Double-layer capacitance and polarization potential of baked carbon anodes in cryolite-alumina melts

Baked carbon anodes with varying apparent densities and baking temperatures were tested in Na₃AlF₆–Al₂O_3(sat) melts at 1010° C. The double-layer capacitance (C _dl) was used as an indicator of the wetted surface area. For unpolarized anodes,C _dl increased with increasing time of immersion and reached a constant level after 1.5–2h. The values decreased with increasing polarization potential in the range 1–1.5 V positive to aluminium. TheC _dl of polished samples increased markedly during electrolysis, particularly at low current densities. No clear correlation was found betweenC _dl and apparent density. Semi-logarithmic plots of potential versus current could be divided into three segments. The lower two were linear, the ranges and slopes being 0.01–0.1 A cm^–2, 0.20–0.44 V per decade and 0.1–0.5 A cm^–2, 0.18–0.24 V per decade, respectively. At higher current densities the curves bent upwards. The current density corresponding to an overpotential of 0.5 V increased slightly with increasing apparent density, whereas the ohmic voltage drop. at constant current density decreased. The current densities were corrected for differences in wetted surface area on the basis of theC _dl data. The change in baking temperature from 970 to 1100°C had no appreciable effect on the overpotential, whereas samples baked at 1250°C showed a somewhat lower overpotential.     

Shape complexity

The complexity of 3D shapes that are represented in digital form and processed in CAD/CAM/CAE, entertainment, biomedical, and other applications has increased considerably. Much research was focused on coping with or on reducing shape complexity. However, what exactly is shape complexity? We discuss several complexity measures and the corresponding complexity reduction techniques. Algebraic complexity measures the degree of polynomials needed to represent the shape exactly in its implicit or parametric form. Topological complexity measures the number of handles and components or the existence of non-manifold singularities, non-regularized components, holes or self-intersections. Morphological complexity measures smoothness and feature size. Combinatorial complexity measures the vertex count in polygonal meshes. Representational complexity measures the footprint and ease-of-use of a data structure, or the storage size of a compressed model. The latter three vary as a function of accuracy.     

Finally Everyone Can Work With Highly Complex 3D Models

In the past, access to 3D databases was restricted to few specialists having the appropriate CAD skills, software, and graphics hardware. The availability of inexpensive graphics support on personal computers, the Internet’s impact on private and commercial communication, and the emergence of multimedia standards provide the basis for linking CAD databases with other personal productivity and communication tools and for making them accessible to everyone at home, in schools, in hospitals, or in the industry. For example, employees that have no design expertise, customers, and suppliers would benefit from having an easy access to the 3D databases of a company for: collaborative design review, 3D-based multi-media problem reports, collaborative problem solving and tracking, online training and documentation, internet-based part purchasing and subcontracting, demonstration to customers, or advertising. This presentation will address three of the key issues that have so far limited the non-specialist’s access to 3D databases. First-time or occasional non-expert users need to become instant experts in 3D navigation through Virtual Environments or in the interactive manipulation of digital 3D models, so that they may immediately focus on their tasks, and not waste precious time learning and fighting an unnatural user interface. Immersive VR is not the panacea – other more effective techniques show promise. The data complexity found in commercial CAD databases, especially in the automotive, aerospace, and construction industries, significantly exceeds the capabilities of any interactive graphics system. This situation is not likely to change, since the growth of the complexity and availability of 3D models outpaces the performance improvement of personal computers. Research on the automatic simplification of 3D models and on the use of levels of detail to accelerate the rendering of distant portions of the scene is growing rapidly. The still limited bandwidth of internet communication channels prohibits a pervasive access to large amounts of 3D data. Recent 3D compression techniques reduce the storage requirements for polyhedral 3D models by two orders of magnitudes.     

Cost optimal and nearly zero (nZEB) energy performance calculations for residential buildings with REHVA definition for nZEB national implementation

This study determined cost optimal and nearly zero energy building (nZEB) energy performance levels following the REHVA definition and energy calculation methodology for nZEB national implementation. Cost optimal performance levels – meaning the energy performance leading to minimum life cycle cost – were calculated with net present value method according to the cost optimal draft regulation. The seven-step procedure was developed to conduct cost optimal and nZEB energy performance levels calculations in systematic and robust scientific fashion. It was shown that cost optimal primary energy use can be calculated with limited number of energy simulations as only four construction concepts were simulated and cost calculated. The procedure includes the specification of building envelope components based on specific heat loss coefficient and systems calculation with post processing of energy simulation results, without the need to use iterative approach or optimization algorithm. Model calculations were conducted for Estonian reference detached house to analyse the difference between the cost optimal and nZEB energy performance levels. Cost optimal energy performance level of Estonian reference detached house was 110 kW h/(m² a) primary energy including all energy use with domestic appliances and it was significantly lower than the current minimum requirement of 180 kW h/(m² a).     

OCTOR: Subset selection in recursive pattern hierarchies

We present an approach for encoding and specifying selections in recursive pattern hierarchies. Hierarchies of patterns of features or other modeling entities are used in architectural and mechanical CAD to eliminate laborious repetitions from the design process. Yet, often a subset of the repeated occurrences in the model must be edited. Specifying a desired selection is often tedious and difficult. Our approach addresses these two drawbacks simultaneously, offering an effective and intuitive solution which requires only two mouse-clicks to specify any one of a wide range of selections, including selections in recursive hierarchies. The selection has a concise representation that is simple to compute.     

Shared Memory Programming in Metacomputing Environments: The Global Array Approach

The performance of the Global Array shared-memory nonuniform memory-access programming model is explored in a wide-area-network (WAN) distributed supercomputer environment. The Global Array model is extended by introducing a concept of mirrored arrays that thanks to the caching and user-controlled consistency of the shared data structure scan reduce the application sensitivity to the network latency. Latencies and bandwidths for remote memory access are studied, and the performance of a large application from computational chemistry is evaluated using both fully distributed and also mirrored arrays. Excellent performance can be obtained with mirroring if even modest (0.5 MB/s) network bandwidth is available.     

Optimizing the topological and combinatorial complexity of isosurfaces

Since the publication of the original Marching Cubes algorithm, numerous variations have been proposed for guaranteeing water-tight constructions of triangulated approximations of isosurfaces. Most approaches divide the 3D space into cubes that each occupy the space between eight neighboring samples of a regular lattice. The portion of the isosurface inside a cube may be computed independently of what happens in the other cubes, provided that the constructions for each pair of neighboring cubes agree along their common face. The portion of the isosurface associated with a cube may consist of one or more connected components, which we call sheets. The topology and combinatorial complexity of the isosurface is influenced by three types of decisions made during its construction: (1) how to connect the four intersection points on each ambiguous face, (2) how to form interpolating sheets for cubes with more than one loop, and (3) how to triangulate each sheet. To determine topological properties, it is only relevant whether the samples are inside or outside the object, and not their precise value, if there is one. Previously reported techniques make these decisions based on local—per cube—criteria, often using precomputed look-up tables or simple construction rules. Instead, we propose global strategies for optimizing several topological and combinatorial measures of the isosurfaces: triangle count, genus, and number of shells. We describe efficient implementations of these optimizations and the auxiliary data structures developed to support them.