Crystallographic structure of sea ice along a salinity gradient and environmental control of microalgae in the brine cells

Duplicate ice cores were taken in southeastern Hudson Bay (Canadian Arctic), at 8 stations along a transect, from the mouth of the Great Whale River to saline waters 25 km offshore. One core from each station was cut into 10-cm sections, which were melted at room temperature for determinations of salinity, nutrients, algal pigments and taxonomic composition of the microalgal assemblages. On the second core, thin sections were cut every 2 cm to optically determine the ratio of brine/gas pockets to the total sections. Relative volumes of brine in melted samples were computed from the observed ratios and calculated values, and used to convert per unit brine volume the concentrations of pigments, nutrients and algal cells, initially measured per unit volume of melted sample.Salinity of the melted samples indicate the presence of freshwater beneath the sea ice close to shore throughout the growth season, and the vertical salinity profiles suggest intermittent intrusions of freshwater. Nutrient concentrations in the brine were high, suggesting that microalgae in the brine cells were generally not nutrient limited. Concentrations of ice algae in the brine cells reached relatively high values, that are often of the same order as those reported for eutrophic marine waters. Statistical analyses identified the rate of ice growth as the most important factor controlling the vertical and horizontal distributions of algal biomass and taxonomic composition in the sea ice, along the salinity gradient. Higher biomasses were generally associated with slower ice growth and also possibly lower grazing activity. In addition, the rate of ice growth influenced the distribution of microalgal assemblages, with nitrogen limitation potentially playing a secondary role in some instances.     

The Chatter of Semi-Active On-Off Suspensions and its Cure

Semi-active suspensions are those in which otherwise passively generated damper forces are modulated using feedback control and small amounts of control effort. Recently it was discovered that two-stage, ON-OFF, semi-active control would chatter between the ON and OFF states in a manner similar to bang-bang, active control systems. This chatter is dependent upon the switching algorithm. This paper describes the dynamics of this chatter and suggests alternative control policies for its cure.     

Modelling route choice behaviour in multi-modal transport networks

The paper presents new findings on the influence of multi-modal trip attributes on the quality and competitiveness of inter-urban multi-modal train alternatives. The analysis covers the entire trip from origin to destination, including access and egress legs to and from the train network. The focus is on preferences for different feeder modes, railway station types and train service types as well as on the relative influence of time elements and transfer penalties. Data from dedicated surveys are used including individual objective choice sets of 235 multi-modal homebound trips in which train is the main transport mode. The observed trips have origins and destinations within the Rotterdam–Dordrecht region in The Netherlands with an average total trip time of 50 minutes. Hierarchical Nested Logit models are estimated to take account of unobserved similarities between alternatives at the home-end and the activity-end of the trip respectively, resulting in two-level nesting structures which differentiate between intercity (IC) and non-intercity railway station types at the upper level and between transit and private access modes at the lower level. In order to reflect the multi-dimensional structure of the data a more advanced so-called Multi-Nested GEV model according to the Principles of Differentiation has been estimated which significantly improves the explanatory power and stresses the importance of the home-end of the multi-modal trip.     

Contents Volume 32 2005

Contents Volume

Contents Volume 32 2005     

Rural mobility and mode choice: Evidence from the 2001 National Household Travel Survey

This article uses data from the 2001 National Household Travel Survey (NHTS) to compare travel behavior in rural and urban areas of the U.S. As expected, the car is the overwhelmingly dominant mode of travel. Over 97% of rural households own at least one car vs. 92% of urban households; 91% of trips are made by car in rural areas vs. 86% in urban areas. Regardless of age, income, and race, almost everyone in rural areas relies on the private car for most travel needs. Mobility levels in rural areas are generally higher than in urban areas. That results from the more dispersed residences and activity sites in rural areas, which increase trip distances and force reliance on the car. Somewhat surprisingly, the rural elderly and poor are considerably more mobile than their urban counterparts, and their mobility deficit compared to the rural population average is strikingly less than for the urban elderly and poor compared to the urban average. Data limitations prevented a measurement of accessibility, however, and it seems likely that rural areas, by their very nature, are less accessible than urban areas, especially for the small percentage of car-less poor and elderly households.     

FLUENT软件对管壳式换热器壳程流体数值模拟方法可行性的验证

鉴于单弓形折流板管壳式换热器应用的广泛性,对传统单弓形折流板管壳式换热器进行了数值模拟分析,验证了FLUENT软件应用到管壳式换热器壳程流体数值模拟方法的可行性,并为换热器其它壳程支撑结构的研究提供了一定的理论依据.     

Statistical characteristics of transitional queue conditions in signalized arterials

The performance of signalized arterials is related to queuing phenomena. The paper investigates the effect of transitional traffic flow conditions imposed by the formation and dissipation of queues. A cross-recurrence quantification analysis combined with Bayesian augmented networks are implemented to reveal the prevailing statistical characteristics of the short-term traffic flow patterns under the effect of transitional queue conditions. Results indicate that transitions between free-flow conditions, critical queue conditions that exceed the detector’s length, as well as the occurrence of spillovers impose a set of prevailing traffic flow patterns with different statistical characteristics with respect to determinism, nonlinearity, non-stationarity and laminarity. The complexity in critical queue conditions is further investigated by introducing two supplementary regions in the critical area before spillover occurrence. Results indicate that the supplementary information on the transitional conditions in the critical area increases the accuracy of the predictive relations between the statistical characteristics of traffic flow evolution and the occurrence of transitions.     

以截面平衡原理分析各种桥型结构体系

从材料力学中构件截面抗弯、抗剪平衡原理出发，分析桥梁的横截面、纵向桥型体系的受力特点，并以梁的受力方式来理解常用的纵向受力体系，如拱、斜拉、悬索等结构体系，称之为“等代梁”。从而将常见桥型分为两类：按正弯矩设计的体系，如简支梁、拱桥、悬索桥；按负弯矩设计的体系，如T构、连续梁、连续刚构和斜拉桥。提出了以组合体系来解决目前常见大跨径梁式桥常见病害的看法。     

Evaluation of transit operations: data applications of Tri-Met's automated Bus Dispatching System

In this paper, archived Automatic Vehicle Location and Automatic Passenger Counter data are used to evaluate actual bus running time variation in relation to scheduled service for Tri-Met, the transit provider for the Portland, Oregon metropolitan area. Given observed variation in running times, scheduled recovery times are found to be generally (though not universally) excessive. This results in an under-investment of resources in revenue service relative to non-revenue service. Analysis of trip level data reveals that bus operators are an important source of running time variation after controlling for such factors as route design, time of day and direction of service, and passenger activity.     

The existence, uniqueness and computation of an arc-based dynamic network user equilibrium formulation

In this paper, a dynamic user equilibrium traffic assignment model with simultaneous departure time/route choices and elastic demands is formulated as an arc-based nonlinear complementarity problem on congested traffic networks. The four objectives of this paper are (1) to develop an arc-based formulation which obviates the use of path-specific variables, (2) to establish existence of a dynamic user equilibrium solution to the model using Brouwer's fixed-point theorem, (3) to show that the vectors of total arc inflows and associated minimum unit travel costs are unique by imposing strict monotonicity conditions on the arc travel cost and demand functions along with a smoothness condition on the equilibria, and (4) to develop a heuristic algorithm that requires neither a path enumeration nor a storage of path-specific flow and cost information. Computational results are presented for a simple test network with 4 arcs, 3 nodes, and 2 origin–destination pairs over the time interval of 120 periods.