Comparison of Molecular Dynamics and Binary Collision Approximation Simulations for Atom Displacement Analysis

Molecular dynamics (MD) and binary collision approximation (BCA) computer simulations are employed to study surface damage by single ion impacts. The predictions of BCA and MD simulations of displacement cascades in amorphous and crystalline silicon and BCC tungsten by $1 keV Ar^+$ ion bombardment are compared. Single ion impacts are studied at angles of $50^{\circ}, 60^{\circ} and 80^{\circ}$ from normal incidence. Four parameters for BCA simulations have been optimized to obtain the best agreement of the results with MD. For the conditions reported here, BCA agrees with MD simulation results at displacements larger than $5 \mathring{A}$ for amorphous Si, whereas at small displacements a difference between BCA and MD arises due to a material flow component observed in MD simulations but absent from a regular BCA approach due to the algorithm limitations. MD and BCA simulation results for crystalline W are found to be in a good agreement even at small displacements, while in crystalline Si there is some difference due to displacements in amorphous pockets.Engineering and Applied Science

Large scale flood risk mapping in data scarce environments: An application for Romania

Large-scale flood risk assessment is essential in supporting national and global policies, emergency operations and land-use management. The present study proposes a cost-efficient method for the large-scale mapping of direct economic flood damage in data-scarce environments. The proposed framework consists of three main stages: (i) deriving a water depth map through a geomorphic method based on a supervised linear binary classification; (ii) generating an exposure land-use map developed from multi-spectral Landsat 8 satellite images using a machine-learning classification algorithm; and (iii) performing a flood damage assessment using a GIS tool, based on the vulnerability (depth-damage) curves method. The proposed integrated method was applied over the entire country of Romania (including minor order basins) for a 100-year return time at 30-m resolution. The results showed how the description of flood risk may especially benefit from the ability of the proposed cost-efficient model to carry out large-scale analyses in data-scarce environments. This approach may help in performing and updating risk assessments and management, taking into account the temporal and spatial changes in hazard, exposure, and vulnerability

Protest blues : public opinion on the policing of protest in South Africa

Abstract: The policing response to rising protest action in the country has received increased attention in the last decade. This is particularly due to concerns over confrontations during which protestors have been arrested, injured and in some instances killed by the police. Despite the criticism voiced by various stakeholders about the manner in which the police manage crowd gatherings, relatively little is known about the views of South African adults on the policing of protest action and the factors that shape such attitudes. To provide some insight, the study presented in this article draws on data from a specialized module on protest‐related attitudes and behaviour that was fielded as part of the 2016 round of the Human Sciences Research Council’s South African Social Attitudes Survey (SASAS) series. This nationally representative survey included specific questions probing the public’s overall evaluation of the performance of the police in dealing with protests and the justifiability of the use of force in policing protest action. The paper will present the national pattern of results based on these measures and then determine the extent to which there exist distinct underlying socio‐demographic cleavages. A combination of bivariate and multivariate analysis will also be undertaken to provide an understanding of the role of the perceived effectiveness, acceptability and reported participation in protest (especially disruptive and violent actions) in shaping views regarding policing protest. The article concludes with a discussion that reflects on the implications of the research for the policing of protest action in future, taking into account the appreciable rise in the incidence of protest since the mid‐2000s as well as the mounting tensions between state institutions and communities over the political, moral and constitutional arguments for and against such actions

Argon cluster impacts on layered silicon, silica, and graphite surfaces

Seven structures of covalently bonded materials are used as targets of 6 keV Ar12 cluster bombardment in classical molecular dynamics simulations. Energy deposition, cratering and Ar ranges are compared and remarkable differences are found between the structures. In particular, bombardment of a thin 2 nm silica layer on top of the Si(111) surface is shown to behave quite differently from bombardment of pure Si

Optimization of large amorphous silicon and silica structures for molecular dynamics simulations of energetic impacts

A practical method to create optimized amorphous silicon and silica structures for molecular dynamics simulations is developed and tested. The method is based on the Wooten, Winer, and Weaire algorithm and combination of small optimized blocks to larger structures. The method makes possible to perform simulations of either very large cluster hypervelocity impacts on amorphous targets or small displacements induced by low energy ion impacts in silicon.Engineering and Applied Science

Crater functions for compound materials: A route to parameter estimation in coupled-PDE models of ion bombardment

During the ion bombardment of targets containing multiple component species, highly-ordered arrays of nanostructures are sometimes observed. Models incorporating coupled partial differential equations, describing both morphological and chemical evolution, seem to offer the most promise of explaining these observations. However, these models contain many unknown parameters, which must satisfy specific conditions in order to explain observed behavior. The lack of knowledge of these parameters is therefore an important barrier to the comparison of theory with experiment. Here, by adapting the recent theory of “crater functions” to the case of binary materials, we develop a generic framework in which many of the parameters of such models can be estimated using the results of molecular dynamics simulations. As a demonstration, we apply our framework to the recent theory of Bradley and Shipman, for the case of Ar-irradiated GaSb, in which ordered patterns were first observed. In contrast to the requirements therein that sputtered atoms form the dominant component of the collision cascade, and that preferential redistribution play an important stabilizing role, we find instead that the redistributed atoms dominate the collision cascade, and that preferential redistribution appears negligible. Hence, the actual estimated parameters for this system do not seem to satisfy the requirements imposed by current theory, motivating the consideration of other potential pattern-forming mechanisms.Engineering and Applied Science

Safer_RAIN: A DEM-based hierarchical filling-&-spilling algorithm for pluvial flood hazard assessment and mapping across large urban areas

The increase in frequency and intensity of extreme precipitation events caused by the changing climate (e.g., cloudbursts, rainstorms, heavy rainfall, hail, heavy snow), combined with the high population density and concentration of assets, makes urban areas particularly vulnerable to pluvial flooding. Hence, assessing their vulnerability under current and future climate scenarios is of paramount importance. Detailed hydrologic-hydraulic numerical modeling is resource intensive and therefore scarcely suitable for performing consistent hazard assessments across large urban settlements. Given the steadily increasing availability of LiDAR (Light Detection And Ranging) high-resolution DEMs (Digital Elevation Models), several studies highlighted the potential of fast-processing DEM-based methods, such as the Hierarchical Filling-&-Spilling or Puddle-to-Puddle Dynamic Filling-&-Spilling Algorithms (abbreviated herein as HFSAs). We develop a fast-processing HFSA, named Safer_RAIN, that enables mapping of pluvial flooding in large urban areas by accounting for spatially distributed rainfall input and infiltration processes through a pixel-based Green-Ampt model. We present the first applications of the algorithm to two case studies in Northern Italy. Safer_RAIN output is compared against ground evidence and detailed output from a two-dimensional (2D) hydrologic and hydraulic numerical model (overall index of agreement between Safer_RAIN and 2D benchmark model: sensitivity and specificity up to 71% and 99%, respectively), highlighting potential and limitations of the proposed algorithm for identifying pluvial flood-hazard hotspots across large urban environments