Cylindrical micro-pressure wave radiation from tunnel portals in deep cuttings

What is it about?

In high-speed railways, micro-pressure waves (MPWs) emitted from tunnel portals can cause significant audible sound and window rattling. An MPW is typically a spherical wave. However, in infinitely deep cuttings, an MPW becomes a cylindrical wave that affects larger areas. In this study, cylindrical MPWs emitted toward infinitely deep cuttings were analytically and numerically investigated. Analytical expressions were introduced, and a far-field approximation using the half derivative of velocity at the tunnel exit was established.

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Photo by Florian Olivo on Unsplash

Photo by Florian Olivo on Unsplash

Why is it important?

The gradual decrease of the cylindrical MPW around the tunnel exit allows it to travel further compared to the spherical MPW. This phenomenon is caused by the superposition of many reflected MPWs between the cutting faces. The strength of the main source term for the cylindrical MPW is demonstrated to be approximately proportional to √(Δp_in (∂p_in/∂t)_max ), whereas that for the spherical MPW is proportional to (∂p_in/∂t)_max. Here, p_in and Δp_in represent the pressure and magnitude of the compression wave at the tunnel exit, respectively.