First normal stress difference and crystallization in a dense sheared granular fluid

The first normal stress difference (${\mathcal N}_1$) and the microstructure in a dense sheared granular fluid of smooth inelastic hard-disks are probed using event-driven simulations. While the anisotropy in the second moment of fluctuation velocity, which is a Burnett-order effect, is known to be the progenitor of normal stress differences in {\it dilute} granular fluids, we show here that the collisional anisotropies are responsible for the normal stress behaviour in the {\it dense} limit. As in the elastic hard-sphere fluids, ${\mathcal N}_1$ remains {\it positive} (if the stress is defined in the {\it compressive} sense) for dilute and moderately dense flows, but becomes {\it negative} above a critical density, depending on the restitution coefficient. This sign-reversal of ${\mathcal N}_1$ occurs due to the {\it microstructural} reorganization of the particles, which can be correlated with a preferred value of the {\it average} collision angle $\theta_{av}=\pi/4 \pm \pi/2$ in the direction opposing the shear. We also report on the shear-induced {\it crystal}-formation, signalling the onset of fluid-solid coexistence in dense granular fluids. Different approaches to take into account the normal stress differences are discussed in the framework of the relaxation-type rheological models.Comment: 21 pages, 13 figure

Alluvial fans, landslides and Late Quaternary climatic change in the wet tropics of northeast Queensland

Extensive alluvial‐fan and debris‐flow deposits occur along the base of the escarpment of the east Australian highlands in the wet tropics of northeast Queensland. Luminescence and radiocarbon dating show that these deposits accumulated between 27 ka and 14 ka, which was the driest phase of climate during the last full glacial cycle. Climatic desiccation and reduced plant cover, along with a continuation of discrete high‐magnitude rainfall events, were the principle causes of this phase of enhanced slope instability. Landslide activity and alluvial‐fan development have continued throughout the Holocene, but probably to a lesser extent and magnitude because of the amelioration of climate and the re‐establishment of forests throughout the region

Long-term natural variability of tropical cyclones in Australia

Numerous late Holocene records of tropical cyclones have been collected from tropical northern Australia. They are in the form of multiple shore parallel sedimentary ridges deposited over the past 6,000 years and an 800 year long annual resolution oxygen isotope record from a calcium carbonate cave stalagmite. The sedimentary ridges are composed of coral fragments, or shell and sand or pure sand. Numerical models relating surge height and tropical cyclone central pressure were used to determine the intensity of the tropical cyclone responsible for deposition of the ridges at each site. The results suggest that in the majority of cases these features were deposited by very high magnitude events. The results suggest that extrapolation from short instrumental and historical records, which is the method commonly used to assess risk from this hazard, substantially underestimate the risk from this hazard. This is confirmed for the Cairns region by the 800 year long high resolution isotope record which suggests that tropical cyclone activity in northeast Queensland has been in a phase of quiescence since before European settlement of the region in approximately AD 1870. Comparisons between the short and long-term records suggest that non-stationarity may be an inherent feature of the long-term natural variability of tropical cyclones in this region