306,582 research outputs found
Inducible defenses and the dynamics of planktonic food chains
No full textContains fulltext :
30214.pdf (Publisher’s version ) (Open Access)Radboud Universiteit Nijmegen, 19 oktober 2007Promotor : Donk, E. van Co-promotores : Mooij, W.M., Vos, M.144 p
Complex-systems design methodology for systems-engineering collaborative environment
No full textIn the last decades man-made systems have gained in overall complexity and have become more articulated than before. From an engineering point of view, a complex system may be defined as one in which there are multiple interactions between many different elements of the system and many different disciplines concurring to its definition. However, the complexity seen from the system perspective is only partial. In more general terms complexity does not only regard the system per se, but it is also related to the whole life-cycle management of the system. This encompasses all the activities needed to support the program development from the requirements definition to the verification, validation and operation of the system in the presence of a large number of different stakeholders. These two interrelated views of complexity, being bottom-up in the first case and top-down in the second, both converge to the system defined as an entity formed by a set of interdependent functions and elements that complete one or more functions defined by requirements and specifications. Systems Engineering processes have been increasingly adopted and implemented by enterprise environments to face this increased complexity. The purpose is to pursue time and cost reduction by a parallelization of processes and activities, while at the same time maintaining high-quality standards. From the life-cycle management point of view the tendency has been to rely more and more on software tools to formally applying modelling techniques in support of all the activities involved in the system life-cycle from the beginning to the end. The transition from document-centric to model-centric systems engineering allows for an efficient management of the information flow across space and time by delivering the right information, in the right place, at the right time, and to the right people working in geographically-distributed multi-disciplinary teams. This standardized implementation of model-centric systems engineering, using virtual systems modelling standards, is usually called Model Based Systems Engineering, MBSE. On the other side, looking at the problem from the perspective of the system as a product, the management of complexity is also experiencing a radical modification. The former adopted approach of sequentially designing with separate discipline activities is now being replaced by a more integrated approach. In the Aerospace-Engineering domain, for instance, designing with highly integrated mathematical models has become the norm. Already from the preliminary design of a new system all its elements and the disciplines involved over the entire life-cycle are taken into account, with the objective of reducing risks and costs, and possibly optimizing the performance. When the right people all work as a team in a multi-disciplinary collaborative environment, the MBSE and the Concurrent Engineering finally converge to the definition of the system. The main concern of the engineering activities involved in system design is to predict the behavior of the physical phenomena typical of the system of interest. The development and utilization of mathematical models able to reproduce the future behavior of the system based on inputs, boundary conditions and constraints, is of paramount importance for these design activities. The basic idea is that before those decisions that are hard to undo are made, the alternatives should be carefully assessed and discussed. Despite the favorable environment created by MBSE and Concurrent Engineering for the discipline experts to work, discuss and share knowledge, a certain lack of engineering-tool interoperability and standardized design methodologies has been so far a significant inhibitor, (International Council on Systems Engineering [INCOSE], 2007). The systems mathematical models usually implemented in the collaborative environments provide exceptional engineering-data exchange between experts, but often lack in providing structured and common design approaches involving all the disciplines at the same time. In most of the cases the various stakeholders have full authority on design issues belonging to their inherent domain only. The interfaces are usually determined by the experts and manually fed to the integrated models. We believe that the enormous effort made to conceive, implement, and operate MBSE and Concurrent Engineering could be consolidated and brought to a more fundamental level, if also the more common design analytical methods and tools could be concurrently exploited. Design-space exploration and optimization, uncertainty and sensitivity analysis, and trade off analysis are certainly design activities that are common to all the disciplines, consistently implemented for design purposes at the discipline-domain level. Bringing fundamental analysis techniques from the discipline-domain level to the system-domain level, to exploit interactions and synergies and to enable an efficient trade-off management is the central topic discussed in this chapter. The methodologies presented in this chapter are designed for their implementation in collaborative environments to support the engineering team and the decision-makers in the activity of exploring the design space of complex-system, typically long-running, models. In Section 2 some basic definitions, terminology, and design settings of the class of problems of interest are discussed. In Section 3 a test case of an Earth-observation satellite mission is introduced. This satellite mission is used throughout the chapter to show the implementation of the methods step by step. Sampling the design space is the first design activity discussed in Section 4. Then in Section 5 and Section 6 a general approach to compute sensitivity and to support the engineering team and decision makers with standard visualization tools are discussed. In Section 7 we provide an overview on the utilization of a unified sampling method for uncertainty and robustness analysis. Finally, we conclude the chapter providing some recommendations and additional thoughts in Section
Re-entry Shape Optimisation Using the Axisymmetric Analogue Method with Modified Newtonian Technique Resolved Inviscid Flowfield
No full textRe-entry shape and trajectory optimisation studies typically require hundreds to thousands of flow solutions to resolve the heat transfer and skin friction. Due to the fact that full CFD simulations and even Euler simulations are typically very expensive, this work presents a development of a much simpler technique, in which the modified Newtonian approximation is coupled with the axisymmetric-analogue-based viscous method. Afterwards, the developed technique is applied to the shape optimisation study of the DART module originally conducted by Sudmeijer and Mooij in 2002. Large differences in heat fluxes are observed mainly owing to the fact that the present method estimates the transition region from approximate transition criteria while the original study assumes the transition point to be fixed at the interface between the cone and the flare. This emphasises the sensitivity of design optimisation studies on delicate parameters such as the location of transition, which typically cannot be accurately predicted during the conceptual phase. Finally, the limitations of the current method are identified and the areas in which the technique will be improved further are outlined.Virtual/online event due to COVID-19Astrodynamics & Space Mission
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
Orthogonal-Array based Design Methodology for Complex, Coupled Space Systems
No full textThe process of designing a complex system, formed by many elements and sub-elements interacting between each other, is usually completed at a system level and in the preliminary phases in two major steps: design-space exploration and optimization. In a classical approach, especially in a company environment, the two steps are usually performed together, by experts of the field inferring on major phenomena, making assumptions and doing some trial-and-error runs on the available mathematical models. To support designers and decision makers during the design phases of this kind of complex systems, and to enable early discovery of emergent behaviours arising from interactions between the various elements being designed, the authors implemented a parametric methodology for the design-space exploration and optimization. The parametric technique is based on the utilization of a particular type of matrix design of experiments, the orthogonal arrays. Through successive design iterations with orthogonal arrays, the optimal solution is reached with a reduced effort if compared to more computationally-intense techniques, providing sensitivity and robustness information. The paper describes the design methodology in detail providing an application example that is the design of a human mission to support a lunar base
Aerocapture Guidance: Numerical and Artificial Intelligence Solutions
No full textAerospace EngineeringAstrodynamics and Space Mission
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
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