Source field superposition analysis of two-dimensional potential problems

The source field superposition method for problems governed by Laplace's equation involves representing the potential field in a given solution domain as a superposition of fields generated by a number of sources. These sources are located outside the solution domain, and in the case of two-dimensional singly-connected finite domain problems can be uniformly distributed around the perimeter of a circle enclosing the physical domain. Some test problems are used to make detailed comparisons with the boundary element method. The results show that remarkable accuracy can be achieved, often to five or six significant figures, with very little computational effort relative to other numerical methods. In contrast to the boundary element method, however, geometric corners require no special treatment, and high aspect ratio of the solution domain is not a significant limitation.     

Surging in screw extruders

Output uniformity is one of the main factors limiting the maximum output obtainable from a single screw plasticating extruder, and is adversely affected by surging. Several causes of surging have been identified, perhaps the most important being instabilities in the melting process. These are caused by periodic break-up of the bed of compacted solid polymer formed in the screw channel. Solid bed break-up is shown, both experimentally and theoretically, to be associated with rapid acceleration of the bed in the downstream direction parallel to the screw flight. A novel method of measuring solid bed velocity and hence acceleration is described. The theoretical model of the melting process is shown to be capable of predicting this acceleration reliably, and therefore the tendency for a particular combination of screw design, material and operating conditions to cause surging.     

Stress singularities in composite materials with an arbitrarily oriented crack meeting an interface

The plane, bending and antiplane strain elastostatic problems for two bonded half planes containing an arbitrarily oriented semi-infinite crack meeting the interface, are examined. A simple analogy between the interfacial continuity conditions for the plane and bending problems is derived and its implications are discussed. Using the eigenfunction-expansion method the crack tip stress singularities for various crack orientations and material combinations are compared for the three problems. It is shown that, under certain conditions, singularities are no longer present in the stress field. In the case of the antiplane strain problem subsequent paths of crack propagation are predicted.

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Résumé On examine des problèmes élasto-statiques en déformation plane, de flexion, et antiplanaire rencontrés dans deux demi-plans solidaires comportant, dans leur interface, une fissure semi-infinie orientée arbitrairement. On tire une analogie simple entre les conditions de continuité et interfaciales qui se présentent dans le cas des problèmes plans et flexionnels; ses implications sont discutées. En utilisant une méthode d'expansion d'une eigenfonction, on peut comparer les singularités de contraintes aux extrémités de la fissure, pour diverses orientations de la fissure et de combinaisons de matériaux différents. On montre que, sous certaines conditions, le champ de contraintes peut ne plus présenter de singularités. On peut par ailleurs prédire les chemins de la propagation d'une fissure dans le cas d'une déformation antiplanaire.

     

The resistivity of mixed valence compound Cs2Au2Cl6 at high pressure and low temperatures

Resistivity measurements of the mixed valence compound Cs₂Au^IAu^IIICl₆ at quasihydrostatic pressures up to ～120 kbar and temperatures down to ～10 K show a resistivity drop of about 9 decades and a semiconductor to metal transition at ～60 kbar. The results will be discussed, especially with respect to a possible continuous transition from the tetragonal distorted to the cubic perovskite structure.     

The Effectiveness of the Land-Drainage Consent System

ABSTRACT This paper presents the results of a study into the effectiveness of the land-drainage consents system. The investigation studied eighteen main river reaches, each 2 km in length, in the north-east Thames region. All works within the main channel and the 8 m margin, subject to the regulations in the Water Resources Act 1991, were recorded. The impact that each of these works might have had upon the environment, including the ability of the river to pass the peak flow and the potential for an increase in pollution, has been assessed and an overall impact rating for each reach has been determined. Each of the works identified on the reaches has been cross-referenced with consent records held by the National Rivers Authority. Results that indicate that a high proportion (92%) of works do not have consent under the present system.     

Bromide therapy in refractory canine idiopathic epilepsy

On a retrospective basis, the response to adding chronic oral bromide (BR) to phenobarbital (PB) administration in 23 refractory canine idiopathic epileptics between 1986 and 1991 was studied. The mean age for an observed first seizure was 24 months (range 7 to 72) for all dogs. Thirteen (57%) dogs were males with no breed predisposition observed. All dogs were diagnosed as having idiopathic epilepsy based on normal metabolic and neurologic diagnostic evaluations. Dogs were evaluated before BR therapy for a mean time of 22 months (range 5 to 75 months). Seventeen dogs (74%) received multiple antiepileptic drugs (AEDs) before BR therapy. All animals were maintained on PB at least 4 months before the onset of BR therapy, with a mean trough serum concentration of 37.8 mcg/mL and no improvement in seizure severity or recurrence. Twelve dogs presented with generalized isolated seizures and 11 with generalized cluster seizures (two or more seizures within 24 hours) as their first seizure. The effects of BR therapy were evaluated for a mean time of 15 months (range 4 to 33), with 17 dogs (74%) followed for 12 or more months. The mean BR serum concentration for the 0 to 4 months time period was 117 mg/dL compared with 161 mg/dL for the greater than 4 months period. Overall, response to BR therapy was associated with a reduction in the total number of seizures in 83% of the dogs when compared with their respective pre-BR period. For those followed for 1 year after BR, there was a 53% reduction in the number of seizures compared with the previous 12 months. Furthermore, owners reported a decrease in seizure intensity (65% of dogs) and change to a less severe seizure type (22% of dogs) in those dogs that continued to have seizures. Seizure-free status was obtained in 26% of the dogs with protection continuing up to 31 months in one dog. No correlations could be determined between response to BR and either age of onset of the first seizure or interval from the first AED therapy to BR therapy. Adverse effects of concomitant BR and PB therapy were polydipsia (56% of dogs), polyphagia (30% of dogs), excessive sedation (30% of dogs), and generalized ataxia (17% of dogs). As a result of BR treatment, the PB dosage was reduced in eight dogs (35%). In conclusion, concomitant BR and PB was well tolerated in dogs of this study and was effective in treating refractory canine idiopathic epilepsy, regardless of prior interval of seizure activity or previous treatment.