报告题目:Granular dynamics: opportunities and challenges (special seminar)

报  告  人:Dr. Weitao Sun (ZHOU PEI-YUAN Center for Applied Mathematics)

报告时间:Tuesday, May 28, 2019, 10:30am

报告地点:蒙民伟科技南楼S727

报告摘要:Granular matter, which is made of macroscopic particles, has been thought as the second most ubiquitous substance following water. The granular material can range in size from sands at micrometer to asteroids at kilometer. The most amazing characteristics is that it can form a hybrid state between fluid and solid. Unlike conventional fluid and solid, granular matter shows rich interesting dynamic behaviors, including size segregation and vortex structure formation. Some of the phenomena are believed to be associated with the dynamics of Saturn’s dense rings and the mantle convection inside a planet like the Earth. Despite granular dynamics play substantially important roles in many scientific field, the mechanism remains poorly understood. Although hydrodynamic models have been successfully applied to rapid granular flows, it may not be sufficiently general to accurately predict remarkable difference between grains and fluids behaviors, especially when the mean free path of the particle collisions is comparable to or smaller than the particle size.

In this talk, I reported the existence of a quasi-symmetric segregation-convection pattern in tightly filled granular sphere and disks rotating around a horizontal axis in experiments. The convection patterns are different from conventional fluid in that the vortex roll is segregated into alternating small-large particle bands, from inner core to the periphery. Amazingly, there exists mass transportation among the vortex rolls, which instead of destroying but maintain the segregated vortex structure. The global segregation-convection structure comes into a stable state after a quick initialization stage (a few hours). While local scale grain migration and mass circulation persists for long-term evolution (from days to a few weeks). What’s more striking is that two clear radial/tangential flux gaps emerge at a distance of about 1/3 and 0.88 radius from the disk center, which separates the granular disk into concentric rings. We realize that the formation of crust-mantle-core structure inside a planet like the Earth and the newly discovered ringlets in Saturn’s ring are in no ways as simple as the formation of gaps in a rotating granular sphere. However, the analogy is so strong that it naturally suggests a very interesting question: can vortexes and gaps be spontaneously generated in a rotating granular sphere/disk with the size of planet? This calls for a unified granular dynamics theory which would be applicable both in dilute particulate rings and in densely packed particle system.