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[en] The response to a localized perturbation of an excitable medium under stirring by chaotic advection is investigated. It is found that below a critical stirring rate a localized perturbation produces a coherent global excitation of the system. For very slow stirring, however, the coherence of the global excitation is gradually lost. We propose a simple model to describe the effect of the flow on the excitable dynamics, and explain the observed behavior as a consequence of a steady excited filament state found in the reduced problem
[en] The stirring of a body of viscous fluid using multiple stirring rods is known to be particularly effective when the rods trace out a path corresponding to a nontrivial mathematical braid. The optimal braid is the so-called 'pigtail braid', in which three stirring rods execute the usual 'over-under' motion associated with braiding (plaiting) hair. We show how to achieve this optimal braiding motion straightforwardly: one stirring rod is driven in a figure-of-eight motion, while the other two rods are baffles, which rotate episodically about their common centre. We also explore the extent to which the physical baffles may be replaced by flow structures (such as periodic islands).
[en] The direct observation of a thermoelectric magnetohydrodynamic (TEMHD) flow has been achieved and is reported here. The origin of the flow is identified based on a series of qualitative tests and corresponds, quantitatively, with a swirling flow TEMHD model. A theory for determining the dominant driver of a free-surface flow, TEMHD or thermocapillary (TC), is found to be consistent with the experimental results. The use of the analytical form for an open geometry develops a new dimensionless parameter describing the ratio of TEMHD to TC generated flows.
[en] Full-scale mixing and solid suspension studies have been conducted to determine the optimum agitator design for precipitators used in plutonium processing. Design considerations include the geometry of precipitator vessels, feed locations, flow patterns, and product requirements. Evaluations of various agitator designs are based on their capabilities: (1) to achieve uniform mixing of reactants in minimum time, (2) to suspend the slurry uniformly throughout the vessel, and (3) to minimize power consumption without inducing air entrainment. Tests of full-scale agitator designs showed that significant improvements in mixing, solid suspension, and energy consumption were achieved
[en] In this paper, it is analysed the way in which the thermodynamic results obtained in an isothermal titration microcalorimeter, with continuous injection of a liquid and a variable mass are affected. Two aspects are presented, the first one refers to a variation in the baseline that takes place when the liquid mass is increased in the laboratory cell, this variation is due to the change in the thermal coupling between the stirrer and its contact with the thermostat. The second aspect is the analysis of the effect of the stirring process and the injection velocity on the homogenization of the remaining mixture in the laboratory cell. The aim of the study of these two previous aspects is to increase the accuracy of the thermodynamic measures carried out. The variation of the baseline affects in a percentage from 1 to 2% of the total energy developed. It is also shown the necessity of a stirring process according to the injection velocity in order to obtain a homogeneous mixture at every instant. This fact allows to carry out a deconvolution of the calorimetric signal and to obtain directly the power developed in the mixture process in terms of the amount of mixed substance [Meas. Sci. Technol. 1 (1990) pp. 1285-1290; J. Thermal Analysis 41 (1994) pp. 1385-1392]
[en] The estimate of free surface velocity at the billet mold in magnetohydrodynamic calculation is important for steel surface quality. It is considered that the free surface velocity in the electromagnetic stirring operation is transferred through the wall, a boundary of the molten steel. Then the wall friction phenomena between the wall and the molten steel are modeled so as a wall friction, force is a function of wall velocity. The calculated velocity distribution in consideration of the wall friction in which the friction coefficient is estimated as 0.03 is in good agreement with the measured value. [copyright] 2001 American Institute of Physics
[en] In this paper, some preliminary results achieved in the evaluation of the homogenization time at a vacuum pan for massecuite b and seed preparation , using two approaches of the radiotracer method, are presented. Practically no difference between the on line, using small size detector and the sampling methods, in mixing studies performed at the high-grade massecuite was detected. Results achieved during the trials performed at the vacuum station show that the mechanical agitation in comparison with normal agitation improves the performance of mixing in high-grade massecuite b and in seed preparation at the vacuum pan
[en] Early, yet still often-cited, mathematical models for electromagnetic stirring (EMS) in continuous casting are re-examined and found to contain a surprising anomaly: the solutions obtained were not unique. Analysis for the case of a round billet under rotary EMS shows how to avoid this behavior, whilst still making use of the experimental data that motivated the original models. The relevance of this result for current-day modeling of EMS is highlighted, particularly in the context of modulated EMS.
[en] To improve the procedure for electrochemical niobium polishing, it is suggested to use pressurization of electrolytic bath and pumping of liberated hydro.oen, that prevents electrolyte inflow and retards the process of electropolishing and retval of noxious vapors of sulphuric and hydrofluoric acids. It is also suggested to change mechanical mixing of the solution for dry nitrogen bubbling, which increases the electrolyte lifetime 3-4 times and accelerates the process of electropolishing. The quality of surface treated according to the new procedure is satisfactory
[en] A cultured network of neurons coupled with a multi-electrode-array (MEA) recording system has been a useful platform for investigating various issues in neuroscience and engineering. The neural activity supported by the system can be sensitive to environmental fluctuations, for example, in the medium’s nutrient composition, ph, and temperature, and to mechanical disturbances, yet this issue has not been the subject. Especially, a normal practice in maintaining neuronal cell cultures involves an intermittent sequence of medium exchanges, typically at a time interval of a few days, and one such sudden medium exchange is unavoidably accompanied by many unintended disturbances. Here, based on a quantitative time-series analysis of synchronized bursting events, we explicitly demonstrate that such a medium exchange can, indeed, bring a huge change in the existing neural activity. Subsequently, we develop a medium perfusion-stirring system and an ideal protocol that can be used in conjunction with a MEA recording system, providing long-term stability. Specifically, we systematically evaluate the effects of medium stirring and perfusion rates. Unexpectedly, even some vigorous mechanical agitations do not have any impacts on neural activity. On the other hand, too much replenishment (e.g., 1.8 ml/day for a 1.8-ml dish) of neurobasal medium results in an excitotoxicity.