Estelle Bongini

Synthesis of Noise of Operating Vehicles: Development within SILENCE of a Tool with Listening Features

The European Integrated project SILENCE is dedicated to the reduction of railway and road noise in urban areas. Within this context, SNCF and LMA collaborate in the sub-project B in order to develop a pass-by sound simulation software. This global modelling tool will support parametric studies on the reduction of the noise of a train or a car pass-by, by providing standard indicators (time signature, sound pressure level) and sound samples. It will be used to determine the best combination of optimised sources, developed by manufacturers, in order to reduce the global pass-by noise. In the software, each simulation is based upon the definition of the acoustic sources and the pass-by scenario. Each physical acoustic source on the vehicle is represented by one or several point sources. These point sources radiate either pure tones or broadband noise. A dedicated algorithm is used to simulate each type in an efficient way to reduce the computation time. The characteristics of the sources are obtained either from numerical models, or from standstill and pass-by antenna measurements (carried out in the SILENCE project). As a large source such as a cooling system can not be modelled by several point sources located at close positions, a radiation pattern is allocated to the point source. Dedicated studies are in progress to measure the radiation pattern of classical sources on a train (two series of measurements on a 1:14 scale-model and on a train).

Vibration Mitigation by Innovative Low Stiffness Rail Fastening Systems for Ballasted Track

The collaborative work presented in this paper is part of the European project RIVAS, dedicated to the mitigation of ground-borne vibrations from rail traffic. In this paper, focus is made on innovative low stiffness fastening systems designed for ballasted track. These kinds of systems reduce ground-borne vibration by the filtering effect provided by their low stiffness in comparison to the global track stiffness. Numerical simulations performed as a first step confirmed this expected behaviour: ground vibration is strongly attenuated above a cut-off frequency at which resonance occurs. Following these results, two fastening systems designed by Pandrol (member of the RIVAS consortium) were chosen for testing, first in laboratory and then in a commercial track. The laboratory tests mainly consisted in the evaluation of the systems stiffness in realistic conditions (single sleeper in a ballast box), using an appropriate excitation. Combined with acceleration measurements, those tests confirmed the main results expected from simulations but also raised specific issues for the testing of low stiffness fastening systems. In order to be tested in a commercial track in France, some preliminary work was required on these systems; it is also presented in this paper.

A Comparison of Predicted and Measured Ground Vibrations due to High Speed, Passenger, and Freight Trains

In March 2008, the SNCF has launched a blind prediction test for for railway-induced ground vibration. The aim was to compare predictions from different numerical models to measured ground vibrations at a site along the LGV Atlantique and a site along the Paris-Bordeaux line. Apart from vibration measurements, SNCF has also performed tests and collected data for the determination of the input parameters of the prediction models. Based on these input parameters, the free-field vibrations have been predicted by means of a numerical model that has been developed at K.U.Leuven. This model takes into account the dynamic interaction between the train, the track and the soil. The track geometry is assumed to be invariant with respect to the longitudinal direction, allowing for an efficient numerical solution of the dynamic track–soil interaction problem in the frequency-wavenumber domain. Although a good qualitative agreement has been obtained between the predicted and measured vibration velocities, the ratio between the predicted and measured running RMS values of the vibration velocity is sometimes as high as 3 or 5.

Impact of the Heterogeneity of the Ballast on the Dynamical Behavior of the Ballast-Soil System

This paper discusses the dynamical behavior of a randomly-fluctuating heterogeneous continuum model of the ballast. The Young’s modulus is modeled as a random field parameterized by its average, its variance and a correlation model representing non-interpenetrating spheres. A numerical model of the ballast and the surrounding soil is then constructed based on an efficient implementation of an explicit spectral element solver on a large cluster of computers. This model allows to describe numerically the wave field generated in the ballast and soil by the passage of a train, as well as to construct dispersion equations for the ballast-soil model. The influence of heterogeneity is discussed by comparison with a similar model where the ballast is assumed homogeneous. Different values of the soil mechanical parameters are considered and compared. Finally, potential consequences for the design of the ballast are discussed.

Development of a Test Procedure for Stiffness Measurements Appropriate to Ground-Borne Noise Modelling

A simple test procedure is proposed to measure the stiffness of rail fastening systems specifically for the purpose of providing suitable input data to predictive models for ground-borne noise. It is therefore intended to provide stiffness data that is appropriate to the frequencies, amplitudes and types of fastening systems that are relevant to studies of ground-borne noise. The stiffness of the fastening system is found to be dependent on vibration amplitude within the range expected in track. The proposed test procedure allows stiffness measurements to be made over a suitable range of amplitudes and would represent only a small addition to existing standard tests for fastening systems in terms of the resources required.

Outdoor sound propagation for high-speed moving sources

This paper deals with modeling of sources in motion in time-domain solvers. In the context of transportation noise, acoustic sources are complex. Indeed, they are in motion, and they are generally not compact. Equivalent point sources are often used to simplify the problem. Heuristic methods are then applied to handle acoustic propagation over complex sites. Besides, time-domain solutions of the linearized Euler equations have proved to be an attractive approach to study outdoor sound propagation, and can then be used to validate these models. However point sources in arbitrary motion are difficult to account for in these approaches. Distributed volume sources can be used instead. First, influence of the spatial support of the source on the acoustic field is investigated. The case of a harmonic source moving at a constant speed is studied. Then, simulations of a broadband moving source above a rigid ground surface in a three-dimensional geometry are presented, and ground effect is highlighted.