排序方式: 共有13条查询结果,搜索用时 328 毫秒
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Nadia Creignou Phokion Kolaitis Bruno Zanuttini 《Journal of Computer and System Sciences》2008,74(7):1103-1115
We give a quadratic algorithm for the following structure identification problem: given a Boolean relation R and a finite set S of Boolean relations, can the relation R be expressed as a conjunctive query over the relations in the set S? Our algorithm is derived by first introducing the concept of a plain basis for a co-clone and then identifying natural plain bases for every co-clone in Post's lattice. In the process, we also give a quadratic algorithm for the problem of finding the smallest co-clone containing a Boolean relation. 相似文献
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Maria A. Founti Dimitrios I. Katsourinis Dionysios I. Kolaitis 《Numerical Heat Transfer, Part B: Fundamentals》2013,63(1):51-68
An “in-house” computational fluid dynamics code implementing a Euler-Lagrange approach is extended by incorporating the Euler-Euler (two-fluid model) approach, to improve prediction capabilities of flow and thermal characteristics of turbulent evaporating sprays. The performance of both approaches is assessed by comparing predictions with experimental data for a variety of evaporating-spray test cases. The applicability of the Euler-Lagrange and Euler-Euler approach is established in an isopropyl alcohol–air turbulent flow, in which characteristic droplet quantity predictions are in satisfactory overall agreement with measurements. The evaporating spray characteristics are then predicted under “stabilized cool flame” conditions and the computational results are compared to experimental data for a nonreactive case and a reactive case. In both cases, the velocity and thermal fields are successfully captured by both approaches. Overall, the article demonstrates an approach toward the development of performance-based computational tools. 相似文献
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We establish a dichotomy in the complexity of computing the consistent answers of a Boolean conjunctive query with exactly two atoms and without self-joins. Specifically, we show that the problem of computing the consistent answers of such a query either is in P or it is coNP-complete. Moreover, we give an efficiently checkable criterion for determining which of these two possibilities holds for a given query. 相似文献
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During a fire event, ventilated facade systems may contribute to external fire spreading to the upper floors of a building via the facade, thus representing a significant risk. In this frame, the performance of a typical ventilated façade system under fire conditions is experimentally investigated, using a full-scale compartment-facade test rig. Two alternative façade configurations are examined and comparatively assessed, namely a plain façade (PF) and a ventilated façade (VF) system. Emphasis is given on the estimation of the thermal characteristics of the developed Externally Venting Flames (EVF) and the thermal boundary conditions developing on the façade's exposed surface. An extensive set of sensors was installed at the interior of the fire compartment, the façade systems and the exterior of the test configurations. Analysis of the experimental data suggests that even though gaseous combustion products managed to penetrate the air cavity of the VF system, no consistent flaming conditions were established. On the unexposed face of both PF and VF systems, temperatures remained constantly below 180°C throughout the duration of both fire tests. The Eurocode correlations are assessed against the obtained experimental data; certain parameters, such as EVF length, width and centreline temperature, are found to be under-estimated by the Eurocode methodology. 相似文献
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We investigate the infinitary logic L∞ωω, in which sentences may have arbitrary disjunctions and conjunctions, but they involve only finite numbers of distinct variables. We show that various fixpoint logics can be viewed as fragments of L∞ωω, and we describe a game-theoretic characterization of the expressive power of the logic. Finally, we study asymptotic probabilities of properties expressible in L∞ωω on finite structures. We show that the 0–1 law holds for L∞ωω, i.e., the asymptotic probability of every sentence in this logic exists and is equal to either 0 or 1. This result subsumes earlier work on asymptotic probabilities for various fixpoint logics and reveals the boundary of 0–1 laws for infinitary logics. 相似文献
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We investigate here NP optimization problems from a logical definability standpoint. We show that the class of optimization problems whose optima are definable using first-order formulae coincides with the class of polynomially bounded NP optimization problems on finite structures. After this, we analyze the relative expressive power of various classes of optimization problems that arise in this framework. Some of our results show that logical definability has different implications for NP maximization problems than it has for NP minimization problems, in terms of both expressive power and approximation properties. 相似文献