Coupled axial-radial segregation in rotating drums with high fill levels

Axial segregation and radial segregation within the granular bed in a rotating drum with a fill level > 50% were experimentally investigated. The axial segregated patterns and the radial segregated patterns were coupled within the bed, resulting in observation of new segregated patterns at the plane along the rotational shaft. The segregated patterns are different from the previous axial band and radial core patterns and show coupled new patterns, including the "cross" patterns, the "band-core-in-core" patterns, and other geometric patterns. The new segregation patterns are strong functions of the fill level and the rotational speed of the drum. The formation of the axial-radial coupled patterns was attributed to the fill level, the free flowing surface of the bed. A virtual drum hypothesis is proposed to explain the formation of the complex segregated patterns.