Journal of Computational Acoustics

Advances in computational aeroacoustics (CAA) depend critically on the availability of accurate, nondispersive, least dissipative computation algorithm as well as high-quality numerical boundary treatments. This paper focuses on the recent developments of numerical boundary conditions. In a typical CAA problem, one often encounters two types of boundaries. Because a finite computation domain is used, there are external boundaries. On the external boundaries, boundary conditions simulating the solution outside the computation domain are to be imposed. Inside the computation domain, there may be internal boundaries. On these internal boundaries, boundary conditions simulating the presence of an object or surface with specific acoustic characteristics are to be applied. Numerical boundary conditions, both external or internal, developed for simple model problems are reviewed and examined. Numerical boundary conditions for real aeroacoustic problems are also discussed through specific examples. The paper concludes with a description of some of the much-needed research in numerical boundary conditions for CAA.

Electrical discharge activity within an electrical power apparatus has detrimental effects on the insulating components of the system, and thus its prognosis and diagnosis are of utmost importance. Traditional electrical methods have been replaced by newer techniques such as acoustic detection. Among these, acoustic methods possess certain inherent advantages, especially where complex electrical systems like GIS are involved. The major concerns therein are the estimation of the magnitude of the discharge level as well as location of the source of discharge. The present work addresses, on a theoretical basis, problems connected with identification, measurement and location of electrical discharges, in particular in GIS. Models for the inception and propagation of p.d. within GIS have been proposed. This study, based essentially on a numerical simulation model, corroborates many of the experimental findings reported in the existing literature, and in addition attempts to suggest newer directions in magnitude estimation and location of p.d.

We validate, using a coupled finite element/boundary element computer code, a recently-developed¹ series solution for the structural acoustics problem of scattering from two submerged spherical elastic shells. Although the general purpose computational tools for acoustic scattering have never been restricted to single scatterers, the availability of the series solution provides, for the first time, the mutual validation of both exact and numerical approaches for a multiple elastic scatterer problem. The excellent agreement between the two solutions presented thus allows this problem to be added to the short list of existing benchmark structural acoustics problems possessing an analytic solution. For the purposes of this comparison, the direction of incidence is taken as parallel with the axis joining the two shells. The numerical solution uses the NASHUA code, which couples a finite element shell model of the two shells with a boundary element model of the surrounding fluid. The exact solution is found by expanding in terms of classical modal series and uses the addition theorem for the spherical wave functions. The exact solution requires coupling coefficients that are expressed in terms of sums of products of Wigner 3-j symbols (or Clebsch-Gordan coefficients).

Normal-mode expansions for Green's functions are derived for ocean–bottom systems. The bottom is modeled by Kirchhoff and Reissner–Mindlin plate theories for elastic and poroelastic materials. The resulting eigenvalue problems for the modal parameters are investigated. Normal modes are calculated by Hankel transformation of the underlying equations. Finally, the relation to the inverse problem is outlined.