Nuestros investigadores

Asier Alonso Pazos

Publicaciones científicas más recientes (desde 2010)

Autores: Giménez, José Germán; Baeza, L., (Autor de correspondencia)
ISSN 0042-3114  Vol. 57  Nº 12  2019  págs. 1822 - 1846
In this paper the two-dimensional contact problem is analysed through different mesh topologies and strategies for approaching equations, namely; the collocation method, Galerkin, and the polynomial approach. The two-dimensional asymptotic problem (linear theory) associated with very small creepage (or infinite friction coefficient) is taken as a reference in order to analyse the numerical methods, and its solution is tackled in three different ways, namely steady-state problem, dynamic stability problem, and non-steady state problem in the frequency domain. In addition, two elastic displacements derivatives calculation methods are explored: analytic and finite differences. The results of this work establish the calculation conditions that are necessary to guarantee dynamic stability and the absence of numerical singularities, as well as the parameters for using the method that allows for maximum precision at the minimum computational cost to be reached.
Autores: Casanueva, C., (Autor de correspondencia); Perez, J.; et al.
Revista: WEAR
ISSN 0043-1648  Vol. 436  2019 
This work presents the initial steps given in order to obtain a comprehensive physical damage model for the specific case of wheel rail contact wear, which would be able to relate contact conditions, material properties and wear rates. The main advantage of a physical damage wear model is that wheelset and rail manufacturers can perform simulations in order to improve and optimise material properties for different operational cases. The work in this paper focuses on delaminative wear, starting with the importance and modelling of rough contact, and a comparison against classic smooth contact models.
Autores:  Kari, L. ; et al.
ISSN 0042-3114  Vol. 56  Nº 4  2018  págs. 529 - 552
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.
Autores: Gil-Negrete, Nere;
ISSN 0003-682X  Vol. 133  2018  págs. 144 - 156
Air radiation caused by vibrating structural systems is a fundamental problem in acoustic engineering with a wide application range. Exact analytical solutions are possible for some simple structures but for complex ones analytical expressions are not easily established. In this paper a methodology for computing the acoustic radiation caused by any type of flat structural system is presented. A general equation is established in modal terms, including the interaction between the structure and the surrounding fluid through pressure fluctuations. These are defined using a potential function, whose description is based on Hankel or Green baffled functions depending on the radiating surface dimension. Solving the obtained equation modal displacements caused by a specific excitation are known. Through these displacements the radiated power by the structure as well as the energy stored are quantified. Relating both the coupling loss factors between the structural system and the surrounding fluid are determined. In order to achieve the statistical diffuse field assumption coupling loss factor values are averaged both in frequency and in space. Some examples are analysed and it is shown that accurate results can be obtained applying the developed methodology in simple structures. In complex configurations it is validated using the computed coupling loss factors to estimate the transmission loss values and compare them with the experimental ones.
Autores: Erviti, L. M., (Autor de correspondencia); Giménez, José Germán;
ISSN 0042-3114  Vol. 55  Nº 12  2017  págs. 1884 - 1908
The application of combined subspace identification methods to land vehicles would allow the modal parameters of the vehicle to be obtained, while it is in operation, thus improving the vehicle modal characterisation. However, when a land vehicle is circulating through a straight line, the excitations applied to the different axles are identical among them but with a certain time delay. The presence of repeated excitations with different time delays implies that the past and future subspaces include common vectors that may lead to problems in the application of the method. To analyse its influence, an index called the reduction coefficient resulting from excitations (RCE) has been defined. RCE evaluates the amount of information eliminated in the oblique projection due to the effect of repeated excitations. The use of RCE enables the selection of analysis parameters, particularly the sampling frequency, the signals grouping and some specific parameters of the subspace identification methods.
Autores: Garcia-Ramirez, P.; Vicente, Carmen; Arriazu, Elena; et al.
ISSN 0390-6078  Vol. 102  2017  págs. 50 - 50
Autores: Aizpun, M.; Vinolas, J.;
ISSN 0954-4097  Vol. 230  Nº 3  2016  págs. 879 - 890
A more widespread use of the dynamic simulation of rail vehicles would become possible if the validation of the model was secured and the model parameters were known to be accurate. This paper proposes an analytical methodology for the objective identification of parameter values used in modelling rail vehicles, using the results of a bogie rotational resistance test defined in the acceptance process of railway vehicles (EN 14363). This methodology also takes into account the variability of the measuring process by providing a probabilistic estimation of the identified parameters. The methodology is experimentally validated using test results obtained for an Intercity vehicle. Seven model parameters can be accurately estimated: longitudinal and lateral stiffness of the air-spring secondary suspension, damping and force-speed characteristics of the anti-yaw dampers, longitudinal and lateral stiffness of the emergency spring, and lateral/longitudinal friction coefficient of the emergency spring; they all show excellent correlation with the component tests.
Autores: Guiral, A.; Giménez, José Germán;
ISSN 0022-460X  Vol. 355  2015  págs. 284 - 304
Vehicle-track interaction in the mid- and high-frequency range has become an important issue for rolling-stock manufacturers, railway operators and administrations. Previous modelling approaches have been focused on the development of flexible wheelset-track systems based on the assumption that the unsprung masses are decoupled from the high-frequency dynamic behaviour of carbody and bogies. In this respect, the available flexible wheelset models account for gyroscopic and inertial effects due to the main rotation but are, in general, developed from the viewpoint of inertial spaces and consequently restricted to the study of tangent layouts. The aim of this paper is to present the formulation of a flexible rotating wheelset derived within the framework of a non-inertial vehicle moving reference frame. This brings a double advantage; on the one hand, the formulation is not restricted to tangent tracks, but is also suitable for the study of transition curves and curve negotiation. On the other hand, the use of a vehicle moving reference frame allows the introduction of the hypothesis of small displacement for the degrees of freedom of the wheelset. This hypothesis is not applied to the pitch angle, as it is associated with the main axis of rotation. In addition, unlike previous flexible wheelset models that only consider the rotation around the main axis, all the degrees of freedom will be considered when developing the dynamic equations of motion. Results for the proposed model will be presented and the influence of the inertial and gyroscopic terms not taken into account in previous derived formulations will be evaluated. (C) 2015 Elsevier Ltd. All rights reserved.
Autores: Aizpun, M.; Viñolas, Jordi;
ISSN 0042-3114  Vol. 52  Nº 9  2014  págs. 1139 - 1152
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.
Autores: Viñolas, Jordi;
ISSN 0954-4097  Vol. 228  Nº 4  2014  págs. 408 - 421
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%.
Autores: Guiral, A.; Baeza, L.; et al.
ISSN 0042-3114  Vol. 52  2014  págs. 469 - 487
The wheel-rail contact problem plays an important role in the simulation methods used to solve railway dynamics problems. As a consequence, many different mathematical models have been developed to calculate wheel-rail contact forces. However, most of them tackle this problem purely from a theoretical point of view and need to be experimentally validated. Such validation could also reveal the influence of certain parameters not taken into account in the mathematical developments. This paper presents the steps followed in building a scaled test-bench to experimentally characterise the wheel-rail contact problem. The results of the longitudinal contact force as a function of the longitudinal creepageare obtained and the divergences with respect to Kalker's simplified theory are analysed. The influence of lateral creepage, angular velocity and certain contaminants such as cutting fluid or high positive friction modifier is also discussed.
Autores: Perez, J.; et al.
ISSN 1744-232X  Vol. 20  Nº 4  2013  págs. 312 - 326
This paper presents and validates an appropriate physical damper model to predict the dynamic behaviour of a railway damper for frequencies of up to 200 Hz. The model is computationally efficient and, therefore, appropriate for implementation in NVH-CAE models used to study the transmission of vibration paths to the train body. The model parameters are related to the physical characteristics of the damper (volume of the chambers, piston rod sections, characteristics of the valves, etc.). However, it is shown that in the absence of internal constructive information, which is most often the case, the model parameters can be fitted from a few tests carried out at low frequency (0-20 Hz), based on its theoretical background.
Autores: Gil-Negrete, Nere; et al.
ISSN 0022-460X  Vol. 332  Nº 12  2013  págs. 3032 - 3048
Rubber elements are widely used in the railway industry in order to achieve vibration transmission requirements. Although they are critical components in railway vehicles, their modelling in the dynamic models of railway vehicles is usually relatively simple: it is usual to characterise them using a simple linear model formed by a spring and a viscous dashpot in parallel. In this paper the behaviour of typical rubber elements is analysed and a model that allows more accurately the prediction of its behaviour is proposed. The methodology to implement this model in railway simulation programs is also discussed. (C) 2013 Elsevier Ltd. All rights reserved.
Autores: Lagos, R.F.; Viñolas, Jordi; et al.
ISSN 0954-4097  Vol. 226  Nº F6  2012  págs. 587 - 602
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.
Autores: Gomez, E.; Giménez, José Germán;
ISSN 0888-3270  Vol. 25  Nº 8  2011  págs. 3062 - 3077
The precise experimental measurement of wheel-rail forces is vital both for railway vehicle acceptance processes and for research in vehicle-track interaction. With this aim, dynamometric wheelsets are used. These systems can be based on the installation of sensors either on the axle or on the web of the wheel. Existing measurement methods face a number of difficulties associated with wheel to rail contact, making them low accuracy solutions. One of the main difficulties encountered in methods based on wheel instrumentation is signal variation with wheel rotation. This work proposes a variety of solutions that provide an improvement over existing solutions, laying the starting basis for the development of modern dynamometric wheelsets that meet current requirements for accuracy. (C) 2011 Elsevier Ltd. All rights reserved.
Autores: Gómez, Eduardo; Giménez, José Germán;
ISSN 0042-3114  Vol. 49  Nº 9  2011  págs. 1367 - 1387
This paper deals with the modelling of yaw dampers and determining the influence of the modelling of this component on the results obtained when predicting the dynamic stability of a vehicle. The first part of the work analyses the influence of the yaw damper characteristics on railway dynamic stability. Following this, a physical model of the damper is developed which allows its performance to be reproduced accurately in the whole range of operating conditions the damper is envisaged to operate in. Once obtained, it was found that the computational cost of the model was relatively high. Therefore, a simplified model has been developed. The simplified model allows obtaining accurate results without excessively increasing the time required to perform the simulations. Analysing the results obtained with this model, it has been concluded that with respect to previous model based on conventional approaches, it improves the accuracy of dynamic calculation for the stability assessment. Also, it has been found that the accurate modelling of the yaw damper is critical when dealing with the vehicle's dynamic performance. In the last part of the paper, a special type of yaw damper was studied as well as its effect on the dynamic behaviour of the vehicle.
Autores: González, Jorge; Viñolas, Jordi; et al.
ISSN 0042-3114  Vol. 49  Nº 42401  2011  págs. 199 - 218
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.
Autores: Giménez, José Germán; et al.
ISSN 0042-3114  Vol. 48  2010  págs. 271 - 286
The air spring is one of the components that most affects vehicle comfort. This element usually makes up the main part of the secondary suspension, which introduces both stiffness and damping between the bogie and the car body. Therefore, a deep understanding of this element is necessary in order to study the comfort of a vehicle, the influence of different parameters and the ways to improve it. In this work, the effect of the air spring system on comfort is studied. To accomplish this, a typical pneumatic suspension composition is briefly studied as a first step. Then, the test bench developed to characterise air springs is described, presenting experimental results. Correlation of the results with some theoretical models is also addressed. Afterwards, the effect of the air spring system on comfort is analysed, and finally, improvements from introducing a variable area orifice in the pipe that joints the air spring and the surge reservoir are discussed.