Effects of turbulence on the geometric collision rate of sedimenting droplets. Part 1. Results from direct numerical simulation
By Orlando M. Ayala, B. Rosa, Lian-Ping Wang, Wojciech W. Grabowski. 2008
177 citations recorded by the source corpus.
There have been relatively few studies of turbulent collision rate of sedimenting droplets in the context of cloud physics, for which both the gravitational settling and inertial effects must be simultaneously considered. In this study, direct numerical simulations (DNS) were used to study the geometric collision rates of cloud droplets. Both Stokes drag law and a nonlinear drag law were considered, but the droplet–droplet local aerodynamic interactions were not included. Typical droplet and turbulence parameters of convective clouds were used to determine the flow dissipation rate ϵ, characteristic Stokes numbers, and the nondimensional terminal velocities. DNS results from a large number of runs covering the ϵ range from 10 to 400 cm2 s− 3 and droplet sizes from 10 to 60 μm in radius are presented. These results show that air turbulence can increase the geometric collision kernel by up to 47%, relative to geometric collision by differential sedimentation. This is due to both a moderate enhancement of the radial relative velocity between droplets and a moderate level of pair nonuniform concentration due to local droplet clustering. The turbulence enhancements increase with the flow dissipation rate and flow Reynolds number. Comparisons with related DNS studies show that our results confirm and extend the previous findings. The mean settling velocity of droplets in a turbulent flow was also obtained, showing that a maximum increase relative to the terminal velocity occurs for 20 μm cloud droplets. This agrees with a previous theory based on simple vortex flows and confirms the importance of a new nondimensional parameter τp3g2/ν for sedimenting droplets, where τp is the droplet inertial response time, g is the gravitational acceleration and ν is the air kinematic viscosity. Limitations of DNS and future directions are also noted.
Subfields: Computational Mechanics, Earth-Surface Processes, Ocean Engineering
View Paper: https://www.semanticscholar.org/paper/1a30e28e67f0bd300e93d3ee5869618a4be42692
How researchers see this paper
The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.
In Computational Mechanics
In Earth-Surface Processes
In Ocean Engineering
Citations in the corpus
- Growth of Cloud Droplets in a Turbulent Environment (2013, methodology): “25 cos α , (7) where the fluid acceleration scaling is taken from Zaichik et al. (2003), which incorporates flow intermittency (see also Ayala et al. 2008a, section 2.1).”
- The effect of Reynolds number on inertial particle dynamics in isotropic turbulence. Part 1. Simulations without gravitational effects (2015, background): “, see Ayala et al. 2008), and thus our primary focus is on the collision rates of smaller (St . 3) particles. K̂(d) is independent of ρp/ρf for 1 . St 6 3, in agreement with the findings of Voßkuhle et al. (2014). In this case, while both g(d) and S −‖/uη are dependent on d, these two quantities hav”
- The effect of Reynolds number on inertial particle dynamics in isotropic turbulence. Part 2. Simulations with gravitational effects (2015, background): “…velocity (Wang & Maxey 1993; Yang & Lei 1998; Yang & Shy 2005; Ireland & Collins 2012; Bec et al. 2014; Good et al. 2014) and the collision frequency (Franklin, Vaillancourt & Yau 2007; Ayala et al. 2008; Onishi, Takahashi & Komori 2009; Woittiez et al. 2009; Rosa et al. 2013; Bec et al. 2014).”