Index of content:
Volume 121, Issue 4, April 2007
- STRUCTURAL ACOUSTICS AND VIBRATION 
Theoretical foundations of apparent-damping phenomena and nearly irreversible energy exchange in linear conservative systems121(2007); http://dx.doi.org/10.1121/1.2697030View Description Hide Description
This paper discusses a class of unexpected irreversible phenomena that can develop in linear conservative systems and provides a theoretical foundation that explains the underlying principles. Recent studies have shown that energy can be introduced to a linear system with near irreversibility, or energy within a system can migrate to a subsystem nearly irreversibly, even in the absence of dissipation, provided that the system has a particular natural frequency distribution. The present work introduces a general theory that provides a mathematical foundation and a physical explanation for the near irreversibility phenomena observed and reported in previous publications. Inspired by the properties of probability distribution functions, the general formulation developed here is based on particular properties of harmonic series, which form the common basis of linear dynamic system models. The results demonstrate the existence of a special class of linear nondissipative dynamic systems that exhibit nearly irreversible energy exchange and possess a decaying impulse response. In addition to uncovering a new class of dynamic systemproperties, the results have far-reaching implications in engineering applications where classical vibration damping or absorption techniques may not be effective. Furthermore, the results also support the notion of nearly irreversible energy transfer in conservative linear systems, which until now has been a concept associated exclusively with nonlinear systems.
On the existence of localized shear horizontal acoustic waves in a piezoelectric plate with two semi-infinite same/different coatings121(2007); http://dx.doi.org/10.1121/1.2641812View Description Hide Description
In this paper, the existence theorem of localized shear horizontal acoustic waves in a piezoelectric plate with two semi-infinite same/different coatings is established. Some properties of the waves in the waveguidestructures are also discussed. The results show that the waveguides have some advantages and provide more choice for the designs of acoustic devices.
Using cross correlations of turbulent flow-induced ambient vibrations to estimate the structural impulse response. Application to structural health monitoringa)121(2007); http://dx.doi.org/10.1121/1.2710463View Description Hide Description
It has been demonstrated theoretically and experimentally that an estimate of the impulse response (or Green’s function) between two receivers can be obtained from the cross correlation of diffuse wave fields at these two receivers in various environments and frequency ranges: ultrasonics, civil engineering, underwater acoustics, and seismology. This result provides a means for structural monitoring using ambient structure-borne noise only, without the use of active sources. This paper presents experimental results obtained from flow-induced random vibration data recorded by pairs of accelerometers mounted within a flat plate or hydrofoil in the test section of the U.S. Navy’s William B. Morgan Large Cavitation Channel. The experiments were conducted at high Reynolds number ( million) with the primary excitation source being turbulent boundary layer pressure fluctuations on the upper and lower surfaces of the plate or foil. Identical deterministic time signatures emerge from the noise cross-correlation function computed via robust and simple processing of noisemeasured on different days by a pair of passive sensors. These time signatures are used to determine and/or monitor the structural response of the test models from a few hundred to a few thousand Hertz.