1,720,982 research outputs found
Novel shape parametrization technique applied to the optimization of a supersonic ORC turbine cascade
No full textShape-optimization techniques are nowadays an essential tool in the design chain of turbomachinery to strike the highperformance targets demanded by modern applications. Although the geometric parametrization may affect significantly the optimization cost and ultimately the optimization outcome, the selection of control points and of the subsequent design space is usually based on heuristic considerations. This paper proposes a cost-efficient parametrization procedure based on the ANOVA analysis of B-Spline control points. To tackle the extremely large computational burden arisen from the use of several control points, a surrogate strategy is implemented, testing four different methods. The optimization relies on surrogate-assisted evolutionary strategies coupled with an experimentally validated CFD solver. The technique is applied to a supersonic Organic Rankine Cycle turbine cascade, which features a converging-diverging bladed channel. It is shown that only the diverging section of the suction side as well as the adjacent region of unguided turning have the major impact on the aerodynamic performance. These findings enable to select an optimal distribution of mobile control points in the optimization block, leading to significant savings in computational cost. Finally, three optimizations are carried out, varying locally the number of control points; results are widely discussed in terms of both optimization outcomes and optimizer robustness
Three-dimensional shape optimization of a centrifugal compressor stage for supercritical carbon dioxide power systems
Full text linkThe design of a centrifugal compressor for supercritical carbon dioxide power cycle must consider non-ideal gas effects and the possible occurrence of two-phase flows. Shape optimization techniques, combined with computational fluid-dynamic (CFD) simulations, can produce optimized designs while inherently coping with such peculiar flow characteristics. This study presents a three-dimensional shape optimization of a compressor stage composed of the impeller and the vaneless diffuser, whose compression starts close to the critical point. Impeller blade angle distributions and meridional channel are parameterized with Bezier control points, enabling local shape control within the optimization routine. The pinch of the vaneless diffuser is also optimized. The experimentally validated CFD solver considers both non-ideal effects and two-phase homogeneous flows, assuming thermodynamic equilibrium and a barotropic fluid. The constrained optimization problem is addressed using genetic algorithms. To mitigate computational costs, Kriging surrogates for the objective function and constraints are trained using a limited number of CFD results. The optimized geometry shows an appreciable efficiency increase (1.1 percentage points) while delivering the design pressure ratio. Although performing better at the design condition, the operating range of the compressor is altered by the optimization. Future optimizations that include both design and off-design operating points in the definition of the objective function and constraints may mitigate this problem
High-Fidelity Shape Optimization of Non-Conventional Turbomachinery by Surrogate Evolutionary Strategies
No full textThis paper presents a novel tool for the shape optimization of turbomachinery blade profiles operating with fluids in non-ideal thermodynamic conditions and in complex flow configurations. In novel energy conversion systems, such as organic Rankine cycles or supercritical CO2 cycles, the non-conventional turbomachinery layout as well as the complex thermodynamics of the working fluid complicate significantly the blade aerodynamic design. For such applications, the design of turbomachinery may considerably benefit from the use of systematic optimization methods, especially in combination with high-fidelity computational fluid dynamics (CFD), as it is shown in this paper. The proposed technique is implemented in the shape-optimization package FORMA (Fluid-dynamic OptimizeR for turbo-Machinery Aerofoils) developed in-house at the Politecnico di Milano. FORMA is constructed as a combination of a generalized geometrical parametrization technique based on B-splines, a CFD solver featuring turbulence models and arbitrary equations of state, and multiple surrogate-based evolutionary strategies based on either trust-region or training methods. The application to the re-design of a supersonic turbine nozzle shows the capabilities of applying a high-fidelity optimization, consisting of a 50% reduction in the cascade loss coefficient and in an increased flow uniformity at the inlet of the subsequent rotor. Two alternative surrogate-based evolutionary strategies and different fitness functions are tested and discussed, including nonlinear constraints within the design process. The optimization study reveals relevant insights into the design of supersonic turbine nozzles as well on the performance, reliability, and potential of the proposed design technique
Impact of shape-optimization on the unsteady aerodynamics and performance of a centrifugal turbine for ORC applications
Full text linkThis paper presents the results of the application of a shape-optimization technique to the design of the stator and the rotor of a centrifugal turbine conceived for Organic Rankine Cycle (ORC) applications. Centrifugal turbines have the potential to compete with axial or radial-inflow turbines in a relevant range of applications, and are now receiving scientific as well as industrial recognition. However, the non-conventional character of the centrifugal turbine layout, combined with the typical effects induced by the use of organic fluids, leads to challenging design difficulties. For this reason, the design of optimal blades for centrifugal ORC turbines demands the application of high-fidelity computational tools. In this work, the optimal aerodynamic design is achieved by applying a non-intrusive, gradient-free, CFD-based method implemented in the in-house software FORMA (Fluid-dynamic OptimizeR for turboMachinery Aerofoils), specifically developed for the shape optimization of turbomachinery profiles. FORMA was applied to optimize the shape of the stator and the rotor of a transonic centrifugal turbine stage, which exhibits a significant radial effect, high aerodynamic loading, and severe non-ideal gas effects. The optimization of the single blade rows allows improving considerably the stage performance, with respect to a baseline geometric configuration constructed with classical aerodynamic methods. Furthermore, time-resolved simulations of the coupled stator-rotor configuration shows that the optimization allows to reduce considerably the unsteady stator-rotor interaction and, thus, the aerodynamic forcing acting on the blades
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Computational and Experimental Assessment of a MW-Scale Supercritical CO2 Compressor Operating in Multiple Near-Critical Conditions
Full text linkThis work illustrates the results of a wide experimental campaign in the frame of the EU-funded project sCO2-Flex, which focused on the investigation of a MW-scale sCO2 compressor operating in plant-representative conditions. The experimental tests were carried out for four temperature levels between 304.15 K and 309.15 K at a fixed pressure of 79.79 bar, hence covering an extended thermodynamic region close to the critical point. The experimental results are thoroughly discussed with the support of steady computational fluid-dynamics simulations, assuming homogeneous flows and thermodynamic equilibrium for the two-phase flow description. Changing the upstream total state, two peculiar variabilities in the compressor pressure ratio and choking flow rate are experimentally and computationally observed. While the former is mainly related to the single-phase flow thermodynamics, the latter originates from the onset of two-phase flows. As the simulations predict the experimental choking with a maximum error of 3%, the corresponding two-phase speed of sound is analyzed to infer the underlying equilibria between phases. It is found that, for the tested conditions, two-phase flows quickly achieve thermodynamic equilibrium, and non-equilibrium or metastable effects arguably play a marginal role in the process
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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