A modal approach based on perfectly matched layers for the forced response of elastic open waveguides

Abstract

This paper investigates the computation of the forced response of elastic open waveguides with a numerical modal approach based on perfectly matched layers (PML). With a PML of infinite thickness, the solution can theoretically be expanded as a discrete sum of trapped modes, a discrete sum of leaky modes and a continuous sum of radiation modes related to the PML branch cuts. Yet with numerical methods (e.g. finite elements), the waveguide cross-section is discretized and the PML must be truncated to a finite thickness. This truncation transforms the continuous sum into a discrete set of PML modes. To guarantee the uniqueness of the numerical solution of the forced response problem, an orthogonality relationship is proposed. This relationship is applicable to any type of modes (trapped, leaky and PML modes) and hence allows the numerical solution to be expanded on a discrete sum in a convenient manner. This also leads to an expression for the modal excitability valid for leaky modes. The physical relevance of each type of mode for the solution is clarified through two numerical test cases, a homogeneous medium and a circular bar waveguide example, excited by a point source. The former is favourably compared to a transient analytical solution, showing that PML modes reassemble the bulk wave contribution in a homogeneous medium. The latter shows that the PML mode contribution yields the long-term diffraction phenomenon whereas the leaky mode contribution prevails closer to the source. The leaky mode contribution is shown to remain accurate even with a relatively small PML thickness, hence reducing the computational cost. This is of particular interest for solving three-dimensional waveguide problems, involving two-dimensional cross-sections of arbitrary shapes. Such a problem is handled in a third numerical example by considering a buried square bar.

Numerical modelling of wave scattering by local inhomogeneities in elastic waveguides embedded into infinite media

“…In the latter case, the ingoing modal coefficients are obtained from the modal decomposition of the forced response (see Ref. [33]), further propagated analytically and enforced to the appropriate cross-section boundary. The subscript + denotes outgoing (scattered) modes.…”

“…These modes are often called PML modes (or Berenger modes). Although PML modes are non-intrinsic to the physics (they resonate mainly in the PML region), their modal superposition enables to accurately reconstruct the forced response of an elastic open waveguide [33]. Nevertheless, their contribution remains unclear as far as scattering problems are considered.…”

Investigation and application of nonlinear ultrasonic guided wave in closed and open waveguides

“…is the general orthogonality relation in open waveguides[111], which is similar with Auld's real orthogonality relation[102].…”

“…In addition, it has been demonstrated that the PML 5. Nonlinear Ultrasonic Guided Waves in Open Waveguides modes mainly oscillate in the PML region and only contribute to long-term diffraction, geometrical decay of the field [111] which is of little interest in the context of NDT, so that the sums can only run over all trapped and leaky modes.…”

“…The general orthogonality relation is given by [111] Q n,m = 0, if n = −m. wheref (1) • n A dΩ havingf (1) = e i2k l zf (1) ands (1) = e i2k l zŝ (1) represents the wavenumber of the primary wave which is…”