Giacomo Squicciarini

The effect of temperature on railway rolling noise

The stiffness and damping of railpads in a railway track are affected by changes in the temperature of the surrounding environment. This results in the rolling noise radiated by trains increasing as the temperature increases. This paper quantifies this effect for a ballasted track equipped with natural rubber railpads and also studies the behaviour of a cork-reinforced rubber railpad. By means of measurements in a temperature-controlled environment, it is shown that the shear modulus of the natural rubber increases by a factor of six when the temperature is reduced from 40 ℃ to −20 ℃. The loss factor increases from 0.15 at 40 ℃ to 0.65 at −20 ℃. The shear modulus of the cork-reinforced rubber increases by a factor of 10, and the loss factor shows the typical trend of transition between rubbery and glassy regions. The railpad stiffness estimated from decay rate measurements at different temperatures is shown to follow the same trend. Field measurements of the noise from passing trains are performed for temperatures between 0 ℃ and 35 ℃; they show an increase of about 3–4 dB. Similar results are obtained from predictions of noise using the measured dependence of pad stiffness.

Curve Squeal in the Presence of Two Wheel/Rail Contact Points

A real case of a city tramcar generating very high curve squeal noise levels has motivated research in this area considering the simultaneous presence of two contact points. In this paper, available measurements are summarised in a qualitative manner in order to highlight the most important frequencies involved and a theoretical model in the frequency domain is developed with the aim of predicting curve squeal tones. Good matching is found between numerically predicted and measured unstable frequencies and a peculiar shift toward higher frequencies is found both in measurements and predictions.

A State-of-the-Art Review of Curve Squeal Noise: Phenomena, Mechanisms, Modelling and Mitigation

Curve squeal is an intense tonal noise occurring when a rail vehicle negotiates a sharp curve. The phenomenon can be considered to be chaotic, with a widely differing likelihood of occurrence on different days or even times of day. The term curve squeal may include several different phenomena with a wide range of dominant frequencies and potentially different excitation mechanisms. This review addresses the different squeal phenomena and the approaches used to model squeal noise; both time-domain and frequency-domain approaches are discussed and compared. Supporting measurements using test rigs and field tests are also summarised. A particular aspect that is addressed is the excitation mechanism. Two mechanisms have mainly been considered in previous publications. In many early papers the squeal was supposed to be generated by the so-called falling friction characteristic in which the friction coefficient reduces with increasing sliding velocity. More recently the mode coupling mechanism has been raised as an alternative. These two mechanisms are explained and compared and the evidence for each is discussed. Finally, a short review is given of mitigation measures and some suggestions are offered for why these are not always successful.

Estimating the Performance of Wheel Dampers Using Laboratory Methods and a Prediction Tool

Wheel and rail dampers are well known mitigation measures against rolling noise. A combination of laboratory measurements and computations seems the most efficient way to determine their effect. The DEUFRAKO project STARDAMP had the aim of supporting the transfer of wheel and rail dampers from the research phase to their regular application. One goal of the project was the development of a prediction tool that is dedicated to the estimation of the efficiency of wheel and rail dampers. The input data relies on laboratory measurements that are relatively easy to perform. This paper focuses on the wheel damper part. Rail dampers are addressed in a companion paper.

Numerical Analysis of the Dynamic Response of a 5-Conductor Expanded Bundle Subjected to Turbulent Wind

The paper describes a numerical model, i.e. a design tool, to determine the dynamic response of bundle conductors to turbulent wind excitation. The structure is modeled by means of finite element method and the force field exerted on each cable, as function of cables relative position and velocity, which was derived by experimental measurement at Politecnico di Milano Wind Tunnel. The proposed methodology has been applied to a real case to define the spacers number and position along the span to avoid large conductors displacements.

Estimating the performance of rail dampers using laboratory methods and software predictions

Rail dampers are designed to reduce the rail component of rolling noise by increasing the attenuation with distance along the rail (decay rate, DR). There is no standardized method to assess the performance of rail dampers. The method described here, developed during the Franco-German STARDAMP project, uses laboratory tests and computer simulation to avoid the need for expensive and time-consuming field trials. The premise of the method is that the DRs of a damped track can be found from summing the DRs of a short-section of damped ‘freely supported’ rail and the DRs of an undamped track. Reasonable predictions of the decay rates of a test track have been made using this method. Software has been produced that implements TWINS-like predictions of rolling noise with and without rail dampers to predict the damper effect. The effect of rail pad stiffness on the effectiveness of rail dampers has been considered for track constructions typical in the UK and a regional train travelling at 120 km/h. For track fitted with ‘soft’ 120 MN/m rail pads, the dampers are predicted to reduce the total level by 2.5 dB(A) while with the ‘stiff’ 800 MN/m pads a 0.7 dB(A) reduction is expected.

Influence of ground impedance on the sound radiation of a railway sleeper

A railway track consists of rails attached to sleepers (cross ties) which are laid in ballast. The sleeper provides support for the rail and transfer loads to the ballast and subgrade. Due to the wheel/rail interaction the rail is induced to vibrate and this vibration is transmitted to the sleepers; both the rail and the sleepers radiate sound. Existing models used to predict the sound radiation from the sleeper consider this to be completely embedded in a rigid ground; in reality, however, the sleeper is surrounded by, or embedded to some extent, in the ballast. It is therefore necessary to take these conditions into account in order to obtain a more realistic model. This paper investigates the influence of the ground in close proximity to the sleeper on its sound radiation. A 1/5 scale concrete sleeper is analyzed by using the boundary element method in 3-D. Ground absorption is introduced in terms of its acoustic impedance using the Delany-Bazley model and its effects on the sleeper radiation are predicted. Finally, the numerical results are validated by experimental results using a 1/5 scale model.

Rail roughness and rolling noise in tramways

Companies which manage railway networks have to cope continually with the problem of operating safety and maintenance intervention issues related to rail surface irregularities. A lot of experience has been gained in recent years in railway applications but the case of tramways is quite different; in this field there are no specific criteria to define any intervention on rail surface restoration. This paper shows measurements carried out on some stretches of a tram network with the CAT equipment (Corrugation Analysis Trolley) for the principal purpose of detecting different states of degradation of the rails and identifying a level of deterioration to be associated with the need for maintenance through rail grinding. The measured roughness is used as an input parameter into prediction models for both rolling noise and ground vibration to show the potential effect that high levels of roughness can have in urban environment. Rolling noise predictions are also compared with noise measurements to illustrate the applicability of the modelling approach. Particular attention is given to the way the contact filter needs to be modelled in the specific case of trams that generally operate at low speed. Finally an empirical approach to assess vibration levels in buildings is presented.

Statistical description of wheel roughness

Wheel roughness measurements available from several different campaigns are presented in terms of average levels and dispersion. The dependence on factors such as brake type and whether the wheel is powered or trailing is also addressed. A method to decide how many wheels from a train are to be measured is then presented. Finally, the main outcomes are described from a round robin test aimed at assessing the effect on wheel roughness measurements of adopting different equipment, used independently by different teams.

Method for obtaining the wheel-rail contact location and its application to the normal problem calculation through ‘CONTACT’

ABSTRACT This work presents a robust methodology for calculating inter-penetration areas between railway wheel and rail surfaces, the profiles of which are defined by a series of points. The method allows general three-dimensional displacements of the wheelset to be considered, and its characteristics make it especially suitable for dynamic simulations where the wheel–rail contact is assumed to be flexible. The technique is based on the discretisation of the geometries of the surfaces in contact, considering the wheel as a set of truncated cones and the rail as points. By means of this approach, it is possible to reduce the problem to the calculation of the intersections between cones and lines, the solution for which has a closed-form expression. The method has been used in conjunction with the CONTACT algorithm in order to solve the static normal contact problem when the lateral displacement of the wheelset, its yaw angle and the vertical force applied in the wheelset centroid are prescribed. The results consist of smooth functions when the dependent coordinates are represented as a function of the independent ones, lacking the jump discontinuities that are present when a rigid contact model is adopted. Example results are shown and assessed for the normal contact problem for different lateral and yaw positions of the wheelset on the track.