Radiosity for dynamic scenes in flatland with the visibility complex

The radiosity method is particularly suitable for global illumination calculations in static environments. Nonetheless, recent applications of image synthesis such as architectural simulation or lighting design require the ability to modify environments. Previous methods have attempted to deal with dynamic environments (environments where the geometry, the material properties, etc., can change)but still suffer some limitations in the case of moving objects. One of the main problems remaining is the efficient and accurate detection of which form factors must really be recomputed, since their calculation is the most time-consuming part of the radiosity method. To correctly understand and solve this problem, we start with a method in 2D for polygonal scenes using the visibility complex. It is a powerful data structure representing the visibility relationships between objects in the plane. We have developed and implemented an algorithm which uses this structure to efficiently compute the discontinuity mesh and the form factors for static scenes. We also propose an extension to our algorithm to efficiently update only the modified form factors when an object is moving. This approach enhances our understanding and will hopefully lead to efficient solutions in 3D.     

Requirements capture and IS methodologies

Abstract. The problem of not using a methodological approach during the development process is well known and its implications for organizations clearly identified. This paper reports on a survey of information systems practice carried out to provide recent evidence about the use of development methodologies by different organizations. Data from 72 projects was obtained through a mail survey within the UK. The focus of the analysis is on the discovery of potential relationships between the use of methodologies for requirements capture and analysis and the characteristics of the projects     

轻便X荧光仪上应用特/散法的探讨

本文针对轻便X射线荧光仪,探讨了特/散法的校正机理,并主要结合地质粉末样品测量,讨论了方法的应用条件及有关影响因素。     

Environmental change in river channels: a neglected element. Towards geomorphological typologies,standards and monitoring

Rivers integrate the impacts of change in atmospheric and terrestrial systems; they then deliver these to the coast. En route geomorphological processes create dynamic and diverse habitats, both in-stream and in riparian/floodplain ecotones. The dynamics of channel change conflict with human resource development, the outcome is that many river and riparian environments have been significantly modified, complicating the interpretation of change. Collection of geomorphological data on both form and process has to date been overwhelmingly an academic pursuit; standard measurement networks and long-term monitoring have, as a result been largely absent-as in the Environmental Change Network (ECN), despite the emerging requirements of legislation such as the EU Water Framework Directive. In this paper, we utilise a unique set of repeat channel surveys and long-term bed-load sediment yields to provide guidance on both definitions of change and those variables and survey techniques which might form the basis, in future, of improved national-scale monitoring. The Environment Agency's River Habitat Surveys suggest the basis for channel typologies that could structure a sampling framework and rationalise the variables to be monitored. We also point to the value of more detailed geomorphological procedures in use at the catchment/project scale-Catchment Baseline Surveys and Fluvial Audits-as a standardised basis for monitoring the detail of change in the fluvial sediment system. A perfect opportunity to lay foundations for such monitoring activity has been provided in England and Wales by the winter floods of 2000/2001.     

Mode II intersonic crack propagation in poroelastic media

A crack is steadily running in an elastic isotropic fluid-saturated porous solid at an intersonic constant speed c. The crack tip speeds of interest are bounded below by the slower between the slow longitudinal wave-speed and the shear wave-speed, and above by the fast longitudinal wave-speed. Biot’s theory of poroelasticity with inertia forces governs the motion of the mixture. The poroelastic moduli depend on the porosity, and the complete range of porosities n ∈ [0, 1] is investigated. Solids are obtained as the limit case n = 0, and the continuity of the energy release rate as the porosity vanishes is addressed. Three characteristic regions in the plane (n, c) are delineated, depending on the relative order of the body wave-speeds. Mode II loading conditions are considered, with a permeable crack surface. Cracks with and without process zones are envisaged. In each region, the analytical solution to a Riemann–Hilbert problem provides the stress, pore pressure and velocity fields near the tip of the crack. For subsonic propagation, the asymptotic crack tip fields are known to be continuous in the body [Loret and Radi (2001) J Mech Phys Solids 49(5):995–1020]. In contrast, for intersonic crack propagation without a process zone, the asymptotic stress and pore pressure might display a discontinuity across two or four symmetric rays emanating from the moving crack tip. Under Mode II loading condition, the singularity exponent for energetically admissible tip speeds turns out to be weaker than 1/2, except at a special point and along special curves of the (n, c)-plane. The introduction of a finite length process zone is required so that 1. the energy release rate at the crack tip is strictly positive and finite; 2. the relative sliding of the crack surfaces has the same direction as the applied loading. The presence of the process zone is shown to wipe out possible first order discontinuities.     

An overview of a hybrid crack element and determination of its complete displacement field

The hybrid crack element (HCE) is one of the most accurate and convenient finite elements for the direct calculation of the stress intensity factor (SIF) and coefficients of the higher order terms of the Williams expansion. It is formulated from a simplified variational functional using truncated asymptotic crack tip displacement and stress expansions and interelement boundary displacements compatible with the surrounding regular elements. However, the exclusion of the rigid body modes in the truncated asymptotic displacements creates jumps between these displacements and element compatible boundary displacements. In this study, an overview of the HCE is given. Furthermore, the rigid body modes excluded in its formulation are recovered by minimizing the jumps via a least squares method. Limitations of the boundary collocation method (BCM) widely used for predicting these terms, as well as the complete displacements are also investigated.     

Embedded strong discontinuity finite elements for fractured geomaterials with variable friction

The strong discontinuity approach to modelling strain localization, combined with an enhanced strain element, has been used for more than a decade to model strain localization in materials including geomaterials. Most implementations of enhanced strain elements in the post-localization regime use very simple constitutive formulations along the discontinuity, such as linear softening or a constant friction coefficient. However, the softening relations can be much more complex for geomaterials. For rocks this softening is induced by micro-fractures coalescing into macroscopic cracks during a narrow time interval called ‘slip weakening.’ During this interval the cohesive resistance on the nucleating crack decays to zero while the frictional resistance increases. Furthermore, research has shown that the coefficient of friction for these materials is not constant, but in fact is a function both of the slip speed and the state of the material, including wear, temperature, and other factors. In this paper we augment the modelling capabilities of an enhanced strain element by incorporating a cohesive softening law and a popular rate- and state-dependent friction model commonly used for describing the constitutive properties of rocks and rock-like materials sliding along the fractured surface. Copyright © 2007 John Wiley & Sons, Ltd.     

Application of distance sampling techniques for diving ducks on Lake St. Clair and western Lake Erie

Lake St. Clair and western Lake Erie are important migration staging areas for diving ducks including canvasbacks (Aythya valisineria), redheads (Aythya americana), and lesser and greater scaup (Aythya affinis and Aythya marila). Starting in 1983, the Michigan Department of Natural Resources (MDNR) attempted to census diving ducks on the United States portion of Lake St. Clair throughout autumn migration; however, in 2010 the MDNR expanded the traditionally surveyed area to include all of Lake St. Clair and a portion of western Lake Erie. The idea of achieving a census over the expanded study area was unrealistic, and instead distance sampling techniques were adopted in an effort to generate statistically valid estimates of detection probabilities and abundances for diving ducks during spring and autumn migration. We found distance sampling techniques to be a viable option for estimating diving duck abundance as long as flock size is accounted for as a covariate affecting the detection function. Diving ducks were generally more abundant on our study area during autumn migration with a mean of 306,327 ducks/survey (SE = 40,729) compared to an average spring abundance of 91,053 ducks/survey (SE = 19,175). Peak abundance occurred on 20 November 2012 with an estimated 596,335 diving ducks on Lake St. Clair and western Lake Erie. Ultimately, our methodology could be used to establish long-term, standardized data collection techniques and applied to conservation planning for waterfowl in the Great Lakes region.     

River wildlife databases and their value for sensitive environmental management

A survey over a 3-year period, which commenced in 1987, of the plant and bird communities along 1200 km of river in Lincolnshire and South Humberside, Eastern England, is described. Analysis of data from 247 km of river recorded a great diversity of wildlife, comprising 570 species of plant and 80 species of breeding birds. The management value of such surveys is discussed and these include the preparation of detailed maps of each 500m section of river, which illustrate the habitat structure, and plant and bird communities. A maintenance procedure for each 500 m section of river is then agreed between conservation and operations personnel, preventing areas of wildlife importance from being accidentally destroyed. An index which combines both the diversity and rarity of the plant and bird communities is proposed as a measure of conservation importance. This index allows sections of river to be graded according to their conservation value into one of five groups. The surveys also enable predictions to be made of the effects of management actions on the wildlife and provide information by which fundamental reviews of operational activities can be carried out.     

The effect of stream regulation on the physico-chemical properties of the palmiet river,South Africa

The effect of regulation, by multiple impoundment, on ‘discontinuity’ (recovery) ‘distances’ (sensu Ward and Stanford, 1983) for various physico-chemical attributes of the Palmiet River (a short, steep gradient, cool temperate, acid, low-nutrient system) was investigated. Physico-chemical features were measured monthly, from February 1986 to August 1987, at 11 sites along the river, but concentrating on two zones: one below an upper-reach impoundment, Nuweberg Dam, and one below a middle-reach impoundment, Arieskraal Dam. Median total suspended solids, nitrates, nitrites and ammonium all increased below both dams with varying levels of ‘recovery’. Median spot temperatures, pH and alkalinity increased below Nuweberg, but decreased below Arieskraal Dam, whereas the converse was true for soluble reactive phosphate. Nuweberg had no effect on conductivity and total dissolved solids, while Arieskraal caused a slight decrease. The river remained well oxygenated down its length. Distinct seasonal variations were recorded for: flow rate, with flow reversal below Nuweberg and flow constancy below Arieskraal; temperature, with seasonal shifts and dampening of annual temperature range below both dams; alkalinity, with seasonal shifts below Arieskraal, and nitrites and nitrates, but not for any other attributes. At least partial recovery was noted in all cases, but in some instances, recovery was incomplete to the estuary. The implications of such alterations, for invertebrates, in terms of reduction in habitat diversity as well as for food supply must be considerable and studies are currently under way to determine community structure and diversity.