117,291 research outputs found
Microtomography of Soil and Soot Deposits: Analysis of Three-Dimensional Structures and Surface Morphology
No full textThe detrimental effects generated by the gas turbine fouling phenomenon are well known. Due to soil and soot particles ingestion, gas turbines experience performance drops related to greater fuel consumption and even lower efficiency. These effects are related to the modification of the shape and surface roughness of relevant surfaces (compressor and turbine blades and vanes, especially) due to the presence of a thin layer generated by micro/nano-sized particle adhesion. Such contaminants are swallowed by the unit and, as a function of the operating conditions, adhere to the surface, causing a sort of dangerous coating to the surface. In this work, a microtomography analysis of the deposited layer is reported. The deposited layer has been generated using micro-sized soil and soot powders under specific impact conditions and substrate surface roughness similar to those in the cold section of a gas turbine compressor. The microtomography analysis has been carried out using the beamline at the ELETTRA Sincrotrone research center. Thanks to the resolution of the beamline, the detection of the threedimensional internal structure of the soil and soot layers has revealed that within the layer, the structure is characterized by discontinuities. Soot and soil particles, even characterized by similar diameter distributions and test conditions, generate layer structures that differ by the magnitude, orientation, location of the internal discontinuities, and surface morphology (i.e., roughness). The comprehension of the packing process allows to understand the adhesion process and define general guidelines to predict the fouling phenomenon
Microtomography of Soil and Soot Deposits: Analysis of Three-Dimensional Structures and Surface Morphology
No full textThe detrimental effects generated by the gas turbine fouling phenomenon are well known. Due to soil and soot particles ingestion, gas turbines experience performance drops related to greater fuel consumption and even lower efficiency. These effects are related to the modification of the shape and surface roughness of relevant surfaces (compressor and turbine blades and vanes, especially) due to the presence of a thin layer generated by micro/nanosized particle adhesion. Such contaminants are swallowed by the unit and, as a function of the operating conditions, adhere to the surface, causing a sort of dangerous coating to the surface. In this work, a microtomography analysis of the deposited layer is reported. The deposited layer has been generated using microsized soil and soot powders under specific impact conditions and substrate surface roughness similar to those in the cold section of a gas turbine compressor. The microtomography analysis has been carried out using the beamline at the ELETTRA Sincrotrone research center. Thanks to the resolution of the beamline, the detection of the three-dimensional internal structure of the soil and soot layers have revealed that within the layer, the structure is characterized by discontinuities. Soot and soil particles, even characterized by similar diameter distributions and test conditions, generate layer structures that differ by the magnitude, orientation, location of the internal discontinuities, and surface morphology (i.e., roughness). The comprehension of the packing process allows us to understand the adhesion process and define general guidelines to predict the fouling phenomenon
An Evolutionary Variant of Q-learning for Global Optimization
No full textThe design of complex aeronautical systems requires the solution of multidisciplinary design optimization (MDO) problems. Despite the considerable technological progress of the last decades, MDO is still computationally expensive and involves a large number of high-fidelity simulations to evaluate the objective functions. This paper describes a derivative-free optimization method, based on reinforcement learning, for global unconstrained optimization problems. Specifically, an evolutionary variant of the well-known Q-learning (QL) algorithm, namely the Evolutionary Q-learning (EVQL), is developed to reduce the computational cost needed to find the optimal solution. Both single- and multi-agent formulations’ performance are assessed on six analytical benchmark problems, showing how EVQL outperforms the QL in terms of learning process acceleration and accuracy. Furthermore, EVQL is also compared with a deterministic particle swarm optimization, providing comparable results. Finally, the EVQL algorithm is used for the solution of an aeroacoustic problem, pertaining to the identification of optimal engine installation to maximize noise shielding. Specifically, the multi-agent EVQL is used to find the minimum of the insertion loss (IL), conditional to the noise source location and the Mach number. The IL is given by a surrogate model, based on stochastic radial basis functions, and trained by a boundary element method solver. The EVQL has a double use for the present application: a) it is used to find the minimum of the objective function, b) it defines where to add new training points based on the surrogate uncertainty, which is used to define algorithm trust regions. to find the optimal solution of the surrogate function and to identify new samples to update the metamodel
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
Erosive wear behavior of a high-chromium cast iron: effect of different destabilization heat treatments
No full textHigh-chromium cast irons are typically used in high demanding applications, where low production costs and wear performance are key factors. The excellent resistance of these alloys to both abrasion and erosion results from the overall microstructural features, i.e. type, morphology and distribution of hard primary and secondary carbides, along with the matrix constituents. More specifically, it has been suggested that erosive wear resistance is strongly affected by the behavior of the matrix. Hence, according to the specific chemical composition, the microstructure can be tuned, e.g. by increasing its hardness and toughness, to enhance the lifetime of such wear- resistant materials. Heat treatments performed to destabilize the austenite promote its transformation into martensite, the coalescence and thickening of secondary carbides thus increasing the overall hardness of the material.
The present study investigates the effects of different destabilization heat treatments performed on a hypereutectic white cast iron, with 21 % Cr and 4 % Nb, applied as an hardface coating to a low carbon steel substrate to improve its erosive wear resistance. The hardfacing plate was made by the open-arc welding of a flux- cored wire: high-chromium cast iron electrodes were deposited onto the substrate. The as-received and the heat- treated material was analyzed through optical and scanning electron microscopy, X-Ray diffraction and hardness tests to determine the relationship between microstructural variations and heat-treatment parameters. The erosive resistance was evaluated per the ASTM G76 standard in a purpose-built air blast test rig: experiments were performed with clinker powder (with a d90 of 9.73 μm) as erodent particles at an impingement angle of 90°. The tribological performance was assessed through weight loss measurements and analyses of the worn surfaces. The results suggest that erosion is dependent on carbide volume fraction and hardness along with the matrix microstructure: the hardest martensitic matrix gave the greatest erosion resistance
Successful treatment of pure red cell aplasia in systemic lupus erythematosus with cyclosporin A : a case report
No full textWe report a patient with longstanding systemic lupus erythematosus (SLE) who developed pure red cell aplasia (PRCA). This condition is rare in connective tissue diseases and is reported in 32 previous cases of SLE in literature. Our patient recovered, apparently in response to treatment with high dosage of corticosteroids, but relapse occurred when the prednisone dosage was tapered down to 10 mg/day. The patient was successfully treated with cyclosporin A with no recurrence of the disease in the last 2 years
Square Dancing with the Stars to Enhance Dynamic Hirschman Linkages?
No full textIn this Presidential Address, the author takes the reader on a reconnaissance of his life and time as a regional scientist. He points out scenery he found scintillating along the way, hoping that some may pick up the banner and chew on a few of the ideas for a while. He suggests a revisit to Albert O. Hirschman’s notion of key sectors and more empirical analysis related to Marcus Berliant’s and Masahisa Fujita’s notion of knowledge creation and transfer.Presidential Address, San Antonio, Texas, March 29, 2014 (53rd Meetings of the Southern Regional Science Association
Appropriate Similarity Measures for Author Cocitation Analysis
No full textWe 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
Analysis of soil and soot deposits by X-ray computed microtomography
No full textEngineering systems continuously interact with nano-sized particles coming from natural or artificial sources. Considering nano-sized contaminants the detrimental effects are known as soiling and fouling phenomena. The objective of the present work is to gather morphological and three-dimensional quantitative information of discontinuities such as voids and pores within the nano-sized particle layer and in correspondence to the substrate/layer interface using microtomography. Deposits are obtained through impact tests using fine powder (soil and soot), and there are realized on a low-density material substrate characterized by different surface roughness values. The packing process and the discontinuities at the substrate-to-layer interfaces explain macroscopic effects as detachment and spallation but at the same time, give the possibility to understand more in detail the process related to the deposition mechanisms involved in this type of test defining general guidelines to predict the fouling phenomenon and to improve the capability of the removal process
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