C.J.C. Jones

A theoretical model for ground vibration from trains generated by vertical track irregularities

A model is developed for predicting ground vibrations due to vertical track irregularities. This model incorporates vehicles, a track and a layered ground, and uses the moving axle loads and the vertical rail irregularities as its inputs. Outputs include the dynamic wheel–rail forces and the displacement power spectra of the track and the ground surface. Results from this model are presented for a single-axle vehicle model and a British Mark 3 passenger coach running on different tracks (a ‘lighter ballasted track’, a ‘heavier ballasted track’ and a slab track) at different speeds (25, 60 and 83 m/s). Based on these results, the effects of track structure, vehicle speed and frequency range on the observed vibration levels are identified. The different roles of the moving axle loads and the roughness-induced dynamic loads are indicated, at different frequencies and for train speeds below and above the lowest ground wave speed.

A comparison of a theoretical model for quasi-statically and dynamically induced environmental vibration from trains with measurements

This paper presents comparisons between a theoretical ground vibration model and measured data at three sites. The model, which is briefly outlined here, encompasses both the quasi-static and dynamic mechanisms of excitation. The vertical dynamics of a number of vehicles travelling at a constant speed on an infinite track are coupled to a semi-analytical model for a three-dimensional layered ground. This model is also used to demonstrate the roles of the two components of vibration at different frequencies and for train speeds below and above the lowest ground wave speed. It is found that, in most practical cases, the dynamic component gives rise to the higher level of vibration.

A theoretical study on the influence of the track on train-induced ground vibration

An investigation is presented on the nature of train-induced ground vibration propagation. It is based on a theoretical model for the track and a layered ground. Results are given of the responses of the ground and track to a moving harmonic or quasi-static load on the rails. The dispersion characteristics of the propagating modes of vibration in the track and the ground are presented and the excitation of vibration in the ground via the track is discussed in relation to these propagating wavenumbers. An important feature of the coupled system is the coincidence of a propagating wavenumber in the track and the ground that gives rise to the main peak in the vibration spectrum in the frequency range of interest. It has been observed, in some cases, that when the train speed reaches a value close to the speed of propagating waves in the ground, the response to the quasi-static axle loads of the train reaches a peak. The relationship between this critical speed and the wave speeds in the track and ground is considered in order to investigate the effectiveness of controlling this peak response load speed by increasing the bending stiffness of the track/embankment structure or by reducing its mass. It is found that such treatments may, or may not, have a significant effect depending on the ground stiffness and layering. For the multiple quasi-static moving axle loads of a train the loading has strong, closely spaced harmonic components. The effect on the vibration spectrum of the superposition of vibration from multiple axles is shown to lead to the reinforcement or suppression of some frequencies as a function of axle spacing and speed. This is demonstrated with calculated results.

Brief report: The development and psychometric properties of an observer-rated measure of engagement with mental health services

Engagement with mental health services is important if people with serious, long-standing mental health problems are to be adequately supported in the community. However, engagement of this client group is notoriously difficult. While research is needed to address reasons for disengagement, there are no existing measures of the engagement concept. This paper presents the development of an observer-rated, multi-item engagement measure, together with evidence in support of its reliability and validity. The measure is quick and easy to use, with significant clinical and research value.Engagement with mental health services is important if people with serious, long-standing mental health problems are to be adequately supported in the community. However, engagement of this client group is notoriously difficult. While research is needed to address reasons for disengagement, there are no existing measures of the engagement concept. This paper presents the development of an observer-rated, multi-item engagement measure, together with evidence in support of its reliability and validity. The measure is quick and easy to use, with significant clinical and research value.

Surface vibration propagation over a layered elastic half-space with an inclusion

Abstract The transmission of vibrations, and its reduction, in the far-field of the surface of the ground due to a harmonic load acting over a strip, is investigated theoretically. A possible vibration attenuation device that has shown some promise is the “wave impedance block” (WIB). The principle of this is to modify the modal wave propagation regime of the ground by introducing an artificial stiffened layer (inclusion) under the load. The ground is modelled as an elastic layered half-space and the inclusion is under the load within the layered half-space. The performance of the inclusion in impeding wave transmissions at a number of receiver positions is studied and measured in terms of insertion loss analogous to the study of noise barrier designs in outdoor sound propagation. A numerical model is presented which enables the wave-field in the region of the inclusion to be determined. This is based on an integral equation formulation of the problem which is solved using a boundary element approach. It is shown that an inclusion has a beneficial effect at low frequencies. However, when the wavelength becomes short compared with the depth and width of the inclusion, adverse effects occur at some frequencies which are still observed in the far-field. The possible causes are difficult to analyse as radiation, scattering and transmission of the shear and compression waves due to the inclusion must be taken into account. Various configurations of inclusion are studied and results presented so that some preliminary conclusions may be derived.

Investigation into the validity of two-dimensional models for sound radiation from waves in rails

The sound radiation from a vibrating rail can be predicted using a two-dimensional model under certain conditions. This paper explores these conditions and shows that, if the decay rate of vibration along the rail becomes large or the wavelength in the rail becomes small, it becomes necessary to allow for three-dimensional radiation characteristics. In practice, however, noise from a rail can be predicted using a two-dimensional model for frequencies above about 250 Hz, and even where three-dimensional effects become important, these can be allowed for by simple correction terms. When the wavelength in the rail approaches that of acoustic waves in air, the angle between the direction of sound radiation from the rail and the normal to the rail increases, in some cases to more than 45 degrees. This must be accounted for if the performance of noise barriers is to be calculated using a two-dimensional approach.

Extended Validation of a Theoretical Model for Railway Rolling Noise Using Novel Wheel and Track Designs

A theoretical model for railway rolling noise, TWINS, was first developed some years ago and was previously validated against field measurements for conventional wheel and track designs. This model has subsequently been used in the design of noise-reducing wheels and tracks. An outcome of the recent Silent Freight and Silent Track projects was a series of novel designs that were tested in a comprehensive field experiment. Alongside this development, the theoretical model has been updated to improve accuracy and include new features. The results of 34 wheel/track combinations that were measured in field experiments are compared with corresponding predictions using the improved model. It is found that the mean difference between measured and predicted overall A-weighted sound pressure levels is less than 2 dB while the standard deviation is 1.9 dB. The improved accuracy of the model is also shown by a reanalysis of the original validation experiments.

Experimental and Theoretical Investigation of Railway Wheel Squeal

Abstract The tangential contact forces that arise at the interface between the wheel of a railway vehicle and the rail provide all the traction, braking, and guidance required by the vehicle. These forces are the result of microslip or creepage and can become unstable exciting vibration of the wheel, particularly at frequencies corresponding to the wheels axial (and radial) modes. Although theories exist for predicting these creep forces and their relationship to creepages, most lack experimental verification in the characterization of the falling friction coefficient during unsteady squealing. This paper presents some new results from a project which aims to develop a complete, validated model of curve squeal noise generation accounting for friction characteristics, excitation due to unstable forces between the wheel and rail and vehicle dynamic behaviour. The model includes wheel and track dynamic response and acoustic radiation. As part of the project, a twin disc rig has been modified to provide experimental data for the validation of the model and measurements were made of the lateral force and dynamic response of the rollers due to varying amounts of lateral creepage during squealing. The main feature of the twin disc rig compared with previous research [1] is that the contact force measuring system measures the contact forces at the web of the rollers and therefore close to the contact patch and through a slipring arrangement enables the lateral vibration of both rollers in relation to squeal be measured with relative ease. In this paper, detailed descriptions of the twin disc rig and the test method developed are given. An outline of the squeal model is also presented. Results from the tests have been compared with the prediction from the squeal model and with available theories and showed good agreement.

Ground vibration from trains: experimental parameter characterization and validation of a numerical model

Trains running on railway tracks on the surface of the ground or in tunnels induce vibrations in the ground which propagate away from the track. These may be experienced as feelable vibration or as audible rumbling noise in the buildings nearby, both of which are difficult to control. As the properties of the ground differ widely between locations they must be characterized for a particular site in order to make reliable predictions. This article describes field measurements of the vibration at two sites with soft clay soil in Southern England. The properties of the ground material, including its layered structure, have been identified from comparisons between results of a layered ground model and measurements obtained using impact excitation. Presentation in the wavenumber-frequency domain is particularly helpful for this purpose. Measurements of vibrations from passing trains are then compared with predictions using a semianalytical model for ground vibration from trains and good agreement is found.

Ride comfort of high-speed trains travelling over railway bridges

The ride comfort of high-speed trains passing over railway bridges is studied in this paper. A parametric study is carried out using a time domain model. The effects of some design parameters are investigated such as damping and stiffness of the suspension system and also ballast stiffness. The influence of the track irregularity and train speed on two comfort indicators, namely Sperlings comfort index and the maximum acceleration level are also studied. Two types of railway bridges, a simple girder and an elastically supported bridge are considered. Timoshenko beam theory is used for modelling the rail and bridge and two layers of parallel damped springs in conjunction with a layer of mass are used to model the rail-pads, sleepers and ballast. A randomly irregular vertical track profile is modelled, characterized by its power spectral density (PSD). The ‘roughness’ is generated for three classes of tracks. Nonlinear Hertz theory is used for modelling the wheel-rail contact. The influences of some nonlinear parameters in a carriage-track-bridge system, such as the load-stiffening characteristics of the rail-pad and the ballast and that of rubber elements in the primary and secondary suspension systems, on the comfort indicators are also studied. Based on Galerkins method of solution, a new analytical approach is developed for the combination between the rigid and flexural mode shapes, which could be used not only for elastically supported bridges but also other beam-type structures.