In this paper is reported how the Brownian motion of colloids (beads) immersed in a liquid can be affected by the local properties of the fluid and how, in turn, it can alter them. Near- critical binary mixtures are known to exhibit important concentration fluctuations when nearing their liquid-liquid critical point (CP). Photon-beating spectroscopy measurements show that colloids immersed in such a critical mixture of isobutyric acid and water exhibit a slow-down of their Brownian motion due to the fluctuation-induced critical enhancement of viscosity. However, as the mixture temperature is brought close to the CP temperature, the fluctuations lifetime becomes large enough such that the
fluctuations can be in turn strongly deformed by the shear flow around the colloids. Mean-field behavior, where the influence of fluctuations can be ignored, follows. Viscosity does not increase anymore and colloids motion ceases to slow down.
The discovery of fire has contributed to the emergence of humanity. It is then the scientific mastery of this same combustion that allowed the man to propel himself on the moon or more modestly to operate the engine of your car. Who was not fascinated by fireworks, a candle or a fire? Many physical and chemical phenomena occur there that this article will allow you to discover. The combustion results from a set of chemical reactions that releases energy. The different chemical bodies involved are essentially an oxidant, a reducing agent and the products of combustion. These bodies are in motion, diffuse into each other and the heat produced by chemical reactions spreads. The combustion is first approached in a gaseous medium. There are diffusion flames and premix flames. We first study the laminar case, then the action of turbulence on these flames. Then, a few heterogeneous cases are presented, such as the combustion of plates, sprays and powders.
The purpose of this study is to develop a computational code for solving the equations governing the flow of fluids in a vertical axis wind turbine. In order to check the effectiveness of our code, we made an application on the Savonius rotor. The equations that define the flow of fluids in a rotating wheel are that of continuity, Navier-Stokes and energy. These equations are solved numerically by the explicit Lax-Wendroff finite difference method followed by the addition of a time-corrected artificial viscosity. The precision in the space and in the time of our scheme is second order and the stability of the numerical computation is ensured by the CFL condition which imposes a constrain on the time step. The results are presented by pressure and Mach curves.
For pure substances, the liquid-vapor phase separation interface disappears at the critical point. This property and others have attracted the interest of many scientists. This article is an update of the text of the invited conference on Critical Fluids presented at a seminar of Saint-Gobain-Recherche Aubervilliers on June 14, 2007. Thermostatics is first presented; then successively the thermodynamics of fluids near the critical point; the piston effect, a specific mode of heat transfer; the expansion of a "drop" at the critical pressure; the behavior of a supercritical fluid pocket
immersed in a high temperature environment; finally boiling near the critical point. It is partly a resumption of texts issued by various authors cited.
The discrete mechanics formalism and equations are considered in the present work in order to establish the role played by representative motion equations on the study of turbulence in fluids. In particular, a set of differences related to the turbulent pressure, the dynamics of vorticity in two spatial dimensions, the turbulent dissipation or the divergence of acceleration are discussed compared to the classical continuous media and Navier-Stokes equations. A second part is devoted to presenting on a first example, the rigid rotational motion, the differences between discrete and continuum mechanics. A last section is devoted to simulating the turbulent channel flow at turbulent Reynolds number of Reτ = 590. It is demonstrated that discrete mechanics allow to recover accurately the mean velocity profiles of reference DNS and also to provide scale laws of the whole mean velocity profile from the wall to the center of the channel.
Safety issues in nuclear power plant involve complex turbulent bubbly flows. To predict the behavior of these flows, the two-fluid approach is often used. Nevertheless, this model has been developed for the simulation of small spherical bubbles, considered as a dispersed field. To deal with bubbles with a large range of sizes, a multifield approach based on this two-fluid model has been proposed. A special treatment, called the Large Bubble Model (LBMo), has been implemented and coupled to the dispersed model. However, only laminar and isothermal flows were considered in previous papers. Thus, here, Large Eddy Simulations (LES) are investigated to model turbulence effects. For this purpose, the two-fluid model equations are filtered to highlight the specific subgrid terms. Then, an a priori LES study using filtered Direct Numerical Simulation (DNS) results is detailed. This analysis allows classifying these terms according to their relative weight and then concentrating the modelling efforts on the predominant ones. Five different turbulence models are compared. These results are finally used to perform true LES on a turbulent two-phase flow. Moreover, in order to tackle non-isothermal flows occurring in industrial studies, a new heat transfer model is implemented and validated to deal with phase change at large interfaces using the Large Bubble Model.