The variational gaussian approximation revisited

The variational approximation of posterior distributions by multivariate gaussians has been much less popular in the machine learning community compared to the corresponding approximation by factorizing distributions. This is for a good reason: the gaussian approximation is in general plagued by an Omicron(N)(2) number of variational parameters to be optimized, N being the number of random variables. In this letter, we discuss the relationship between the Laplace and the variational approximation, and we show that for models with gaussian priors and factorizing likelihoods, the number of variational parameters is actually Omicron(N). The approach is applied to gaussian process regression with nongaussian likelihoods.     

Computational fluid dynamic analysis of a guide tube in a PWR

Security in nuclear power plants demands severe limitations of the maximal drop time of rod cluster control assemblies. In February 1995, several assemblies of the Chinese plant in Daya Bay failed to comply with these requirements. Electricité De France undertook a research program to get a better insight of this problem since the plant has been built by French and also because the French new four-loops N4 reactor was equipped with the same guide tubes. This paper is limited to a numerical study of the influence of the pressure forces applied to control rods and due to flow circulation through the guide tubes. After a validation test case, a first calculation has been carried out on a simplified N4 guide tube. The sensitivity of the pressure forces to transverse flow and to modifications of the geometry has been determined. The program has been extended to guide tubes used in 1300-MW reactors and similar computations have been done. To make simulations more representative, a global computation of the flow in the whole upper internals plenum (UIP) will be achieved to provide accurate boundary conditions for local calculations with better resolution.     

The effect of oak tannin (Quercus robur) and hops (Humulus lupulus) on dietary nitrogen efficiency,methane emission,and milk fatty acid composition of dairy cows fed a low-protein diet including linseed

The objective of this study was to test the effects of inclusion of hop pellets (HP) and oak tannin extracts (OT) alone or in combination on N efficiency, methane (CH₄) emission, and milk production and composition in 2 experiments with dairy cows fed low-N rations supplemented with linseed. In both experiments, 6 lactating Holstein cows were assigned to 3 dietary treatments in a 3 × 3 duplicated Latin square design (21-d periods). Cows were fed a total mixed ration at a restricted level to meet their nutrient requirements. In experiment 1, 169 g dry matter (DM) of OT or 56 g DM of HP was included separately in the control diet (C1). In experiment 2, the additives were included together (OT-HP) in the control diet (C2) similar to C1. Diet C2 was compared with a control without linseed (C0). In experiment 1, the supplementation of the control diet with OT decreased urinary N excretion by 12%. In experiment 2, the combination of OT and HP decreased urinary N by 7%. Oak tannin extracts and HP alone or in combination did not influence the daily enteric CH₄ production of cows. Cows fed diet C0 produced 17% more enteric CH₄ daily than those fed diet C2. Intake of diet C2, which contained 6.7% extruded linseed on a DM basis (experiment 2), decreased the sum of 6:0 to 14:0 fatty acids (?16%) and palmitic acid (?26%) and increased the stearic acid (+50%), oleic acid (+36%), vaccenic acid (trans-11 18:1; +285%), rumenic acid (cis-9,trans-11 18:2; +235%), and α-linolenic acid (+100%) in milk fat. The supplementation of diet C2 with the OT-HP mixture further improved the milk's fatty acid composition. Intake of the OT alone increased α-linolenic acid by 17.7% (experiment 1). The results of this study show that at the economically acceptable dose we tested, hops had no effect on urinary N excretion, CH₄ emission, milk production, and milk composition. By contrast, supplementation of diets with oak tannin extract can be considered for reducing urinary N excretion. The combination of oak tannin and hops had no more effect than oak tannin alone except on the milk fatty acid profile, which was favorably influenced from a nutritional point of view.     

Comparison Between Robust and Stochastic Optimisation for Long-term Reservoir Management Under Uncertainty

Long-term reservoir management often uses bounds on the reservoir level, between which the operator can work. However, these bounds are not always kept up-to-date with the latest knowledge about the reservoir drainage area, and thus become obsolete. The main difficulty with bounds computation is to correctly take into account the high uncertainty about the inflows to the reservoir. In this article, we propose a methodology to derive minimum bounds while providing formal guarantees about the quality of the obtained solutions. The uncertainty is embedded using either stochastic or robust programming in a model-predictive-control framework. We compare the two paradigms to the existing solution for a case study and find that the obtained solutions vary substantially. By combining the stochastic and the robust approaches, we also assign a confidence level to the solutions obtained by stochastic programming. The proposed methodology is found to be both efficient and easy to implement. It relies on sound mathematical principles, ensuring that a global optimum is reached in all cases.     

Characterization of liquid crystals impregnated micrometric diffraction gratings for an active near-IR light extraction control

Microsystem Technologies - Due to their narrow reflection peak as well as their compact structure, guided mode resonance filters (GMRFs) are attractive for many applications. In this work, we will...     

Experimental tests of proton beam localization

The depth of penetration of heavy charged-particle therapy beams is sensitive to the density of tissues traversed. Maximum depth of dose contours will vary appreciably as the beam passes through bone, muscle, lung, and air or gas. Calculations suggest that beam activation of the short-lived positron-emitting isotope 15O in vivo will permit localization of proton therapy beams with resonable detected-event density and dose. Preliminary tests of this method indicate that the beam can be located at depth with a typical dose of 15 rad, using a large field-of-view positron camera on-ling. This technique is also applicable to other heavy charged-particle beams, negative pions, and heavy ions.     

Comparative study of pressure-correction and Godunov-type schemes on unsteady compressible cases

Two pressure-correction algorithms are studied and compared to an approximate Godunov scheme on unsteady compressible cases. The first pressure-correction algorithm sequentially solves the equations for momentum, mass and enthalpy, with sub-iterations which ensure conservativity. The algorithm also conserves the total enthalpy along a streamline, in a steady flow. The second pressure-correction algorithm sequentially solves the equations for mass, momentum and energy without sub-iteration. This scheme is conservative and ensures the discrete positivity of the density. Total enthalpy is conserved along a streamline, in a steady flow. It is numerically verified that both pressure-correction algorithms converge towards the exact solution of Riemann problems, including shock waves, rarefaction waves and contact discontinuities. To achieve this, conservativity is compulsory. The two pressure-correction algorithms and the approximate Godunov scheme are finally compared on cases with heat source terms: all schemes converge towards the same solution as the mesh is refined.     

Discrepancies in Flood Modelling Approaches in Transboundary River Systems: Legacy of the Past or Well-grounded Choices?

Flood modelling in transnational rivers requires efficient cross-border collaboration among the riparian countries. Currently, each country/region usually uses a different hydraulic modelling approach, which may hinder the modelling of the entire river. For the sake of accurate and consistent river modelling there is a necessity for the establishment of a framework that fosters international collaborations. This study investigates the current hydraulic modelling approach across the whole length of the River Meuse, the main course of which crosses three north-western European countries. The numerical models used by French, Belgian, and Dutch agencies and authorities were interconnected by exchanging boundary conditions at the borders. At the central part of the river, the Belgian hydraulic model assumed steady flow conditions, while the rest of the river was modelled in unsteady mode. Results for various flood scenarios revealed a distinctive pattern of water depths at the Belgian-Dutch border. To clarify whether this is a bias induced by the change in modelling approach at the border (steady vs. unsteady), we remodelled a stretch of the river across the Belgian-Dutch border using a consistent unsteady modelling approach. The steady and unsteady approaches led to similar patterns across the border, hence discarding the hypothesis of a bias resulting from a change in the employed model. Instead, the pattern in water depths was attributed to a change in the topography of the Meuse Valley, where there is a transition from a narrow steep corridor with limited water storing capacity in Ardennes massif to wide floodplains in the Dutch lowlands. The associated flood damping for the 100-year discharge is less than 1% in the Ardennes and exceeds 15% in the Dutch lowlands. It can be inferred that the current differences in regional hydraulic modelling approaches for the River Meuse are generally well-grounded and not just a legacy of the past.

     

New organic discotic materials for photovoltaic conversion

Two-layer organic photovoltaic cells have been fabricated using a triphenylene ether as a hole transporting material, and perylene derivatives as electron transporting materials. Three devices were studied and showed external quantum efficiencies of around 3%. These results are interpreted in the context of electrochemical measurements that provide the ionization potential and electron affinity. Furthermore, the high-exciton diffusion lengths and absorption coefficients contribute to the high-observed photocurrents. The organic/organic interface was found to be the main origin of the photocurrent generation. However, the photovoltaic parameters were found to be dependent also on the ITO/organic interface. In particular, we show that ITO treatments with argon plasma and UV–ozone modify the open-circuit voltage.