Diffuse Reverberant Ultrasonics in Time-varying Media
R. L. Weaver*
National Science Foundation, CMS 97-01142

Reverberant ultrasound in low-damping materials survives long after the pulse has terminated. Typical rays can survive for periods of the order of 105 cycles in, for example, aluminum, and therefore explore the sample thoroughly. Such fields are complex random interference patterns whose phase and amplitude are, because of the long ray path lengths, strong functions of such parameters as temperature, strain, and geometry (e.g. crack opening). This project seeks to ascertain to what extent the variation of these fields with sundry quasi-static parameters can be modeled and understood, and then perhaps used for materials characterization.

Diffuse Ultrasonics and Materials Characterization
R. L. Weaver,* O. Lobkis, I. Rozhkov
National Science Foundation, CMS 97-01142

The diffuse ultrasonics of polycrystalline materials is studied with a view toward ultimate applications in robust ultrasonic characterization of microstructures and flaw detection in the midst of grain noise. The main focus is on the validation of ultrasonic radiative transfer formulations of multiple diffuse scattering in polycrystals. Of particular interest is the transition from the simple single-scattering limit model used by many, through the complicated regime in which typical rays have scattered a few times, to the once again simple diffusion limit in which typical rays have scattered many times. Theoretical and numerical work is complemented by experimental work done elsewhere.

Diffuse Ultrasonics in Strongly Scattering Media
R. L. Weaver,* O. Lobkis
National Science Foundation, CMS 97-01142

Ultrasound in materials with extremely strong scattering microstructures, with mean free paths of the order of wavelengths, is studied. Examples include solid foams, slurries, and polycrystalline aggregates of crystallites of strong anisotropy. The particular focus is on energy-density fluctuations and energy transport as one approaches the Anderson localization transition.

Flow Diagnostics and Noise Generation in a Fan-Coil Assembly
R. L. Weaver,* S. Balachandar,* J. C. Dutton* (Mech. & Indus. Engr.), A. M. Jacobi, S. E. Zeller
U of I Air Conditioning and Refrigeration Center

In the air-conditioning and refrigeration industries, acoustic noise strongly influences human comfort, customer satisfaction, and general system quality. Unfortunately, the underlying noise source and the responsible components are not always well understood. This pilot project brings together researchers in fluid dynamics and acoustics in an attempt to understand the flow features responsible for the generation of objectionable noise in an axial fan and coil heat-exchanger unit. We will make measurements of flow, of radiated acoustic noise, and of near-field dynamic pressure, in an attempt to locate and characterize the actual sources of the noise.

Laser-generated Stress Waves for Determination of Thin-Film Interfacial Properties
R. L. Weaver,* N. R. Sotos, J. Wang
Campus Research Board

We investigate, both theoretically and experimentally, the generation of high-amplitude compression waves due to the sudden deposition of heat from a YAG laser pulse in a thin metallic film between two solids. The resulting pulse, with a duration of 10 nsec, and a strain amplitude of the order of 1%, is measured using laser interferometry. Particular issues of concern include the effects of nonlinearity in the wave propagation and the corresponding development of shocks, and mode conversion at oblique interfaces with consequent generation of high-amplitude shear waves. It is anticipated that each of these effects will be important in ultimate application to the testing, by high-speed stress loading, of thin-film coatings.