Bloqueio, cristalização, deformação e fluidização em meios granulares

Author(s)

Fernando David Cuñez Benalcazar

• Document type: Doctoral Thesis
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Faculdade de Engenharia Mecânica
• Defense date: 2021-06-09

Examining board members

Erick de Moraes Franklin; Douglas J Jerolmack; Rafael Gabler Gontijo; Luben Cabezas Gómez; Carlos Eduardo Keutenedjian Mady

• Advisor: Erick de Moraes Franklin

Abstract

This Ph.D. thesis evaluates the behavior of granular materials when the jamming transition begins in two different applications: narrow fluidized beds and the onset of sediment transport, by using laboratory experiments and numerical simulations. In the case of narrow fluidized beds, which are a suspension of grains submitted to an ascending fluid flow and whose ratio between the tube and particles diameter is less than 10, there is a spatial and temporal evolution of different patterns, crystallization and jamming, that emerge due to high confinement effects. For a monodisperse distribution of grains, the results show that it is possible to observe the formation of granular plugs that propagate with characteristic lengths and celerities at a given fluid velocity; however, if the fluid velocity is decreased with a given deceleration, grains tend to crystalize and to be jammed even for fluid velocities higher than those for incipient fluidization, supposed to maintain the grains fluidized. For a bidisperse distribution of grains, solid particles coexist and segregation by diameter and density occurs, leading eventually to a layer inversion inside the granular bed. Finally, the high confinement effects can be intensified or modified if some grains are bonded together. The results provide, for the first time, new insights into the length scales and celerities of the granular plugs, fluidization conditions, crystallization, and jamming in this type of fluidized beds. For horizontal flows, on the onset of sediment transport, the propagation of an "unjamming" front, which marks the boundary between the bed-load and the creeping regimes, is examined. Repeated shear cycles give rise to an aging effect, as the bed-load layer thins over progressive cycles to an eventual steady state. The results show that the aging results from two effects: an isotropic component due to compaction, and an anisotropic component connected to grain fabric structures aligned to resist the applied shear. This is demonstrated by reversing the direction of shear, where the bed recovers some of its initial mobility. Beyond a critical shear rate, grains are suspended, resulting in dilation and the destruction of anisotropic granular fabrics. The results explain puzzling observations of bed-load hysteresis effects seen in natural rivers and delineate the transport regimes under which memory is created and destroyed in sedimentary beds (AU)

• FAPESP's process: 16/18189-0 - Liquid-fluidized beds in narrow pipes: dynamics and instabilities
• Grantee: Fernando David Cúñez Benalcázar
• Support Opportunities: Scholarships in Brazil - Doctorate