The production of uniformly sheared streams by means of double gauzes in wind tunnels: a mathematical analysis

The methods which have been used to design gauzes for the production of uniformly sheared streams in wind tunnels are analysed. The analysis of the results which have been obtained with single gauzes show that the poor theoretical evaluations of the shear rate that have been reported in the literature seem to be related to the different empirical expressions which have been assumed for the lift coefficient. This analysis indicates that the empirical relation established by Dryden and Schubauer (1947) gives satisfactory results, as long as mean mean angle of inclination of the gauze is taken into account in evaluating the resistance coefficient. The theoretical analysis of these flows shows that streams with “high” shear rates can only be obtained with single gauzes when “high” drag screens are used. This has the disadvantage of producing large scale turbulent motion downstream from the gauze. As an alternative to this single gauze approach, and to the semi-empirical technique used by Woo et al. (1981) of building a double gauze with non-uniform porosity, a theoretical analysis of the flow through double gauzes of uniform porosity leads to a useful method of designing gauzes for specific shear rates of high intensity. The method was used in the design of a double gauze that produced a stream with a non-dimensional shear rate of 1.73 (Woo et al. obtained 1.8), which is quite uniform and shows close agreement with the theoretical value. The results of this analysis indicate that the most appropriate method of producing high shear rates with low turbulence intensities is to use a set of several gauzes of low resistance. Modifications to the theoretical expressions obtained that take into account the presence of the additional gauzes might be a valuable improvement in the designing of gauzes for highly sheared streams with low turbulence intensities.     

High-speed laser cutting of superposed thermoplastic films: thermal modeling and process characterization

Common thermoplastic films used in the packaging industry have a thickness lower than 100 μm, and present low absorption to CO₂ laser radiation. This characteristic renders the use of cutting parameters, predicted by models developed for thicker thermoplastics inappropriate. In addition, the usual procedures involve the use of an assisting gas, responsible for removing the melted material, which, when processing thin films, induces changes in position in the material. A new theoretical model describing the temperature distribution on thin thermoplastic material during laser cutting was later developed. The heat conduction was solved analytically by the Green function method and heating and cooling thermal stress evolution was taken into consideration. The laser beam diameter over the samples provides the possibility of obtaining two cut operations: a simple cut, on beam focus, and a cut with welding, defocusing the beam. Engineering parameters predicted by the model were applied to cutting superposed high- and low-density polyethylene and polypropylene samples, transparent and white, with thicknesses between 10 and 100 μm, and experimentally validated.Proper modeling and the introduction of a reflective substrate under the samples allowed the improvement of process efficiency and the achievement of cutting operations up to 20 m s⁻¹, and cut with welding up to 14 m s⁻¹; an order of magnitude of improvement on industrial speeds previously attained for this operation.     

Finite volume computation of the turbulent flow over a hill employing 2D or 3D non-orthogonal collocated grid systems

A general numerical method for the solution of the complete Reynolds-averaged Navier-Stokes equations for 2D or 3D flows is described. The method uses non-orthogonal co-ordinates, Cartesian velocity components and a pressure-velocity-coupling algorithm adequate for non-staggered grid systems. The capability of the method and the overall performance of the κ–? eddy viscosity model are demonstrated by calculations of 2D and 3D flow over a hill. Solution error estimations based on fine grids, e.g. 320 × 192 control volumes, together with comparisons with standard turbulence model modifications, low-Reynoldsnumber or streamline curvature effects, have allowed the investigation of model drawbacks in predicting turbulent flows over surface-mounted hills.     

Electroosmotic Phenomena in Fractures

Electroosmotic phenomena in fractures have been investigated in the linear limit for various double-layer thicknesses. The effects of the geometrical parameters were systematically studied for deterministic sinusoidal and random self-affine fractures. The numerical results show a definite influence of the surface amplitude on electroosmotic processes. For self-affine fractures, the roughness or Hurst exponent has a much larger incidence than the correlation parameter between the two surfaces. All the electroosmotic coupling coefficients can be gathered in a single relationship which depends on a characteristic length scale Lambda, applicable to every configuration. Such a length was already found useful for porous media, but the relationship is different. Copyright 2000 Academic Press.     

Studies under Continuous Irradiation of Photochromic Spiro[fluorenopyran‐thioxanthenes]

New indeno‐fused spiro[benzopyran‐thioxanthenes] were synthesized (see 3a – d in Scheme 3) and their photochromic properties evaluated under continuous irradiation (Table 1). When submitted to irradiation for several minutes with a Xe lamp, the system behaved as one constituted by two open colored forms with different thermal bleaching rates and different susceptibilities to degradation. An increase in irradiation time led to significant degradation and to the apparent predominance of the open colored form with the faster bleaching rate.     

Bromine,arsenic, cadmium,and lead in several key food groups: an assessment of relative risk

ABSTRACT

This study endeavoured to provide an overview of the bromine, Br, arsenic (As, a metalloid) and metal (cadmium, Cd, and lead, Pb) relative risk associated with the consumption of relevant food groups (lean fish, fatty fish, bivalves, cephalopods, fresh fruit, dried fruit, and green vegetables) in Portugal. Though As, Cd, and Pb have been frequently studied, a comprehensive overview applying the same state-of-the-art methodology to a wide array of foods is missing. Besides, there is a large deficit of content information and risk assessment studies on Br. Thus, it is important to apply a mathematically realistic and innovative methodology (extreme value theory) to recent and accurate databases for the aforementioned food groups and elements.

Best fits to Br, As (total), Cd, and Pb concentrations were attained through different functions, ranging from the normal distribution to the Weibull distribution. A semi-probabilistic risk assessment approach on the basis of four alternative scenarios (one monthly meal; one weekly meal; three weekly meals; one daily meal) showed low relative risk regarding Br and Pb in the selected food groups, including bivalves, which contained the highest Br and Pb contents. With respect to total As, high relative risk values were calculated for seafood, but it should be emphasized that this As is overwhelmingly organic, thus presenting low risk. For Cd, substantial relative risk associated with bivalves’ and cephalopods’ consumption, particularly with one or more weekly meal(s), was found. However, the current study enables us to compare between the relative risk of the studied food groups and scenarios, and other studies encompassing the whole diet and following long-term dietary patterns are needed for the assessment of absolute risk.     

The important role of crystallinity and amylose ratio in thermal stability of starches

Journal of Thermal Analysis and Calorimetry - The thermal features of potato, banana, corn and cassava starches were correlated with structural properties. These starches were characterized by SEM,...