Noninvasive Measurement of Particle-Settling Velocity and Comparison with Stokes’ Law
Publication: Journal of Environmental Engineering
Volume 140, Issue 2
Abstract
An image-analysis technique was applied for noninvasive measurements of settling rate of polystyrene latex spheres of 6, 10, 20, 50, 100, and 160 μm diameters in a quiescent environment. A CCD (charged couple device) camera and strobe light were used to capture in situ images of the particles, which were analyzed using image-analysis software. To overcome the challenge of noninvasive measurement of very slow-moving particles, variable pixel resolutions were chosen depending on settling speed so that the separation of the particle images in a given camera exposure time was sufficient to determine the temporal displacements of the particles with precision. Particle-settling behavior was not disrupted for any kind of sample handling, and measurements were made without hindering particle settling and allowing particle movement under gravity alone. Measurements were compared with settling rates calculated with Stokes’ law, and the validity of Stokes’ law under low particle Reynolds number () was investigated for solid and spherical particles. While the smaller particles settled at a rate closely predicted by Stokes’ law, the larger particles settled slightly more slowly than was predicted. This effect is likely due to an increase in the drag coefficient with relatively larger-particle Reynolds number, compared to the value used in Stokes’ law.
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Acknowledgments
The authors greatly appreciate the IAGLR-C.S. Mott scholarship, Mark Diamond Research Foundation (SUNY at Buffalo), and CH2MHILL for assistance during various phases of the manuscript preparation. Prof. Joseph F. Atkinson of SUNY at Buffalo contributed in developing the concept of this study and design of the experimental setup. The authors would also like to thank Prof. Atkinson for suggestions/comments throughout the preparation of this article. Also, comments from anonymous reviewers are greatly appreciated.
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Published In
Journal of Environmental Engineering
Volume 140 • Issue 2 • February 2014
Copyright
© 2013 American Society of Civil Engineers.
History
Received: May 16, 2011
Accepted: Oct 3, 2013
Published online: Oct 5, 2013
Published in print: Feb 1, 2014
Discussion open until: Apr 19, 2014
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