Journal of Computational Acoustics

The linear elastodynamic equations are ill-posed for models which contain high contrast density discontinuities. This paper presents a quasi-linear superset of the linear equations that is well-posed for this situation. The extended system contains a conservation of mass equation and a quasi-linear convective term in the momentum equation. Density, momentum, and stress are the field variables in the quasi-linear system, which is cast in a first order form. Using a Lax–Wendroff finite difference approximation, the utility of the quasi-linear system is demonstrated by modeling underwater acoustic scattering from a truncated ice sheet. The model contains air, ice, and water with a density contrast between air and ice or water of O(10³). Superlinear convergence of the Lax–Wendroff scheme is demonstrated for his heterogeneous medium problem.

Several radiation boundary conditions for inhomogeneous acoustic media are investigated. Previous investigators have developed various approximate radiation conditions and have studied their accuracy by calculating an effective reflection coefficient for plane waves incident on such radiating boundaries. In this paper, it is shown that effective reflection coefficients can be calculated for a class of parabolic approximations to the Helmholtz equation. These results are valid for vertically heterogeneous media. Comparison of these radiation conditions is given through numerical examples.

A relatively straightforward Boundary Element Method (BEM) for the numerical solution of the exterior Helmholtz problem is specified in a tutorial fashion. The algorithm employs the Combined Helmholtz Integral Equation Formulation (CHIEF) and then Singular Value Decomposition (SVD) to solve the resulting system. Its accuracy and convergence characteristics are examined, and compared to the simplest boundary element method for exterior acoustics, the Helmholtz Integral Equation Formulation or HIEF. Boundary element and auxiliary (CHIEF) point requirements to obtain BEM solutions of a desired accuracy are described. This particular CHIEF algorithm is found to largely avoid the numerical difficulties of the HIEF technique while retaining theoretical and practical implementation simplicity.

In this paper, a new method for the numerical solution of the two-dimensional parabolic equation of Tappert in horizontally-stratified media is presented. This method uses orthogonal cubic spline collocation for the semidiscretization with respect to depth, and the resulting system of differential-algebraic equations is solved using the NAG Library routine D02NNF. In the present application, orthogonal spline collocation, which has been exceedingly effective in the approximate solution of a broad class of problems, has the advantage that it systematically incorporates the requisite interface conditions. The state-of-the-art routine D02NNF, which implements backward differentiation formulas, has been used successfully in the solution of related problems.

A new algorithm for nonlinear plane-wave propagation is presented. The algorithm uses a novel time domain representation to account for nonlinearity, while accounting for absorption in the frequency domain. The new algorithm allows for accurate representations of diffractive shockwave propagation in the framework of an existing nonlinear beam propagation model using far fewer harmonics (and thus time) than alternative algorithms based on a frequency domain solution to Burgers' equation. The new algorithm is tested against the frequency domain solution to Burgers' equation in a variety of cases and then used to model a focused ultrasonic piston transducer operating at very high source intensities.