The role of particle-flow interactions in cloud microphysics

by Pijush Patra

Wednesday 17 May 2023, 15:15 → 16:00 Europe/Stockholm

Albano Building 3

zoom link : https://stockholmuniversity.zoom.us/j/622224375

 

The collision of particles sedimenting in a flow field is 
relevant to many environmental and industrial processes, such as droplet 
growth in warm clouds and the aggregation of aerosol particles in 
industrial settings. The evolution of the drop size distribution in 
clouds depends on the collision rate between the drops, where the 
combined effects of background flow, gravity, and interparticle 
interactions drive the collision dynamics. A study of this problem may 
explain the condensation-coalescence bottleneck (or the ‘size gap’ of 15 
- 40 microns droplets) in warm rain formation, where neither 
condensation nor gravitational collision alone is the dominant growth 
mechanism. We have focused on studying the collision dynamics of 
particle pairs subject to a background flow (simple shear flow, 
turbulent flow) and gravity, incorporating hydrodynamic and 
interparticle interactions.

In the first problem, we have studied the role of Brownian motion in the 
coagulation of bidisperse like-charged spherical particles interacting 
via non-continuum hydrodynamics, van der Waals, and electrostatic 
interactions. We have found that electrostatic interactions can enhance 
the collision rate between like-charged Brownian particles while 
interacting through non-continuum hydrodynamics, and their charge ratio 
is high. To consider the effect of a background flow, we have studied 
the collision of hydrodynamically interacting particle pairs settling in 
a laminar simple shear flow. By incorporating non-continuum 
hydrodynamics, van der Waals interactions, and the coupled driving 
forces of sedimentation and simple shear, our work provides collision 
efficiency results relevant to flows of suspensions through vertical 
pipes and channels, sampling of aerosols, and the transport of 
particulate matter in riser reactors. Finally, we have also investigated 
collision dynamics of sub-Kolmogorov interacting spheres rapidly 
settling in a homogeneous isotropic turbulent flow. This work relied on 
formulating an equation for the pair probability PDF and solving it 
semi-analytically.