Modeling and Dynamics of Friction Wedge Dampers in Railroad Freight Trucks

A mechanical model for the vertical response of the bolster and friction-wedge suspension elements in a railroad freight truck is developed. The model incorporates linear stiffnesses and dry friction contact between rigid elements. Numerical bifurcation diagrams for sticking events, obtained for parameterizations of the amplitude and frequency of a harmonic track excitation, and for various friction laws, reveal distinct parameter-dependent dynamic responses: strongly nonlinear stick-slip oscillations, observed away from period-1 resonance; and weakly nonlinear, nearly harmonic responses, observed near the period-one resonance. The analytical method of harmonic balance is applied to develop a first order approximation for the period-1 response, and, as should be expected, is found to correspond well with the numerical results in the large amplitude limit and near the resonance location. Given that such weakly nonlinear, large amplitude, period-1 responses are not common for the speeds and system parameters of most freight vehicles, we conclude that friction-wedge models based on some form of equivalent linearization may be inadequate for simulating freight car dynamics in many cases of practical importance.     

The allocation of buses in heavily utilized networks with overlapping routes

Many transit systems outside North America are characterized by networks with extensively overlapping routes and buses frequently operating at, or close to, capacity. This paper addresses the problem of allocating a fleet of buses between routes in this type of system; a problem that must be solved recurrently by transit planners. A formulation of the problem is developed which recognizes passenger route choice behavior, and seeks to minimize a function of passenger wait time and bus crowding subject to constraints on the number of buses available and the provision of enough capacity on each route to carry all passengers who would select it. An algorithm is developed based on the decomposition of the problem into base allocation and surplus allocation components. The base allocation identifies a feasible solution using an (approx.) minimum number of buses. The surplus allocation is illustrated for the simple objective of minimizing the maximum crowding level on any route. The bus allocation procedure developed in this paper has been applied to part of the Cairo bus system in a completely manual procedure, and is proposed to be the central element of a short-range bus service planning process for that city.     

Some promising innovations in taxicab operations

Several shared-ride passenger transportation services which could be offered by taxicabs are currently prohibited in U.S. cities by state or local regulations. Although relaxation of these regulations has been advocated on numerous occasions, little regulatory change has occurred to date. Moreover, the taxicab industry has rarely been considered as a potential provider for publicly subsidized transportation services. This paper discusses several promising innovations in taxicab service provisions and subsidy mechanisms which have been implemented or proposed recently in U.S. cities. The paper argues for more well-designed experimentation with such innovations as a means of stimulating greater interest on the part of planners, policy-makers, regulators, and public transportation operators in broadening the role of taxicabs in urban transportation.     

Taxicab deregulation: Economic consequences and regulatory choices

The regulation of taxicab services is receiving an increasing amount of attention by city governments. At issue are the questions of whether local regulations should limit the supply of taxicabs and whether the regulations should control taxi fares.Recently, deregulation has become a popular suggestion; however, little empirical or theoretical evidence has existed to indicate the effects of taxi deregulations. This paper discusses these effects within a framework of eight regulatory scenarios involving different price, entry, and industry concentration factors. The analysis provides support for a public brokerage function.The preparation of this paper was supported, in part, by a contract from the Urban Mass Transportation Administration, U.S. Department of Transportation, Washington, DC.     

同侧肾缺血及对侧肾切除时留存肾DNA合成的研究

幼年小白鼠经体内~3H-TdR参入法液闪测定,检测一侧肾缺血及对侧肾切除后留存肾DNA合成的影响。结果表明:肾缺血、肾切除24h后均促进留存肾DNA合成。而一侧肾缺血同时对侧肾切除留存肾脏~3H-TdR参入量升高最明显。并探讨了一侧肾缺血及对肾切除后留存肾的代偿机理。     

Attitudinal models of modal choice: The multinomial case for selected nonwork trips

Attitudinal multinomial logit models of modal choice are presented for four nonwork activities: major grocery shopping, shopping for odds and ends, shopping for personal goods and visiting friends and acquaintances. Explanatory variables are individuals' beliefs about attributes of four modal alternatives: bus, car, taxi and walking. Factor analysis is employed to identify latent dimensions of perception of the modal alternatives and to eliminate problems of multicollinearities in model estimation. Models are estimated using data obtained for a sample of residents of Buffalo, New York. Planning implications of the methodology are assessed.This author is presently Systems Planner with Applied Resource Integration, Ltd., Boston, Massachusetts.     

A Driver Model Based on the Cerebellar Model Articulation Controller

A model of driver behavior is described which is based on a current theory of neurophysiological processes occurring in the cerebellum. The model learns to control the vehicle through experience, provides discontinuous ramp steer inputs to the vehicle, accepts discontinuous input data, and is applicable to all control situations.

The model is implemented on a simple simulation model of a car and learning is accomplished by the use of an explicit driver model which drives the vehicle along a specified trajectory.     

Experimental and 3D,embedded modeling for diesel engine SCR deposit

Nowadays, the selective catalytic reduction (SCR) is a promising solution to fulfill stringent nitrogen oxide (NO_X) emission standards enforced by worldwide regulation bodies for lean burning engines. However, in the practical operation, urea deposits occur under unfavorable conditions, which bring about a failure of urea dosing strategy, affect the SCR system de-NOx performance and lowering the fuel economy by increasing the engine back pressure. This paper will present 3D deposit model, which can be used to predict the deposit position. Furthermore, a model-based controls strategy and calibration are designed. The comparison test results of both engine emission tests and vehicle field tests shows there is significant deposits improvement with the embedded model.