Modeling the evolution of natural cliffs subject to weathering. 2, Discrete element approach

The evolution of slopes subjected to weathering has been modeled by assuming Mohr-Coulomb behavior and by using a numerical approach based on the discrete element method (DEM). According to this method, soil and/or rock are represented by an assembly of bonded particles. Particle bonds are subject to progressive weakening, and so the material weathering and removal processes are modeled. Slope instability and material movement follow the decrease of material strength in space and time with the only assumption concerning the weathering distribution within the slope. First, the case of cliffs subject to strong erosion (weathering-limited conditions) and uniform weathering was studied to compare the results of the DEM approach with the limit analysis approach. Second, transport-limited slopes subject to nonuniform slope weathering were studied. Results have been compared with experimental data and other geomorphologic models from the literature (Fisher-Lehmann and Bakker–Le Heux). The flux of material from the slope is modeled assuming degradation both in space and time

Faculty Productivity in Supervising Doctoral Students’ Dissertations at Cornell University

Excerpt] Economists and academic administrators have long been concerned with issues of faculty productivity. For example, sets of studies have addressed whether faculty research productivity is related to faculty salaries, whether gender differences in faculty salaries remain after one controls for research productivity, and whether a negative association between faculty salary and seniority at an institution is due to universities having monopsony power or due to declining faculty research productivity with seniority. To take another example, concern that the ending of mandatory retirement, which became effective for tenured faculty in January 1994, would lead to an aging nonproductive faculty has led other researchers to examine how faculty research and teaching productivity, the latter measured by undergraduate student evaluations, have varied over the life cycle. More recently, researchers studied whether declining research productivity is related to the acceptance of an offer for an early retirement incentive. Finally, other researchers have looked at how faculty research productivity varies across cohorts, finding that when a scientist enters the labor market has a substantial effect on his or her productivity over the life cycle and that more recently educated cohorts are not necessarily more productive than earlier cohorts. While some studies have looked at the implicit role that PhD student production has on the quality rankings of PhD programs, to our knowledge no studies have focused on how the distribution of PhD student supervisory responsibilities varies across faculty members at a university. Our study uses data on all PhDs produced during a 7-year period at Cornell University to illustrate how researchers can study whether the degree of inequality in PhD student supervision across faculty members within a broad field of study, varies across fields, as well as what the determinants are of differences in PhD student supervision responsibilities across individual faculty members within each broad field. Of particular concern to us, given the elimination of mandatory retirement, is how faculty members’ productivity in the supervision of PhD students varies over their life cycles

Mesh sensitivity in discrete element simulation of flexible protection structures

The Discrete Element Method (DEM) has been employed in recent years to simulate flexible protection structures undergoing dynamic loading due to its inherent aptitude for dealing with inertial effects and large deformations. The individual structural elements are discretized with an arbitrary number of discrete elements, connected by spring-like remote interactions. In this work, we implement this approach using the parallel bond contact model and compare the numerical results at different discretization intervals with the analytical solutions of classical beam theory. Successively, we use the same model to simulate the punching test of a steel wire mesh and quantify the influence of a different number of elements on the macroscopic response

Video vehicle detection at signalised junctions: a simulation-based study

Many existing advanced methods of traffic signal control depend on information about approaching traffic provided by inductive loop detectors at particular points in the road. But analysis of images from CCTV cameras can in principle provide more comprehensive information about traffic approaching and passing through junctions, and cameras may be easier to install and maintain than loop detectors, and some systems based on video detection have already been in use for some time. Against this background, computer simulation has been used to explore the potential of existing and immediately foreseeable capability in automatic on-line image analysis to extract information relevant to signal control from images provided by cameras mounted in acceptable positions at signal-controlled junctions. Some consequences of extracting relevant information in different ways were investigated in the context of an existing detailed simulation model of vehicular traffic moving through junctions under traffic-responsive signal control, and the development of one basic and one advanced algorithm for traffic-responsive control. The work was confined as a first step to operation of one very simple signalcontrolled junction. Two techniques for extraction of information from images were modelled - a more ambitious technique based on distinguishing most of the individual vehicles visible to the camera, and a more modest technique requiring only that the presence of vehicles in any part of the image be distinguished from the background scene. In the latter case, statistical modelling was used to estimate the number of vehicles corresponding to any single area of the image that represents vehicles rather than background. At the simple modelled junction, each technique of extraction enabled each of the algorithms for traffic-responsive control of the signals to achieve average delays per vehicle appreciably lower than those given by System D control, and possibly competitive with those that MOVA would give, but comparison with MOVA was beyond the scope of the initial study. These results of simulation indicate that image analysis of CCTV pictures should be able to provide sufficient information in practice for traffic-responsive control that is competitive with existing techniques. Ways in which the work could be taken further were discussed with practitioners, but have not yet been progressed

Bistability and instability of dark-antidark solitons in the cubic-quintic nonlinear Schroedinger equation

We characterize the full family of soliton solutions sitting over a background plane wave and ruled by the cubic-quintic nonlinear Schroedinger equation in the regime where a quintic focusing term represents a saturation of the cubic defocusing nonlinearity. We discuss existence and properties of solitons in terms of catastrophe theory and fully characterize bistability and instabilities of the dark-antidark pairs, revealing new mechanisms of decay of antidark solitons.Comment: 8 pages, 10 figures, accepted in PR

Tracing a relativistic Milky Way within the RAMOD measurement protocol

Advancement in astronomical observations and technical instrumentation implies taking into account the general relativistic effects due the gravitational fields encountered by the light while propagating from the star to the observer. Therefore, data exploitation for Gaia-like space astrometric mission (ESA, launch 2013) requires a fully relativistic interpretation of the inverse ray-tracing problem, namely the development of a highly accurate astrometric models in accordance with the geometrical environment affecting light propagation itself and the precepts of the theory of measurement. This could open a new rendition of the stellar distances and proper motions, or even an alternative detection perspective of many subtle relativistic effects suffered by light while it is propagating and subsequently recorded in the physical measurements.Comment: Proceeding for "Relativity and Gravitation, 100 Years after Einstein in Prague" to be published by Edition Open Access, revised versio

Testing dark matter and geometry sustained circular velocities in the Milky Way with Gaia DR2

Flat rotation curves in disk galaxies represent the main evidence for large amounts of surrounding dark matter. Despite of the difficulty in identifying the dark matter contribution to the total mass density in our Galaxy, stellar kinematics, as tracer of gravitational potential, is the most reliable observable for gauging different matter components. This work tests the flatness of the MW rotation curve with a simple general relativistic model suitable to represent the geometry of a disk as a stationary axisymmetric dust metric at a sufficiently large distance from a central body. Circular velocities of unprecedented accuracy were derived from the Gaia DR2 data for a carefully selected sample of disk stars. We then fit these velocities to both the classical, i.e. including a dark matter halo, rotation curve model and a relativistic analogue, as derived form the solution of Einstein's equation. The GR-compliant MW rotational curve model results statistically indistinguishable from its state-of-the-art DM analogue. This supports our ansatz that a stationary and axisymmetric galaxy-scale metric could "fill the gap" in a baryons-only Milky Way, suggestive of star orbits dragged along the background geometry. We confirmed that geometry is a manifestation of gravity according to the Einstein theory, in particular the weak gravitational effect due to the off-diagonal term of the metric could mimic for a "DM-like" effect in the observed flatness of the MW rotation curve. In the context of Local Cosmology, our findings are suggestive of a Galaxy phase-space as the exterior gravitational field of a Kerr-like source (inner rotating bulge) without the need of extra-matter.Comment: Acknowledgments and references updated; 18 pages, 2 figures, improved version after referee's comment

A computational toy model for shallow landslides: Molecular Dynamics approach

The aim of this paper is to propose a 2D computational algorithm for modeling of the trigger and the propagation of shallow landslides caused by rainfall. We used a Molecular Dynamics (MD) inspired model, similar to discrete element method (DEM), that is suitable to model granular material and to observe the trajectory of single particle, so to identify its dynamical properties. We consider that the triggering of shallow landslides is caused by the decrease of the static friction along the sliding surface due to water infiltration by rainfall. Thence the triggering is caused by two following conditions: (a) a threshold speed of the particles and (b) a condition on the static friction, between particles and slope surface, based on the Mohr-Coulomb failure criterion. The latter static condition is used in the geotechnical model to estimate the possibility of landslide triggering. Finally the interaction force between particles is defined trough a potential that, in the absence of experimental data, we have modeled as the Lennard-Jones 2-1 potential. In the model the viscosity is also introduced and for a large range of values of the model's parameters, we observe a characteristic velocity pattern, with acceleration increments, typical of real landslides. The results of simulations are quite promising: the energy and the time triggering distributions of local avalanches shows a power law distribution, analogous to the observed Gutenberg-Richter and Omori power law distributions for earthquakes. Finally it is possible to apply the method of the inverse surface displacement velocity [Fukuzono 1985] for predicting the failure time

Rest frame optical properties of Lyman-alpha emitters from the HETDEX survey

HETDEX is a blind spectroscopic survey that is using use 78 fiber based integral field units to search for Lyman Alpha Emitting galaxies (LAEs) in the high redshift universe. HETDEX will discover about half a million LAEs to measure cosmological parameters of the young universe. An important ingredient to this is the precise quantification of the velocity offsets of the Lyα emission form their corresponding host systems. The latter can be measured from other optical emission lines. Here we present the results from VLT/KMOS near-infrared spectroscopic follow-ups of 8 Lyman-alpha emitters (LAEs) at z = 2.1 - 2.5 in the COSMOS field discovered by HETDEX. Observations are performed in the HK band. For these eight LEAs we detect rest-frame optical nebular lines Hα, [OIII]λλ4960,5008, for three of them we detect Hβ and for one [NII]λ6585. For non detected lines we measure a 1σ upper limit. We derive LAEs physical properties, including the Lyα velocity offset, star formation rate (SFR), gas-phase metallicity. Seven LAEs show a velocity shifts of Lyα relative to the systemic redshift ranging between +126 and +367 km/s with an average of +281 km/s. By matching KMOS3D and HETDEX catalogs we measure velocity offsets for 2 more LAEs finding a mean velocity shift of +247 km/s. The velocity offsets we measure from these two independent samples are compatible to Song et al. (2014). The Lyα velocity offsets show a moderate correlation with the measured SFR. We show that Lyα radiative transfer effects influence the correlation function measurements at s < 10 Mpc/h