Tesis doctoral de Roberto Arévalo Turnes
Microscopic dynamics in silo discharge and force networks in a rigid granular medium. the microscopic and macroscopic dynamics of granular media in some typical geometries have been analyzed by means of soft particle molecular dynamics simulations. The silo discharge by gravity is an example of widely studied topic in which, however, many questions remain unanswered. In this work, the flow as a function of the exit orifice have been analyzed showing that accepted laws in the literature are valid only for large orifices, while deviations appear for small apertures. The analysis of flow fluctuations reveals a change of behavior for large and small orifices: fluctuations are gaussian for the former and non-gaussian with fat tails for the latter. This change of behavior is related to the formation of arches that render the flow intermittent. Based in the existence of arches, a modification of the flow law is proposed that is able to fit the data in both the region of large and small orifices. the arches that appear in granular packing are further analyzed generating configurations with different densities and coordination numbers. This is achieved by a tapping process that is applied to a packing until a stationary state is observed. The analysis of stationary configurations reveals that the packing undergoes a transition from an ordered state at low excitation to a disordered state at high tapping. The transition is smooth showing a gradual decrease of density and coordination number as excitation is increased, at the same time the number of arches in the system grows. Furthermore, arches also reflect the order/disorder transition: arches sizes follow an exponential distribution in the ordered system and a gaussian in the disordered state. the analysis of structural properties in granular media is extended using the concepts of the field of complex networks. The system chosen for this study is an isotropically compressed granular medium which is known to undergo a transition of critical character from a liquid-like to a solid-like rigid state. From the granular packing a network is defined such that every grain with at least one contact is a node and contacts become edges. The networks are analyzed as a function of a parameter fu such that only contacts bearing a for larger than fu are edges. The study carried out reveals that the topological properties of the force network are robust against system size, polidispersity of the grains, friction coefficient and geometry of the compression cell. An important result is that the contact network can be divided into to subnetworks with different structure and function: a linear subnetwork composed of large forces and a polygonal subnetwork composed of small forces. The linear subnetwork constitutes the response of the system to external loads, while the polygonal subnetwork gives stability to the linear chains.
Datos académicos de la tesis doctoral «Dinámica microscópica en la descarga de silos y redes de fuerza en un medio granular rígido«
- Título de la tesis: Dinámica microscópica en la descarga de silos y redes de fuerza en un medio granular rígido
- Autor: Roberto Arévalo Turnes
- Universidad: Navarra
- Fecha de lectura de la tesis: 27/02/2009
Dirección y tribunal
- Director de la tesis
- Diego Martín Maza Ozcoidi
- Tribunal
- Presidente del tribunal: vicente Garzo puertos
- ignacio Pagonabarraga mora (vocal)
- raúl Cruz hidalgo (vocal)
- Mª del carmen Miguel lópez (vocal)
