Volume 109, Issue 2, February 2001
Index of content:
- GENERAL LINEAR ACOUSTICS 
109(2001); http://dx.doi.org/10.1121/1.1315290View Description Hide Description
Excitation and propagation of non-axisymmetric guided waves in a hollow cylinder is studied by using the normal mode expansion method (NME). Different sources such as angle beam, tube end excitation with normal beam, and comb transducer possibilities are discussed based on the derivations of the NME method. Numerical calculations are focused on the case of angle beam partial loading. Based on the NME method, the amplitude coefficients for all of the harmonic modes are obtained. Due to the difference of phase velocities for different modes, the superimposed total field varies with propagating distances and hence makes particle displacement distribution patterns (angular profile) change with distance. This varying non-axisymmetric angular profile of guided waves represents a nonuniform energy distribution around the hollow cylinder and thus has an impact on the inspection ability of guided waves. The angular profiles of an angle beam source are predicted by theory and then verified by experiments. The predicted angular profiles also provide information for determining the transducer location to find defects in a certain position on the hollow cylinder.
109(2001); http://dx.doi.org/10.1121/1.1336137View Description Hide Description
This paper presents a novel algorithm and numerical results of sound wave propagation. The method is based on a least-squares Legendre spectral element approach for spatial discretization and the Crank–Nicolson [Proc. Cambridge Philos. Soc. 43, 50–67 (1947)] and Adams–Bashforth [D. Gottlieb and S. A. Orszag, Numerical Analysis of Spectral Methods:Theory and Applications (CBMS-NSF Monograph, Siam 1977)] schemes for temporal discretization to solve the linearized acoustic field equations for sound propagation. Two types of NASA Computational Aeroacoustics (CAA) Workshop benchmark problems [ICASE/LaRC Workshop on Benchmark Problems in Computational Aeroacoustics, edited by J. C. Hardin, J. R. Ristorcelli, and C. K. W. Tam, NASA Conference Publication 3300, 1995a] are considered: a narrow Gaussian sound wave propagating in a one-dimensional space without flows, and the reflection of a two-dimensional acoustic pulse off a rigid wall in the presence of a uniform flow of Mach 0.5 in a semi-infinite space. The first problem was used to examine the numerical dispersion and dissipation characteristics of the proposed algorithm. The second problem was to demonstrate the capability of the algorithm in treating sound propagation in a flow. Comparisons were made of the computed results with analytical results and results obtained by other methods. It is shown that all results computed by the present method are in good agreement with the analytical solutions and results of the first problem agree very well with those predicted by other schemes.
Improved solution for the vortical and acoustical mode coupling inside a two-dimensional cavity with porous walls109(2001); http://dx.doi.org/10.1121/1.1340648View Description Hide Description
This work presents an improved solution to a former study that analyzes the oscillatory motion of gases prescribed by vortico-acoustical mode coupling inside a two-dimensional porous cavity. The physical problem arises in the context of an oscillating gas inside a rectangular enclosure with wall transpiration, sublimation, or sweating. Previously, a multiple-scale solution was derived for the temporal field. The asymptotic formulation was based on an unconventional choice of scales. Its accuracy was also commensurate with the size of a parameter that captured the effect of small viscosity. Currently, an exact solution is derived and compared to the previous formulation. A simple WKBJ solution is also constructed for validation purposes. Unlike both asymptotic formulations, the exact solution remains accurate regardless of the range of physical parameters.
109(2001); http://dx.doi.org/10.1121/1.1337958View Description Hide Description
The fundamental azimuthal modes of a constricted annular resonator are investigated. It is found that a given mode of an unconstricted resonator splits into two separate modes in the constricted resonator. One mode is of a higher frequency and has a pressure antinode centered in the constricted region. The other mode is of a lower frequency and has a pressure node centered in the constricted region. The resonance frequency of the higher-frequency modes increases linearly with a decrease in the constricted to unconstricted area ratio, whereas the lower frequency drops nonlinearly. Measurements and theory match to within 0.5% when end corrections and thermo-viscous losses are included in the system model. It was found that end correction impedances derived by mode-matching techniques were the only ones accurate enough to match the measurements and computation to within the error bounds.