Formation of particle clouds. (31st March 2014)

• Record Type: Journal Article
• Authors: Zhao, B.; Law, A. W. K.; Adams, E. E.; Er, J. W.
• Is Part Of: Journal of fluid mechanics. Volume 746(2014:May)
• Page Start: 193
• Page End: 213
• Publication Date: 2014-03-31
• Subjects: convection, -- plumes/thermals; Fluid mechanics -- Periodicals; 532.005
• DOI: 10.1017/jfm.2014.121
• Languages: English
• ISSNs: 0022-1120

Abstract

Abstract: 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 ( $\mathit{Ra} > {10^3}$ ) or, equivalently, the source cloud number was small ( $\mathit{Nc} < 3.2 \times 10^{-2}$ ). 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 $\mathit{Nc}$ values ranging from $2.9 \times 10^{-3}$ to $5.9 \times 10^{-2}$ 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.