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Proceedings Paper

Optical Particle Sizing For Hydrodynamics Based On Near Forward Scattering
Author(s): Y. C. Agrawal; J. B. Riley
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Paper Abstract

In the study of boundary layer dynamics in the ocean, the stratification induced by suspended matter enters through the parameter cnwn where cn and wn are, respectively, the concentration and fall velocity of size class n. In this paper, an optical arrangement for instantaneous measurement of the particle size distribution will be described. The basic principle is that the near forward scattering by a particle is approximated quite well by the Fraunhofer diffraction through a circular aperture. Thus, when light is scattered by particles illuminated by a collimated laser beam, the observed intensity distribution in the Fourier transform plane of a coaxial lens is the incoherent sum of the Airy patterns from all particles, weighted by area square. An inversion of this observed intensity distribution produces the size distribution. The inversion, based on a Titshmarch-Bateman formula is analytic, and has been known in the literature to reduce to a Fourier transform of the angular intensity distribution, weighted by the third power of the scattering angle, i.e. o3.I(s). In this paper the information content of the measured intensity distribution is discussed, in particular, the size resolution and minimum and maximum size of particles about which information is contained in the observations are described. Furthermore, the concept of a Nyquist size and aliasing is introduced. By enclosing the scattering volume when the decay of the size distribution is differentiated in time, the fall velocity, hence mass density, at each size can be deter-mined separately. The above reduction of the observations generates the relevant parameter cnwn and also permits the direct observation of the fluctuations in concentrations, which are theoretically predicted to follow the -5/3 power law but have never before been verified. Theoretical background and analysis of synthetic data will be presented.

Paper Details

Date Published: 27 September 1984
PDF: 9 pages
Proc. SPIE 0489, Ocean Optics VII, (27 September 1984);
Show Author Affiliations
Y. C. Agrawal, Woods Hole Oceanographic Institution (United States)
J. B. Riley, Woods Hole Oceanographic Institution (United States)

Published in SPIE Proceedings Vol. 0489:
Ocean Optics VII
Marvin A. Blizard, Editor(s)

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