Mechanisms of particle lifting and entrainment in dust devils

Dust devils are known to easily lift a large quantity of dust particles into the atmosphere even in the absence of a strong mean wind. Once these particles are lifted off they remain suspended for a long time. Therefore dust devils are integral to the dust cycle in the atmosphere which in turn influences climate by affecting the planet's radiative balance. The objective of this work is to understand the mechanisms that govern particle lifting and entrainment in dust devils. In the first stage of the work, we simulated characteristic features of an established dust devil (persistent vortical column with a turbulent, low-pressure core) in a computational model of a laboratory vortex generator. Simulations resolving the vortex flow were carried out at a radial Reynolds number of 18,000 and swirl ratio of 0.33 using a filtering-based LES model. In the second stage, we look at the vortex flow's role in particles entrainment from a particle bed on the ground. We focus on mechanisms resulting from particle-fluid interactions and particle-particle interactions during the initial liftoff and subsequent entrainment by the vortical structure. We use an Euler-Lagrange method with four-way coupling to introduce particles with Stokes number in the range St = 0.2 − 0.6 to the flow. The particle bed spanning the vortex core is placed on the ground as the simulation begins. The particle forcing model includes drag, lift, pressure gradients, collisions, and hydrodynamic torque, and their individual roles in the entrainment processes are identified. Our work quantifies and correlates the instantaneous local particle resuspension and deposition fluxes with instantaneous local flow structures. Results also show how interaction between particles and flow structures depend on particle size.