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- ItemOpen AccessA cognition-centered personalization framework for cultural-heritage applications
Τμήμα Ηλεκτρολ. Μηχαν. και Τεχνολ. Υπολογ. (ΔΔ)Ράπτης, Γεώργιος; Raptis, GeorgeCultural-heritage applications provide the visitors with high volume of cultural information and therefore there is a need for personalizing the visit experience to help visitors understand and perceive the provided cultural content according to the characteristics of their cognitive style. However, the current research and design attempts do not consider the diversity of the visitors, in terms of their cognitive styles, as an important design and evaluation factor. Cognitive styles influence the way people seek, collect, organize, process, and recall information and are expected to have an impact on the cultural-heritage domain, which is enriched in cultural information. The results presented in the PhD thesis revealed that visitors with different cognitive styles behave differently when interacting with cultural-heritage applications. Theses differences influence the visit experience, in terms of content understanding and immersion. The current cultural-heritage applications do not consider the diversity of the visitors in terms of cognitive styles, and thus, they unintentionally led to experience imbalances between visitors with different cognitive styles. The PhD thesis presents CogniCHeF, which is a framework that aims to help designers to deliver personalized cultural-heritage applications tailored to the visitors’ cognitive styles, aiming to offer them an enhanced visit experience. Moreover, the results presented in the PhD thesis revealed that the cognitive styles can be elicited automatically with high accuracy during the first stages of interaction between the visitors and the cultural-heritage applications through the use f eye-tracking data. Furthermore, the results presented in the PhD thesis revealed that CogniCHeF can be efficiently used by cultural-heritage designers to implement personalized applications tailored to the visitors’ cognitive styles. Finally, the results presented in the PhD thesis revealed that cognition-centered personalization improved the overall visit experience.
- ItemOpen AccessA constructionist view of complex interactions between inflection and derivation: the case of SMG and Griko
Τμήμα Φιλολογίας (ΔΔ)(2014-09-15) Κουτσούκος, Νικόλαος; Ράλλη, Αγγελική; Booij, Geert; Iacobini, Claudio; Γαβριηλίδου, Ζωή; Μανωλέσσου, Ιώ; Ευθυμίου, Αγγελική; Ξυδόπουλος, Γεώργιος; Koutsoukos, NikolaosOne of the most difficult, but -at the same time- interesting questions in morphological theory is the relation between inflection and derivation. This question lies at the heart of the problem of the architecture of the morphological component and thus raises important issues such as: (a) the relation between the lexicon and the grammar, and (b) the model which best accounts for the relevant facts. In the present thesis, the aim is to examine certain morphological phenomena which reveal the close relation between the two processes, and to show that both should be accounted for in the morphological component. Three aspects of the problem are exhaustively discussed, drawing data from Standard Modern Greek and Griko: (a) the relation between conversion and inflectional classes in Standard Modern Greek, (b) the evolution of derivational affixes into inflectional ones in Griko and (c) the appearance of inflection inside derivation in both Griko and Standard Modern Greek. The analysis of the data is given within a Construction Morphology (CM) framework. CM offers important insights into the problem since it has a strong lexicalist orientation with both inflection and word formation within the lexicon, and the proposed word-formation schema (construction) is applicable to both derivation and inflection. The CM framework provides effective solutions to the problems discussed in the relevant chapters and paves the way for the analysis of similar phenomena. It should be mentioned that the contribution of the present thesis to the relevant discussion can be described in two points: (a) it presents some data that have not been described before and offers an up-to-date analysis, (b) it examines the relation between the two processes within the CM framework.
- ItemOpen AccessA contribution to the theoretical study and numerical calculation of edge diffraction
Τμήμα Μηχανολόγων και Αεροναυπηγών Μηχαν. (ΔΔ)Νικολάου, Πέτρος; Μενούνου, Πηνελόπη; Μενούνου, Πηνελόπη; Σκαρλάτος, Δημήτριος; Ταρουδάκης, Μιχαήλ; Μουρτζόπουλος, Ιωάννης; Παπαδόπουλος, Πολύκαρπος; Πολύζος, Δημοσθένης; Κωστόπουλος, Βασίλειος; Nikolaou, PetrosThe subject of the present thesis is the sound field around solid obstacles and specifically, the diffraction field generated as sound reaches the edges of the obstacles. Consider, for example, the edge of a half-plane or of a wedge. Diffraction is important in many practical problems, such as noise barriers, sonic boom propagation around buildings, room acoustics, volcanic explosions or ancient theater acoustics. The present work aims to derive new analytical solutions and/or to extend existing models in both time and frequency domain. The purpose of the work is to provide new physical insight in the study of diffraction and to accelerate its computation. The analysis begins in the frequency domain, where an existing analytical model, the Directive Line Source Model (DLSM), is extended in areas where it was not valid before. The new model has a unified form for all types of incident radiation, being exact for plane incident waves and approximate for cylindrical and spherical incident waves. It is shown that the diffraction field can be interpreted as radiation from a directional line source (as stipulated by the original DLSM) irrespective of the receiver proximity to a shadow boundary, provided that the directivity of the virtual line source is appropriately modified. The properties of the new directivity function are investigated, its parameters recast and appropriate simpler asymptotic forms presented. Based on the new directivity function a new, precisely defined separation of the diffraction field around the shadow boundaries is proposed, which also provides a computational advantage for large scale simulations. Further, the proposed reformulation of the diffraction solution and of its parameters enables the application of the model to two cases of practical interest: (i) directional sound sources and (ii) diffraction by wedges, where the proposed formulation is considerably faster to compute than well-established solutions. In time domain a new solution in the form of impulse response is also derived as the Fourier transform of the proposed frequency domain solution. The proposed impulse response, as its frequency counterpart, has a unified form for all types of incident signals, is exact for plane incident signals, and approximate for cylindrical and spherical incident signals. The investigation of the derived formulation leads to the derivation of a generator curve that embodies the impulse response at any source–receiver configuration. The generator curve is function of a single variable, namely the diffraction number. The diffraction number is a universal diffraction parameter, which translates the generator curve into impulse response at all times and all source-receiver locations, according to a condition termed the similarity condition. The employment of the generator curve can provide considerable computational benefit compared to direct computations. A separation of the diffracted signal into precisely determined time stages is also proposed. Further, for the case of the spherical incident signal the concept of the generator curve is also extended for the case of the exact solution. The exact solution is embodied in the same generator curve as the approximate solution but with a different diffraction number and similarity condition. A separation of the diffracted signal based on the exact impulse response is also proposed. The work continues in the time domain with the study of the convolution between the impulse response and the incident signal, which is used to obtain the diffracted signal or diffraction response. The primitive functions of the proposed impulse response are employed to: (i) prove that the convolution of the impulse response with any bounded signal is bounded for all times, (ii) obtain analytically the diffraction response, as combination of elementary functions, for any incident signal approximated piecewise by fitting polynomials, (iii) improve the performance of the numerical convolution by orders of magnitude, and (iv) handle the convolution of very sparsely sampled incident signals. The analysis also handles the diffraction of spherical signals incident on edges of finite length. Existing finite length diffraction theory and the proposed impulse response for the infinite edge are combined to form a new impulse response for the finite edge. The new impulse response has a simple analytical form and as opposed to other analytical solutions does not require integration along the edge length to compute the diffracted signal. Because it is based on the impulse response for infinite edges, it inherits all afore mentioned benefits associated with its primitive functions. Furthermore, it offers substantial computational benefit compared to traditional integration formulas along the edge. In the frequency domain the Fourier transform of the new finite length impulse response can be approximated analytically. The resulting frequency formula does not require integration along the edge and offers physical insight on exactly how much different portions of the edge contribute to the total diffraction field.
- ItemOpen AccessA layerwise spectral finite element method for the prediction of stresses and delamination crack growth in composite structures subjected to static and impact loading
Τμήμα Μηχανολόγων και Αεροναυπηγών Μηχαν. (ΔΔ)Σιορίκης, Δημήτριος; Siorikis, DimitrisA sophisticated numerical tool is developed for the prediction of delamination crack onset and propagation in sandwich and laminated composite structures under static and impact loading, which is capable of reducing numerical complexities involved in classical fracture mechanics approaches with substantially improved efficiency and computational savings. The proposed method incorporates: i) novel layerwise mechanics for the accurate prediction of the complex displacement and stress fields through the thickness of the laminate and across ply interfaces of highly heterogeneous composite structures in conjunction with ii) a multi-node spectral finite element model (SFE) with high-order spatial approximation in the plane of the structure and with integration points collocated with the nodes, leading to improved prediction of interlaminar stresses and fracture energy at the nodes, and at the same time yielding a consistent diagonal or nearly diagonal mass matrix, and (iii) inherent capability for the simulation of impact events through the combination of a fast explicit time-integration scheme and contact laws Initially, a newly developed cubic spline layerwise theory (CSLT) combined with a spectral finite element is presented for the fast and precise prediction of through the thickness stress fields at free edges, ply interfaces, resin-rich layers and artificial delaminations. The conducted validations revealed the capabilities and advantages of the present method over other competitive solid FEA based models in critical and highly demanding computational problems, establishing the present model as an attractive candidate for the analysis of delamination propagation problems. Within the framework of the finite element method in conjunction with fracture mechanics theory, the virtual crack closure technique (VCCT) and a cohesive zone model (CZM), the two most famous methods of delamination modelling, are implemented and elaborated. The previously established CSLT is further enriched with additional degrees of freedom (DoFs), representing the resultant discontinuities in the displacement field induced by the presence of delamination cracks, which allows the modelling of delamination crack onset and growth by simply activating the delamination degrees of freedom without FE remeshing. Strain energy release rate is predicted by adapting the VCCT method to rely exclusively on the damage DoFs. Finally, an iterative solution method is developed that takes advantage of the damaged DoFs to quickly predict the crack propagation without remeshing and the proposed numerical scheme is applied to mode I, II and mixed-mode static delamination fracture problems and is validated vs. reference literature solutions, experimental results and standard 2D plane-strain FE models. When a pre-crack exists and the fracture process zone is negligible compared to the specimen size, the VCCT is still a favorable candidate to predict delamination. However, the VCCT scheme suffers from several limitations, such as a-priori knowledge of the crack path, which is very restrictive for the modelling of impact-induced delamination in healthy structures, and the inability to model material softening present in ductile adherents and finite-thickness adhesive joints. In order to address the previous shortcomings of the VCCT approach a cohesive zone approach is also formulated. In this context, a variable kinematic layerwise theory is developed to reduce the numerical complexities involved in classical cohesive approaches, which employs cubic Hermite splines for the composite laminates and the core and linear interpolation functions for the thin resin-rich layers in connection with continuum damage mechanics for predicting and tracking the debonding propagation. The layerwise mechanics and continuum damage mechanics are integrated into a SFE for laminated composite strips, termed thereafter as cohesive-layerwise spectral finite element (CLW-SFE) and the developed computational model is applied to analyze global mode I, mode II, and mixed-mode fracture problems and is correlated with experimental results and reference high-fidelity finite element models for composite sandwich and laminated composite structures. Finally, the previous CLW-SFE is extended to enable the modelling of delamination growth in laminated and sandwich plates under impact loading. The variable kinematic layerwise strip theory is extended in three dimensions to address and explore the three-dimensional generated delamination footprints in laminated and sandwich plates and is subsequently integrated into a time-domain SFE combined with an explicit time integration scheme to model impact loading responses. Admissible contact laws for both rigid and deformable (hailstone) impactors are investigated numerically and experimentally to provide realistic and representative impact loads for laminated composite and sandwich plates. Green Lagrange non-linear strain terms are incorporated into the non-linear explicit impact dynamics to account for large displacements and rotations. The convergence, stability and accuracy of the proposed numerical framework are verified for the entire spectrum of the impact characterization diagram. Obtained results of the developed numerical code are correlated with experimental data and reported high-fidelity finite element models, demonstrating crucial enhancements in computational speed and meshing process due to elimination of cohesive layer parameter calibration, alleviation of finite element aspect ratio constraints, block-diagonal mass matrix and high-convergence rates.
- ItemEmbargoA managerial accounting framework for the performance measurement and management of profit and cost centers of food – retailing organizations in dynamic environments
Τμήμα Διοίκησης Επιχειρήσεων (ΔΔ)Λαμπρόπουλος, Ιωάννης; Lampropoulos, IoannisThe present thesis develops an operational efficiency managerial accounting framework for retailing networks in dynamic environments. The research is motivated by the fact that during adverse economic conditions, retailers focus on improving the operations of their stores, making operational efficiency a strategic priority for them. Also, when the economic environment gradually begins to improve, the management needs the appropriate measurement tool to properly evaluate the efficiency effects of alternative growth modes implemented, either through the opening of new stores or acquiring other store networks under a merger and acquisition (M&A) strategy. The benchmarking analysis based on the advanced bootstrapped DEA tool, is motivated and illustrated utilizing a unique set of micro-data on 106 retail stores of a representative Greek Super Market (S/M) retailing network that operated under a highly recessionary and competitive environment, undertaking a systematic monthly-based analysis over two recessionary years (2012-2013) with a gradual improvement of the operating environment the year after (2014). The proposed managerial framework with the joint use of bootstrap DEA, second-stage regression analysis, radar analysis and decision tree algorithm: a) locates properly the efficiently best and worst performers over time b) identifies the main sources of inefficiency c) proposes efficiency improvements at the operational and strategic level. Additionally, efficiency measurements in different dimensions (i.e. operational and profit) show that efficiency is unequally distributed over recession and between different types of store sizes (i.e., large stores being more efficient than small and medium ones) while higher operational efficiency tends to be related to higher profit efficiency. Also, the undertaken efficiency analysis during normal economic conditions provides evidence that compared to organic growth, M&A result in lower operational efficiency. The proposed performance managerial framework offers a managerial tool for evaluating the efficiency of S/M retailing networks in turbulent times while it provides significant managerial implications for growth purposes when economic conditions are normalized.
- ItemOpen AccessA method for the generation of assembly information from product design : applications in automotive industry
Τμήμα Μηχανολόγων και Αεροναυπηγών Μηχαν. (ΔΔ)Πίντζος, Γεώργιος; Μούρτζης, Δημήτριος; Χρυσολούρης, Γεώργιος; Δέντσορας, Αργύριος; Ανυφαντής, Νικόλαος; Καρακαπιλίδης, Νικόλαος; Αδαμίδης, Εμμανουήλ; Τατσιόπουλος, Ηλίας; Pintzos, GeorgiosAssembly planning methods have been in the centre of industrial and academic research for many decades, since manual assembly costs may often account for even half of the total manufacturing expenses. Existing and emerging manufacturing trends, such as mass customization and personalization, require fast responses when it comes to the conception and realization of the relevant manufacturing systems. Although, work methodologies have been proposed and applied, such as concurrent engineering, gaps still exist between product development and manufacturing. Current Product Lifecycle Management (PLM) systems focus on the coordination of activities among engineers of different disciplines without, however, being capable of providing actual decision support functionality to decision makers. Moreover, solutions for the different phases of assembly planning have been proposed, without however taking into account the holistic nature of assembly planning that spans the different engineering phases. In this work, a method is proposed to enable the generation of assembly information, relevant to different phases of a product’s lifecycle, from product data available during its design. The approach is accompanied by three applications focusing on assembly planning, assembly execution and End of Life (EoL) product handling. The core method is an algorithm that generates assembly precedence relations between all components of a product. The algorithm uses collision / interference detection as well as constraints and information extraction as the main means to identify the assembly relations. The results of the algorithm are then used in three different applications: End of Life value estimation, Assembly Line Balancing and Simulation and Assembly Instructions Generation. The applications target both engineers that work in overlapping phases of product and production development as well as the manual assembly operators during execution of the actual processes. The Assembly Precedence Diagram Generation (APDG) algorithm was developed as an add-on to a widely-used CAD software. The developed application has a minimum input requirement which concerns the selection of the base part(s) as well as configuring the algorithm’s performance through different options. The performance of the algorithm has been tested by data provided by the automotive industry. The rest of the applications were developed using three different approaches. More specifically, the applications relevant to assembly line planning were developed as part of a collaborative web-platform that can also extract and use information residing in engineering files. The assembly instruction generation and visualisation was developed as a separate module and an EoL indicators calculator was developed as a macro application. The development of the applications relevant to assembly line planning involved the elaboration of a new method integrating Discrete Event Simulation (DES) with Assembly Line Balancing (ALB). All proposed applications have been tested on the basis of industrial data and the respective results are provided using time and cost related indicators. The results highlight the benefits of applying the developed methods in industrial environments. More specifically, the effectiveness of the APDG in generating assembly information for use in both production planning and EoL evaluation of products is presented, as well as the added value of the proposed integrated approach (assembly line balancing and simulation) during production design.
- ItemOpen AccessA multi parametric measurement and control system implemented on flexible substrates with printing technologies
Τμήμα Φυσικής (ΔΔ)Μπαρμπάκος, Δημήτριος; Barmpakos, DimitriosIn this doctoral dissertation, materials for development of electronic devices via printing processes were evaluated. More specifically, two silver-based inks with different sintering requirements (chemical and thermal sintering), a conductive polymer (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)), an ethyl-cellulose stabilized graphene dispersion in cyclohexanone – terpineol and a water-based functionalized reduced graphene oxide ink were used for developing humidity, temperature and strain sensors on paper and polyimide substrate. Also, electrical contact methods were evaluated; pre-patterned copper tracks on polyimide were interfaced with inkjet-printed graphene and silver nanoparticle structures via direct over-printing and commercial connectors. A comprehensive literature review was performed for identifying possible material – substrate combinations that could lead to cost-effective, repeatable production of sensors at a mass scale via inkjet printing. Two resistive output humidity sensors, one with Ag-nanoparticle based electrodes and one with PEDOT:PSS on paper substrate were developed, demonstrating the usage of paper substrate as a humidity sensing film with a mean resistance change of one order of magnitude per 10 %rH in the range of 0 to 30 %rH and a mean resistance change of one order of magnitude per 20 %rH in the range 30 – 90 %rH. Ag nanoparticle and PEDOT:PSS-based temperature sensors on paper, and graphene and f-rGO temperature sensors on polyimide substrate were developed and evaluated for their electrical characteristics and their Thermal Coefficients of Resistance (TCR) were extracted as 9.389×10-4 oC-1, -0.0139 oC-1, -1.94x10-3 oC-1 and -1.64x10-2 oC-1, respectively. Graphene-based materials exhibited good thermal cycling endurance and a mean response time of 2.47 s (graphene) and 2.94 s (f-rGO). Ag nanoparticle and PEDOT:PSS-based strain sensors were evaluated for creating a complete sensing platform alongside the aforementioned sensors; gauge factors of GFAgTensile = 0.4259 and GFPEDOT:PSSTensile = 0.1422 for tensile strain and GFAgCompress = -0.1572 and PEDOT:PSS GFPEDOT:PSSCompress = 0.1448 for compressive experiments. Also, long-term mechanical strain sensing was evaluated with a mechanical XY precision stage; samples were bended for 1000 cycles and resistance was measured in parallel. Ag nanoparticle-based strain sensor outperformed PEDOT:PSS in terms of mechanical endurance. 2D thermal flow sensors were studied and two approaches for fabrication were investigated: firstly, a 2D thermal flow sensor consisting of discrete SMT platinum elements which acted both as micro-heaters and sensing elements, was developed on prelaminated polyimide with copper tracks. Capabilities to detect flow amplitude with two modes of operation (namely, constant current and constant temperature) and angle of attack were assessed via a custom measurement setup. Transfer of technology to a fully printed device was performed via screen printing of active components consisting of activated carbon and BaTiO3 which acted as heaters and sensing elements on PET substrate. The same modes of operation were evaluated and characteristics regarding flow detection capabilities were extracted. For the SMT based flexible device on polyimide, an artificial neural network was also developed for accompanying and correcting the measurements offline. Both flow sensors were capable of detecting flows in the range of 0 to 25 SLPM, while constant temperature mode was evidently more sensitive (12.6 mW/(m/s) versus -2.80 mW/(m/s) for constant current mode) for the SMT sensor. Different methods for electrical interfacing between printed lines and traditional Cu-based electronics on polyimide was also performed. It was concluded that depending on the application, commercial mechanical connectors such as Amphenol FPC and Clincher can provide adequate electrical interfacing. Also, as also evaluated by finite element analysis, the engagement points and measurement method are of major importance in electrical field formation and current density.
- ItemOpen AccessA new framework for structuring and deploying advanced personalized and ubiquitous healthcare services
Τμήμα Ηλεκτρολ. Μηχαν. και Τεχνολ. Υπολογ. (ΔΔ)(2014-08-03) Φέγγου, Μαρία-Άννα; Λυμπερόπουλος, Δημήτριος; Λυμπερόπουλος, Δημήτριος; Κωτσόπουλος, Σταύρος; Κουκιάς, Μιχαήλ; Δενάζης, Σπύρος; Σγάρμπας, Κυριάκος; Αναστασόπουλος, Γιώργος; Παναγιωτάκης, Γιώργος; Fengou, Maria-AnnaThis dissertation focuses on the design of personalized and ubiquitous healthcare (PUH) services in the medical sector. It is proposed a new framework for structuring and deploying advanced PUH services in the right place, time and manner. According to this PUH framework, diverse types of entities (subjects and objects) are involved during the execution of each service. Each type of subject (e.g. patient, caregiver (doctor or nurse), volunteer, patient’s friend or relative, etc) or object (medical unit, medical files manager, etc) performs predefined actions according to the patient’s current health status and an agreed plan of actions. In the provision of PUH services, the permissible actions of each entity (or a group of entities) are determined by the “entity’s profile”, which is created in advance for this purpose. This work considers also the “group profile” that determines the behavior and the attributes of a specific group of entities. The PUH framework provides also mechanisms for the management of the patient’s context (environmental, living, etc) and the content of subject’s profile. Different profiling mechanisms are activated according to the subject’s type. Structurally, the proposed framework is an expansion of the framework that is utilized by the Next Generation Networks (NGN) (known as OSA/Parlay architecture) for the reliable provision of multipoint – multimedia communication services. The PUH framework distinguishes four classes of mechanisms: a) the conventional ETSI/Parlay mechanisms handling the communication among the entities that are involved during the execution of a PUH service, b) the mechanisms handling the acquisition of bio and contextual data by sensor networks deployed around the patient (e.g. Body Area Network), c) the mechanisms for the management of profiles data (e.g. availability, preferences, capabilities, care giving role, activities) and d) the security mechanisms. For the representation of these classes of mechanisms, an appropriate ontology has been created. Based on the above PUH framework, an entire profile management system is proposed. This system uses data from patient/caregivers profiles (or group profiles) in order to deploy a PUH service that will be able to support multi-party group-working schemes with well defined behaviors of all involved entities, devices and services. The technology of smart cards has selected as the appropriate technology for creating, accessing and enriching the subjects profiles. A prototype of the proposed profile management system is implemented using the cloud computing technology (Windows Azure platform). We have considered that parts of a profile are located in different service providers. The efficiency of the developed profile management system is evaluated in terms of time response in order to select the appropriate healthcare provider by simulating system's response to real world scenarios.
- ItemOpen AccessA numerical approach for the shape optimization of woven fabric composite structural elements
Τμήμα Μηχανολόγων και Αεροναυπηγών Μηχαν. (ΔΔ)Κουμπιάς, Αντώνιος; Παντελάκης, Σπυρίδων; Παντελάκης, Σπυρίδων; Τσερπές, Κωνσταντίνος; Λαμπέας, Γεώργιος; Gibson, Geoff; Κερμανίδης, Θεώδορος; Σαραβάνος, Δημήτριος; Παπαδόπουλος, Χρήστος; Koumpias, AntoniosIn the present thesis a novel numerical approach for the optimization of composite structures fabricated from woven composite materials is developed. The aim is to increase the ultimate strength of the structure while at the same time decreasing its weight. The numerical approach is based on a combination of the numerical algorithm of progressive damage modelling (PDM), along with shape optimization (SO) in an iterative subroutine. PDM, which is comprised of three steps, namely stress analysis, failure analysis and material property degradation, is used to predict the initiation and propagation of failure in the structure. During the phase of SO certain geometrical parameters are varied within limits in order to minimize the stresses that lead the structure to ultimate failure as indicated by PDM results. Finally the resulting geometry is solved with PDM to ensure the enhancement in the ultimate strength and the decrease in ultimate weight. Within the frame of this approach, a new methodology for the numerical modeling and the simulation of mechanical behavior of woven composite materials is proposed. The highly inhomogeneous nature of woven composite materials in the micro-scale is taken under consideration to create accurate representative volume element (RVE) FE models which represent the actual material. Then PDM is used for the simulation of their mechanical response. The calculated properties, in terms of stiffnesses and strengths, are then inserted as inputs in the global FE model of the composite structure. Additionally, the reliability and applicability of a continuum damage model (CDM), in comparison with cohesive zone model (CZM), are assessed in order to use the CDM for the modeling of the adhesive’s mechanical behavior. The mentioned numerical approach is applied in an H-shaped joining element fabricated from two different woven composite materials for the loading case of tension. In the first case NCF composite is used while in the second case the joint is made of 3D fully interlaced weave (FIW) composite. The purpose of the H-shaped element is the joining of two composite plates via the method of adhesive bonding.
- ItemOpen AccessA numerical methodology for predicting the mechanical properties of unidirectional composite laminates with pores : evaluation of porous CFRP specimens using X-ray CT data and an artificial neural network
Τμήμα Μηχανολόγων και Αεροναυπηγών Μηχαν. (ΔΔ)Σταμόπουλος, Αντώνιος; Τσερπές, Κωνσταντίνος; Παντελάκης, Σπυρίδων; Δέντσορας, Αργύριος; Ανυφαντής, Νικόλαος; Λαμπέας, Γεώργιος; Φιλιππίδης, Θεόδωρος; Λούτας, Θεόδωρος; Stamopoulos, AntoniosIn the present thesis, a numerical methodology was developed to predict the mechanical properties of unidirectional carbon fiber reinforce polymers (CFRPs) using data from NDT tests. To achieve the goal of the thesis, X-Ray CT scan data analysis was conducted, optical microscopy was utilized, multi-scale FE models were created, mechanical tests were conducted and Artificial Neural Networks were trained by utilizing the developed FE models. Originally, UD CFRP plates with different pore content were manufactured by implementing a variety of autoclave pressures. The pressure values were the optimum, most often implemented and lowest respectively. To this end, four different types of plates of the same material were manufactured. The pores developed in the samples cut from these plates was characterized by means of X-Ray computed tomography. Prior to the final porosity quantification, a parametrical study was conducted for defining the most efficient parameters set on the software’s defect detection module. In parallel, optical microscopy was utilized for defining the large void’s dimensions in the sample with the highest pore content. By comparing the measurements obtained by the optical microscope and the XCT, the defect detection parameters were properly calibrated and validated. Small pores and micro-pores were also attributed to the analysis. At the end of this framework, porosity developed in all four samples was characterized. Subsequently, a numerical methodology was developed which employs the pore’s characteristics. Due to the pore’s dimension high ratio between the smallest and bigger observed, the methodology was designed in three distinct levels, namely the epoxy resin with small pores, porous epoxy resin with large voids and CFRP specimens. The output of each level was the input of the consecutive. The methodology was implemented in four porosity levels’ pores characteristics resulting the mechanical properties of the CFRP laminate, namely the transverse strength and stiffness, short beam strength, flexural strength and modulus. The results in terms of mechanical behavior were validated against mechanical tests. All the matrix dominated properties appeared to be highly affected by the pores and voids leading to a degradation up to 14% of the interlaminar shear strength and roughly 20% of the in-plane shear properties. After having validated the numerical methodology, an Artificial Neural Network was developed to link the pore’s characteristics defined by means of XCT and the mechanical properties of the porous UD CFRP laminate. To this end, a number of 30 different autoclave pressure scenarios were created excluding the 2 out of 4 actual cases, thus 30 pores dataset. After simulating the corresponding mechanical tests with the use of the numerical methodology developed previously, the ANN was trained utilizing the 30 input and output data. The two excluded cases which correspond to the most often implemented autoclave pressures, validated the accuracy of the developed ANN. The main conclusion of the thesis is that the developed numerical methodology can accurately predict the mechanical properties of porous CFRP laminates. Additionally, the ANN based on this methodology is equally accurate and capable of predicting the degraded mechanical properties of the CFRP material using a pore’s characteristics dataset in a direct, effective and fast manner. Consequently, both two methods –after minor modifications- may be used as tools for the scopes of designing and optimizing materials saving cost and increasing the design’s effectiveness.
- ItemOpen AccessA wavelet domain numerical method for the prediction of transient dynamic response in composite structures with a focus on wave propagation
Τμήμα Μηχανολόγων και Αεροναυπηγών Μηχαν. (ΔΔ)Νάστος, Χρήστος; Σαραβάνος, Δημήτριος; Σαραβάνος, Δημήτριος; Πολύζος, Δημοσθένης; Φασόης, Σπήλιος; Λαμπέας, Γεώργιος; Λούτας, Θεόδωρος; Καράμπαλης, Δημήτριος; Μανώλης, Γεώργιος; Nastos, ChristosAn efficient numerical method is developed for the simulation of transient dynamic response from one dimensional rod to three dimensional thick laminated composite and sandwich plate structures involving very high frequencies and wave numbers. The proposed method incorporates Daubechies wavelet and scaling functions for the interpolation of the in-plane displacements in the finite domain of the structure, hence is termed as finite wavelet domain method. The developed numerical method further exploits some advantageous mathematical properties provided by wavelet analysis, such as the orthogonality and compact support of wavelet and scaling functions, which lead to diagonal or near diagonal consistent mass matrices and high sparsity stiffness matrices. Hence, an uncoupled equivalent discrete spatial dynamic system is formulated, synthesized and rapidly solved in the wavelet domain using an explicit time integration scheme. The in plane wavelet interpolation is further combined with the development of a high order laminate strip theory and a cubic spline laminate plate theory, resulting in the construction of innovative wavelet based elements for the accurate prediction of symmetric and anti-symmetric wave modes. The aforementioned theories are extended to multiple layer theories for the efficient prediction of high frequency transient phenomena, even in structures with high heterogeneity which are crucial for structural health monitoring purposes. Various guided wave propagation cases are investigated and the presented numerical results are compared with respective results acquired by analytical, semi-analytical, finite element and time spectral element methods. The proposed method yielded higher convergence rates and substantial reductions in computational effort compared to the traditional finite element and to the respective time spectral finite element methods. The computational gains are quantified in terms of the number of nodes required for a converged solution and of the required computational operations. A further improvement on the proposed method, which results from the multiresolution property is also presented. One and two dimensional pioneering multiresolution wavelet based elements are constructed by using Daubechies family wavelet and scaling functions as basis functions. The transient response is predicted by hierarchical explicit time integration schemes, which are composed of different components and are thoroughly described on present dissertation. Homogeneous, inhomogeneous and periodic structures cases are investigated and both further reduced the computational demands and enhanced the localization capabilities, prove the superiority of the multiresolution analysis, with respect to all the previously mentioned numerical methods.
- ItemOpen AccessA'-dependencies in Greek : a movement approach to resumption
Τμήμα Φιλολογίας (ΔΔ)Γεωργίου, Ρένος; Georgiou, RenosThe present thesis focuses on gap and resumptive A’-dependencies in Greek and its main goal is to investigate the syntactic representation and the syntax-semantics mapping that gives rise to clitic-resumption in A’-dependencies. The main claim of the present thesis is that resumptive dependencies are derived by movement and that resumptive clitics mark copies (‘traces’) that are interpreted as variables of type e (ReMIV – Resumption Marks Individual Variables). This proposal captures both the distribution of resumptive clitics in A’-dependencies and the syntactic/semantic behavior of resumptive and non-resumptive dependencies. Regarding the distribution of resumption in Greek A’-dependencies, it is shown that resumptive clitics are obligatory in some contexts (e.g., CLLD), excluded in others (e.g., Topicalization), while in some contexts they are optional (e.g., which-questions). In line with ReMIV, it is proposed that obligatory clitic-resumption indicates that movement chains that are headed by individual-denoting phrases (e.g., CLLD) always leave behind a copy/variable of type e, which triggers clitic-resumption. On the other hand, movement chains that are headed by property-denoting phrases (e.g., Topicalization) cannot map onto individual traces, thus clitics are excluded. Last, clitic-resumption in wh-questions is optional, because wh-questions can be either interpreted through λ-abstraction over individual variables (resumptive wh-questions) or through some other semantic strategy (abstraction over choice functions, total reconstruction) which gives rise to gap wh-questions. Moreover, resumptive dependencies (e.g., CLLD, resumptive wh-questions) do not show all the standard properties of A’-movement chains. For example, they are restricted to wide- scope readings and they are insensitive to WCO-effects. According to the movement analysis of resumptive dependencies proposed here, these properties stem from the individual-type of copies/traces in resumptive dependencies. Finally, the thesis proposes that resumptive clitics enter the derivation as definite determiners. Building on the assumption that copies turn into individual variables through a trace conversion rule, (Fox 2002), it is proposed that resumptive clitics realize the indexed determiner introduced by this rule. This line of analysis captures both the correlation between resumptive clitics and individual variables in the trace position (as predicted by ReMIV) and the morphological resemblance between clitics and definite articles in Greek.
- ItemOpen AccessAb initio κβαντοχημική μελέτη των διαμοριακών επιδράσεων μορίων ιδιαζούσης σημασίας για τις περιβαλλοντικές επιστήμες
Τμήμα Χημείας (ΔΔ)(2010-08-02T09:17:17Z) Χασκόπουλος, Αναστάσιος; Μαρούλης, Γεώργιος; Μαρούλης, Γ.; Κουτσούκος, Π.; Ντάλας, Ε.; Σίμος, Θ.; Κοσμά-Μυλωνά, Α.; Ζδέτσης, Α.; Σάμιος, Ι.; Chaskopoulos, Anastasios-
- ItemOpen AccessAcoustic energy harvesting with emphasis on low frequencies
Τμήμα Ηλεκτρολ. Μηχαν. και Τεχνολ. Υπολογ. (ΔΔ)Παπαδάκος, Χαράλαμπος; Μουρτζόπουλος, Ιωάννης; Φακωτάκης, Νίκος; Καλύβας, Γρηγόριος; Πολύζος, Δημοσθένης; Μενούνου, Πηνελόπη; Πυργιώτη, Ελευθερία; Μπίρμπας, Μιχαήλ; Papadakos, CharalamposThis work presents novel findings and results in the field of acoustic energy harvesting with emphasis on low frequencies. At the beginning of this work, ambient acoustic power density is estimated and a comparison between the potential of sound and that of other ambient energy sources exploited for energy harvesting is performed. The interest in AEH is justified and the corresponding state of the art is presented. Prior the analysis, the motivation of this work is described in detail and the main objectives are set. The analysis considers the piezoelectric transducer to be the transduction element for this study. Initially, it provides a lumped element model (LEM) for evaluating the AEH potential of such transducers directly coupled to the sound field and utilizing such a framework, it demonstrates the inefficiency of such approach for AEH of practical interest. Subsequently, a novel method and device is proposed for improving the coupling of such transducers and the sound field and enhancing the potential of AEH. For this case of indirect coupling, the analysis provides a modified LEM compared to the direct coupling case. At a next level, a novel biomimetic device that mimics the function and the structure of the human middle ear is proposed for implementing such indirect coupling of piezoelectric transducers and the sound filed for AEH. Such device can potentially ensure sufficient electric power for driving low power applications in the range nW - mW. Next, in order to provide an AEH proof - of - concept case, this study focuses on loudspeaker enclosures since the sound pressure level (SPL) therein can be significantly high during operation, as it is shown via tests performed for realistic 3D virtual prototype models, which are validated also via measurements performed inside real - life loudspeaker systems. Specific examples of 3D virtual prototype models of loudspeaker enclosures are combined with the proposed biomimetic device and a piezoelectric transducer and are tested for evaluating the in - Box AEH potential. The test results indicate that the proposed biomimetic device may operate as a harvester and the same time as a passive absorber. These test results further show that the output voltage of the proposed AEH can be adequate for charging a rechargeable battery or for driving an electronic load, while the electric power delivered to a load resistance may be up to units of mW. These results presented in this work are discussed in detail and the framework for future work and applications is presented.
- ItemOpen AccessAdaptive planning and control in cyber-physical production systems
Τμήμα Μηχανολόγων και Αεροναυπηγών Μηχαν. (ΔΔ)Νικολάκης, Νικόλαος; Nikolakis, NikolaosThe objective of this work is the study of the production systems under the scope of cyber-physical production systems (CPPSs) and in terms of enabling their reconfiguration towards increased automation and flexibility to volatile working conditions as well as market demands. Towards that end, methods corresponding to different layers of the ISA95 automation pyramid, mapped to the layers of the 5C architecture for cyber-physical systems, have been connected integrating different layers of the architecture and tested in the under investigation CPPSs. First, the dynamic closed-loop control of a CPPS has been studied for enabling and controlling safe human-robot collaborative assembly operations. An initial implementation is evaluated in a specific use case and its results are compared regarding the use of one or more sensors. The comparison is performed in terms of system response time to detect human presence within a predefined safety zone. Afterwards and considering the manual assembly operations, their adaptive planning and reconfiguration of the assembly station are discussed. The implementation of the digital-twin approach is presented to digitally close the loop between physical and virtual system thus enabling a cost-effective improvement of the planning, commissioning as well as the entire lifecycle of human-based production processes. A case study from the warehouse, intra-factory logistics operation, in the white goods industry, demonstrates the feasibility of the proposed approach. Also, as part of the third case study, a holistic framework for reconfigurable cyber-physical production systems is discussed, enabled by container technologies. The presented approach enhances flexibility in a cyber-physical production system (CPPS) through the dynamic reconfiguration of the automation system and the production schedule, based on occurring events. The proposed solution has been implemented on a software framework and applied in a small scale CPPS coming from the automotive industry. Finally, the contribution of the integration 5C layers for the implementation, deployment, and reconfiguration of CPPS functionalities converting conventional manufacturing processes to smart ones has been assessed via a set of CPPS indicators in terms of contribution to their automation level.
- ItemOpen AccessAdaptive polarization mode dispersion equalizers for coherent optical communications systems
Τμήμα Ηλεκτρολ. Μηχαν. και Τεχνολ. Υπολογ. (ΔΔ)(2010-11-01T07:32:17Z) Μαντζούκης, Νικόλαος; Ρούδας, Ιωάννης; Ρούδας, Ιωάννης; Τζες, Αντώνιος; Μπερμπερίδης, Κωνσταντίνος; Γκίκας, Δημήτριος; Μπίρμπας, Αλέξιος; Καμαλάκης, Θωμάς; Βλάχος, Κυριάκος; Mantzoukis, NikolaosPolarization mode dispersion (PMD) arises as a result of the birefringence in optical fibers, due to inherent asymmetries and deformities from external stresses. The spectral components of the input optical pulse propagate with different group velocities. Consequently, pulse duration increases leading to intersymbol interference between consequent symbols, leading to performance reduction of the coherent systems. In order to compensate for the PMD, we use adaptive linear PMD equalizers. Due to the dynamic and random nature of PMD, it is crucial for a system designer to efficiently simulate the PMD-induced outage probabilities of 10-5. Because of this stringent requirement, it is computationally costly to use the conventional Monte Carlo methods. To overcome this hurdle, Importance Sampling methods, such as the multicanonical Monte Carlo method have been applied in the past in order to efficiently reduce the simulation time required to estimate the statistics of these rare events. The multicanonical Monte Carlo method does not require any prior knowledge of which rare events contribute significantly to the PMD-induced outages. In essence, multicanonical Monte Carlo simulations adaptively bias the input random variables with a priori unknown weights. The PMD emulation model consists of a concatenation of birefringent sections, simulated based on MMC. The objective of this dissertation is to apply, for the first time, the multicanonical Monte Carlo method to accurately and efficiently evaluate the performance of adaptive, blind, feed-forward PMD equalizers employed in coherent polarization division multiplexed (PDM) quadrature phase-shift keying (QPSK) systems in all order PMD emulation model. In the exclusive presence of PMD, we demonstrated that the half-symbol-period-spaced adaptive electronic equalizers, based on the constant modulus algorithm (CMA) equalizers perform slightly better than the decision directed least mean square (DD-LMS) counterparts at links with larger PMD values, whereas the opposite holds true for the low PMD regime. Due to their distinguishable performance in different regimes of the PMD, they provided an even better performance when running DD-LMS after a first round of CMA-based equalization than using either one of the equalization algorithms stand alone. Finally, the joint presence of PMD and intermediate frequency offset or PMD and random differential phase carrier shifts slightly worsened the performance of the coherent PDM QPSK systems, independently of the equalizer. Although these random differential carrier phase shifts are typically omitted in similar PMD studies in intensity modulated/direct detection (IM/DD) systems, they should be taken into account in due to the phase sensitivity of the PDM QPSK coherent systems.
- ItemOpen AccessAdvanced and complete functional series time-dependent ARMA (FS-TARMA) methods for the identification and fault diagnosis of non-stationary stochastic structural systems
Τμήμα Μηχανολόγων και Αεροναυπηγών Μηχαν. (ΔΔ)(2013-02-01) Σπυριδωνάκος, Μηνάς; Φασόης, Σπήλιος; Φασόης, Σπήλιος; Τζες, Αντώνης; Μπερμπερίδης, Κώστας; Μπούντης, Τάσσος; Αντωνιάδης, Ιωάννης; Μανατάκης, Μανώλης; Σακελλαρίου, Ιωάννης; Spiridonakos, MinasNon-stationary signals, that is signals with time-varying (TV) statistical properties, are commonly encountered in engineering practice. The vibration responses of structures, such as traffic-excited bridges, robotic devices, rotating machinery, and so on, constitute typical examples of non-stationary signals. Structures characterized by properties that vary with time are generally referred as TV structures and their vibration-based identification under normal operating conditions is a significant and challenging problem. An important class of parametric methods for the solution of this problem is based on Functional Series Time-dependent AutoRegressive Moving Average (FS-TARMA) models. These models have parameters that explicitly depend on time, with the dependence described by deterministic functions belonging to specific functional sub-spaces. The focus of the present thesis is on the development of complete and advanced FS-TARMA methods that will offer important improvements in overcoming drawbacks of existent methods and will further foster practical use and application of FS-TARMA models in non-stationary vibration analysis. The specific objectives of the thesis are: a) The introduction of a novel class of Adaptable FS-TARMA (AFS-TARMA) models and the development of a method for their effective identification. AFS-TARMA models are adaptable in the sense that they are not based on basis functions of a fixed form, but instead, they use basis functions with a-priori unknown properties that may adapt to the specific random signal characteristics. b) The postulation of a vector FS-TARMA method for output-only structural identification and the development of effective tools for both model parameter estimation and model structure selection. c) The introduction of a statistical method for vibration-based fault diagnosis in TV structures. d) The presentation of a thorough review on FS-TARMA models covering both theoretical and practical aspects of the model parameter estimation and structure selection problems with special emphasis being placed on promising recent methods. The methods that are developed in each chapter of this thesis are validated through their application in both numerical and experimental case studies and comparisons with currently available non-stationary signal identification methods. The results of the study demonstrate the new methods' applicability, effectiveness, and high potential for parsimonious and accurate identification and dynamic analysis of TV structures.
- ItemOpen AccessAdvanced functional and sequential statistical time series methods for damage diagnosis in mechanical structures
Τμήμα Μηχανολόγων και Αεροναυπηγών Μηχαν. (ΔΔ)(2013-02-01) Κοψαυτόπουλος, Φώτης; Φασόης, Σπήλιος; Φασόης, Σπήλιος; Σαραβάνος, Δημήτριος; Δερματάς, Ευάγγελος; Σακελλαρίου, Ιωάννης; Μπούντης, Αναστάσιος; Παπαδόπουλος, Χρήστος; Φιλιππίδης, Θεόδωρος; Kopsaftopoulos, FotisThe past 30 years have witnessed major developments in vibration based damage detection and identification, also collectively referred to as damage diagnosis. Moreover, the past 10 years have seen a rapid increase in the amount of research related to Structural Health Monitoring (SHM) as quantified by the significant escalation in papers published on this subject. Thus, the increased interest in this engineering field and its associated potential constitute the main motive for this thesis. The goal of the thesis is the development and introduction of novel advanced functional and sequential statistical time series methods for vibration based damage diagnosis and SHM. After the introduction of the first chapter, Chapter II provides an experimental assessment and comparison of vibration based statistical time series methods for Structural Health Monitoring (SHM) via their application on a lightweight aluminum truss structure and a laboratory scale aircraft skeleton structure. A concise overview of the main non-parametric and parametric methods is presented, including response-only and excitation-response schemes. Damage detection and identification are based on univariate (scalar) versions of the methods, while both scalar (univariate) and vector (multivariate) schemes are considered. The methods' effectiveness for both damage detection and identification is assessed via various test cases corresponding to different damage scenarios, multiple experiments and various sensor locations on the considered structures. The results of the chapter confirm the high potential and effectiveness of vibration based statistical time series methods for SHM. Chapter III investigates the identification of stochastic systems under multiple operating conditions via Vector-dependent Functionally Pooled (VFP) models. In many applications a system operates under a variety of operating conditions (for instance operating temperature, humidity, damage location, damage magnitude and so on) which affect its dynamics, with each condition kept constant for a single commission cycle. Typical examples include mechanical structures operating under different environmental conditions, aircrafts under different flight conditions (altitude, velocity etc.), structures under different structural health states (various damage locations and magnitudes). In this way, damage location and magnitude may be considered as parameters that affect the operating conditions and as a result the structural dynamics. This chapter's work is based on the novel Functional Pooling (FP) framework, which has been recently introduced by the Stochastic Mechanical Systems \& Automation (SMSA) group of the Mechanical Engineering and Aeronautics Department of University of Patras. The main characteristic of Functionally Pooled (FP) models is that their model parameters and innovations sequence depend functionally on the operating parameters, and are projected on appropriate functional subspaces spanned by mutually independent basis functions. Thus, the fourth chapter of the thesis addresses the problem of identifying a globally valid and parsimonious stochastic system model based on input-output data records obtained under a sample of operating conditions characterized by more than one parameters. Hence, models that include a vector characterization of the operating condition are postulated. The problem is tackled within the novel FP framework that postulates proper global discrete-time linear time series models of the ARX and ARMAX types, data pooling techniques, and statistical parameter estimation. Corresponding Vector-dependent Functionally Pooled (VFP) ARX and ARMAX models are postulated, and proper estimators of the Least Squares (LS), Maximum Likelihood (ML), and Prediction Error (PE) types are developed. Model structure estimation is achieved via customary criteria (Bayesian Information Criterion) and a novel Genetic Algorithm (GA) based procedure. The strong consistency of the VFP-ARX least squares and maximum likelihood estimators is established, while the effectiveness of the complete estimation and identification method is demonstrated via two Monte Carlo studies. Based on the postulated VFP parametrization a vibration based statistical time series method that is capable of effective damage detection, precise localization, and magnitude estimation within a unified stochastic framework is introduced in Chapter IV. The method constitutes an important generalization of the recently introduced Functional Model Based Method (FMBM) in that it allows, for the first time in the statistical time series methods context, for complete and precise damage localization on continuous structural topologies. More precisely, the proposed method can accurately localize damage anywhere on properly defined continuous topologies on the structure, instead of pre-defined specific locations. Estimator uncertainties are taken into account, and uncertainty ellipsoids are provided for the damage location and magnitude. To achieve its goal, the method is based on the extended class of Vector-dependent Functionally Pooled (VFP) models, which are characterized by parameters that depend on both damage magnitude and location, as well as on proper statistical estimation and decision making schemes. The method is validated and its effectiveness is experimentally assessed via its application to damage detection, precise localization, and magnitude estimation on a prototype GARTEUR-type laboratory scale aircraft skeleton structure. The damage scenarios considered consist of varying size small masses attached to various continuous topologies on the structure. The method is shown to achieve effective damage detection, precise localization, and magnitude estimation based on even a single pair of measured excitation-response signals. Chapter V presents the introduction and experimental assessment of a sequential statistical time series method for vibration based SHM capable of achieving effective, robust and early damage detection, identification and quantification under uncertainties. The method is based on a combination of binary and multihypothesis versions of the statistically optimal Sequential Probability Ratio Test (SPRT), which employs the residual sequences obtained through a stochastic time series model of the healthy structure. In this work the full list of properties and capabilities of the SPRT are for the first time presented and explored in the context of vibration based damage detection, identification and quantification. The method is shown to achieve effective and robust damage detection, identification and quantification based on predetermined statistical hypothesis sampling plans, which are both analytically and experimentally compared and assessed. The method's performance is determined a priori via the use of the analytical expressions of the Operating Characteristic (OC) and Average Sample Number (ASN) functions in combination with baseline data records, while it requires on average a minimum number of samples in order to reach a decision compared to most powerful Fixed Sample Size (FSS) tests. The effectiveness of the proposed method is validated and experimentally assessed via its application on a lightweight aluminum truss structure, while the obtained results for three distinct vibration measurement positions prove the method's ability to operate based even on a single pair of measured excitation-response signals. Finally, Chapter VI contains the concluding remarks and future perspectives of the thesis.
- ItemOpen AccessAdvanced three dimensional digital tomosynthesis studies for breast imaging
Τμήμα Ιατρικής (ΔΔ)(2015-07-07) Μαλλιώρη, Ανθή; Παλληκαράκης, Νικόλαος; Παλληκαράκης, Νικόλαος; Νικήτα, Κωνσταντίνα; Buliev, Ivan; Καρδαμάκης, Δημήτριος; Κουτσούρης, Δημήτριος; Κωσταρίδου, Ελένη; Μπράβου, Βασιλική; Malliori, AnthiThe current thesis is focused on the study of tomosynthesis techniques applied on breast imaging, in order to improve the detection of breast lesions. Breast Tomosynthesis (BT) is a pseudo-three-dimensional (3D) x-ray imaging technique that provides reconstructed tomographic images from a set of angular projections taken in a limited arc around the breast, with dose levels similar to those of a two-view conventional mammography. Simulation studies and clinical trials suggest that BT is very useful for imaging the breast in an attempt to optimize the detection and characterization of lesions particularly in dense breasts and has the potential to reduce the recall rate. Reconstruction algorithms and acquisition parameters are critical for the quality of reconstructed slices. The aim of this research is to explore tomosynthesis modalities for breast imaging and evaluate them against existing mammographic techniques as well as to investigate the effect of reconstruction algorithms and acquisition parameters on the image quality of tomosynthetic slices. A specific aim and innovation of the study was to demonstrate the feasibility of combining BT and monochromatic radiation for 3D breast imaging, an approach that had not been studied thoroughly yet. For the purposes of this study a computer-based platform has been developed in Matlab incorporating reconstruction algorithms and filtering techniques for BT applications. It is fully parameterized and has a modular architecture for easy addition of new algorithms. Simulations studies with the XRayImaging Simulator and experimental work at ELETTRA Synchrotron facilities in Trieste, Italy, have been performed using software and complex hardware phantoms, of realistic shape and size, consisting of materials mimicking the breast tissue. The work has been carried out in comparison to conventional BT and mammography and demonstrates the feasibility of the studied new technique and the potential advantages of using BT with synchrotron modality for the detection of breast low- and high-contrast breast lesions such as masses and microcalcifications (μCs). Evaluations of both simulation and experimental tomograms demonstrated superior visibility of all reconstructed features using appropriately optimized filtered algorithms. Moreover, image quality and evaluation metrics are improved with extending the acquisition length for the masses. The visualization of μCs was found less sensitive to this parameter due to their high inherent contrast. Breast tomosynthesis shows advantages in visualizing features of small size within phantoms of increased thickness and especially in bringing into focus and localizing low-contrast masses hidden in a highly heterogeneous background with superimposed structures. Monochromatic beams can result in better tissue differentiation and in combination with BT can lead to improvement of features’ visibility, better detail and higher contrast. Monochromatic BT provided improved image quality at lower incident exposures, compared to conventional mammography, concerning mass detection and visibility of borders, which is important for their characterization, especially when they are spiculated. Overall it has been proved that while reducing the radiation dose, monochromatic beams combined with BT, result in an improvement of image quality. These findings are encouraging for the development of a tomosynthesis system based on monochromatic beams.
- ItemOpen AccessAirport business excellence : a holistic approach to performance management
Τμήμα Διοίκησης Επιχειρήσεων (ΔΔ)Παράσχη, Παρασκευή; Γεωργόπουλος, Αντώνιος; Γεωργόπουλος, Αντώνιος; Καλδής, Παναγιώτης; Παπαθεοδώρου, Ανδρέας; Πέππας, Παύλος; Πολυχρονίου, Παναγιώτης; Παπαδημητρίου, Δήμητρα; Τσαγκανός, Αθανάσιος; Paraschi, ParaskeviAviation business is a primary impetus for the global economy and airports pose a central position in the aviation system. Air transport restructuring of the last three decades has increased the demand for performance improvement in all nodes of the air transport pipeline. Nevertheless, airports are still treated more like infrastructures rather than enterprises, with their efficiency almost exclusively measured in operational and financial terms. The current study is the first known attempt to deal with airport performance in a holistic manner. We approach airports as integrated business ecosystems and we borrow concepts from ecology science to explore their interacting components. We first investigate airports biotic factors, i.e. all living airport participants and their relations. To do so, we develop a novel Airport Business Excellence Model (ABEM) which assesses airport performance using twelve groups of criteria: Leadership, Strategy, HRM, Suppliers and Resources, Partners and Customers, Processes, Products and Services, People results, Operational results, Quality results, Customer results, Society results and Financial results. ABEM was applied to a pluralistic sample of 143 airports in 92 states all over the world. The total performance score derived from ABEM application was subsequently associated with abiotic determinants of the external environment, in order to draw the overall picture of Airport Ecosystem Performance. The study results indicate that airport biotic factors weave a complex nexus of interactions, with Leadership and Strategy displaying the broader impact. Rather surprisingly, Financial results were not considered of primary importance contrary to the People and Operational results that were estimated as more crucial to achieving airport excellence. Regarding the environmental abiotic factors, our findings further support and extend the existing literature, concluding that low seasonality (50-60%), a big airport size between 60 and 80 million WLUs and a combination of Light-handed/monitoring with Dual Till regulation scheme have a positive contribution to the airport performance. On the contrary, hosting no airlines as home base, having a mixed ownership/public management or public ownership/private management, the absence of any Performance Management System and the combination of public ownership with RoR regulatory framework have a negative influence. Finally, we studied the joint impact of the biotic and abiotic performance determinants, finding that the contribution of the software factors is more significant than the effect of the business surroundings to the overall performance of the airport ecosystem.