A multi-axis time domain SEM–PML for modelling wave excitation, propagation and scattering in fluid-filled pipes immersed in fluid

Finite element models coupled with Perfectly Matched Layers (PMLs) have been used to model time-domain wave propagation in fluid-filled pipes. This is a fluid-structure interaction problem, which becomes computationally challenging when the pipes are further immersed in fluid, as the domain extends to infinity in both the radial and axial directions. The time domain excitation, propagation, and scattering of guided waves from a point source in such systems have rarely been studied theoretically. This article proposes a high-order Spectral Element Method (SEM) incorporating multi-axis PMLs that operate in both the fluid and structural domains. This development efficiently truncates the modelling domain in at least two dimensions (radial and axial) and seamlessly integrates fluid–solid coupling from unstretched regions to the PML regions. The proposed SEM–PML model is validated against an enlarged SEM model and semi-analytical finite element (SAFE) predictions, with results showing excellent agreement. It is found that when a point source is located in the inner fluid, a specific fluid-type wave mode is excited and is dominant in the inner fluid, while a structural “trapped” wave mode concentrates its energy in the pipe wall. Additionally, a radiation mode, appearing as the bulk-fluid wave, is excited and shown to couple across the inner fluid, pipe wall, and outer fluid domains. The SEM–PML model is also used to study wave scattering from a defect in the coupled system. Results indicate that both fluid and structure type guided waves can be effectively utilised for non-destructive inspection of such defects.