Estimating the poroelastic properties of cracked materials

We present a comparative study of the ability of some micromechanics estimates to predict the overall properties of heterogeneous materials. We focus mainly on cracked materials, for which this task is difficult and many estimates fail. We study particularly the interaction direct derivative estimate, proposed by Zheng and Du, which is an approximation of the generalized self-consistent scheme, but has the very convenient property to be always explicit. A modified version of this estimate, called full-range IDD by Zheng and Du, yields good results when comparing all poromechanical coefficients predicted by the estimate to finite element simulations of a 2D cracked material in plane strain, up to crack density factors of 1 for aligned cracks and 0.60 for randomly oriented cracks. The accuracy of finite element computations of the overall moduli is also commented by plotting the convergence of the average of the properties as well as the confidence intervals on these averages.     

Collapses and Watersheds in Pseudomanifolds of Arbitrary Dimension

This work is settled in the framework of abstract simplicial complexes. We propose a definition of a watershed and of a collapse (i.e., a homotopic retraction) for maps defined on pseudomanifolds of arbitrary dimension. Then, we establish two important results linking watersheds and homotopy. The first one generalizes a property known for distance transforms in a continuous setting to any map on pseudomanifolds: a watershed of any map is a subset of an ultimate collapse of the support of this map. The second result establishes, through an equivalence theorem, a deep link between watershed and collapse of maps: any watershed of any map can be straightforwardly obtained from an ultimate collapse of this map, and conversely any ultimate collapse of the initial map straightforwardly induces a watershed.     

Central generation of grooming motor patterns and interlimb coordination in locusts

Coordinated bursts of leg motoneuron activity were evoked in locusts with deefferented legs by tactile stimulation of sites that evoke grooming behavior. This suggests that insect thoracic ganglia contain central pattern generators for directed leg movements. Motoneuron recordings were made from metathoracic and mesothoracic nerves, after eliminating all leg motor innervation, as well as all input from the brain, subesophageal ganglion, and prothoracic ganglion. Strong, brief trochanteral levator motoneuron bursts occurred, together with silence of the slow and fast trochanteral depressor motoneurons and activation of the common inhibitor motoneuron. The metathoracic slow tibial extensor motoneuron was active in a pattern distinct from its activity during walking or during rhythms evoked by the muscarinic agonist pilocarpine. Preparations in which the metathoracic ganglion was isolated from all other ganglia could still produce fictive motor patterns in response to tactile stimulation of metathoracic locations. Bursts of trochanteral levator and depressor motoneurons were clearly coordinated between the left and right metathoracic hemiganglia and also between the mesothoracic and the ipsilateral metathoracic ganglia. These data provide clear evidence for centrally generated interlimb coordination in an insect.     

Diffusion of dextran in concentrated solutions

A free-diffusion method has been developed for the determination of the intradiffusion coefficient ('self-diffusion coefficient') of a polymer in highly concentrated solutions. A fraction of the polymer is labelled with a small amount of light-absorbing substituent. The diffusion of this labelled species, present in low concentration, is followed in the presence of a high concentration of unlabelled material with the aid of absorption optics in the analytical ultracentrifuge. The diffusion proceeds over a boundary at which the difference in concentration of unlabelled material is varied. The average concentration of total polymer and the concentration of the labelled material are, however, constant. From theoretical considerations it is shown that by extrapolation of the diffusion coefficient so obtained to zero concentration difference of total material, the intradiffusion coefficient of the polymer at that concentration is obtained. The procedure also permits the ordinary translational diffusion coefficient to be estimated. The method has been applied to two dextran fractions with weight-average molecular weights of 19000 and 150000, which were labelled with fluorescein groups. As expected, the intradiffusion coefficient decreases with increasing polymer concentration, the decrease being more pronounced for the high-molecular-weight material. This decrease in the diffusion rate of dextran is, however, less than the corresponding decrease in the sedimentation rate which proteins with similar hydrodynamic parameters experience in dextran solutions. This agrees with the hypothesis that flexible linear polymers move through a network as chains rather than as hydrodynamic spheres. By combining measurements of the ordinary diffusion coefficient and the intradiffusion coefficient, it is possible to calculate the thermodynamic properties (as expressed by the virial expansion) of the system. This method is of particular importance in studies on concentrated solutions of high-molecular-weight polymers.     

A prototype approach to information system design and development

Many organisations view the development of an information processing system as a linear process consisting of an ordered sequence of steps from conception to hand-over and make no provision for alternative strategies. While excellent in many circumstances, the linear strategy is not the only possible approach. This paper discusses alternatives, with particular reference to a prototype strategy, which has some important advantages.     

A physically-based computer model of aggregate electric waterheating loads

Electric water heaters have been the focus of several previous studies because of their pervasiveness in power systems and their consequent potential importance when considering conservation through more efficient design and operation of the heaters. Also, because such devices are associated with an energy storage capability, they are often considered within load management by direct device control programs. Finally, they tend to be responsible for persistent system load transients in a cold load pickup situation. Understanding of the above issues can be greatly enhanced with the availability of a computer model of aggregate electric water heating loads. A physically-based such model is presented and its dynamic properties are investigated via numerical simulation under various operating conditions and parameter configurations. The results are analyzed in the paper     

Partial demineralization of maple sap by electrodialysis: impact on syrup sensory and physicochemical characteristics

In this project, samples of osmosed maple saps were demineralized to 12.5 and 25% levels by electrodialysis (ED). The effect of this treatment on the composition and the physicochemical and sensory properties of maple syrups obtained from demineralized maple sap was evaluated. The ED technology was efficient to decrease levels of malic acid and calcium in osmosed maple saps. Effectively, 38% and 24% decreases in malic acid and calcium respectively were reached for ED with a demineralization level of 25% without any changes in the other measured components of osmosed maple saps. The demineralization process had no effect on the yield of syrups produced and on their characteristics: no significant difference was observed during sensory analysis and viscosity. Moreover, the percentage of light transmission of syrups produced from demineralized osmosed saps was higher than for the control. This work suggests that ED could be a potential technology to decrease or avoid sugar sand formation during maple syrup production. Copyright © 2007 Society of Chemical Industry     

Tracking and MAP reconstruction of line scratches in degraded motion pictures

A suitable detection and reconstruction approach is proposed for removing line scratches from degraded motion picture films. The detection procedure consists of two steps. First, a simple 1D-extrema detector provides line scratch candidates. Unlike impulsive distortions, which appear randomly in an image, line artifacts persist across several frames. Furthermore, the detection process is complicated by the fact that lines occur as a natural part in interesting scenes. Therefore, we add a validation step for separating possible line defects from false detections. It consists in tracking the potential line artifacts over the frames using a Kalman filter. A new Bayesian restoration technique, dealing with both low and high frequencies around and inside the detected deteriorations, is investigated to achieve a nearly invisible reconstruction of damaged areas. Accepted: 13 August 2001