history effects can be neglected compared to the gravity and flow entrainment effects due to the viscous drag when one wants to determine the long-term trajectories of suspended particles.  相似文献   
3.
ABSTRACT

The increasing availability of devices to capture the position of moving objects (and other environmental information) leads to a very large amount and variety of mobility data. In order to obtain important information about the objects, their behavior or the environment of the objects, an automatic analysis is required. This article highlights current research questions in the context of the analysis of mobility data and presents them on the basis of work carried out at the Institute of Cartography and Geoinformatics (ikg) at Leibniz University of Hannover, Germany. A focus is put on the analysis and exploitation of information from Mobile Mapping vehicles.  相似文献   
4.
The calculation of air passenger flow density and the recognition of heat airspace in terminal areas of large hub airports is a new challenging research in the intelligent era, that is, using big data can automatically generate air passenger flow and basic rules. Aimed for the air passenger flow density in airport and its relationship between occupation and airspace, based on the establishment of the Beijing International Airport 1 h flight, which consists of basic parameters-latitude, longitude and height, additional parameters-dogleg and speed of trajectories, time parameters, a spatio-temporal data set by clustering trajectories and calculation of aircraft trajectories points was made up. Then, heat cloud map of flight trajectories under 4D conditions was generated. Cell was used to identify the basic parameters and time parameter of heat airspace; grid graphs of flight trajectories were used to identify additional parameters and explain the influence on heat airspace; probability fitting graphs were used to verify the accuracy of 4D results. The conclusion is as follows: there are two areas of Beijing International Airport, which have the high density and at two different heights there also exist hot peaks; flight trajectories and speed of trajectories also affect the heat airspace. The variation of heat airspace caused by different flight occupancy time in 4D recognition was revealed. The method realized the 4D heat cloud map of flight trajectories, which is better for detailed and accurate information construction, expression of spatial changes, and more the parameters of density and visualization of temporal and spatial range, which can assist the automatic dynamic airspace sectorization and decision making on airspace allocation, and also have a definite reference meaning in alleviating the contradiction between the current air traffic demand and limited airspace.  相似文献   
5.
Maladaptive trajectories of change in Makira, Solomon Islands   总被引:2,自引:0,他引:2  
Trajectories of change are dynamic processes of individual, group and/or societal responses to change which create further change and responses, with outcomes that reflect the cumulative properties of those processes. Understanding trajectories of change is an important initial step for designing appropriate adaptation strategies because even though responses may enable people to cope with change in the short term, the accumulated responses of individuals can generate undesirable maladaptive outcomes over longer periods of time. This paper examines trajectories of change in Kahua, Solomon Islands, where people have traditionally relied on subsistence activities and have in the past been subsistence affluent. Participatory methods, including 76 focus group discussions in 38 communities with 821 individuals, were used to determine changes in the region and its drivers. A conceptual model was developed of the underlying feedback processes within the Kahua social–ecological system. The results show that communities are facing rapid and extensive changes. Most changes, however, are being driven by the two key drivers of population growth and a strong desire for monetary prosperity that act synergistically to generate stress in communities. People are generally responding by focusing on income generation, which is reinforcing stress in communities and resulting in maladaptive trajectories of change. The results suggest development policy in the Solomon Islands needs to: (1) take the challenges of population growth much more seriously; (2) place greater effort on development activities that reduce per capita impact on the environment; (3) improve management of the high expectations for monetary prosperity; (4) increase emphasis on wellbeing aspects of development rather than income generation per se, and (5) better align development with existing adaptation strategies to ensure that vulnerability to future global change does not increase.  相似文献   
6.
A statistical study has been carried out of the availability of favourable flight opportunities to near-Earth asteroids with orbits similar to the Earth's. Emphasis is given to rendezvous-type mission profiles employing two-burn impulsive transfers. Velocity-optimized Lambert trajectories for a sample of 27 actual objects were calculated and compiled in a database. The velocity and flight time statistics of the resulting 1200 different solutions covering a period of 11 years have been investigated and discussed. Comparison with typical flight profiles to the Moon and near planets has revealed flight opportunities to 5 objects within a decade from the present requiring less ΔV than favourable flight opportunities to Mars or Venus. One of the objects involved, 1999 AO10, can be rendezvoused with using a total velocity increment that is smaller than that required to establish a lunar orbiter. The use of slow flybys for the most scientifically appealing targets is illustrated through an example trajectory involving the C-class binary object 1996 FG3. The challenges and opportunities for doing science in proximity to such small objects are also discussed.  相似文献   
7.
Mobile devices are becoming very popular in recent years, and large amounts of trajectory data are generated by these devices. Trajectories left behind cars, humans, birds or other objects are a new kind of data which can be very useful in the decision making process in several application domains. These data, however, are normally available as sample points, and therefore have very little or no semantics. The analysis and knowledge extraction from trajectory sample points is very difficult from the user's point of view, and there is an emerging need for new data models, manipulation techniques, and tools to extract meaningful patterns from these data. In this paper we propose a new methodology for knowledge discovery from trajectories. We propose through a semantic trajectory data mining query language several functionalities to select, preprocess, and transform trajectory sample points into semantic trajectories at higher abstraction levels, in order to allow the user to extract meaningful, understandable, and useful patterns from trajectories. We claim that meaningful patterns can only be extracted from trajectories if the background geographical information is considered. Therefore we build the proposed methodology considering both moving object data and geographic information. The proposed language has been implemented in a toolkit in order to provide a first software prototype for trajectory knowledge discovery.  相似文献   
8.
Among well-mixed multi-dimensional Lagrangian stochastic (LS) dispersion models, we observe that those in poorest agreement with observations produce spiralling trajectories, with an associated reduction in dispersion. We therefore investigate statistics of increments d ' to the orientation '= arctan(W'/U') of the Lagrangian velocity-fluctuation vector – as a possible means to distinguish the better LS models within the well-mixed class. Zero-spin models, having d' = 0, are found to provide best agreement with observations. It is not clear however, whether imposition of the zero-spin property selects (in conjunction with the well-mixed condition) a unique model.  相似文献   
9.
Whether in classical networks such as meteorological networks of in more recent ones of atmospheric chemistry, a wealth of data is at hand. These data have been evaluated in a manner depending on the purpose of the network. However, much more information is hidden in these time series and waits for discovery. Only the imagination of scientists is needed. Four examples are given which lead to new information about the atmospheric aerosol and the behaviour of the atmosphere. These examples are: Atmospheric turbidity from sunshine recordings, Meteorological drainage area from the variance of observations, Location of point sources from air mass trajectories, and Total vertical ozone from turbidity measurements.  相似文献   
1
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号