Numerical Simulation on Air Flow and Dust Transportation of Dust Devils

摘要

Large eddy simulation (LES) with dynamic sub-grid scale model is used to solve the turbulence of atmosphere (gas phase) flow field for dust devil. The transport model of dust particles in the gas phase field of dust devils is also studied based on Lagrangian reference frame. The evolution of dust devils can be divided by three stages, developing, developed and vanishing stage. The sand dust has no effect on the atmosphere flow field in the developing stage and it is only in the developed stage of dust devils that the movement of sand dust was studied. With grids stretched in the radial and vertical directions, the computational domain for the atmosphere flow of dust devils is cylindrical with radius R=50m and height H=150m, which is suitable to utilize a reasonable fine grid resolution to resolve the dominant turbulence in the dust devil core and analyze the fine flow structure of dust devils. The atmosphere flow of dust devil in the a quasi-steady state was firstly simulated to indicate the characters of the gas phase field in the developed stage, including the prediction of the diminished vertical velocities, the lower pressure and higher temperature in the vortex core. In dust devils, the near-surface air parcels flow into the center and then reflect upward to the vortex core, which results in a rapid outward-leaning swirling updraft in the corner. Dust devils have a high-speed horizontal wind near the surface and high-speed vertical wind above the surface to make the dust lifting. Dust devils are visualized by the dust lifting in the gas phase field. The transport mechanism of dust particles in the gas phase field of dust devils was studied based on Lagrangian reference frame. The effect of horizontal and vertical wind on dust lifting is specified that the horizontal wind can only make dust creep and move near the surface and the vertical wind plays an important role in dust lifting. The tracks of dust grains with different materials and diameters are calculated to show that the material and diameter of the particle determine the outer profile of dust devils and the interactions between large and small dust grains in the air field make the flow more complex and turbulent.