Modal coupling analysis of the acoustic wave scattering from blockage in a pipe

Acoustic sensing system deployed on an autonomous platform (also referred to as robot) for accurate condition monitoring and fault detection in pipes requires the knowledge of wave scattering from various in-pipe faults or the robot itself. Existing solutions to estimate wave scattering tend to either be constrained to the plane wave regime or be computationally expensive outside this range. There has been a lack of work to apply analytical modal coupling methods to study wave scattering from a non-symmetric cross-sectional change in a pipe beyond the plane wave regime. This paper proposes an efficient three-dimensional (3D) modal coupling method to predict wave scattering from a cross-sectional change in a pipe in the frequency range beyond the plane wave regime. The trapped modes induced by a 3D axisymmetric or non-axisymmetric cross-sectional change in an air-filled pipe are estimated using modal coupling analysis. The derived analytical model is validated against numerical simulations and measurements. It agrees with a finite element simulation with Comsol Multiphysics in the 0.01<kR<4 frequency range (k being the wavenumber and R being the pipe radius) within 15 %, but it is approximately 600 times faster than the Comsol simulation making it attractive for the deployment on sensors with limited computer power that can be used for autonomous inspection of buried pipes.