Control of silver-polymer aggregation mechanism by primary particle spatial correlations in dynamic fractal-like geometry

Silver nanocrystals have been prepared by reacting silver nitrate with ascorbic acid in aqueous solution containing a low concentration of a commercial polynaphtalene sulphonate polymer (Daxad 19). Various crystalline morphologies have been obtained simply by tuning the reaction temperature. We have investigated the nanoparticle formation mechanism at three different temperatures by in situ and time resolved Small Angle X ray Scattering measurements. By modeling the scattering intensity with interacting spherical particles in a fractal-like polymer-Ag matrix, we found signatures of nucleation, growth and assembly of primary particles of about 15-20 nm. We observed how the time evolution of both spatial correlations between primary particles and the dynamic fractal geometry of the polymer-Ag matrix could influence and determine both the aggregation mechanism and the morphology of forming nanostructures in solution

Roche lobe effects on the atmospheric loss of "Hot Jupiters"

Observational evidence of a hydrodynamically evaporating upper atmosphere of HD209458b (Vidal-Madjar et al. 2003; 2004) and recent theoretical studies on evaporation scenarios of ``Hot Jupiters'' in orbits around solar-like stars with the age of the Sun indicate that the upper atmospheres of short-periodic exoplanets experience hydrodynamic blow-off conditions resulting in loss rates of the order of about 10^10 - 10^12 g s^-1 (Lammer et al. 2003; Yelle 2004; Baraffe et al. 2004; Lecavlier des Etangs et al. 2004; Jaritz et al. 2005, Tian et al. 2005; Penz et al. 2007). By studying the effect of the Roche lobe on the atmospheric loss from short-periodic gas giants we found, that the effect of the Roche lobe can enhance the hydrodynamic evaporation from HD209458b by about 2 and from OGLE-TR-56b by about 2.5 times. For similar exoplanets which are closer to their host star than OGLE-TR-56b, the enhancement of the mass loss can be even larger. Moreover, we show that the effect of the Roche lobe raises the possibility that ``Hot Jupiters'' can reach blow-off conditions at temperatures which are less than expected (< 10000 K) due to the stellar X-ray and EUV (XUV) heating.Comment: 4 pages, 2 figures, submitted to A&

A Semi-implicit Treatment of Porous Media in Steady-State CFD

There are many situations in computational fluid dynamics which require the definition of source terms in the Navier–Stokes equations. These source terms not only allow to model the physics of interest but also have a strong impact on the reliability, stability, and convergence of the numerics involved. Therefore, sophisticated numerical approaches exist for the description of such source terms. In this paper, we focus on the source terms present in the Navier–Stokes or Euler equations due to porous media—in particular the Darcy–Forchheimer equation. We introduce a method for the numerical treatment of the source term which is independent of the spatial discretization and based on linearization. In this description, the source term is treated in a fully implicit way whereas the other flow variables can be computed in an implicit or explicit manner. This leads to a more robust description in comparison with a fully explicit approach. The method is well suited to be combined with coarse-grid-CFD on Cartesian grids, which makes it especially favorable for accelerated solution of coupled 1D–3D problems. To demonstrate the applicability and robustness of the proposed method, a proof-of-concept example in 1D, as well as more complex examples in 2D and 3D, is presented

Гидродинамический подшипник скольжения: моделирование упругих деформаций

This article deals with a new approach for calculation of self-consistent pressure distribution and surface deflection for a lubricated journal bearing. This approach is based on the numerical solution of the 2-D Reynolds’ equation for the lubrication layer, numerical calculation of the surface deformations by the 3-D ANSYS package and Fourier series expansion for the compliance matrix. A simple analytical approximation is found for the obtained compliance matrix, which can be used for heavy loaded journal bearings. The compliance matrix is implemented into the iterative procedure for calculation of self-consistent pressure distribution and surface deflection in the contact zone. Results of calculations are presented for the particular journal bearingВ статье рассмотрен новый подход к вычислению самосогласованного распределения давления и деформации поверхности для цилиндрического подшипника скольжения. Предлагаемый метод основан на численном решении 2-мерного уравнения Рейнольдса для смазочного слоя, вычислении деформации поверхности с помощью 3-мерного пакета АНСИС с использованием разложения Фурье для вычисления матрицы податливости. Найдена простая аналитическая аппроксимация для матрицы податливости, которая может применяться для расчета тяжело нагруженных подшипников скольжения. Найденная матрица податливости используется в итеративной процедуре для расчета самосогласованного распределения давления и прогиба поверхности в зоне контакта. Представлены результаты расчета конкретного подшипника скольжени