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Development of a mathematical model for the dynamic optimization of batch reactors, and MINLP synthesis of plug-flow reactors in complex networks
Marcel Ropotar, Zdravko Kravanja, 2007, izvirni znanstveni članek

Opis: This paper describes the development of a robust and efficient reactor model suitable for representing batch and plugflow reactors (PFRs) in different applications. These would range from the nonlinear (NLP) dynamic optimization of a stand-alone batch reactor up to the mixed-integer nonlinear (MINLP) synthesis of a complex reactor network in overall process schemes. Different schemes for the Orthogonal Collocation on Finite Element (OCFE) and various model formulations, in the case of MINLP model, were studied in order to increase the robustness and efficiency of the model. A deterministic model for known kinetics was obtained for batch and PFR reactors and extended for uncertainties in process parameters and reaction kinetics when the kinetics is unknown. Different variations of the developed model were applied to certain problems, as examples. The first motivating example was the dynamic optimization of batch reactor and the second the MINLP synthesis of a process scheme for the production of allyl chloride. The NLP version of the model with moving finite elements was found to be the most efficient for representing a batch reactor in the dynamic optimization example, and PFR trains in the process synthesis example.
Ključne besede: batch reactor, orthogonal collocation, off-line optimization, on-line optimization, flexibility, uncertainty
Objavljeno: 31.05.2012; Ogledov: 1116; Prenosov: 3
.pdf Polno besedilo (520,87 KB)

QoS-oriented design of embedded systems with specification PEARL
Roman Gumzej, Wolfgang A. Halang, 2007, izvirni znanstveni članek

Opis: Only recently have methodical tools adequate to design real-time systems been formally introduced in design methodologies. Naturally, they were present fromthe beginning, but due to the large diversity of embedded systemsć areas of deployment, specially dedicated formalisms have been developed and used. High-level language programming and integration of modeling formalisms into design methods eased the development of more complex real-time applications. With the emerging object-oriented programming languages and design methods, their integration into larger information systems has become more transparent. It was the UML methodology, however, which eventually merged also the design methods and concepts of real-time systems into a consistent whole. It took a large consortium and a long process to persuade industry of the benefits the new integral methodology can offer. On the other hand, there are some trade-offs, and there are some features not completely covered, yet. Here, a different, more straightforward approach to program and design (embedded) real-time systems is presented. Since it emerged from the real-time community, it includes most features relevant there. Independent of the UML profile for schedulability, performance and time specification, a profile was devised for use in PEARL-oriented UML design. The strengths of the mentioned language and design methods for QoS-oriented design of (embedded) real-time systems are emphasised throughout this article.
Ključne besede: real time systems, embedded systems, quality of service, co-design, co-simulation, safety, dependability, security, timeliness, determinism, readiness, reliability, portability, flexibility, specification PEARL, UML profiles, patterns
Objavljeno: 31.05.2012; Ogledov: 1142; Prenosov: 3
URL Polno besedilo (0,00 KB)

Application of group technology in complex cluster type organizational systems
Slobodan Morača, Miodrag Hadžistević, Igor Drstvenšek, Nikola Radaković, 2010, izvirni znanstveni članek

Opis: The aim of this research was to contribute to the development of structural design procedures of complex - cluster type organizational systems. Industrial clusters can help companies to improve their own market positions, effectiveness, productivity and product quality. Organization of the production process in a company is an extremely complex process itself, and when it is transferred to the cluster level, the result is a complex task which is difficult to solve. For that purpose, this paper analyses the conditions and possibilities that would enable those structures to adapt to changes in the surroundings - flexibility and management adequacy of production and organizational structures - by lowering the degree of complexity. For the time being, no simple models which would enable an increase of process effectiveness in complex organizational units like clusters have been developed. One of the possible solutions which would decrease the complexity of flows and increase process effectiveness within an industrial cluster is the application of Group approach.
Ključne besede: industrial clusters, group technology, planning, work cells, complexity, flexibility
Objavljeno: 31.05.2012; Ogledov: 881; Prenosov: 2
URL Polno besedilo (0,00 KB)

Optimum design of plane timber trusses considering joint flexibility
Simon Šilih, Miroslav Premrov, Stojan Kravanja, 2005, izvirni znanstveni članek

Opis: The paper presents the optimization of metal-plate-connected plane timber trusses with special emphasis on joint flexibility. The optimization was performed by the non-linear programming approach. Since various truss design parameters such as type of truss configuration, span/depth ratio, number and type of diagonal and vertical members and type of joint connections simultaneously affect each other, it is proposed that all of these parameters should be considered simultaneously in a single mathematical model. An optimization model for cost optimization of timber trusses was thus developed. The economic objective function for minimizing the structure's self-manufacturing costs was defined, subjected to the design, stress and deflection (in)equality constraints. The finite element equations were as the equality constraints defined for the calculation of the internal forces and the deflections of the structure. The stiffness matrix of the structure was composed by considering fictitiously decreased cross-section areas of all the flexibly connected elements. Constraints for the dimensioning of the timber members were determined in accordance with Eurocode 5 in order to satisfy the requirements of both the ultimate and the serviceability limit states. The cross-section dimensions and the number of fasteners were defined as independent optimization variables. A numerical example demonstrates the applicability of the optimization approach presented as well as the influence of the fasteners' flexibility on the optimal self-manufacturing costs.
Ključne besede: civil engineering, timber structures, trusses, fasteners, flexibility, optimization, non-linear programming
Objavljeno: 01.06.2012; Ogledov: 777; Prenosov: 13
URL Polno besedilo (0,00 KB)

Identification of critical points for the design and synthesis of flexible processes
Zorka Novak-Pintarič, Zdravko Kravanja, 2008, izvirni znanstveni članek

Opis: Optimization problems for the design and synthesis of flexible chemical processes are often associated with highly discretized models. The ultimate goal of this work is to significantly reduce the set of uncertain parameter points used in these problems. To accomplish the task, an approach was developed for identifying the minimum set of critical points needed for flexible design. Critical points in this work represent those values of uncertain parameters that determine optimal overdesign of process, so that feasible operation is assured within the specified domain of uncertain parameters. The proposed approach identifies critical values of uncertain parameters a-priori by the separate maximization of each design variable, together with simultaneous optimization of the economic objective function. During this procedure, uncertain parameters are transformed into continuous variables. Three alternative methods are proposed within this approach: the formulation based on Karush-Kuhn-Tucker (KKT) optimality conditions, the iterative two-level method, and the approximate one-level method. The identified critical points are then used for the discretization of infinite uncertain problems, in order to obtain the design with the optimum objective function and flexibility index at unity. All three methods can identify vertex or even nonvertex critical points, whose total number is less than or equal to the number of design variables, which represents a significant reduction in the problem's dimensionality. Some examples are presented illustrating the applicability and efficiency of the proposed approach, as well as the role of the critical points in the optimization of design problems under uncertainty.
Ključne besede: chemical processing, process synthesis, flexibility, design, critical point, vertex, nonvertex
Objavljeno: 01.06.2012; Ogledov: 756; Prenosov: 3
URL Polno besedilo (0,00 KB)

A strategy for MINLP synthesis of flexible and operable processes
Zorka Novak-Pintarič, Zdravko Kravanja, 2004, izvirni znanstveni članek

Opis: Abstract This paper presents a sequential two-stage strategy for the stochastic synthesis of chemical processes in which flexibility and static operability (the ability to adjust manipulated variables) are taken into account. In the first stage, the optimal flexible structure and optimal oversizing of the process units are determined in order to assure feasibility of design for a fixed degree of flexibility. In the second stage, the structural alternatives and additional manipulative variables are included in the mathematical model in order to introduce additional degrees of freedom for efficient control. The expected value of the objective function is approximated in both stages by a novel method, which relies on optimization at the central basic point (CBP). The latter is determined by a simple set-up procedure based on calculations of the objective functionćs conditional expectations for uncertain parameters. The feasibility is assured by simultaneous consideration of critical vertices. The important feature of the proposed stochastic model is that its size depends mainly on the number of design variables and not on the number of uncertain parameters. The strategy is illustrated by two examples for heat exchanger network synthesis.
Ključne besede: chemical processing, process synthesis, MINLP, mixed integer nonlinear programming, flexibility, operability, controllability, steady state model
Objavljeno: 01.06.2012; Ogledov: 1229; Prenosov: 3
URL Polno besedilo (0,00 KB)

Fluctuating excitability : a mechanism for self-sustained information flow in excitable arrays
Matjaž Perc, 2007, izvirni znanstveni članek

Opis: We show that the fluctuating excitability of FitzHugh-Nagumo neurons, constituting a diffusively coupled excitable array, can induce phase slips that lead to a symmetry break yielding a preferred spreading direction of excitatory events, thus enabling persistent self-sustained and self-organized information flow in a periodic array long after a localized stimulus perturbation has sized. Possible oscillation frequencies of the information-carrying signal are expressed analytically, and necessary conditions for the phenomenon are derived. Our results suggest that cellular diversity in neural tissue is crucial for maintaining self-sustained and organized activity in the brain even in the absence of immediate stimuli, thus facilitating continuous evolution of its mechanisms for information retrieval and storage.
Ključne besede: physics, complex systems, dynamical systems, flexibility, chaos, chaotic systems, oscillations, perturbation
Objavljeno: 07.06.2012; Ogledov: 806; Prenosov: 1
URL Polno besedilo (0,00 KB)

Eva Lorenčič, 2013, diplomsko delo

Opis: Unemployment negatively affects the government budget and inflicts costs on the unemployed individuals as well as on the society as a whole in many ways. The direct costs of unemployment for the state budget arise due to the benefits paid out to the unemployed, and due to foregone direct and indirect taxes. On the level of an individual, nonmonetary costs are a multiple of the monetary ones, which indicates that employment plays a central role in a person’s well-being. Unemployment has a spill-over effect on the society as a whole and affects not only the unemployed, but also the employed population, particularly in the areas experiencing high unemployment rates. We take the case study of Germany, the country often used as a quintessence of how to overcome the problem of massive unemployment. Before the four Hartz reforms, which were implemented in years 2003, 2004 and 2005, Germany was characterized as the »sick man of Europe« owing to its exploding levels of unemployment, dysfunctional labor market, unsustainable social budget, and rising public debt. As the reforms were put into force, the situation in the German labor market underwent a 360° turnaround and became portrayed as the »German economic miracle«. The true effects of the reforms fully transpired in the economic and financial crisis of 2008/2009, when most countries experienced escalating unemployment rates, while Germany kept them at bay. Studies show that flexible working-time arrangements and short-time work have played a major role in inhibiting lay-offs in German companies during the recent crisis, and hence in preventing a spike in unemployment rates, which we use as one of the points that validate our Model of employment. The government subsidies made it possible for companies to reduce the working hours of their employees instead of laying them off. An important feature of the German labor market is a comparatively high percentage of part-time workers (around 25% in years 2006–2011), which also explains the low unemployment rates Germany has been recording since the reforms were introduced. The reforms cut the amount and duration of unemployment benefits, which gave the unemployed more incentives to find a job. A negative aspect of the reform may be an increased divide between the core work force and the marginal workers, which intensifies social differences and is currently on the agenda of German politics. Also some researches stress the importance of social cohesion and that the risks and opportunities in the labor market should be more evenly distributed. In the accompanying explanations to the Model of employment, some of the successful measures of Germany’s Hartz reforms may be spotted, while at the same time we avoid the »mistakes« of these reforms, thus making sure that social differences among people would narrow down upon the real-life implementation of the Model. When the number of job vacancies sharply exceeds the number of unemployed, and when there are poor prospects of new job openings in the near future, the only viable solution to the problem of unemployment is to reduce the working hours of the existing employees. The Model of employment that we develop represents a possible solution to the problem of unemployment and thus addresses the need to reduce the costs of unemployment to the unemployed individual, to the state budget, and to the society as a whole. The crux of the Model is a re-distribution of the total number of workplaces and total annual working hours among the »new actively employed population«, which comprises all persons having the potential to become or stay employed or self-employed in a given year. The model allows for a flexible arrangement of working hours – from four and up to eight hours per employee per day. Our solution will release the working hours, make them available to the currently unemployed, and hence ensure a more equitable distribution of income and work load amongst the population. In addition, as the workers will be less tired and stressed-out, their productivity will in
Ključne besede: unemployment, labor force, labor market, part-time employment, full-time employment, labor market flexibility, costs of unemployment, Hartz reforms
Objavljeno: 30.08.2013; Ogledov: 1011; Prenosov: 97
.pdf Polno besedilo (1,23 MB)

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