Dilatancy in slow granular flows

When walking on wet sand, each footstep leaves behind a temporarily dry impression. This counterintuitive observation is the most common illustration of the Reynolds principle of dilatancy: that is, a granular packing tends to expand as it is deformed, therefore increasing the amount of porous space. Although widely called upon in areas such as soil mechanics and geotechnics, a deeper understanding of this principle is constrained by the lack of analytical tools to study this behavior. Using x-ray radiography, we track a broad variety of granular flow profiles and quantify their intrinsic dilatancy behavior. These measurements frame Reynolds dilatancy as a kinematic process. Closer inspection demonstrates, however, the practical importance of flow induced compaction which competes with dilatancy, leading more complex flow properties than expected

The Geometrical Structure of Disordered Sphere Packings

The three dimensional structure of large packings of monosized spheres with volume fractions ranging between 0.58 and 0.64 has been studied with X-ray Computed Tomography. We search for signatures of organization, we classify local arrangements and we explore the effects of local geometrical constrains on the global packing. This study is the largest and the most accurate empirical analysis of disordered packings at the grain-scale to date with over 140,000 sphere coordinates mapped. We discuss topological and geometrical ways to characterize and classify these systems, and discuss implications that local geometry can have on the mechanisms of formation of these amorphous structures.Comment: 15 pages; 16 figure

Local and Global relations between the number of contacts and density in monodisperse sphere packs

The topological structure resulting from the network of contacts between grains (\emph{contact network}) is studied for large samples of monosized spheres with densities (fraction of volume occupied by the spheres) ranging from 0.59 to 0.64. We retrieve the coordinates of each bead in the pack and we calculate the average coordination number by using three different methods. We show that, in the range of density investigated, the coordination number is larger than 4 and it increases with the packing fraction. At local level we also observe a positive correlation between local packing fraction and number of neighbors. We discover a dependence between the local densities of configurations with few neighbors in contact and the global sample-denities. This might indicate that local configurations with small number of neighbors are able to deform plastically when the sample is compactifying. PACS: 45.70.-n, Granular Systems; 45.70.Cc, Static sandpiles; Granular Compaction.Comment: 10 pages, 6 figure

Online social sports networks as crime facilitators

Emerging technologies such as broadband services and mobile and wireless technologies create not only benefits for the community but also risks (Choo, Smith & McCusker, 2007). The implications of these developments should be evaluated to make any necessary changes to policing, policy and legislation. This study investigates the risk of disclosure of confidential information via online public exercise routes. The study identified in particular whether a) people inadvertently disclose their home address more often indirectly via online sports tracking networks than directly via other means and whether b) gender and age play a role in this disclosure. In addition, an analysis of the temporal characteristics of runs was performed to establish the window of opportunity for a home burglary and whether running is temporally predictable by hour of day or day of week. A total of 513 RunKeeper users were selected from the Dutch cities of Enschede and Nijmegen. 231 runners (45.03%) were located via RunKeeper and 122 (23.78%) via other Internet (i.e. non-social sports network) sources. It was found that a statistical difference exists between the indirect and direct disclosure of addresses; more runners disclose their home address via online sports tracking networks than via other sources. Furthermore, it was found that age played a role in the direct disclosure of addresses but not in the indirect disclosure. Older users more often disclosed their home address directly than younger ones. Conversely, gender plays a role in the indirect disclosure but not in the direct disclosure. Men more often disclosed their home address indirectly than women. Regarding temporal characteristics, it was found that the window of opportunity for a burglary is approximately 1 hour. Furthermore, the `within subject' analysis suggests that the starting hour of the run is the most predictable temporal characteristic, followed by the duration of the run and the day of the week. This research ultimately shows the extent to which the unique combination of spatial and temporal information available in online sports tracking networks can enable criminals to predict where a potential target lives and when he or she will be out running

X-ray observation of micro-failures in granular piles approaching an avalanche

An X-ray imaging technique is used to probe the stability of 3-dimensional granular packs in a slowly rotating drum. Well before the surface reaches the avalanche angle, we observe intermittent plastic events associated with collective rearrangements of the grains located in the vicinity of the free surface. The energy released by these discrete events grows as the system approaches the avalanche threshold. By testing various preparation methods, we show that the pre-avalanche dynamics is not solely controlled by the difference between the free surface inclination and the avalanche angle. As a consequence, the measure of the pre-avalanche dynamics is unlikely to serve as a tool for predicting macroscopic avalanches