Some complexity and approximation results for coupled-tasks scheduling problem according to topology

We consider the makespan minimization coupled-tasks problem in presence of compatibility constraints with a specified topology. In particular, we focus on stretched coupled-tasks, i.e. coupled-tasks having the same sub-tasks execution time and idle time duration. We study several problems in framework of classic complexity and approximation for which the compatibility graph is bipartite (star, chain,. . .). In such a context, we design some efficient polynomial-time approximation algorithms for an intractable scheduling problem according to some parameters

Finding robust solutions to stable marriage

We study the notion of robustness in stable matching problems. We first define robustness by introducing (a,b)-supermatches. An (a,b)-supermatch is a stable matching in which if a pairs break up it is possible to find another stable matching by changing the partners of those a pairs and at most b other pairs. In this context, we define the most robust stable matching as a (1,b)-supermatch where b is minimum. We show that checking whether a given stable matching is a (1,b)-supermatch can be done in polynomial time. Next, we use this procedure to design a constraint programming model, a local search approach, and a genetic algorithm to find the most robust stable matching. Our empirical evaluation on large instances show that local search outperforms the other approaches

Scheduling stretched coupled-tasks with compatibilities constraints : model, complexity and approximation results for some class of graphs

We tackle the makespan minimization coupled-tasks problem in presence of compatibility constraints. In particular, we focus on stretched coupled-tasks, {\it i.e.}coupled-tasks having the same sub-tasks execution time and idle time duration. We study severals problems in frame works of classic complexity and approximation for which the compatibility graph $G_c$ is bipartite (star, chain, $\ldots$) In such context, we design some efficient polynomial-time approximation algorithms according to difference parameters of the scheduling problem. When $G_c$ is a $k$-stage bipartite graph, we propose, among other, a $\frac{7}{6}$-approximation algorithm when $k=1$, and a $\frac{13}{9}$-approximation algorithm when $k=2$.\

New models for two variants of popular matching

We study the problem of matching a set of applicants to a set of posts, where each applicant has an ordinal preference list, which may contain ties, ranking a subset of posts. A matching M is popular if there exists no matching M0 where more applicants prefer M0 to M. Several notions of optimality are studied in the literature for the case of strictly ordered preference lists. In this paper we address the case involving ties and propose novel algorithmic and complexity results for this variant. Next, we focus on the NP-hard case where additional copies of posts can be added in the preference lists, called Popular Matching with Copies. We define new dominance rules for this problem and present several novel graph properties characterising the posts that should be copied with priority. We present a comprehensive set of experiments for the popular matching problem with copies to evaluate our dominance rules as well as the different branching strategies. Our experimental study emphasizes the importance of the dominance rules and characterises the key aspects of a good branching strategy

High solar flux heating of upflow bubbling fluidized bed circulating in opaque vertical tube - 3d numerical simulation

Current solar Heat Transfer Fluids (HTF) have a limited working temperature (\u3c 600 °C) and present operational risks. We proposed to use air-fluidized Dense Particle Suspensions (DPS), also called Upflow Bubbling Fluidized Bed (UBFB), in tubes as a new HTF and storage medium in the frame of the so-called CSP2 FP7 European project (http://www.csp2-project.eu/). UBFB can operate up to the solid sintering temperature, thus improving the plant efficiency, it has no lower temperature limitation and is riskless. The DPS capacity to extract heat from a tube absorber exposed to concentrated solar radiation was demonstrated on a single-tube experimental receiver that was tested at the focus of the CNRS 1 MW solar furnace in Odeillo. The DPS flowed upward through the absorber tube (i.d. 3.6 cm) that passed through a 50 cm high cavity where it was exposed to concentrated solar flux that heated the DPS. The tube wall-to-DPS Heat Transfer Coefficient (HTC) first values were calculated by Flamant et al. (1). A stable outlet temperature of 750 °C was reached with a metallic tube and a particle reflux in the near tube wall region was evidenced by Benoit et al. (2). In this paper, the UBFB behavior is studied using the multiphase flow code NEPTUNE_CFD (3). Hydrodynamics of SiC Geldart A-type particles (40 µm Sauter diameter) and heat transfer imposed by a thermal flux at the wall are coupled in 3D numerical simulations. The convective/diffusive heat transfer between the gas and dispersed phase, and the inter-particle radiative transfer (Rosseland approximation) are accounted for. The numerical and experimental results are compared in order to validate the model. The heat exchange between the particles close to the tube wall and those in the tube center is characterized. Please click Additional Files below to see the full abstract

Utilisation d'algorithmes d'approximation en Programmation Par Contraintes

National audienceDans cet article, nous présenterons les travaux prélimi-naires menés sur l'utilisation d'algorithmes d'approxima-tion en Programmation Par Contraintes afin d'améliorer le calcul de bornes lors de la résolution de problèmes d'optimisation sous contraintes. L'objectif de nos travaux est d'étudier plus particulièrement quels algorithmes d'ap-proximation présentent suffisamment de flexibilité pour être utilisés en Programmation Par Contraintes, et comment les utiliser au sein d'un propagateur qui mettra à jour les bornes de la variable-objectif à chaque noeud de l'espace de recherche. Enfin l'idée sera d'appliquer cette approche à plusieurs familles de problèmes d'optimisation afin d'en extraire une généralisation

3D numerical simulation of upflow bubbling fluidized bed in opaque tube under high flux solar heating

Current solar Heat Transfer Fluids (HTF) only work below 600°C. We proposed to use air-fluidized Dense Particle Suspensions (DPS), also called Upflow Bubbling Fluidized Bed (UBFB), in tubes as a new HTF and storage medium in the frame of the so-called CSP2 FP7 European project. UBFB can operate up to the solid sintering temperature (1400 °C for SiC particles), thus improving the plant efficiency and cost of produced kWh. The DPS capacity to extract heat from a tube absorber exposed to concentrated solar radiation was demonstrated and the first values of the tube wall-to-DPS heat transfer coefficient were measured. A stable outlet temperature of 750 °C was reached with a metallic tube, and a particle reflux in the near tube wall region was evidenced. In this paper, the UBFB behavior is studied using the multiphase flow code NEPTUNE_CFD. Hydrodynamics of SiC Geldart A-type particles and heat transfer imposed by a thermal flux at the wall are coupled in 3D numerical simulations. The convective/diffusive heat transfer between the gas and dispersed phase, and the inter-particle radiative transfer (Rosseland approximation) are accounted for. Simulations and experiments are compared. The temperature influence on the DPS flow is analyzed