Solution of the Boussinesq equations by means of the finite element method

A finite element method is presented for the computation of flows that are influenced by buoyancy forces. The accuracy of several finite elements is studied by solving the Bénard problem and determining the critical Rayleigh number. It is found that the accuracy is greatly enhanced if the shape functions satisfy a certain requirement that arises from the physical nature of the problem

Why Do Cascade Sizes Follow a Power-Law?

We introduce random directed acyclic graph and use it to model the information diffusion network. Subsequently, we analyze the cascade generation model (CGM) introduced by Leskovec et al. [19]. Until now only empirical studies of this model were done. In this paper, we present the first theoretical proof that the sizes of cascades generated by the CGM follow the power-law distribution, which is consistent with multiple empirical analysis of the large social networks. We compared the assumptions of our model with the Twitter social network and tested the goodness of approximation.Comment: 8 pages, 7 figures, accepted to WWW 201

Toepassingsmogelijkheden van Modified Atmosphere Packaging (MAP) in rozen en sierheesters

Boomkwekerijgewassen, zoals rozen en siergewassen, zijn in diverse stadia van de keten gevoelig voor kwaliteitsverlies. Dit kan uitdroging zijn door vochttekort, verrotting bij vochtovermaat of ongewenst uitlopen van het product. Uit eerder onderzoek is gebleken dat toepassing van zogenaamde Modified Atmosphere folies (MA, zie kader) al dan niet in combinatie met bepaalde vulmiddelen dergelijke problemen sterk kan verminderen en daarmee de houdbaarheid van producten kan verlengen. Dit rapport geeft de resultaten van een onderzoek naar de mogelijkheden van toepassing van MA-folie bij bewaring van rozen en siergewassen in verschillende delen van de keten

Statistical Mechanics of Semi-Supervised Clustering in Sparse Graphs

We theoretically study semi-supervised clustering in sparse graphs in the presence of pairwise constraints on the cluster assignments of nodes. We focus on bi-cluster graphs, and study the impact of semi-supervision for varying constraint density and overlap between the clusters. Recent results for unsupervised clustering in sparse graphs indicate that there is a critical ratio of within-cluster and between-cluster connectivities below which clusters cannot be recovered with better than random accuracy. The goal of this paper is to examine the impact of pairwise constraints on the clustering accuracy. Our results suggests that the addition of constraints does not provide automatic improvement over the unsupervised case. When the density of the constraints is sufficiently small, their only impact is to shift the detection threshold while preserving the criticality. Conversely, if the density of (hard) constraints is above the percolation threshold, the criticality is suppressed and the detection threshold disappears.Comment: 8 pages, 4 figure

The Chemical Origins of Plasma Contraction and Thermalization in CO2 Microwave Discharges

Thermalization of electron and gas temperature in CO2 microwave plasma is unveiled with the first Thomson scattering measurements. The results contradict the prevalent picture of an increasing electron temperature that causes discharge contraction. It is known that as pressure increases, the radial extension of the plasma reduces from ∼7 mm diameter at 100 mbar to ∼2 mm at 400 mbar. We find that, simultaneously, the initial nonequilibrium between ∼2 eV electron and ∼0.5 eV gas temperature reduces until thermalization occurs at 0.6 eV. 1D fluid modeling, with excellent agreement with measurements, demonstrates that associative ionization of radicals, a mechanism previously proposed for air plasma, causes the thermalization. In effect, heavy particle and heat transport and thermal chemistry govern electron dynamics, a conclusion that provides a basis for ab initio prediction of power concentration in plasma reactors

Resolving discharge parameters from atomic oxygen emission

A method is proposed to spatially resolve discharge parameters from experimental measurements of emission intensity and 1D numerical simulations including an O atom collisional-radiative model. The method can be used for different plasmas and conditions. Here, contracted microwave discharges for CO2 conversion are studied at intermediate to high pressures (100–300 mbar). Radial profiles of electron density (ne) are used as input in the model and corrected to successfully simulate the measured Gaussian profiles of emission intensity of the 777 nm transition (I777). As a result, radially-resolved parameters inaccessible in experiments, such as ne, power density (Pabs), electron temperature (Te), electric field and reaction rates, are numerically-obtained for several conditions. ne and Pabs approximately follow Gaussian profiles that are broader than that of I777. For pressures below 150 mbar, the difference in full width at half maximum is typically a factor 1.6. This consists in a phenomenon of optical contraction, which is due to concave profiles of O molar fraction and Te. The implications of the simulated profiles on the study of plasmas for CO2 conversion are discussed and it is shown that these profiles allow to explain high reactor performances at low pressures