Results 1 - 10 of 83141
Results 1 - 10 of 83141. Search took: 0.055 seconds
|Sort by: date | relevance|
[en] The role of prominent Soviet physicist B I Davydov in the development of our understanding of diffusion is briefly reviewed, with emphasis on the ideas he put forward in the 1930s: introducing additional partial derivatives into diffusion equations and extending diffusion concepts to phase space. (from the history of physics)
[en] The equations describing diffusion on a heterogeneous lattice for low concentrations are considered taking into account lattice site blocking. It is shown that lattice site blocking cannot be disregarded in the case of a strongly heterogeneous lattice even for low concentrations. It is established that the equation with a fractional time derivative holds only in a bounded time interval. Anomalous diffusion, which is described by the equation with a fractional time derivative at the initial stage, must be described over long time periods by an ordinary diffusion equation with a concentration-dependent diffusion coefficient
[en] An increasing number of natural phenomena do not fit into the relatively simple description of diffusion developed by Einstein a century ago. As all of us are no doubt aware, this year has been declared 'world year of physics' to celebrate the three remarkable breakthroughs made by Albert Einstein in 1905. However, it is not so well known that Einstein's work on Brownian motion - the random motion of tiny particles first observed and investigated by the botanist Robert Brown in 1827 - has been cited more times in the scientific literature than his more famous papers on special relativity and the quantum nature of light. In a series of publications that included his doctoral thesis, Einstein derived an equation for Brownian motion from microscopic principles - a feat that ultimately enabled Jean Perrin and others to prove the existence of atoms (see 'Einstein's random walk' Physics World January pp19-22). Einstein was not the only person thinking about this type of problem. The 27 July 1905 issue of Nature contained a letter with the title 'The problem of the random walk' by the British statistician Karl Pearson, who was interested in the way that mosquitoes spread malaria, which he showed was described by the well-known diffusion equation. As such, the displacement of a mosquito from its initial position is proportional to the square root of time, and the distribution of the positions of many such 'random walkers' starting from the same origin is Gaussian in form. The random walk has since turned out to be intimately linked to Einstein's work on Brownian motion, and has become a major tool for understanding diffusive processes in nature. (U.K.)
[en] Here we consider an unsteady detonation with diffusion included. This introduces an interaction between the reaction length scales and diffusion length scales. Detailed kinetics introduce multiple length scales as shown though the spatial eigenvalue analysis of hydrogen-oxygen system; the smallest length scale is ∼ 107 m and the largest ∼ 10-2 m; away from equilibrium, the breadth can be larger. In this paper, we consider a simpler set of model equations, similar to the inviscid reactive compressible fluid equations, but include diffusion (in the form of thermal/energy, momentum, and mass diffusion). We will seek to reveal how the complex dynamics already discovered in one-step systems in the inviscid limit changes with the addition of diffusion.
[en] This work is devoted to investigate explicit solutions of the time-fractional diffusion equations with external forces by considering various diffusion coefficients and an absorbent rate. Besides, the 2nth moment related to such an equation is also discussed. Consequently, the diffusion type can be determined from the mean-square displacement. In addition, a rich class of diffusive processes, including normal and anomalous ones, can be obtained
[en] Modern notions in the field of theory and practice of the obtaining of complex diffusion coatings are presented. Rational procedures carbo-chromizing, chromo-nitridation, chromo-boriding and titaniding to obtain the surfaces with special physicochemical properties for the use under complex conditions of exploitaion are presented. The results of metallographical, physical, chemical, mechanical and other methods of investigation of complex coatings are given. Examples of the application of such coatings and modern equipment for their obtaining are described
[en] Description is given of a treatment for porous metallic filters used in isotope separation of UF6 with a view to improve their mechanical and corrosion resistance. The filters are introducted in presence of a Al base cement into a heat and oxidation resisting receptacle. The closed receptacle is filled up with an inert gas and heated in a furnace between 800 and 10000C during one hour
[fr]On decrit un procede de traitement des filtres metalliques poreux utilises dans le separation isotopique de l'hexafluorure d'uranium en vue d'ameliorer leur resistance mecanique et leur resistance a la corrosion. Selon ce procede on place les filtres en presence d'un cement a base d'aluminium dans un recipient ferme resistant a la chaleur et a l'oxydation, et on place l'interieur du recipient en atmosphere de gaz inerte et on le chauffe dans un four entre 8000C et 10000C pendant environ une heure
[en] We study the voter model and related random copying processes on arbitrarily complex network structures. Through a representation of the dynamics as a particle reaction process, we show that a quantity measuring the degree of order in a finite system is, under certain conditions, exactly governed by a universal diffusion equation. Whenever this reduction occurs, the details of the network structure and random copying process affect only a single parameter in the diffusion equation. The validity of the reduction can be established with considerably less information than one might expect: it suffices to know just two characteristic timescales within the dynamics of a single pair of reacting particles. We develop methods to identify these timescales and apply them to deterministic and random network structures. We focus in particular on how the ordering time is affected by degree correlations, since such effects are difficult to access by existing theoretical approaches.
[en] Diffusion of contaminants can play a significant if not dominant role in many applications encountered within the field of environmental geotechnics. The objective of this paper is to provide an overview of the important role diffusion plays in such applications. The presentation proceeds from a historical perspective, beginning with the recognition in the late 1970s to early 1980s that diffusion may be an important process in assessing contaminant migration through low-permeability barriers in waste containment applications. Data from the literature and simplified model simulations are used to illustrate under what conditions diffusion is important, and the significance of diffusion is illustrated with respect to different barrier components and types of barriers used in waste containment applications. The barriers considered include natural clays, compacted clay liners, geomembranes, geosynthetic clay liners, composite liners, vertical cutoff walls, subaqueous caps for contaminated sediments, and highly compacted bentonite buffers for high-level radioactive waste containment. The significance of semi-permeable membrane behavior on liquid-phase diffusion through bentonite-based barriers also is highlighted. The potential importance of matrix diffusion as an attenuation mechanism for contaminant transport is illustrated, and the roles of both liquid-phase and gas-phase diffusion under unsaturated conditions are discussed. Finally, the role of diffusion in terms of remediation applications is illustrated via an example analysis illustrating the impact of reverse matrix or back diffusion on the effectiveness of pump-and-treat remediation, as well as via a summary of several diffusion-based models commonly used to describe the leaching of contaminants from a variety of stabilized-solidified waste forms. (author)