Perforated exit regions for the reduction of micro-pressure waves from tunnels

• Perforated exit tubes can reduce pressure fluctuations radiated from rail tunnels.
• The total area of holes in the tubes is more important than their size and distribution.
• A balance is achievable between radiation through the holes and through the portal.
• The radiation depends upon the steepness and amplitude of internal pressure waves.

The effectiveness of long, perforated exit regions in reducing pressure disturbances from railway tunnels is assessed. Such disturbances always occur, but their amplitudes are usually small. For the particular case of high speed trains, they can reach levels that would cause annoyance in the absence of suitable counter-measures. This risk is especially large in the case of long tunnels. The mechanisms causing the disturbances are described and the potential effectiveness of perforated exit regions as a counter-measure is demonstrated. It is shown that the effectiveness is sensitive to the number, size and distribution of pressure relief holes along the exit region, but that the most important parameter is the combined area of all of the holes. This parameter controls the balance between external disturbances alongside the perforated region and disturbances beyond the exit portal. It is also shown that the amplitudes of the external disturbances are strongly dependent upon the amplitude and duration of wavefronts arriving at the exit region as well as upon their steepness. This contrasts with the behaviour found for tunnels with simple exit portal regions.