PARTICLE IRREGULARITY AND AGGREGATION EFFECTS IN AIRBORNE SUSPENSIONS AT AUDIO- AND LOW ULTRASONIC FREQUENCIES
✍ Scribed by Q. WANG; K. ATTENBOROUGH; S. WOODHEAD
- Book ID
- 102613051
- Publisher
- Elsevier Science
- Year
- 2000
- Tongue
- English
- Weight
- 497 KB
- Volume
- 236
- Category
- Article
- ISSN
- 0022-460X
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✦ Synopsis
In the inertial regime of frequency-radius space, irregularity and aggregation of particles can result in values of acoustic attenuation that are signi"cantly di!erent from those predicted by assuming separated smooth spherical particles. Data obtained previously from suspensions of alumina particles and olivine sand in air at audio-frequencies, together with new data obtained at low ultrasonic-frequencies in suspensions of glass beads and silica #our, are compared with predictions. It is shown that neither a coupled phase theory modi"ed to allow for non-spherical shapes nor e!ective radius theories are able to account for these data. Qian [21] has suggested that suspensions may be treated as fractal media and used the acoustic Reynolds number as the fractal dimension in modifying scattering theory. A new fractal modi"cation of multiple-scattering theory for acoustic attenuation is derived. The theory uses T ( is the angular acoustic frequency, T is the dynamic relaxation time of the particles) as a fractal scale. It requires an empirical determination of the di!erence between the fractal dimension of the measured suspension and that of a hypothetical suspension of spheres with the same particle size distribution. However, values obtained at a single frequency also enable "ts with data at other frequencies. The new fractal modi"cation of scattering theory is found to enable better agreement with measured attenuation as a function of concentration for irregular particles than an e!ective radius model. Also, the fractal modi"cation is able to predict the observed frequency dependence at a given concentration rather better than e!ective radius approaches. Moreover, the fractal approach is found to enable discrimination between the e!ects of particle irregularity and aggregation.