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21.
T. X. Wu D. J. Thompson 《Vehicle System Dynamics: International Journal of Vehicle Mechanics and Mobility》2002,37(3):157-174
The wheel/rail contact forces are calculated in the frequency domain using a track model with multiple wheels on the rail. The effects of the wave reflections between the wheels on the contact force are studied. Different pad stiffnesses are used in the calculations to investigate the influence on the contact force. It is shown that the contact force can have up to four main peaks in the frequency region 550-1200?Hz due to the wave reflections between the wheels, so that the wavelengths of short pitch corrugation can be expected to be associated with multiple frequencies. As a conclusion, it is recommended that in a model for predicting short pitch corrugation the effects of multiple wheel/rail interactions need to be included. 相似文献
22.
The Southern Ocean is an extreme environment, where waters are permanently cold, a seasonal ice cover extends over large areas, and the solar energy available for photosynthesis is severely restricted, either by vertical mixing to considerable depths or, especially south of the Antarctic Circle, by prolonged seasonal periods of low or no irradiance. Such conditions would normally lead to low productivity and a water column dominated by recycling processes involving microbial components of pelagic communities but this does not seem to be the case in the Southern Ocean, where there is efficient export to large apex predators and deep waters. This paper investigates the role of large microphagous zooplankton (salps, krill, and some large copepods) in the partitioning of biogenic carbon among the pools of short- and long-lived organic carbon and sequestered biogenic carbon. Large microphagous zooplankton are able to ingest microbial-sized particles and thus repackage small, non-sinking particles into both metazoan biomass and large, rapidly sinking faeces. Given the wide spatio-temporal extent of microbial trophic pathways in the Southern Ocean, large zooplankton that are omnivorous or able to ingest small food particles have a competitive advantage over herbivorous zooplankton. Krill efficiently transfer carbon to a wide array of apex predators and their faecal pellets are exported to depth during occasional brief sedimentation episodes in spring time. Salps may be a significant link towards some fish (directly) and other apex predators (indirectly) and, at some locations (especially in offshore waters) and time, they may account for most of the downward flux of biogenic carbon. Large copepods are a trophic link towards fish and at least one whale species, and their grazing activity generally impedes the export of organic particles to depth. As a result, biogenic carbon is channelled mainly towards apex predators and episodically into the deep ocean. Without these original interactions, Antarctic waters might well be dominated by microbial components and recycling processes instead of active export from the generally small primary producers towards large apex predators. 相似文献
23.
24.
RMS Values for Force, Stroke and Deflection in a Quarter-car Model Active Suspension with Preview 总被引:1,自引:0,他引:1
A. G. Thompson C. E. M. Pearce 《Vehicle System Dynamics: International Journal of Vehicle Mechanics and Mobility》2003,39(1):57-75
Summary In the case of a quarter-car vehicle model incorporating an active suspension, the addition of preview to the system necessitates finding new techniques for the direct computation of rms values for control force, suspension stroke and tyre deflection on a random road. Here we obtain these rms values from the performance index components as derived analytically by means of matrix operations requiring the solution of Lyapunov equations and the use of similarity transformations. Some numerical examples and MATLAB programs are included. 相似文献
25.
Louis Thompson 《运输评论》2013,33(3):311-355
Abstract Rail freight has been generally in decline in many European Union countries in recent years, contrary to European transport policy. State support for railway operations is commonplace in most countries, and this paper establishes the background to targeted rail freight grant funding in Britain. Through desk‐based analysis of Freight Facilities Grant (FFG) awards, together with a survey of recipient companies, the paper assesses the extent to which the planned flows expected from these awards have materialized and it evaluates the role of the grants in influencing rail freight volumes. The evidence suggests that FFG funding has been largely successful, attracting considerable private sector investment. Overall, FFGs have played an important role in developing or retaining rail freight flows, although the processes could be made more transparent and consistent. As other European countries liberalize their rail freight markets as a result of European Union legislation, such targeted funding may be an appropriate alternative to more general government subsidy of freight operations. 相似文献
26.
Transportation specialists, urban planners, and public health officials have been steadfast in encouraging active modes of
transportation over the past decades. Conventional thinking, however, suggests that providing infrastructure for cycling and
walking in the form of off-street trails is critically important. An outstanding question in the literature is how one’s travel
is affected by the use of such facilities and specifically, the role of distance to the trail in using such facilities. This
research describes a highly detailed analysis of use along an off-street facility in Minneapolis, Minnesota, USA. The core
questions addressed in this investigation aim to understand relationships between: (1) the propensity of using the trail based
on distance from the trip origin and destination, and (2) how far out of their way trail users travel for the benefit of using
the trail and explanatory factors for doing so. The data used in the analysis for this research was collected as a human intercept
survey along a section of an off-street facility. The analysis demonstrates that a cogent distance decay pattern exists and
that the decay function varies by trip purpose. Furthermore, we find that bicyclists travel, on average, 67% longer in order
to include the trail facility on their route. The paper concludes by explaining how the distance decay and shortest path versus
taken path analysis can aid in the planning and analysis of new trail systems.
Kevin J. Krizek is an Associate Professor of Planning and Design at the University of Colorado where he directs the Active Communities/Transportation Research Group. His research interests include land use-transportation policies and programs that influence household residential location decisions and travel behavior. He has published in the areas of transportation demand management, travel behavior, neighborhood accessibility, and sustainable development. He earned a Ph.D. in Urban Design and Planning and M.S.C.E. from the University of Washington in Seattle. His master’s degree in planning is from the University of North Carolina at Chapel Hill and his undergraduate degree is from Northwestern University. Ahmed El-Geneidy is a Post-Doctoral research fellow at the Department of Civil Engineering, University of Minnesota and Humphrey Institute of Public Affairs. El-Geneidy’s research interests include transit operations, travel behavior, land use and transportation planning, and accessibility/mobility measures in urban areas. He earned B.S. and M.S. degrees from the Department of Architectural Engineering at the University of Alexandria, Egypt, and continued his academic work at Portland State University, where he received a Graduate GIS Certificate and earned a Ph.D. in Urban Studies from Nohad A. Toulan School of Urban Studies and Planning. Kristin Thompson was a research assistant with ACT and currently works for Metro Transit in Minneapolis, Minnesota. 相似文献
Ahmed El-GeneidyEmail: |
Kevin J. Krizek is an Associate Professor of Planning and Design at the University of Colorado where he directs the Active Communities/Transportation Research Group. His research interests include land use-transportation policies and programs that influence household residential location decisions and travel behavior. He has published in the areas of transportation demand management, travel behavior, neighborhood accessibility, and sustainable development. He earned a Ph.D. in Urban Design and Planning and M.S.C.E. from the University of Washington in Seattle. His master’s degree in planning is from the University of North Carolina at Chapel Hill and his undergraduate degree is from Northwestern University. Ahmed El-Geneidy is a Post-Doctoral research fellow at the Department of Civil Engineering, University of Minnesota and Humphrey Institute of Public Affairs. El-Geneidy’s research interests include transit operations, travel behavior, land use and transportation planning, and accessibility/mobility measures in urban areas. He earned B.S. and M.S. degrees from the Department of Architectural Engineering at the University of Alexandria, Egypt, and continued his academic work at Portland State University, where he received a Graduate GIS Certificate and earned a Ph.D. in Urban Studies from Nohad A. Toulan School of Urban Studies and Planning. Kristin Thompson was a research assistant with ACT and currently works for Metro Transit in Minneapolis, Minnesota. 相似文献
27.
Zooplankton communities were studied in southeastern Beaufort Sea (Arctic Ocean) in September–October 2002. Cluster analysis and non-metric multidimensional scaling revealed three distinct mesozooplankton assemblages. A neritic assemblage occurred on the Mackenzie Shelf and in Franklin Bay, while distinct off-shelf assemblages prevailed in the Cape Bathurst Polynya and on the Beaufort Slope respectively. Over 95% of the mesozooplankton was comprised of eight copepod taxa. Pseudocalanus spp. contributed predominantly to the discrimination of the three assemblages and was the only significant indicator of the Shelf assemblage. Oithona similis, Oncaea borealis, Metridia longa and Calanus hyperboreus were indicators of the Polynya assemblage. Cyclopina sp. and Microcalanus pygmaeus were indicative of the overall off-shelf community (Polynya and Slope assemblages). The importance of omnivores and carnivores increased from the shelf to the polynya and the slope. Station depth and duration of reduced ice conditions during summer (< 50% ice concentration) underpinned the distribution of the assemblages (r2 = 0.71 and 0.45 respectively). The abundance of Pseudocalanus spp. was independent of depth and increased with the duration of reduced ice conditions (rs = 0.438). The abundance of Cyclopina sp., M. pygmaeus and other indicators of the offshore assemblages followed the opposite trend (rs = − 0.467 and − 0.5 respectively). Under continued climate warming, a reduction of the ice cover will affect the biogeography of mesozooplankton on and around the Mackenzie Shelf, to the potential advantage of Pseudocalanus spp. and other calanoid herbivores. 相似文献
28.
Direct Computation of the Performance Index for an Optimally Controlled Active Suspension with Preview Applied to a Half-Car Model 总被引:1,自引:0,他引:1
A. G. Thompson C. E. M. Pearce 《Vehicle System Dynamics: International Journal of Vehicle Mechanics and Mobility》2001,35(2):121-137
A quadratic integral performance index is defined for a linear active preview-type suspension and a series of matrix expressions derived for its evaluation by means of MATLAB or some similar computer program in the case of a unit step input to the system. The computation, which is both fast and accurate compared to simulation, requires the solution of Lyapunov- and Riccati-type equations. Some examples of numerical computation are given and these show excellent agreement with published results. The conclusion features a useful computer program. 相似文献
29.
Transportation - In this paper, an optimisation model for recovery planning of road networks is presented in which both social and economic resilience is aimed to be achieved. The model is... 相似文献
30.
Transportation - Informal minibus services dominate public transportation in Lagos, Nigeria. Local, state, and federal government entities in Nigeria have historically only been able to provide... 相似文献