On track for stability

As trains get faster they become less stable. A collaborative aerodynamic study aims to find an answer.

Rail travellers are familiar with aerodynamic forces — the lateral buffeting felt when a train passes on an adjacent line, or the change in air pressure on entering and exiting a tunnel.

Though the effect on the commuter may be relatively small, faster train speeds mean increasing variable aerodynamic forces could cause problems for trains and the railway infrastructure.

EPSRC-funded research into Aerodynamic Train System Interactions, which begins next year and runs until 2009, aims to review and model knowledge on the unsteady transient aerodynamic forces that affect UK rail transport.

The project is a collaboration between Birmingham University (UoB), led by Prof Chris Baker, and Manchester Metropolitan University (MMU), led by Dr Simon Iwnicki, and will be co-ordinated by Rail Research UK (RRUK). Industrial partners are Bombardier, Network Rail, Siemens and the Rail Safety and Standards Board (RSSB), an independent body that manages technical standards for railway assets and operations.

Dr Paul Allen, from the engineering and technology department at MMU, is one of the principal researchers on the project. ‘As train speeds increase, there will be new challenges facing the UK rail network,’ said Allen. ‘Operating trains at high speeds with body-tilt systems affects their dynamic behaviour. This can reduce the clearance between vehicle and infrastructure, including rails, tunnels and bridges.

‘The real issue is in the longer term, with potential higher-speed lines. By integrating current advanced computer modelling techniques from two engineering fields, it is possible to significantly improve the current level of understanding of vehicle and aerodynamic interactions.’

The transient forces caused by aerodynamic interactions and the pressure pulses that result can lead to high loadings on the vehicle and line-side objects, including passengers standing on platform edges. It can lead to long-term fatigue of structural members and tearing of soft body sides.

‘These transient forces affect the dynamic movements of the vehicle and hence its clearance to the infrastructure,’ said Allen. ‘There are also issues of passenger comfort with pressure changes in tunnels and buffeting causing undesirable vehicle body movements.’

A rare but significant risk is that of overturning due to high winds, especially on exposed parts of the route. High winds were implicated in a train derailment in Japan in December 2005.

Crosswinds can also cause load displacements, particularly for freight trains. Aerodynamic loadings can affect the kinematic envelope — the limiting boundary within which a dynamic train profile must fit — which can cause the vehicle to exceed its conventional envelope, affecting tunnel and bridge clearances.

The researchers will use computer modelling to predict the response of the train when subjected to aerodynamic forces and assess the impact on interactions between train and infrastructure. ‘We will be using VAMPIRE, a proprietary rail vehicle dynamics software package developed by UK railway consultants DeltaRail,’ said Allen. ‘We propose to integrate it with new software modules developed for our specific research, which aim to couple vehicle dynamic simulations with aerodynamic modelling work carried out at Birmingham.’ The researchers anticipate their findings will improve the fundamental understanding of the effects of aerodynamic forces on railway vehicles and the infrastructure and help the rail industry address increasing safety concerns.

The resultant model will help future investigations of aerodynamics-related vehicles and infrastructures. This may influence the RSSB policies on acceptable vehicles and be used in the design of new railways and rolling stock, particularly for high-speed operation. Previous RRUK research is already bringing benefits to the rail industry. MMU researchers recently completed a project with colleagues from Southampton University to examine wheel squeal and roughness growth on the rail head, both of which can cause noise problems.

The work resulted in the development of a model for the prediction of the onset of wheel squeal, a problem that particularly affects city centre light-rail vehicles, including underground trains. The researchers anticipate the squeal model will now be applied to real-life case studies for refinement and validation.

RRUK’s industrial collaborators and mentors are key to its ongoing success. ‘By obtaining industry buy-in, we not only increase our knowledge of rail systems but we can also balance blue-skies research with the requirements of industry,’ said Allen.

The models produced by the RRUK consortium of universities will contribute to ensuring that future rail journeys can take place at higher speeds without compromising on safety.