A Galerkin boundary element method for high frequency scattering by convex polygons

In this paper we consider the problem of time-harmonic acoustic scattering in two dimensions by convex polygons. Standard boundary or finite element methods for acoustic scattering problems have a computational cost that grows at least linearly as a function of the frequency of the incident wave. Here we present a novel Galerkin boundary element method, which uses an approximation space consisting of the products of plane waves with piecewise polynomials supported on a graded mesh, with smaller elements closer to the corners of the polygon. We prove that the best approximation from the approximation space requires a number of degrees of freedom to achieve a prescribed level of accuracy that grows only logarithmically as a function of the frequency. Numerical results demonstrate the same logarithmic dependence on the frequency for the Galerkin method solution. Our boundary element method is a discretization of a well-known second kind combined-layer-potential integral equation. We provide a proof that this equation and its adjoint are well-posed and equivalent to the boundary value problem in a Sobolev space setting for general Lipschitz domains.

Memory-based Embodied Cognition: a computational architecture

At its most fundamental, cognition as displayed by biological agents (such as humans) may be said to consist of the manipulation and utilisation of memory. Recent discussions in the field of cognitive robotics have emphasised the role of embodiment and the necessity of a value or motivation for autonomous behaviour. This work proposes a computational architecture – the Memory-Based Cognitive (MBC) architecture – based upon these considerations for the autonomous development of control of a simple mobile robot. This novel architecture will permit the exploration of theoretical issues in cognitive robotics and animal cognition. Furthermore, the biological inspiration of the architecture is anticipated to result in a mobile robot controller which displays adaptive behaviour in unknown environments.

Impact of mass lumping on gravity and Rossby waves in 2D finite-element shallow-water models

The goal of this study is to evaluate the effect of mass lumping on the dispersion properties of four finite-element velocity/surface-elevation pairs that are used to approximate the linear shallow-water equations. For each pair, the dispersion relation, obtained using the mass lumping technique, is computed and analysed for both gravity and Rossby waves. The dispersion relations are compared with those obtained for the consistent schemes (without lumping) and the continuous case. The P0-P1, RT0 and P-P1 pairs are shown to preserve good dispersive properties when the mass matrix is lumped. Test problems to simulate fast gravity and slow Rossby waves are in good agreement with the analytical results.

Stratospheric Memory and Skill of Extended-Range Weather Forecasts

We use an empirical statistical model to demonstrate significant skill in making extended-range forecasts of the monthly-mean Arctic Oscillation (AO). Forecast skill derives from persistent circulation anomalies in the lowermost stratosphere and is greatest during boreal winter. A comparison to the Southern Hemisphere provides evidence that both the time scale and predictability of the AO depend on the presence of persistent circulation anomalies just above the tropopause. These circulation anomalies most likely affect the troposphere through changes to waves in the upper troposphere, which induce surface pressure changes that correspond to the AO.

Trace element partitioning and soil particle characterisation around mining and smelting areas at Tharsis, Riotinto and Huelva, SW Spain

Trace elements may present an environmental hazard in the vicinity of mining and smelting activities. However, the factors controlling trace element distribution in soils around ancient and modem mining and smelting areas are not always clear. Tharsis, Riotinto and Huelva are located in the Iberian Pyrite Belt in SW Spain. Tharsis and Riotinto mines have been exploited since 2500 B.C., with intensive smelting taking place. Huelva, established in 1970 and using the Flash Furnace Outokumpu process, is currently one of the largest smelter in the world. Pyrite and chalcopyrite ore have been intensively smelted for Cu. However, unusually for smelters and mines of a similar size, the elevated trace element concentrations in soils were found to be restricted to the immediate vicinity of the mines and smelters, being found up to a maximum of 2 kin from the mines and smelters at Tharsis, Riotinto and Huelva. Trace element partitioning (over 2/3 of trace elements found in the residual immobile fraction of soils at Tharsis) and soil particles examination by SEM-EDX showed that trace elements were not adsorbed onto soil particles, but were included within the matrix of large trace element-rich Fe silicate slag particles (i.e. 1 min circle divide at least 1 wt.% As, Cu and Zn, and 2 wt.% Pb). Slag particle large size (I mm 0) was found to control the geographically restricted trace element distribution in soils at Tharsis, Riotinto and Huelva, since large heavy particles could not have been transported long distances. Distribution and partitioning indicated that impacts to the environment as a result of mining and smelting should remain minimal in the region. (c) 2006 Elsevier B.V. All rights reserved.

A preliminary investigation into mining and smelting impacts on trace element concentrations in the soils and vegetation around Tharsis, SW Spain

Toxic trace elements present an environmental hazard in the vicinity of mining and smelting activities. However. the processes of transfer of these elements to groundwater and to plants are not always clear. Tharsis mine. in the Iberian pyrite belt (SW Spain), has been exploited since 2500 BC, with extensive smelting, taking place front the 1850S until the 1920s. Sixty four soil (mainly topsoils) and vegetation samples were collected in February 2001 and analysed by ICP-AES for 23 elements. Concentrations are 6-6300 mg kg(-1) As and 14-24800 mg kg(-1) Pb in soils, and 0.20-9 mg kg(-1) As and 2-195 mg Pb in vegetation. Trace element concentrations decrease rapidly away from the mine. with As and Pb concentrations in the range 6-1850 mg kg(-1) (median 22 mg kg(-1)) and 14-31 mg, kg(-1) (median 43 mg, kg(-1)), respectively, 1 km away from the mine. These concentrations are low when compared to other well-studied mining and smelting areas (e.g. 600 mg kg(-1) As at 8 km from Yellowknife smelter, Canada; >100 mg kg(-1) Pb over 270 km(2) around the Pb-Zn Port Pirie smelter. South Australia: mean of 1419 mg kg(-1) Pb around Aberystwyth smelter, Wales, UK). The high metal content of the vegetation and the low soil pH (mean pH 4.93) indicate the potential for trace element mobility which Could explain the relatively low concentration of metals in Tharsis topsoils and cause threats to plans to redevelop the Tharsis area as an orange plantation.

The social life of ruins: sites of memory and the politics of a Zimbabwean periphery

This article explores conflicts over a series of ruins located within Zimbabwe's flagship National Park. The relics have long been regarded as sacred places by local African communities evicted from their vicinity, and have come to be seen as their ethnic heritage. Local intellectuals' promotion of this heritage was an important aspect of a defensive mobilization of cultural difference on the part of a marginalized minority group. I explore both indigenous and colonial ideas about the ruins, the different social movements with which they have been associated and the changing social life they have given the stone relics. Although African and European ideas sometimes came into violent confrontation - as in the context of colonial era evictions - there were also mutual influences in emergent ideas about tribe, heritage and history. The article engages with Pierre Nora's notion of 'sites of memory', which has usefully drawn attention to the way in which ideas of the past are rooted and reproduced in representations of particular places. But it criticizes Nora's tendency to romanticize pre-modern 'memory', suppress narrative and depoliticize traditional connections with the past. Thus, the article highlights the historicity of traditional means of relating to the past, highlighting the often bitter and divisive politics of traditional ritual, myth, kinship, descent and 'being first'. It also emphasizes the entanglement of modern and traditional ideas, inadequately captured by Nora's implied opposition between history and memory. (c) 2005 Elsevier Ltd. All rights reserved.

Trace element geochemistry from the Birrimian metasediments of the Northern Region of Ghana

Trace element distributions in rock, soil and groundwater from of the Birrimian metasediments and granites located in the Northern Region of Ghana are described. High positive correlations are observed between selected major elements and trace metals (e.g. K2O and Rb, Al2O3 and V, Fe2O3 and V, and K2O and Y) in rocks and soils, and attributed to the presence of major source minerals. Ca and Sr were strongly correlated in groundwater, suggesting greater water-rock interaction. Low association of V with Fe is explained by (i) relatively higher mobility of V as against Fe; (ii) low Fe content in the parent rocks and (iii) variable sources of Fe and V.

Changing state disruption of lip-reading by irrelevant sound in perceptual and memory tasks

Three experiments investigated irrelevant sound interference of lip-read lists. In Experiment 1, an acoustically changing sequence of nine irrelevant utterances was more disruptive to spoken immediate identification of lists of nine lip-read digits than nine repetitions of the same utterances (the changing-state effect; Jones, Madden, & Miles, 1992). Experiment 2 replicated this finding when lip-read items were sampled with replacement from the nine digits to form the lip-read lists. In Experiment 3, when the irrelevant sound was confined to the retention interval of a delayed recall task, a changing-state pattern of disruption also occurred. Results confirm a changing-state effect in memory for lip-read items but also point to the possibility that, for lip-reading, changing-state effects may occur at an earlier, perceptual stage.

Auditory memory and the irrelevant sound effect: further evidence for changing state disruption

Four experiments investigate the hypothesis that irrelevant sound interferes with serial recall of auditory items in the same fashion as with visually presented items. In Experiment 1 an acoustically changing sequence of 30 irrelevant utterances was more disruptive than 30 repetitions of the same utterance (the changing-state effect; Jones, Madden, & Miles, 1992) whether the to-be-remembered items were visually or auditorily presented. Experiment 2 showed that two different utterances spoken once (a heterogeneous compound suffix; LeCompte & Watkins, 1995) produced less disruption to serial recall than 15 repetitions of the same sequence. Disruption thus depends on the number of sounds in the irrelevant sequence. In Experiments 3a and 3b the number of different sounds, the "token-set" size (Tremblay & Jones, 1998), in an irrelevant sequence also influenced the magnitude of disruption in both irrelevant sound and compound suffix conditions. The results support the view that the disruption of memory for auditory items, like memory for visually presented items, is dependent on the number of different irrelevant sounds presented and the size of the set from which these sounds are taken. Theoretical implications are discussed.