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Turbulence is an important mechanism which influences atmospheric flows and has significant impact on many engineering processes. In particular, turbulence plays a key role in cloud dynamics and intensifies the formation of warm rain. Small-scale turbulent motion augments radial relative velocity between droplets and induces droplet accumulation in the low vorticity region [3]. This leads to enhancement of collision-coalescence and in consequence accelerates the growth of the rain drops. Turbulent collision-coalescence plays an important role for droplets of radius from 10 to 60 μm. For larger droplets gravity dominates the motion but turbulence still affects collision rate through altering settling velocity. Relative differences in settling velocity have a direct effect on the collision rate. In this study, we examine numerically several mechanisms that may enhance or reduce the settling velocity of small heavy particles. One of those is preferential sweeping (particles show preference to reside in the downward-sweeping sides of eddies), which occurs for strong turbulence and large particle inertia. This was found in experiments by Aliseda et al. (2002) [1] and Hill (2005) [2] and numerical simulations by Wang and Maxey (1993) [3], and model theory by Davila and Hunt (2001) [4]. On the other hand, vortex trapping or loitering (particles spend more time in upflow than in downflow regions of the flow) could lead to reduction of the settling velocity of Stokes particles. This reduction typically occurs for weak turbulence, weak inertia, but strong sedimenting particles (Nielsen, 1993 [5]). In both cases, the distribution of particles relative to vortical structure is mainly controlled by their inertial interactions with the small-scale turbulence. However, the level of increase or decrease in the average settling velocity depends strongly on the large energetic eddies [6, 7].
Author(s):
Bogdan Rosa
Institute of Meteorology and Water Management - National Research Institute
Poland
Orlando Ayala
Department of Mechanical Engineering, University of Delaware
United States
Hossein Parishani
Department of Mechanical Engineering, University of Delaware
United States
Lian-Ping Wang
Department of Mechanical Engineering, University of Delaware
United States