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A theoretical approach to the study of self-diffusion properties of interacting colloidal particles in suspensions is developed. It is based on the assumption of a model for the memory function Ms(q,t), whose parameters are calculated by using exact short-time conditions for the self-diffusion propagator. Expressions, in terms of microscopic quantities of the system, for the time-dependent self-diffusion coefficient D(t) are derived from three different functional forms for Ms(q,t): a single-exponential function (already introduced in previous communications [J. L. Arauz-Lara and M. Medina-Noyola, J. Phys. A 19, L117 (1986); G. Nägele, M. Medina-Noyola, R. Klein, and J. L. Arauz-Lara, Physica A 149, 123 (1988)], an algebraic function, and a combination of the two. The latter one involves an adjustable parameter (the relative weight of the two modes) which can be determined by fitting the calculated values of D(t) to experimental data at long times. Explicit calculations of D(t) were carried out for systems of hard spheres interacting via a Derjaguin–Landau–Verwey–Overbeek type pair potential. Comparison of our results with Brownian dynamics data [K. J. Gaylor, I. K. Snook, W. van Megen, and R. O. Watts, J. Chem. Soc. Faraday Trans. 2 76, 1067 (1980)] shows that the bimodal model reproduces quite accurately the computer simulation data for D(t) with just one adjustable parameter, suggesting in this way that the actual functional form of the memory function may consist of a fast and a slow decaying mode. [ABSTRACT FROM AUTHOR]

A mixture of charged hard spheres (restricted primitive model) with an attractive Yukawa tail added to represent van der Waals forces between the ions is considered as a model of molten salts. The location of the critical point of the gas–liquid phase transition is followed as a function of the strength A of the attractive term. It is found that the critical density increases with increasing A, but that a quantitative agreement between the theoretical results and the available experimental estimates of the critical constants for alkali halides requires the use of an effective dielectric constant greater than unity. [ABSTRACT FROM AUTHOR]

The static structure and the time-dependent self-diffusion motion of interacting Brownian particles in a model two-dimensional suspension are discussed. For the static structure we report Brownian dynamics results assuming a hard disk plus Yukawa pair potential. The self-diffusion properties of this model system are calculated from two independent theoretical approaches. In order to assess the accuracy of the predictions of these two theories, we also performed Brownian dynamics calculations of the time-dependent self-diffusion coefficient for a wide range of values of both the particle concentration and the pair potential coupling constant. We find that both theories reproduce very well the main features exhibited by the Brownian dynamics data. Quantitatively, there are some discrepancies between both theoretical predictions and the Brownian dynamics results, which are negligible at moderate couplings, but become larger for strongly coupled systems and long times. © 1994 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The static structure of the two-dimensional hard-disk plus Yukawa fluid is studied on the basis of the hypernetted chain approximation. We find that the most relevant features exhibited by the three-dimensional hard-sphere plus Yukawa system are also exhibited by its two-dimensional counterpart. [ABSTRACT FROM AUTHOR]

Numerical modeling is a prediction tool useful for the design of brine discharge configurations and for environmental impact assessments, to prevent the potential impacts of brine in marine environments. Among the existing approaches of governing equations, dimensional analysis formulas and integration models are usually applied to brine discharge simulations, as is the case of the most used commercial tools for modeling brine discharges. Simulation with computational fluid mechanics (CFDs) advanced models has not yet been implemented and only some research studies are available in the literature. In order to improve the knowledge related to the use and degree of feasibility and reliability of these approaches for the modeling of brine discharges, an exhaustive investigation has been carried out, also including validation with high-quality experimental data. An analysis of the most used commercial models--CORMIX, VISUAL PLUMES, and VISJET--has been carried out focusing on brine discharges. As a result, Technical Specification Cards have been developed for each model (Cormix1, Cormix2, Corjet, UM3, and JetLag), including: theoretical basis, capabilities, limitations, sensitivity analysis, and degree of feasibility and reliability, based on its validation with published experimental data. Faced with the commercial model limitations, alternative MEDVSA simulation tools have been developed and are available on the MEDVSA webpage. They are simple models, based mainly on dimensional analysis and integration formulas, which model the behavior of brine in the near and far field region. Codes have been programmed in Matlab and include tools for modeling different discharge configurations and ambient scenarios. An improvement of the interface and results report and recalibration with new experimental data has been carried out in order to provide alternative higher-quality tools for the commercial models. The following tools have been developed and will be explained in this conference: MEDVSA-IJETG, MEDVSA-MJETS, MEDVSA-JET-SPREADING, MEDVSA-JET-PLUME2D, MEDVSA-PLUME2D, and MEDVSA-PLUMED3D. This global investigation on the numerical modeling of brine discharges has been carried out within the MEDVSA project. The R&D project (2009-2011), financed by the Spanish Ministry of the Environment and Rural and Marine Affairs, aims to develop and implement a methodology to reduce the environmental impact of brine discharges. The information, compiled into a methodological guide, and the numerical tools developed are available on the web site of the project (www.medvsa.es). [ABSTRACT FROM AUTHOR]

We assessed the cerebral blood flow velocity response to head-up tilt test in patients with typical neurocardiogenic syncope compared with patients showing postural tachycardia. Fifty patients (21 men) with history of orthostatic intolerance, younger than 50 years (mean 27±10), participated in the study. Transcranial Doppler sonography of the middle cerebral artery, heart rate and brachial blood pressure were recorded during a head-up tilt test. According to the outcome of the test, patients were categorized in two groups: neurocardiogenic syncope (29 patients) and postural tachycardia (21 patients). The clinical history of the two groups was similar. During baseline in the supine position, no differences in haemodynamic parameters were observed. From the first min of tilt, the heart rate was higher in patients with postural tachycardia than in patients with neurocardiogenic syncope. Although, during tilt, the absolute values of the cerebral blood flow parameters were similar in the two groups, throughout tilt, continuous observation of the Doppler recording in patients with postural tachycardia showed intermittent fluctuation of the blood flow velocity, with an oscillatory pattern, which were not observed in the recordings in patients with neurocardiogenic syncope. Comparison of patients with neurocardiogenic syncope, and those with postural tachycardia also showed larger variations of the pulsatility index (P< 0·05) in the postural tachycardia group. These findings support the possibility that abnormalities within the central nervous system play a pivotal role in the pathogenesis of postural tachycardia. [ABSTRACT FROM PUBLISHER]