1,721,181 research outputs found
G. Ardizzon, G.L. Fontana, Il Museo di Macchine “Enrico Bernardi” dell’Università di Padova, in “Patrimonio industriale”, nn. 18-19, 2018 [AIPAI – Edizioni Scientifiche Italiane], pp. 40-46.
No full textAnalisi tridimensionale agli elementi finiti del moto in una girante centrifuga
No full textE' presentato un procedimento agli elementi finiti per l'analisi tridimensionale del moto a potenziale attraverso una girante centrifuga. L'attenzione e` rivolta alla definizione delle condizioni al contorno a valle della girante. Al riguardo un nuovo criterio, basato sul teorema del momento della quantita' di moto, e` suggerito per il calcolo della circolazione palare. I risultati delle applicazioni sono infine confrontati con quelli che derivano da procedimento quasi tridimensionale agli elementi di contorno
Esperienze con un codice di analisi tridimensionale della corrente nelle turbomacchine / Experience with a 3-D finite element program for potential flow analysis of centrifugal runners
No full textThe results of the predictions obtained by means a 3-D finite element code for potential flow analysis, are discussed and compared with experimental measurements taken at the discharge side of a centrifugal runner
Il metodo degli elementi di contorno nell'analisi del moto nelle turbomacchine / Analysis of flow in turbomachines by boundary element method
No full textBoundary element method is used to study the inviscid flow configurations in a centrifugal runner. Wu formulation is employed to define the flow surfaces. In the solution of the blade to blade flow equation the boundary conditions downstream of the runner are set and iteratively modified by the programme itself. Results are discussed and compared with finite element solutions
Modelli di perdita nelle turbomacchine, previsione delle prestazioni e verifica sperimentale
No full textE' stato sviluppato un codice di calcolo per prevedere il comportamento delle turbomacchine anche nelle condizioni fuori progetto. La struttura principale del programma è basata su modelli di perdita monodimensionali, cui è stato accoppiato uno schema non stazionario d'interazione con la voluta. Le curve caratteristiche predette dal codice di calcolo, sono state confrontate con quelle di due pompe commerciali, mentre la distribuzione teorica delle pressioni nell'elemento statorico è stata comparata con rilievi sperimentali eseguiti su una voluta a sezione rettangolare
Advanced Physical Layer Security Techniques for Non-Terrestrial Communications
No full textThe broad use of wireless communications makes it necessary to investigate specifically targeted security solutions. Moreover, differently from wired communications, wireless communications are by nature vulnerable to many threats: of course, since the medium is easily accessible, any malicious user can disrupt the communication by using a
jamming attack, intercept the signal to disclose its content or information about the transmitter or lead a spoofing attack by generating a counterfeit signal or by tampering the transmitted signal.
In many applications, it is not possible to rely on cryptography: for instance, cryptography-based solutions have a considerable computational cost, thus they may not be suited for many wireless applications, where we want to reduce the user energy consumption, such as in a wireless sensor network (WSN) used in an Internet of things (IoT) context. Thus, we resort to physical-layer security (PLS) approaches. Physical layer authentication relies on the collection of the observation about the channel characteristics (e.g., features of the channel impulse response) to tell apart transmissions by legitimate network members from the ones by impersonating attacker. Moreover, PLS mechanisms are also unconditionally secure, since the security is not provided by a computationally hard problem. On the other hand, since theses techniques rely on the channel model, it may be complex to generalize solutions and each context need to be separately analyzed.
This Thesis focuses on the development of physical layer authentication for global navigation satellite systems (GNSSs) and underwater acoustic networks (UWANs). GNSS services are used to provide positioning and timing. However, these services do not (necessary) rely on the data content of GNSS but on the properties of the signals themselves, i.e., phase and Doppler frequency. Indeed PLS can be used to provide authentication at signal level by making the spreading code (or part of it) unpredictable. The contributions of this Thesis are multiple. We propose a novel network-aided authentication protocol, proposing also a verification based on the generalized likelihood ratio test (GLRT). To show its robustness, the scheme is tested against several attacks: among others, we also consider the security code estimation and replay (SCER) attack and the internal code attack. Next, we focus on the problem of position, velocity, and time (PVT) assurance, where we propose a series of consistency checks to enlarge the set of trusted signals to be used for the PVT. We focus then on the problem of providing an authenticated but robust timing service, relying only on Galileo’s commercial authentication service (CAS). Finally we address the problem of message scheduling in GNSS: considering, for instance, an authentication service that need to disseminate a digital signature over the GNSS channels, we study both single and multi-round scheduling solutions that aim at minimizing the maximum and the average latency.
In the last part of the Thesis, we tackle the problem of physical layer authentication for UWANs: underwater acoustic channels (UWACs) are known to decorrelate easily in space, and to have a limited time coherence, thus by extracting relevant channel features, it is possible to distinguish a packet transmitted by a legitimate transmitter from the one sent by a potential attacker. Indeed by having multiple (trusted) cooperating sensors, it is possible to improve the classification procedure. We address this problem by using machine learning (ML) techniques. We will investigate multiple aspects: for instance, for training, each receiver may have at disposal observations only from the legitimate or from both legitimate and attacker channels; the influence of the amount of the information shared by each user; strategies to deal with mobility and time-varying channels.The broad use of wireless communications makes it necessary to investigate specifically targeted security solutions. Moreover, differently from wired communications, wireless communications are by nature vulnerable to many threats: of course, since the medium is easily accessible, any malicious user can disrupt the communication by using a
jamming attack, intercept the signal to disclose its content or information about the transmitter or lead a spoofing attack by generating a counterfeit signal or by tampering the transmitted signal.
In many applications, it is not possible to rely on cryptography: for instance, cryptography-based solutions have a considerable computational cost, thus they may not be suited for many wireless applications, where we want to reduce the user energy consumption, such as in a wireless sensor network (WSN) used in an Internet of things (IoT) context. Thus, we resort to physical-layer security (PLS) approaches. Physical layer authentication relies on the collection of the observation about the channel characteristics (e.g., features of the channel impulse response) to tell apart transmissions by legitimate network members from the ones by impersonating attacker. Moreover, PLS mechanisms are also unconditionally secure, since the security is not provided by a computationally hard problem. On the other hand, since theses techniques rely on the channel model, it may be complex to generalize solutions and each context need to be separately analyzed.
This Thesis focuses on the development of physical layer authentication for global navigation satellite systems (GNSSs) and underwater acoustic networks (UWANs). GNSS services are used to provide positioning and timing. However, these services do not (necessary) rely on the data content of GNSS but on the properties of the signals themselves, i.e., phase and Doppler frequency. Indeed PLS can be used to provide authentication at signal level by making the spreading code (or part of it) unpredictable. The contributions of this Thesis are multiple. We propose a novel network-aided authentication protocol, proposing also a verification based on the generalized likelihood ratio test (GLRT). To show its robustness, the scheme is tested against several attacks: among others, we also consider the security code estimation and replay (SCER) attack and the internal code attack. Next, we focus on the problem of position, velocity, and time (PVT) assurance, where we propose a series of consistency checks to enlarge the set of trusted signals to be used for the PVT. We focus then on the problem of providing an authenticated but robust timing service, relying only on Galileo’s commercial authentication service (CAS). Finally we address the problem of message scheduling in GNSS: considering, for instance, an authentication service that need to disseminate a digital signature over the GNSS channels, we study both single and multi-round scheduling solutions that aim at minimizing the maximum and the average latency.
In the last part of the Thesis, we tackle the problem of physical layer authentication for UWANs: underwater acoustic channels (UWACs) are known to decorrelate easily in space, and to have a limited time coherence, thus by extracting relevant channel features, it is possible to distinguish a packet transmitted by a legitimate transmitter from the one sent by a potential attacker. Indeed by having multiple (trusted) cooperating sensors, it is possible to improve the classification procedure. We address this problem by using machine learning (ML) techniques. We will investigate multiple aspects: for instance, for training, each receiver may have at disposal observations only from the legitimate or from both legitimate and attacker channels; the influence of the amount of the information shared by each user; strategies to deal with mobility and time-varying channels
Progettazione di giranti centrifughe / Design of centrifugal rotors
No full textThe paper discusses single and double curvature centrifugal turbo machine rotor blades which can use a set of profiles, the fundamental characteristics of which were derived from experiments in the wind tunnel of the large experimental laboratory of NACA, now NASA. A criterion is formulated for extending the use of these profiles of four digits to radial flow rotors. The novelty of the method is not in the circumferential design of the profiles but as a plane development of the sections, characteristic of the error triangles method. The results of the numerical investigations, compared with more traditional solutions such as those with multiple arcs of a circle, confirm the efficiency and flexibility of this new procedure. This possibility of adopting profiles of known geometrical configuration has permitted automatic generation of the blade surface, and also further assisted design of even more complex blades such as those of multi-stage machine diffusors
Influence of the blade stacking on the flow through an axial flow runner and predictions of three-dimensional and quasi three-dimensional numerical codes
No full textThe influence of the stacking of the blade sections on the flow through an axial flow runner is investigated by means of quasi and three-dimensional finite element programs.
The results show up higher pressure gradients close to the leading edges, in the sections near the hub, when the stacking point is displaced towards the trailing edge. These gradients become noticeable near the trailing edge when the stacking point moves towards the inlet, even if in an attenuated way these trends reverse in sections close to the shroud.
The above-mentioned effects are highlighted only if three-dimensional codes or quasi three-dimensional codes with more than one hub-to-shroud surface are employed
Hydraulics/turbo machinery research. Effects of mechanical losses on the affinity characteristics of turbopumps
No full textExperiments on three different designs of pumps, one fitted with a hydraulic thrust control bearing, employed various speeds and geometric similarity extending also to the wall surface roughness and the play between the rotor and the stator. The experiments show that the efficiency values can differ in homologous operating conditions, without any clearly defined law of affinity between the power and the shaft. Applying the similarity law to the overall power, disregarding the mechanical losses, however, permits accurate forecasting of the operating parameters thoughout. A recalculation formula considering separately the hydraulic and mechanical efficiencies, with correction base on a statistical analysis, is probably a more realistic solution to the problem
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