Being a bit hopeful?

Impacts of aerodynamic noise on noise mitigation efficiency, part 6

This is the final blog in this short series on the difficulty in mitigating the noise from very high speed trains, due to aerodynamic effects high up on the train body.

In the first blog of the series, Are you taking this seriously? (posted 27 Nov 2012), I quoted HS2 Ltd from the Appraisal of Sustainability:

“To mitigate potential impacts in areas of high operating speeds, there is a need to control aerodynamic noise through advanced rolling stock design. Without first mitigating the source of aerodynamic noise, wayside noise barriers are not likely be as effective or feasible, due to the required increase in barrier height, to provide shielding to the entire train.”

For this last blog, I will return to this statement and consider the feasibility of the aspiration “to control aerodynamic noise through advanced rolling stock design”.

The researchers from the Infrastructure and Engineering Department of SNCF that I first introduced in my blog It’s there for all to see (posted 5 Dec 2012) have investigated this issue within their overall work programme and published their findings in the paper Pass-by noise reduction at 350 kph: a parametric study. We have encountered two of the authors of this paper, Frank Poisson (first named) and Pierre-Etienne Gautier, in previous blogs, but the list of authors also includes two names new to us, A Fortain and Florence Margiocchi. The paper was presented at the World Congress on Railway Research, held in June 2006 at Montreal.

The SNCF researchers used a computer simulation to evaluate the effectiveness of various noise reduction techniques in lessening the noise generated by the sources on the train at pass-by speeds up to 350 kph. Their findings are that the maximum achievable noise reduction is between 4 dB(A) and 5 dB(A) and that, if only aerodynamic noise reduction measures are employed, barely 2 dB(A) improvement can be achieved. They conclude that the most efficient solution is “the reduction of the rolling noise combined with the reduction of the aerodynamic noise of the first bogie”.

Work done in Japan on aerodynamic design improvements to the pantograph is a little more encouraging to the prospects of reducing aerodynamic noise levels from this source. The design program reported in the paper Aerodynamic Noise Reduction in Pantographs by Shape-smoothing of the Panhead and Its Support and by Use of Porous Material in Surface Coverings – I give full reference information for this source in my blog What do they know? (posted 1 Dec 2012) – enables the authors to claim that “the prototype pantograph [resulting from their design work] reduces aerodynamic noise by approximately 4 dB in comparison with the current low-noise pantograph”.

HS2 Ltd appears to be resigned to the fairly trivial noise source reductions that future design and technology improvements may bring. This is evident by the modest claim in paragraph 6.3.5 on page 50 of Appendix 5 to HS2 London to the West Midlands: Appraisal of Sustainability that:

“At operation, there would be a 3 dB reduction in noise emissions at source based on the anticipated noise control improvements in the next generation of high speed rolling stock.”

Contrast this with the performance that HS2 Ltd expects from noise barriers. In the attachment to the HS2 Ltd response of 5th December 2011 to FOI request 11/327, Noise Source Height of High Speed Trains for the Appraisal of Sustainability, the noise reduction of a 3 metre absorptive barrier is predicted to be 10.1 dB(A).

So clearly HS2 Ltd must do what it can to reduce train noise at source when purchasing the rolling stock and designing the track. However, it is unlikely that this will be enough, and noise barriers will be required in many locations. Where such barriers are employed, significant aerodynamic noise sources high up on the trains are likely to render 3 metre barriers insufficiently effective. For HS2 Ltd to bank on not having to increase barrier height “to provide shielding to the entire train” is, in my view, being a bit hopeful.

And as for noise shielding of trains on viaducts, that really does look problematic.


One response to this post.

  1. Posted by ChrisEaglen on December 17, 2012 at 4:27 pm

    The evaluation should be undertaken in the autumn for leaves on rail head which will change the noise effects.

    Also for corrogations on the rail head and wheel flats the noise outputs will change and the spectra.

    Also the rain on pantographs.

    Suggest the weather and seasonal aspects are included in the reviews too.


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