321 research outputs found
Performance Analysis of Publish/Subscribe Systems
The Desktop Grid offers solutions to overcome several challenges and to
answer increasingly needs of scientific computing. Its technology consists
mainly in exploiting resources, geographically dispersed, to treat complex
applications needing big power of calculation and/or important storage
capacity. However, as resources number increases, the need for scalability,
self-organisation, dynamic reconfigurations, decentralisation and performance
becomes more and more essential. Since such properties are exhibited by P2P
systems, the convergence of grid computing and P2P computing seems natural. In
this context, this paper evaluates the scalability and performance of P2P tools
for discovering and registering services. Three protocols are used for this
purpose: Bonjour, Avahi and Free-Pastry. We have studied the behaviour of
theses protocols related to two criteria: the elapsed time for registrations
services and the needed time to discover new services. Our aim is to analyse
these results in order to choose the best protocol we can use in order to
create a decentralised middleware for desktop grid
Study of the NP-completeness of the compact table problem
ISBN 978-5-94057-377-7International audienceThe problem of compact tables is to maximise the overlap when building a word that is to include permutations of every given words (all the words being the same length). This problem is shown to be NP-complete in the general case, and some specific restrictions are studied
COMMENT ON "THE ROLE OF H3O+ IN THE CRYSTAL STRUCTURE OF ILLITE" BY F. NIETO, M. MELINI, AND I. ABAD
The arguments of Nieto et al. (2010) in favor of the incorporation of H3O+ rather than H2O in interlayer positions of illite are disputable. Stoichiometric arguments suggest that the excess water in the Silver Hill illite is in the form of H2O. Moreover, recent thermodynamic models assuming the incorporation of interlayer H2O in illite provide reasonable estimates of temperature and water content using the AEM/TEM analyses of Nieto et al. (2010)
An activity model for phase equilibria in the H2OâCO2âNaCl system
We present a semi-empirical thermodynamic model with uncertainties that encompasses the full range of compositions in H2OâCO2âNaCl mixtures in the range of 10â380 °C and 1â3500 bars. For binary H2OâCO2 mixtures, the activityâcomposition model is built from solubility experiments. The parameters describing interactions between H2O and CO2 are independent of the absolute thermodynamic properties of the end-members and vary strongly non-linearly with pressure and temperature. The activity of water remains higher than 0.88 in CO2-saturated solutions across the entire pressureâtemperature range. In the H2OâNaCl system, it is shown that the speciation of aqueous components can be accounted for by a thermodynamic formalism where activities are described by interaction parameters varying with intensive properties such as pressure and temperature but not with concentration or ionic strength, ensuring consistency with the GibbsâDuhem relation.The thermodynamic model reproduces solubility experiments of halite up to 650 °C and 10 kbar, and accounts for ion pairing of aqueous sodium and chloride ions with the use of associated and dissociated aqueous sodium chloride end-members whose relative proportions vary with salinity. In the H2OâCO2âNaCl system, an activityâcomposition model reproduces the salting-out effect with interactions parameters between aqueous CO2 and the aqueous species created by halite dissolution. The proposed thermodynamic properties are compatible with the THERMOCALC database (Holland and Powell, 2011) and the equations used to retrieve the activity model in H2OâCO2 can be readily applied to other systems, including minerals
Fluid flow and CO2âfluidâmineral interactions during CO2-storage in sedimentary basins
Modelling the progress of geochemical processes in CO2 storage sites is frustrated by uncertainties in the rates of CO2 flow and dissolution, and in the rates and controlling mechanisms of fluidâmineral reactions that stabilise the CO2 in geological reservoirs. Dissolution of CO2 must be controlled by the complexities of 2-phase flow of CO2 and formation brines and the smaller-scale heterogeneities in the permeability in the reservoirs which increase the fluid contact areas. The subsequent fluid mineral reactions may increase storage security by precipitating CO2 in carbonate minerals but the consequences of fluidâmineral reactions on caprock rocks or potential leakage pathways up fault zones are less certain as the CO2-charged brines may either corrode minerals or decrease permeabilities by precipitating carbonates. Observations from CO2-injection experiments and natural analogues provide important constraints on the rates of CO2 and brine flow and on the progress of CO2 dissolution and mineralâfluid reactions. In these experiments brines in contact with the propagating plume appear to rapidly saturate with CO2. Dissolution of the CO2 drives the dissolution of oxide and carbonate minerals, on times scales of days to weeks. These reactions buffer fluid pH and produce alkalinity such that carbonate dissolution moves to carbonate precipitation over time-scales of weeks to months. The dissolution of Fe-oxide grain coatings and the release of Fe to solution is important in stabilising insoluble FeâMgâCa carbonate minerals but the rate limiting step for carbonate mineral precipitation is the transport of CO2-charged brines and silicate mineral dissolution rates. Observations from CO2-EOR experiments and natural analogues suggest that the silicate mineral dissolution reactions are initially fast in the low pH fluids surrounding the CO2 plume but that reaction progress over months to years drives minerals towards thermodynamic equilibrium and dissolution rates slow over 2â5 orders of magnitude as equilibrium is approached. The sluggish dissolution of silicate minerals is likely to preside over the long-term fate of the CO2 in geological reservoirs. Observations from injection experiments and natural analogues suggest that the potentially harmful trace elements mobilised by the drop in pH are immobilised as adsorbed and precipitated phases as fluid pH is buffered across mineral reaction fronts. There are very few observations of caprock exposed to CO2-rich brines. Preliminary examination of core recently recovered from scientific drilling of a natural CO2 accumulation in Utah suggests that the diffusion of CO2 into reservoir caprocks drives dissolution of Fe-oxides but subsequent precipitation of carbonate minerals likely retards the diffusion distance of the CO2. At this site thin siltstone layers are shown to be effective seals to the CO2-charged fluids, which has significant implications for the long term security of CO2 in geological reservoirs
- âŠ