Formation of particle clouds
Law, Adrian Wing-Keung
Adams, E. Eric
Er, J. W.
Date of Issue2014
School of Civil and Environmental Engineering
Nanyang Environment and Water Research Institute
In the literature, it has been conceptualized that a group of dense particles released instantaneously into homogeneous stagnant water would form a circulating vortex cloud and descend through the water column as a thermal. However, Wen & Nacamuli (Hydrodynamics: Theory and Applications, 1996, pp. 1275–1280) observed the formation of particle clumps characterized by a narrow, fast-moving core shedding particles into the wake. They found clump formation to be possible even for particles in the non-cohesive range as long as the source Rayleigh number was large (Ra>103) or, equivalently, the source cloud number was small (Nc<3.2×10−2). This physical phenomenon has not been investigated further since the experiments of Wen and Nacamuli. In the present study, the relationship between Nc and the formation process is examined more systematically. The theoretical support for cloud number dependence is explored by considering flows passing a porous sphere. Here Nc values ranging from 2.9×10−3 to 5.9×10−2 are tested experimentally using particles with different initial masses and grain sizes, from non-cohesive to marginally cohesive. The formation processes are categorized into cloud formation, a transitional regime and clump formation, and their distinct features are presented through qualitative description of the flow patterns and quantitative assessment of the gross characteristics.
Journal of fluid mechanics
© 2014 Cambridge University Press. This paper was published in Journal of Fluid Mechanics and is made available as an electronic reprint (preprint) with permission of Cambridge University Press . The paper can be found at the following official DOI: [http://dx.doi.org/10.1017/jfm.2014.121]. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper is prohibited and is subject to penalties under law.