Strong scattering of ultrasonic waves in fluidized suspensions, wave propagation, spectroscopy techniques and particle dynamics

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Cowan, Michael L.
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This thesis presents the results of three main experimental projects; a study of ultrasonic wave propagation in strongly scattering materials, the use of the results of this first study to develop new techniques in ultrasonic correlation spectroscopy, and the use of these techniques to measure the dynamics of particles in fluidized beds. The first project involves the investigation of wave propagation in random, inhomogeneous materials that scatter ultrasound strongly (i.e. materials where the scattering mean free path is comparable to the ultrasonic wavelength [lambda]). I have measured the velocities of the weak ballistic pulse that travels through these materials without scattering out of the forward direction. Using the results of this wave propagation study, we have developed two new ultrasonic spectroscopy techniques. Dynamic Sound Scattering (DSS) uses singly scattered sound to measure the rms velocity of scatterers. Diffusing Acoustic Wave Spectroscopy (DAWS) uses multiply scattered sound, describedby the diffusion approximation, to measure the relative motion of scatterers. This thesis explains the underlying principles governing these techniques and the ways that these techniques can be implemented in practice to provide powerful new methods for investigating the dynamics of strongly scattering materials. Using DAWS and DSS, I have investigated the dynamics of particles suspended against sedimentation by a fluid flowing upwards in a fluidized bed. Since ultrasonic wavelengths are on the order of millimeters, the suspensions that can be Probed contain particles with diameters on the millimeter scale; this corresponds to a regime that is of interest in industrial applications. Thus DAWS and DSS are well suited to the high Reynolds number, high volume fraction suspensions to which other current techniques are least suited. In our experiments, the velocity fluctuations and velocity correlation lengths are measured as a function of the volume fraction and sample size for a range of Reynolds numbers. At high volume fractions of particles ([straight phi] > 0.15), we find surprisingly large values of the rms velocity fluctuations and instantaneous correlation length, with a different dependence on volume fraction to that seen at low [straight phi]. (Abstract shortened by UMI.)