1,720,954 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Analysis of Oil Lubricated Thrust Collars for Application in Integrally Geared Compressors
Get PDFIntegrally geared compressors (IGCs) comprise of single stage impellers installed on the ends of pinion shafts, all driven by a main bull gear (BG) and shaft system. When compared to single shaft multistage centrifugal compressors, the benefits of IGCs include better thermal efficiency, reduced footprint and simple foundation, dispensing with a high speed coupling, as well as better access for maintenance and overhauls. In IGCs the compression of the process gas induces axial loads on the pinion shafts that are transmitted via thrust collars (TCs) to the main drive shaft and balanced by a single thrust bearing. The TCs, located on either side of pinion gears, slightly overlap with the BG outer diameter to form lentil-shaped lubricant-wetted regions.
A numerical model, based on classical thin film lubrication theory, predicts the force response of lubricated thrust collars for use in integrally geared compressors. The predictive model determines performance parameters such as lubricant flow rate, mechanical power loss, peak pressure, lubricant temperature rise, as well as rotordynamic stiffness and damping coefficients for a lubricated TC and bull gear pair. A Newton-Raphson based iterative procedure determines an equilibrium operating position for a given set of TC and BG operating conditions and geometry. Periodic, dynamic displacements from the equilibrium position renders complex dynamic stiffnesses (H = K+i��C), from which the fluid film force and moment stiffness and damping coefficients are determined.
In a lubricated thrust collar and bull gear, a hydrodynamic pressure builds in the lower half of the lubricated zone and lubricant cavitation occurs in the upper half. The minimum film thickness and peak pressure in the lubricated zone shift location due to the difference in taper angles between the TC and BG surfaces.
For a given applied load, a study on the differences in taper angle between the TC and BG surfaces reveals that current angular tolerances of ��0.1 produce TC/BG pairs with similar performance parameters (power loss, lubricant temperature rise, etc.) and dynamic force and moment stiffness and damping coefficients. Increasing the taper angles of both the TC and BG decreases mechanical power loss and lubricant temperature rise, but also decreases the fluid film axial stiffness and damping coefficients. In addition, static angular misalignments of the TC and BG about the horizontal (x) axis joining the BG and TC centers alters the shape and extent of the lubricant cavitation region. This change alters the load carrying capacity and mechanical power losses of the lubricated element.
For the specific TC/BG pair investigated herein, there exists a maximum difference between the two taper angles for which the mechanical element can support an imposed thrust load. Increasing the speed of the BG (and proportionally the TC speed) increases the mechanical power loss and lubricant temperature rise and decreases the fluid film axial stiffness and damping coefficients. As with most fluid film bearings, increasing the applied load increases the power loss, lubricant temperature rise, and axial stiffness and damping coefficients.
The thesis delivers a predictive tool, yet to be benchmarked against experimental data, that provides insight to both the static and dynamic force performance of a lubricated thrust collar, not currently in the archived literature
Experiments and Predictions with a Foil Thrust Bearing Supported by Metal Mesh Screen
Get PDFModern micro-turbomachines (MTM) often employ light weight rotors operating at speeds in excess of 50 krpm. These turbomachines require radial and thrust bearings capable of handling high speeds, while also often operating in extreme environments (high and low temperature). Flexible structure air bearings, or foil bearings, provide an economical and clean (no oil or contamination) means of rotor support in this niche. To date, the most commonly utilized foil bearing is the bump-type foil bearing, which utilizes thin corrugated foil strips to support a smooth top foil on which hydrodynamic pressure builds. Despite its widespread use, each bump-foil is complex to build and model, requiring of extensive engineering knowledge and experience. As such, researchers continue to search for cheaper and less complex alternatives to the traditional bump-foil structure. Metal mesh, readily available and relatively cheap, is a viable option for radial foil bearings, but has yet to be investigated for use in foil thrust bearings. The dissertation presents the design and manufacture of a novel Rayleigh-step thrust foil bearing, whose top foil is supported by a circular layer (or layers) of compliant metal mesh screen. Static and dynamic load excitation tests (no rotor speed) with the prototype bearing reveal a structural stiffness which increases with the mass ratio of the mesh screen and decreases with the number of mesh layers used to support the top foil. Dynamic load excitation tests, up to 300 Hz., give a material loss factor γ ~ 0.2 for the mesh structure which is relatively unaffected by frequency. Foil bearings rely on structural damping from material hysteresis, and bump-type foil bearings exhibit a material loss factor which decreases quickly with an increasing excitation frequency, thus lending credence to metal mesh screen as a bearing support structure. Despite the promise of the novel Rayleigh-step foil thrust bearing, experiments with rotor speed failed several of the prototype top foils, the root-cause of which is attributed to the waviness of the metal mesh layers. This waviness unevenly bulges the thin top foil towards the spinning collar, causing solid contact between the metal top foil and collar before hydrodynamic pressure builds over the pads to separate the surfaces. The failure of several prototype foils points to the need of a robust coating for foil bearings, which can not be ignored by designers. Tests with a redesigned prototype, incorporating a circumferential taper and segmented pads, proved the bearing concept. A single pad bearing with a 55° arc extent and three layers of 40 OPI (openings per inch) mesh achieved a modest specific load of W/A = 35 kPa (per pad) at a rotor speed Ω = 40 krpm (ω·Rmid ~ 160 m/s). Further tests with a six-pad circumferentially tapered metal mesh foil thrust bearing (MMFTB) determined an ultimate load capacity of W/A ~ 25 kPa for Ω = 40 krpm. At this load capacity, the bearing temperature rise (measured via a thermocouple on the top foil backside) exceeded 110 °C. The decreased load capacity for the six-pad bearing, as compared to the single-pad bearing, results from an uneven distribution of the thrust load between the six pads, as confirmed by temperature measurements on three of the top foil undersides. Similar to the Rayleigh-step top foil, the circumferentially tapered MMFTB suffered from taper height disparities between the pads, attributed to the waviness of the mesh, which is exacerbated by stacking multiple layers. In addition to the experimental work with compliant surface thrust bearings supported by metal mesh screen, the dissertation provides a thermo-elastohydrodynamic model, validated (to some degree) with cases from the literature. However, bearing drag torque measurements and predictions for a six-pad MMFTB do not agree well. A simple Couette-flow approximation for the bearing drag torque shows that the bearing would need to operate with a uniform film thickness (hconst = 2.7 μm) to produce the measured bearing drag torque (Texp = 180 N.mm). This minute film thickness is within typical combined roughness (rotor collar + pad roughness) values, and as such likely does not produce a full fluid film during operation. The simple analysis, along with posttest photographs of the thrust collar surface, corroborates the notion that the test bearing operated in the mixed-lubrication regime, with continuous sliding contact between asperities on the rotor collar and the pads coated with a sacrificial lubricant (MoS2). A further analysis with the current model compares the steady-state performance of a MMFTB with 3 sheets of 40 OPI mesh to that of a well-known BFTB geometry from the literature. Predictions show that the MMFTB has a nearly identical drag toque to that of the BFTB for loads 10 ≤ W/A ≤ 50 kPa and a large rotor speed Ω = 70 krpm (ω·Rmid ~ 279 m/s), although it has a slightly smaller film thickness (and likely a lower ultimate load capacity). Predictions with Ω = 70 krpm (ω·Rmid ~ 279 m/s) and a specific load W/A ≤ 30 kPa show that that the MMFTB operates slightly cooler than the BFTB (ΔTmax ~ 105 vs 115 K) when no cooling flow applies to the bearing. Applying cooling flow (0 ≤ Qcf ≤ 900 LPM) through the bearing center can decrease the peak film and top-foil temperatures by ~ 40 °C, while decreasing the collar temperature by up to 10 °C. For the bearing geometry utilized in the current predictions (see Table 14), a cooling flow rate Qcf ≥ 100 LPM fully supplies the pad leading edge, such that the bearing draws no fluid from the ambient air surrounding the bearing. Increasing the cooling flow beyond this point cools the pads via forced convection in the areas beneath the pad tapers (albeit little). For the MMFTB, the mesh screens under the pad and in the land section obstruct cooling flow, such that the cooling is less effective (when compared to the BFTB). Importantly, utilizing an aluminum collar (with a high thermal conductivity, κ = 130 W/m2K) limits the thermal gradient through the collar thickness to 5 °C, reducing the possibility of thermal bending and a corresponding thermal runaway event. The dissertation adds to the archival literature on gas foil thrust bearings and provides a model for prediction of their performance. The failures detailed herein provide important lessons for foil bearing designers and researchers. Further research is needed to either qualify or disqualify metal mesh sheets as a viable underspring structure for gas foil thrust bearings, although the following document provides several cautions against their use
Variations on the Author
Get PDF“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
Get PDFWe 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
Get PDFWe 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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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