NUMERICAL STUDY OF THE FOMATION EVOLUTION AND STRUCTURE OF DUST DEVIL

Dust devil is a small whirlwind,usually in a short duration,which swirls dust,debris,and sand to a certain heights.In this paper Eulerian/Lagrangian methods and PISO algorithm are employed to simulat edust devil to getits fine structure and its development.During the course of developing of dust devil,the air near the bottom of hot-bubble is heated by the hotground and flows to the central field,and then the swirling energy is strengthened at the center.The sw irling energy of dust devil comes from the swirling energy of hot-bubble and a part of potential energy.The direction of swirl of dustdevil (clockwise or anticlockwise) accords with the direction of swirl of hot-bubble.For dust devils,the tangential velocity profile behaves like Rankine vortex,so it could bereg arded as a kind of Rankine-like vortex.The study indicated that dust devils have four stages as the swirl ratio is increased from zero:weak vortex stage,developing stage,developed stage and vanishing stage.The fine structure of dustdevil in the developed stage could be roughly divided into four zones:the inflow surface layer near the surface,the corner flow region,the core flow region,and the outer flow region.Alot of sand and dust can be lifted by the upward airflow with high velocity,and the tracks of sand particles are different with different diameters.