170,303 research outputs found
Launch Vehicles Multidisciplinary Optimization, a Step from Conceptual to Early Preliminary Design
No full textA recent collaboration between Politecnico di Milano and Universität Bremen within ESA’s PRESTIGE PhD program has stemmed a significant research effort in the field of Multidisciplinary Design Optimization (MDO) for launch vehicles. This work is aimed at the development and integration of optimization algorithms and engineering methods in a software environment capable of assisting in the conceptual and early preliminary design of space launchers, potentially leading to relevant reductions in development effort and life cycle cost. The implemented MDO approach allows in fact efficient exploration of the design space throughout successive global and local, single and multi-objective optimization processes, guided by the engineering experience of the designer. The main obstacle to the successful application of MDO lays in the difficult task of finding a good compromise between models simplicity and accuracy. To tackle this issue, the engineering models were developed in two successive levels of detail, from conceptual to early-preliminary design. The paper is focused on this modelling effort, showing how a critical analysis of the first level’s results was exploited to improve fidelity and functionality. An overview of the conceptual design models is first presented, together with a quantitative assessment of their accuracy and of the impact of the disciplinary errors on global performance indexes. The models selection converged towards well-known disciplinary tools (NASA’s CEA and USAF’s Missile DATCOM), complemented by a set of ad hoc models in the following disciplines: propulsion, geometry, aerodynamics, weights, trajectory, guidance and control, costs and reliability assessment. The validation campaign showed how system-level errors in performance below 20% can be expected, and allowed identifying the most critical modelling aspects to be improved. In a second part, the paper focuses on the model enhancements stemming from the analysis of the conceptual design results, in particular: solid grain geometry and internal ballistics analysis, pressurization systems and engine cycles modelling, simplified structural sizing for all load bearing components, effect of wind and steering losses on the trajectories, and safety-related analyses (boosters/stages impact ellipse determination, upper stage end-of-life strategy). Validation results are presented with a comparison of the conceptual and early preliminary frameworks, highlighting the advantages in terms of accuracy (down to 12% of worst case system error on performance) with a limited increase in computational effort. The foreseen future research lines are finally discussed, especially those aimed at further increasing the design fidelity and at targeting less traditional launch systems, such as manned and reusable vehicles
A First Step Towards SVAGO: the Space Vehicles Analysis and Global Optimization MDO Tool
No full textBranch and Bound Technique to Efficiently Solve Control and System Design Problems with Mixed-Integer Variables Domains
No full textGoing 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
Mitomycin C in highly myopic eyes - Author reply
No full textOphthalmology. 2005 Feb;112(2):208-18; discussion 219.
Mitomycin C modulation of corneal wound healing after photorefractive keratectomy in highly myopic eyes.
Gambato C, Ghirlando A, Moretto E, Busato F, Midena E.
SourceRefractive Surgery Service and Antimetabolite Therapy Research Unit, Department of Ophthalmology, University of Padova, Padova, Italy.
Abstract
PURPOSE: To evaluate the role of topical mitomycin C in corneal wound healing (CWH) after photorefractive keratectomy (PRK) in highly myopic eyes.
DESIGN: Prospective, double-masked, randomized clinical trial.
PARTICIPANTS: Seventy-two eyes of 36 patients affected by high (>7 diopters) myopia.
METHODS: In each patient, one eye was randomly assigned to PRK with intraoperative topical 0.02% mitomycin C application, and the fellow eye was treated with a placebo. Postoperatively, mitomycin C-treated eyes received artificial tears (3 times daily, tapered in 3 months), whereas the fellow eye was treated with fluorometholone sodium 2% and artificial tears (3 times daily, tapered in 3 months).
MAIN OUTCOME MEASURES: Uncorrected visual acuity (UCVA) and best-corrected visual acuity (BCVA), contrast sensitivity, manifest refraction, and biomicroscopy. Contrast sensitivity was determined using the Pelli-Robson chart. Corneal confocal microscopy documented CWH.
RESULTS: Mean follow-up was 18 months (range, 12-36). No side effects or toxic effects were documented. At 12-month follow-up examination, UCVAs (logarithm of the minimum angle of resolution) were 0.4+/-0.48 and 0.5+/-0.53 (P = .03) in mitomycin C-treated eyes and corticosteroid-treated eyes, respectively. At 1 year, corneal haze developed in 20% of corticosteroid-treated eyes, versus 0% of mitomycin C-treated eyes. At 12, 24, and 36 months, corneal confocal microscopy showed activated keratocytes and extracellular matrix significantly more evident in untreated eyes (Ps = 0.004, 0.024, and 0.046, respectively).
CONCLUSION: Topical intraoperative application of 0.02% mitomycin C can reduce haze formation in highly myopic eyes undergoing PRK.
Comment in
Ophthalmology. 2006 Feb;113(2):357; author reply 357-8
Dispelling the Myths Behind First-author Citation Counts
No full textWe 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
- …
