Bombardier on track to design quieter trains

Train manufacturer Bombardier Transportation is addressing the aeroacoustic noise problem of high-speed trains using advanced computational fluid dynamics and computational aeroacoustics.

The European Commission has a vision for a completely integrated train system and is in the process of finalising what it calls its technical specifications for interoperability (TSI).

Once all these have been agreed across Europe, the rail network will operate in compliance with standard specifications and regulations that will eventually create a completely integrated system.

Of the many areas under discussion, noise pollution is causing train manufacturers a particular headache.

New European legislation will force rail operators to reduce the external noise of their trains.

On high-speed trains, aerodynamic noise is the major cause of concern and manufacturers are searching for a solution before the law changes.

Bombardier Transportation, the worlds leading train manufacturer, has a dedicated department called The Centre of Competence for Aerodynamics and Thermodynamics (CoC-ATh) tackling the aeroacoustic noise problem using advanced computational fluid dynamics (CFD) and computational aeroacoustics (CAA).

Head of the CoC-ATh, Dr Alexander Orellano explains the thrust of his work: "We use state-of-the-art predictive computer aided engineering tools such as CFD and CAA to optimise product design for the rail industry.

Our goal is to meet the needs of our clients and at the same time reduce operating and design costs.

The work we undertake is design optimisation and specification during the bid and design phase and problem solving after delivery".

"Our job extends from ensuring the passenger comfort (such as climate control within the train carriage), to directly improving safety, (for example by investigating the impact of wind loads on all our trains, especially those travelling at high-speed)".

The European Commission (EC) has a working group to define the legal requirements and restrictions for future trains.

Bombardier is well prepared to meet the up-coming new standards as the centre has already been working with CFD to help with noise reduction for over three years.

The level of expertise is impressive, and has given Bombardier a distinct head start over its competitors.

Bombardier is enjoying a tactical advantage by combining acoustics with aerodynamics and thermodynamics in one department.

Orellano explains the benefit: "In other companies they have a separate department for acoustics that would be solving noise problems.

The methodology of acoustics is not traditionally numerical, so analysis techniques that we find invaluable, such as CFD, can get ignored.

Even if they do decide to use it, CFD is not something you can pick up in an afternoon.

We have a strong background in using techniques like this, and can put them to use solving important problems such as noise reduction".

The project to reduce the external noise from Bombardier's high-speed trains involved a detailed analysis of the pantographs (the power connection between the overhead lines and the train).

The research concentrated on the process of vortex shedding, where an obstacle in the flow (here the pantograph) causes eddies - and therefore low-pressure concentrations - to appear on alternate sides of the obstruction.

In rapid succession, these eddies are a significant noise contributor.

"When the contact strip is moving with the train at high speed there is pronounced vortex shedding", explains Orellano.

"It is this disturbance around the pantograph that creates the aerodynamic noise.

We need to find ways of reducing this noise to meet the future legal requirements.

Our work aims to develop a method for predicting these aeroacoustic noise sources by studying transient flows".

Dr Tjark Siefkes, senior director of advanced technology at the centre of competence at Bombardier Transportation explains: "The centre of competence is responsible for the standardisation and harmonisation of tools within its fields of expertise.

It ensures that state-of-the-art tools are used for our vehicle projects".

CoC-ATh uses CFD software - Star-CD - from the CD Adapco Group.

"We evaluated other software", says Orellano, "and decided to use Star.

We appreciate CD Adapco's willingness to co-operate with us for specific projects.

The support we get from them is invaluable".

Orellano stresses the distinction between noise source description and noise radiation.

"With CFD, we are trying to predict the level of vortex shedding.

The vortices cause pressure pulses that represent noise sources.

In this way, the CFD gives us a way to isolate the noise generation mechanism.

We use boundary element methods or the Fwacs-William-Hawkings method to predict the resulting far field noise levels; it's the CFD that pinpoints the source".

The accepted wisdom for this type of analysis is to predict the pressure pulses from steady state flow.

The work at CoC-ATh has moved away from this, focusing directly on transient flow behaviour.

Orellano and his team leader, Thomas Rung have been working with Star-CD to develop a sophisticated turbulence-modelling tool called detached eddy simulation (DES).

By working directly with transient flow, the analysis of the vortex shedding is a much more precise way of trying to predict noise sources.

It is hoped that the predictive tool will be able to help in the reduction of noise from other sources.

"In modern trains with distributed traction the pantograph is positioned directly above passenger seats.

The resulting aerodynamic noise power level of the pantograph is so high that it goes through the roof and enters the passenger compartment", explains Orellano.

"So we have an aeroacoustic problem inside the train as well as outside.

Another source of aeroacoustic noise is the heating, ventilation and cooling system (HVAC).

This is quite annoying for passengers and we aim to use transient flow predictions for firstly identifying the problem and later on for evaluating design variants".

As a new tool, the DES method has been put through some rigorous testing and comparative studies.

During a joint co-operation with DaimlerChrysler Research and Technology and the Centre of Competence a test case has been established to assess the predictive accuracy of different methods.

A comparative study has also been done on the pantograph problem by positioning an array of microphones around a high-speed train to pinpoint precisely where the noise was coming from.

"We asked the Technical University of Berlin to evaluate the test cases against the DES method", said Orellano.

"A comparison was also done against the conventional flow simulation method.

The results showed that the DES method is much more suitable for external noise problems".

Dr Orellano is focused on ensuring that Bombardier is ready for the change in noise regulations resulting from the integration of Europe's rail network.

But as an engineer he is not against this new predictive tool for Star-CD being used beyond company limits.

As a result the, DES tool is being offered to other CD Adapco customers, as Michael Dickens, Business Development Manager at the CD Adapco Group explains: "DES in Star-CD is now available to a selective number of our users, including Bombardier and eventually will be on general release so that everyone can benefit from it".

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