Journal of Computational Acoustics

The problems of acoustic waves scattered by scatterer immersed in unbounded domain is an essential ingredient in the study of acoustic-structure interaction. In this paper the problems of acoustic scattering in an infinite exterior region are investigated by using a fractal two-level finite element mesh with self-similar layers in the media which encloses the conventional finite element mesh for the cavity. The similarity ratio is bigger than one so that the fractal mesh extends to infinity. Because of the self-similarity, the equivalent stiffness (mass) matrix of one layer is proportional to the others. By means of the Hankel functions automatically satisfying Sommerfeld's radiation conditions at infinity, the different unknown nodal pressures on different layers are transformed to some common unknowns of the Hankel coefficients. The set of infinite number of unknowns of nodal pressure is reduced to the set of finite number of Hankel's coefficients. All layers have the same matrix dimension after the transformation and the respective matrices of each layer are summed. Due to the proportionality, the infinite number of layers can be summed in closed form as the entries of each matrix are in geometric series. That is, processing one layer is enough to virtually represent a set of infinite number of layers covering an infinity domain. No new elements are created. Numerical examples show that this method is efficient and accurate in solving unbounded acoustic problems.

The sound production by vortex rings is investigated by means of an axisymmetric vortex particle method. The predictions are first calibrated by analyzing the noise generated by steady vortex rings that are described by the analytical solutions of Fraenkel and Norbury. The noise produced by isolated vortex rings for both nominally steady and unsteady cores is then analyzed. For nominally steady cores, computed results indicate that the efficiency of sound radiation decreases as the slenderness parameter is reduced, and the acoustic signals reveal a dominant period that is approximately half the eddy turnover time. For unsteady cores, the amplitude of the radiated sound is substantially higher than that of similar steady rings. When the initial core vorticity distribution is nonuniform, complex internal motion may also occur within the core which is also reflected in the corresponding far-field acoustic signal. Finally, the effect of vortex stretching is analyzed based on computations of two coaxial corotating vortex rings.

The spheroidal beam equation (SBE)¹ is a parabolic equation, which fits for focused, high-frequency beams from a large circular aperture. Partition of the acoustic field is one of the very important aspects of the SBE. But the criterion of division has not been given. In this paper, the criterion and corresponding numerical simulation are introduced. At the same time, the comparison of the SBE solution with Lucas's parabolic equation is fulfilled.

Adaptive techniques can be applied to improve performance of a beamformer in a cluttered environment. The sequential implementation of an adaptive beamformer, for many sensors and over a wide band of frequencies, presents a serious computational challenge. By coupling each transducer node with a microprocessor, in-situ parallel processing applied to an adaptive beamformer on a distributed system can glean advantages in execution speed, fault tolerance, scalability, and cost. In this paper, parallel algorithms for Subspace Projection Beamforming (SPB), using QR decomposition on distributed systems, are introduced for in-situ signal processing. Performance results from parallel and sequential algorithms are presented using a distributed system testbed comprised of a cluster of computers connected by a network. The execution times, parallel efficiencies, and memory requirements of each parallel algorithm are presented and analyzed. The results of these analyses demonstrate that parallel in-situ processing holds the potential to meet the needs of future advanced beamforming algorithms in a scalable fashion.

A simple model of seismic wave attenuation combining anisotropy with anelastic effects is constructed. The anelastic response is based on the Cole–Cole relaxation function. Time-stepping finite-difference and ray-asymptotic methods of numerical solution are discussed.

The vibrational analysis of ships involves the solution of a coupled fluid-structure interaction problem in order to simulate the inertia response of the water to the structural vibrations. The paper describes a coupled FE-BE procedure for the computational solution of this problem. Regarding the structural equations, special emphasis is put on the modeling of damping. Various models of damping, in particular local damping, are discussed and implemented into the FE-model of the ship structure. The computational results are compared to data from large-scale measurements.

An optimization program based on M. J. Box's Complex Method was implemented in a computational design tool for constrained optimization of acoustic environments produced by vibrating structures. The tool can treat interior and exterior environments, and can consider acoustic and structural excitations. The tool integrates finite element and boundary element methods to perform the requisite structural acoustic analyses. The new optimization component described in this paper contains unique additions to Box's original algorithm. The optimizer executes stand-alone, or can be used as a starting point generator for another optimizer, thus creating a hybrid algorithm. This paper describes the new optimizer's algorithm and the results of an example optimization to reduce the interior noise levels within an unstiffened cylindrical shell. Design variables were selected as the shell thicknesses, subject to upper and lower bounds. All analyses were performed at a single excitation frequency. The results demonstrate effective performance of the optimizer, and the potential to obtain significant noise level reductions though the use of other-than-uniform thickness distributions.