Quantile-based optimization under uncertainties using adaptive Kriging surrogate models

Uncertainties are inherent to real-world systems. Taking them into account is crucial in industrial design problems and this might be achieved through reliability-based design optimization (RBDO) techniques. In this paper, we propose a quantile-based approach to solve RBDO problems. We first transform the safety constraints usually formulated as admissible probabilities of failure into constraints on quantiles of the performance criteria. In this formulation, the quantile level controls the degree of conservatism of the design. Starting with the premise that industrial applications often involve high-fidelity and time-consuming computational models, the proposed approach makes use of Kriging surrogate models (a.k.a. Gaussian process modeling). Thanks to the Kriging variance (a measure of the local accuracy of the surrogate), we derive a procedure with two stages of enrichment of the design of computer experiments (DoE) used to construct the surrogate model. The first stage globally reduces the Kriging epistemic uncertainty and adds points in the vicinity of the limit-state surfaces describing the system performance to be attained. The second stage locally checks, and if necessary, improves the accuracy of the quantiles estimated along the optimization iterations. Applications to three analytical examples and to the optimal design of a car body subsystem (minimal mass under mechanical safety constraints) show the accuracy and the remarkable efficiency brought by the proposed procedure

Contribution de la Régénération Naturelle Assistée des ligneux dans l’approvisionnement en bois des ménages dans le département de Magaria (Niger)

Cet article présente les impacts de la Régénération Naturelle Assistée (RNA) sur l’approvisionnement en bois des ménages dans deux terroirs villageois (Arasofoua et Gaounawa) du département de Magaria au Niger. L’objectif visé était d’évaluer la quantité de bois utilisée au niveau des ménages pour les différents usages (bois énergie, construction, et service). La méthodologie utilisée pour atteindre cet objectif a consisté en des enquêtes auprès des ménages sur le type de bois utilisé et leur quantité ainsi que des mesures pour évaluer le changement induit à travers la pratique de la Régénération Naturelle Assistée. Les résultats obtenus montrent que les populations de ces villages arrivent à satisfaire leurs besoins en bois de feu et de service. Cette technique est pratiquée depuis 1994 dans ces villages. Chaque exploitant collecte en moyenne 24,9 et 17,5 m³ de bois par an dans les deux terroirs villageois respectivement en fonction des différents types d’usage (chauffe, construction, ustensiles de cuisine, etc.) et types de bois utilisés. Environ 90,8% des exploitants n’achètent pas le bois. L’autoconsommation moyenne en bois dans les deux villages est de 57,8%. Les transhumants et les coupes frauduleuses, constituent les contraintes majeures de cette pratique. Sensibiliser et former davantage les paysans aux techniques de régénération et impliquer les femmes dans cette nouvelle pratique favoriseraient la constitution de parcs agroforestiers.Mots clés: Innovation, ménages, produits ligneux, Sahel, Nige

Panorama of Onion Production in Tillabéri, A Region of the Far West of Niger

The objective of this study was to evaluate onion production systems in four municipalities along the Niger River. A cross-sectional survey was carried out among seventy-five onion producers distributed over ten perimeters. The results obtained reveal that onion is mainly grown by women (95%). 70% of the farmers own the farmed land, 20% rent and 10% borrow. The cultivation takes place between September and March. The cultivated varieties are ‘Galmi violet’ and ‘Gothèye white’, Onions are essentially transformed into Gabou, a traditional condiment. The areas exploited are generally less than half a hectare. Irrigation relies mainly on the gravity system. NPK fertilizer and urea in combination with manure are the most commonly used. For phytosanitary treatments, producers use pesticides and/or natural products based on aqueous extract. At harvest time, the price per 100 kg bag varies from 8 000 to 15 000 FCFA (12 to 23 €) and from 40 000 to 50 000 FCFA (61 to 76 €) after four months of storage. The average production cost per hectare is 1 208 564 FCFA (1844 €) and that of the net margins 551 857 FCFA (840 €). There is a low correlation between strong investments and net margins. The major constraints faced by producers are the high cost of agricultural inputs, the rapid drying up of water points, limited access to credit, the straying of animals, the collapse of prices during the harvest period and the difficulties of farming storage and conservation