Asier Alonso

Introduction of a friction coefficient dependent on the slip in the FastSim algorithm

One of the main limitations of algorithms relating forces and creepages at the wheel/rail contact is the use of a friction coefficient independent of the slip. This paper overcomes this limitation through a modification of the FastSim algorithm (based on the Simplified Theory of Kalker). A friction law based on the local value of the slip is established and the required formulation of the local slip elsewhere in the contact area is presented. Some difficulties of the method and the solutions adopted by the authors are also presented. Finally, the achieved improvements are shown through comparison of the results obtained both with the original and the modified FastSim algorithms.

Introduction of falling friction coefficients into curving calculations for studying curve squeal noise

In this paper, a method of introducing falling friction coefficients into curving simulations for studying curve squeal noise is presented. Generation of squeal at the wheel of a railway vehicle in a curve is caused by unstable vibration, caused in turn by a lateral wheel/rail force which reduces with increasing lateral creepage. To model vehicle curve squeal, the wheel/rail tangential force in the curving simulation is calculated by a modified version of FASTSIM, which uses a sliding velocity-dependent friction characteristic. Using the falling friction characteristics, a UK passenger vehicle is modelled with SIMPACK. Curving behaviour is simulated for a range of curve radii and cant deficiencies. The wheel/rail contact properties are then obtained to study the possibility of the occurrence of squeal using a frequency-domain method. The methodology in this paper allows various curves, wheel/rail profiles, vehicle speeds and friction characteristics to be taken into account.

Rail vehicle passing through a turnout: analysis of different turnout designs and wheel profiles

In recent years, different systems have been developed in order to improve the dynamic behaviour of railway vehicles when passing through turnouts. Some of these improvements consist in varying the geometry of the switch itself and including moveable crossing vees. It is worth mentioning that they are designed by taking a certain wheel profile into consideration, i.e. it is assumed that the wheel profile does not change. The objective of the current study is to determine the influence that the turnout design has on vehicle dynamics, as well as the influence that the variability in wheel profiles can have on the effectiveness of the different systems. In order to do this, the MBS software Simpack was used to model one vehicle with two different turnouts and four different profiles. The results show that the geometrical design of the turnout has a critical influence on the vehicle/turnout. We also concluded that the wheel profile does not have a significant influence when the vehicle passes through turnouts.

Using the stationary tests of the acceptance process of a rail vehicle to identify the vehicle model parameters

The accuracy of multi-body simulation results relies on the model building process and the accuracy of the model parameters. A more widespread use of vehicle dynamic calculations in the acceptance process would be possible if the validation of the model was secured. This paper proposes a methodology for an objective and direct identification of the values of the parameters of a rail vehicle model, using the results of the stationary tests defined in the acceptance process of railway vehicles (EN14363). The methodology also takes into account the variability of the measuring process by providing a probabilistic estimation of the identified parameters. The methodology is validated using an example of a virtual wheel unloading test (simulation). Four significant model parameters can be accurately calculated: vertical primary and secondary suspension stiffness, stiffness of the anti-roll bar, and height of the null moment point (the lateral/roll coupling effect of the air spring). Finally, a reduction method is shown which decreases the uncertainties of the identified parameters by up to 50%.

A new parameter identification methodology for a modal analysis test of a rail vehicle

The validation of vehicle mathematical models is a key part of the virtual acceptance process since it is essential to ensure a precise representation of the reality. The model validation procedure should include validation of stationary but also dynamic tests. However, parameter identification from on-track tests is a challenging task due to the non-controlled excitation and the great variability of the test results. Thus, an alternative solution by means of a vehicle modal analysis is proposed, developing a parameter identification methodology for dynamic vehicle model parameters. This methodology calculates estimated values of the vehicle model parameters that have an influence on the excited vehicle vibration modes. Moreover, a new criterion for taking into account the effect of the measurement uncertainties on the selection process of the vehicle parameters is developed. Finally, experimental results show that not only estimations of the suspension stiffness parameters can be obtained, but damping values and structural frequencies from the vehicle bodies can also be estimated.

Railway Dynamometric Wheelsets: A Comparison of Existing Solutions and a Proposal for the Reduction of Measurement Errors

Currently, the testing for the acceptance of running characteristics of railway vehicles in Europe is ruled by EN-14363 which is derived in essential parts from UIC 518. This standard is based on present state of the art which is generally applicable for test procedures and the evaluation of stationary and ‘ontrack’ tests. It defines testing scenarios, analysis conditions and experimental measurements, and proposes limiting values for a number of different parameters mainly associated with vehicle safety and ride quality. Variables to be measured are specified for each method. The method referred to as “normal”, which is applied to the cases of the highest level of criticality (high speed and/or high axle loading) requires the measurement of forces transmitted through wheel-rail contact at several wheelsets of the vehicle.

Damper modelling and its implementation in railway simulation program

This chapter presents a mathematical model of a railway hydraulic damper. The objective is to develop a model suitable to be implemented in a railway simulation program. Computational cost should, therefore, be maintained low not to decrease the simulation speed (rate) to unacceptable values.

Distributed support modelling for vertical track dynamic analysis

ABSTRACT The finite length nature of rail-pad supports is characterised by a Timoshenko beam element formulation over an elastic foundation, giving rise to the distributed support element. The new element is integrated into a vertical track model, which is solved in frequency and time domain. The developed formulation is obtained by solving the governing equations of a Timoshenko beam for this particular case. The interaction between sleeper and rail via the elastic connection is considered in an analytical, compact and efficient way. The modelling technique results in realistic amplitudes of the ‘pinned–pinned’ vibration mode and, additionally, it leads to a smooth evolution of the contact force temporal response and to reduced amplitudes of the rail vertical oscillation, as compared to the results from concentrated support models. Simulations are performed for both parametric and sinusoidal roughness excitation. The model of support proposed here is compared with a previous finite length model developed by other authors, coming to the conclusion that the proposed model gives accurate results at a reduced computational cost.

Simple flexible wheelset model for low-frequency instability simulations

As a general rule, the multi-body simulation models used by railway vehicle designers consider the wheelsets to be fully rigid, thus leading to possible errors when calculating the critical speed of the vehicle under study. This article suggests a wheelset model that takes into account wheelset flexibility for the study of dynamic stability. The model is simple to implement, easily parameterised, and can be applied to both conventional and variable gauge wheelsets. The parameters corresponding to wheelset flexibility that most influence the critical speed of high-speed and variable gauge vehicles are also analysed.

Gas dampers: model development and potential ride performance evaluation

The main characteristic of a gas damper (GD) is the use of gas as internal fluid, which offers an alternative to hydraulic shock absorbers. In this paper, a mathematical GD model is presented and validated. The use of a compressible fluid provides GD with a behaviour dependent on velocity, like conventional dampers, but also with a strong dependency on frequency and on stroke amplitude. This dependency allows an improvement in the traditional compromise between comfort and safety. A quarter-car model is used to evaluate the ride performance that can be expected using the GD, focusing on the cited compromise. Results are compared with the ride performance of a hydraulic shock absorber. Finally, a sensitivity study centred on the stiffness of the internal fluid is presented.