| Summary: | The sound generated by high speed trains can be exacerbated by the presence of trackside
structures. Tunnels are the principal structures that have a strong influence on the
noise produced by trains. A train entering a tunnel causes air to flow in and out of the
tunnel portal, forming a monopole source of low frequency sound ["infrasound"] whose
wavelength is large compared to the tunnel diameter. For the compact case, when the
tunnel diameter is small, incompressible flow theory can be used to compute the Green's
function that determines the monopole sound. However, when the infrasound is "shielded"
from the far field by a large "flange" at the tunnel portal, the problem of calculating the
sound produced in the far field is more complex. In this case, the monopole contribution
can be calculated in a first approximation in terms of a modified Compact Green's function,
whose properties are determined by the value at the center of a. disk (modelling the flange)
of a diffracted potential produced by a thin circular disk. In this thesis this potential is
calculated numerically. The scattering of sound by a thin circular disk is investigated using
the Finite Difference Method applied to the three dimensional Helmholtz equation subject
to appropriate boundary conditions on the disk. The solution is also used to examine the
unsteady force acting on the disk.
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