João Pombo

A new wheel–rail contact model for railway dynamics

The guidance of railway vehicles is determined by a complex interaction between the wheels and rails, which requires a detailed characterization of the contact mechanism in order to permit a correct analysis of the dynamic behavior. The kinematics of guidance of the wheelsets is based on the wheels and rails geometries. The movement of the wheelsets along the rails is characterized by a complex contact with relative motions on the longitudinal and lateral directions and relative rotations of the wheels with respect to the rails. A generic wheel–rail contact detection formulation is presented here in order to determine online the contact points, even for the most general three-dimensional motion of the wheelset. This formulation also allows the study of lead and lag flange contact scenarios, both fundamental for the analysis of potential derailments or for the study of the dynamic behavior in the presence of switches. The methodology is used in conjunction with a general geometric description of the track, which includes the representation of the rails’ spatial geometry and irregularities. In this work, the tangential creep forces and moments that develop in the wheel–rail contact area are evaluated using alternatively the Kalker linear theory, the Heuristic nonlinear model or the Polach formulation. The discussion on the benefits and drawbacks of these methodologies is supported by an application to the dynamic analysis of the bogie of the railway vehicle.

Influence of the aerodynamic forces on the pantograph–catenary system for high-speed trains

Most of the high-speed trains in operation today have the electrical power supply delivered through the pantograph–catenary system. The understanding of the dynamics of this system is fundamental since it contributes to decrease the number of incidents related to these components, to reduce the maintenance and to improve interoperability. From the mechanical point of view, the most important feature of the pantograph–catenary system consists in the quality of the contact between the contact wire of the catenary and the contact strips of the pantograph. The catenary is represented by a finite element model, whereas the pantograph is described by a detailed multibody model, analysed through two independent codes in a co-simulation environment. A computational procedure ensuring the efficient communication between the multibody and finite element codes, through shared computer memory, and suitable contact force models were developed. The models presented here are contributions for the identification of the dynamic behaviour of the pantograph and of the interaction phenomena in the pantograph–catenary system of high-speed trains due to the action of aerodynamics forces. The wind forces are applied on the catenary by distributing them on the finite element mesh. Since the multibody formulation does not include explicitly the geometric information of the bodies, the wind field forces are applied to each body of the pantograph as time-dependent nonlinear external forces. These wind forces can be characterised either by using computational fluid dynamics or experimental testing in a wind tunnel. The proposed methodologies are demonstrated by the application to real operation scenarios for high-speed trains, with the purpose of defining service limitations based on train and wind speed combination.

General Spatial Curve Joint for Rail Guided Vehicles: Kinematics and Dynamics

In the framework of multibody dynamics for rail-guided vehicleapplications, a new kinematic constraint is proposed, whichenforces that a point of a body follows a reference path while thebody maintains a prescribed orientation relative to a Frenet frameassociated to the spatial track curve. Depending on the specificapplication, the tracks of rail-guided vehicle are described byanalytical line segments or by parametric curves. For railway andlight track vehicles, the nominal geometry of the track isgenerally done by putting together straight and circular tracksegments, interconnected by transition track segments that ensurethe continuity of the first and second derivatives of the railwayin the transition points. For other applications, the definitionof the track is done using parametric curves that interpolate agiven number of control points. In both cases, the completecharacterization of the tracks also requires the definition of thecant angle variation, which is done with respect to the osculatingplane associated to the curve. The track models for multibodyanalysis must be in the form of parameterized curves, where thenominal geometry is obtained as a function of a parameterassociated to the curve length. The descriptions adopted hereensure, not only that the type of continuity of the original trackdefinition is maintained, but also that no unwanted deviationsfrom the nominal track geometry are observed, which can beperceived in the dynamic analysis as track perturbations. In thiswork different types of track geometric descriptions arediscussed. The application of cubic splines, to interpolate a setof points used to describe the track geometry, leads to undesiredoscillations in the model. The parameterization of analyticalsegments of straight, circular and cubic polynomial track segmentsdoes not introduce such oscillations on the track geometry but itis rather complex for the description of railways with largeslopes or with vertical curves. Splines with tension minimize theundesired oscillations of the interpolated curve that describesthe railway track nominal geometry, but the curve segmentparameters are not proportional to the length of the track. It isproposed here that the nominal geometry of the track is describedby a discreet number of points, which are organized in a tabularmanner as function of a parameter that is the length of the trackmeasured from its origin to a given point. For each entry, thetable also includes the vectors defining the Frenet frames and thederivatives required by the track constraint. The multibody codeinterpolates such table to obtain all required geometriccharacteristics of the track. With applications to a rollercoaster, the suitability of this description is discussed in termsof the choice of original parametric curves used to construct thetable, the size of the length parameter step adopted for the tableand the efficiency of the computer implementation of theformulation.

Mapping Railway Wheel Material Wear Mechanisms and Transitions

In order to develop more durable wheel materials to cope with the new specifications being imposed on wheel wear, a greater understanding of the wear mechanisms and transitions occurring in wheel steels is needed, particularly at higher load and slip conditions. The aim of this work was to draw together current understanding of the wear mechanisms, regimes, and transitions (particularly with R8T wheel material) and new tests on R7T wheel material; to identify gaps in the knowledge; and to develop new tools for assessing wear of wheel materials, such as wear maps, that can be used to improve wear prediction. Wear assessment of wheel materials, as well as wear rates, regimes, and transitions, is discussed. Twin disc wear testing, used extensively for studying wear of wheel and rail materials, has indicated that three wear regimes exist for wheel materials: mild, severe, and catastrophic. These have been classified in terms of wear rate and features. Wear rates are seen to increase steadily initially and then level off, before increasing rapidly as the severity of the contact conditions is increased. Analysis of the contact conditions in terms of friction and slip has indicated that the levelling off of the wear rate observed at the first wear transition is caused by the change from partial slip to full slip conditions at the disc interface. Temperature calculations for the contact showed that the large increase in wear rates seen at the second wear transition may result from a thermally induced reduction in yield strength and other material properties. Comparisons made between discs and actual wheels have provided some support for the theories relating to the transitions observed. Wear maps have been produced using the test results to study how individual contact parameters such as load and sliding speed influence wear rates and transitions. The maps are also correlated to expected wheel—rail contact conditions. This improved understanding of wheel wear mechanisms and transitions will help in the aim of eventually attaining a wear modelling methodology reliant on material properties rather than on wear constants derived from testing.

A Memory Based Communication in the Co-simulation of Multibody and Finite Element Codes for Pantograph-Catenary Interaction Simulation

Summary. Many complex systems require that computational models of different nature are used for their sub-systems. The evaluation of the dynamics of each one of these models requires the use of different codes, which in turn use different time integration algorithms. The work presented here proposes a co-simulation environment that uses an integrated memory shared communication methodology between the multibody and finite element codes. The methodology is general being applicable to the dynamic co-simulation of models running in different codes. The benefits and drawbacks of the proposed methodology and of its accuracy and suitability are supported by the application to a real operation scenario of a highspeed catenary-pantograph system for which experimental test data is available.

Optimization of high-speed railway pantographs for improving pantograph-catenary contact

A crucial system for the operation of high-speed trains is the pantograph catenary interface as it is the sole responsible to deliver electrical power to the train. Being the catenary a stationary system with a long lifespan it is also less likely to be redesigned and upgraded than the pantographs that fit the train vehicles. This letter proposes an optimization procedure for the improvement of the contact quality between the pantograph and the catenary solely based on the redesign of the pantograph head suspension characteristics. A pantograph model is defined and validated against experimental dynamic characteristics of existing pantographs. An optimization strategy based on the use of a global optimization method, to find the vicinity of the optimal solution, followed by the use of a deterministic optimization algorithm, to fine tune the optimal solution, is applied here. The spring stiffness, damping characteristics and bow mass are the design variables used for the pantograph optimization. The objective of the optimal problem is the minimization of the standard deviation of the contact force history, which is the most important quantity to define the contact quality. The pantograph head suspension characteristics are allowed to vary within technological realistic limits. It is found that current high-speed railway pantographs have a limited potential for mechanical improvements, not exceeding 10%–15% on the decrease of the standard deviation of the contact force.

A computational efficient general wheel-rail contact detection method

The development and implementation of an appropriate methodology for the accurate geometric description of track models is proposed in the framework of multibody dynamics and it includes the representation of the track spatial geometry and its irregularities The wheel and rail surfaces are parameterized to represent any wheel and rail profiles obtained from direct measurements or design requirements A fully generic methodology to determine, online during the dynamic simulation, the coordinates of the contact points, even when the most general three dimensional motion of the wheelset with lespect to the rails is proposed This methodology is applied to study specific issues in railway dynamics such as the flange contact problem and lead and lag contact configurations A formulation for the description of the normal contact forces, which result from the wheel-rail interaction, is also presented The tangential creep forces and moments that develop in the wheel-rail contact area are evaluated using Kalker linear theory, Heuristic force method, Polach formulation The methodology is implemented in a general multibody code The discussion is supported through the application of the methodology to the railway vehicle ML95, used by the Lisbon metro company

PantoCat statement of method

The pantograph–catenary dynamic interaction analysis program (PantoCat) addresses the need for a dynamic analysis code able to analyse models of the complete overhead energy collecting systems that include all mechanical details of the pantographs and the complete topology and structural details of the catenary. PantoCat is a code based on the finite element method, for the catenary, and multibody dynamics methods, for the pantograph, integrated via a co-simulation procedure. A contact model based on a penalty formulation is selected to represent the pantograph–catenary interaction. PantoCat enables models of catenaries with multiple sections, including their overlap, the operation of multiple pantographs and the use of any complex loading of the catenary or pantograph mechanical elements including aerodynamic effects. The models of the pantograph and catenary are fully spatial being simulated in tangential or curved tracks, with or without irregularities and perturbations. User-friendly interfaces facilitate the construction of the models while the post-processing facilities provide all quantities of interest of the system response according to the norms and industrial requirements.

Multibody Modeling of Pantographs for Pantograph-Catenary Interaction

In the great majority of railway networks the electrical power is provided to the locomotives by the pantograph-catenary system. From the mechanical point of view, the single most important feature of this system consists in the quality of the contact between the contact wire(s) of the catenary and the contact strips of the pantograph. Therefore not only the correct modeling of the catenary and of the pantograph must be achieved but also a suitable contact model to describe the interaction between the two systems must be devised. The work proposed here aims at enhancing the understanding of the dynamic behavior of the pantograph and of the interaction phenomena in the pantograph-catenary system. The catenary system is described by a detailed finite element model of the complete subsystem while the pantograph system is described by a detailed multibody model. The dynamics of each one of these models requires the use of different time integration algorithms. In particular the dynamics of the finite element model of the catenary uses a Newmark type of integration algorithm while the multibody model uses a Gear integration algorithm, which is variable order and variable time step. Therefore, an extra difficulty that arises in study of the complete catenary-pantograph interaction concerns the need for the cos-imulation of finite element and multibody models. As the gluing element between the two models is the contact model, it is through the representation of the contact and of the integration schemes applied for the finite and multibody models that the co-simulation is carried on. The work presented here proposes an integrated methodology to represent the contact between the finite element and multibody models based on a continuous contact force model that takes into account the co-simulation requirements of the integration algorithms used for each subsystem model.

Modelling tracks for roller coaster dynamics

A methodology for the accurate description of three dimensional track geometries is proposed. Cubic, Akima and shape preserving splines are proposed for track parameterisation. The track description uses Frenet frames that provide the track referential at every point. During dynamic analysis a pre-calculated database is linearly interpolated to form the kinematic constraints that enforce the wheelsets of the vehicle to move along the track with a prescribed angular orientation. This approach allows calculating the reaction forces that develop between the vehicle and the track. The formulation is discussed emphasising the influence of the parameterisation methods to construct the track geometry.