Le cucurbitacee possono essere attaccate da più di 200 malattie, una corretta gestione è dunque fondamentale per ridurre il rischio di perdite di produzione, sia in termini di quantità sia in termini di qualità. Tra le malattie più importanti che attaccano le cucurbitacee ci sono l’oidio e la peronospora delle cucurbitacee. Podosphaera xanthii e Golovinomyces cichoracearum sono gli agenti causali dell’oidio. È stato studiato l’effetto della temperature sulla germinazione dei conidi in ambiente controllato a 6 temperature costanti (da 10 a 35°C, ad intervalli di 5C°) da 3 a 72 ore. La temperature ottimale per la germinazone, l’infezione e la sporulazione è risultata essere rispettivamente 24.4, 25.7 e 21.3°C per P. xanthii e 17.9, 17.3 e 16.2°C per G. cichoracearum. Un modello meccanicistico è stato sviluppato per il rischio che i due agenti patogeni causassero un attacco di oidio. Il modello simula la germinazione su foglie infettate sulla base delle condizioni ambientali di temperatura e umidità relativa. Le equazioni che calcolano la germinazione delle dei conidi di entrambi i funghi sono state sviluppate sulla base di dati presenti in bibliografia. Un secondo modello meccanicistico è stato sviluppato per Pseudoperonospora cubensis, agente causale della peronospora delle cucurbitacee. Il modello calcola la data di comparsa dei sintomi e la probabilità del superamento di determinate soglie di gravità della malattia, sulla base di sotto-processi dell’infezione. Il passaggio da uno stato dell’infezione al successivo è dipendente dalle condizioni ambientali. Entrambi i modelli sono stati validati confrontando i dati ottenuti dai modelli con dati indipendenti rilevati in coltivazioni del nord Italia. / Cucurbits are potentially affected by more than 200 diseases of diverse etiologies, so a good disease management is crucial to reduce the risk of high yield losses in terms of quantity and quality. Among the more important diseases there are powdery and downy mildew. Podosphaera xanthii and Golovinomyces cichoracearum are the causal agents of cucurbit powdery mildew. The effect of temperature on conidial germination was studied in controlled condition at 6 constant temperature (from 10 to 35°C, step 5°C) for 3 to 72 hours. Optima temperature for conidial germination, infection and sporulation were 24.4, 25.7 and 21.3°C respectively for P. xanthii and 17.9, 17.3 and 16.2°C for G. cichoracearum. A mechanistic model was developed for the risk posed by P. xanthii and G. cichoracearum to cause cucurbit powdery mildew. The model simulates germination on infected leaves on the base of environmental conditions of temperature and relative humidity. Equation regulating spore germination of both fungi were developed using published data. Another mechanistic model was develop also for Pseudoperonospora cubensis, causal agent of cucurbit downy mildew. The model calculates the symptoms appearance and the probability of overtaking severity threshold based on sub-processes of infection. Changes from one state of the infection to the following one depend on environmental conditions. Both models were validated by comparing model outputs with independent data sets collected in fields located in the north of Italy.

La presente tesi studia le aspettative in macroeconomia contribuendo alla letteratura esistente sia indagando circa il meccanismo di formazione delle aspettative, sia analizzando come le aspettative a razionalità limitata influenzino la dinamica economica. Nel primo capitolo viene presentato un esperimento nel quale ai soggetti viene chiesto di predire il valore futuro dell'inflazione: a seconda del trattamento, i soggetti possono venire esposti ad un segnale, che mira a stabilizzare l'economia, che fungendo quindi da indicazione prospettica (Forward Guidance). I risultati vengono poi studiati sottolineando il meccanismo di formazione delle aspettative soprattutto in funzione della credibilità del segnale; inoltre viene studiata l'efficacia dello strumento di politica monetaria nella stabilizzazione del sistema economico: si evidenzia come un segnale informativo permetta una sensibile stabilizzazione dell'economia, prevenendo spirali deflazionistiche. Nel secondo capitolo viene sviluppato un modello ad agenti il quale incorpora un meccanismo di formazione delle aspettative a razionalità limitata, derivato da esperimenti precedenti. Inoltre, grazie ad un peculiare processo di aggregazione, viene derivato un modello analiticamente trattabile che permette di studiare il meccanismo di trasmissione di uno shock, isolando gli effetti dovuti all'eterogeneità fra gli agenti e alle aspettative: entrambi gli effetti sono considerevoli ed aiutano nello spiegare la dinamica economica. / The present dissertation analyses expectations in macroeconomics, contributing to the existing literature both studying the expectation formation process, and inquiring how economic dynamic is influenced by boundedly rational expectations. The first chapter presents a learn to forecast experiment in which subject are asked to form expectation regarding the future value of inflation: depending on the treatment, subjects might be exposed to a signal, which possibly aim at stabilizing economy, mimicking the non conventional monetary policy instrument called Delphic Forward Guidance. The collected data are studied trying to recover the underlying expectation formation process highlighting especially the role of credibility of the signal; moreover from the data emerges that informative Forward Guidance helps in stabilizing economy, drastically reducing the probability of deflationary spirals. The second chapter develops an agent-based model, encapsulating a boundedly rational expectation formation process, which had been extrapolated in previous experiments. Moreover benefiting from a specific aggregation procedure, we derive a model characterized by high analytical tractability, allowing hence to study the transmission mechanisms of a shock by insulating the effects due to the heterogeneity among agents and due to expectations: both the effects are sizable and help in understanding the dynamics of the economic system.

In the continuous casting of steel, the Submerged Entry Nozzle (SEN), in particular the SEN geometry, has a primary influence on the flow pattern: the SEN controls the speed, direction and other characteristics of the jet entering the mould. The SEN is however relatively inexpensive to change (in comparison with other continuous casting equipment). Thus; there is a feasible incentive to exactly understand and predict the flow of molten steel through the SEN and into the mould, in order to maximise the quality of the steel by altering the design of the SEN. By changing the SEN geometry and SEN design, the flow pattern in the mould will also change: it is thus possible to obtain an optimum SEN design if (or when) the desired flow patterns and/or certain predetermined temperature distributions are achieved. Expensive and risky plant trials were traditionally utilised to “perfect” continuous casting processes. As opposed to the plant trials, this dissertation is concerned with the Computational Fluid Dynamics (CFD) modelling of the SEN and mould, which, when used in conjunction with the Mathematical Optimiser LS-OPT, will enable the optimisation of the SEN design to achieve desired results. The CFD models are experimentally verified and validated using 40%-scaled (designed and built in-house) and full-scale water model tests. This dissertation proves that the CFD modelling of the SEN and mould can be quite useful for optimisation and parametric studies, especially when automated model generation (geometry, mesh and solution procedures) is utilised. The importance of obtaining reliable and physically correct CFD results is also emphasised; hence the need for CFD model verification using water modelling. / Dissertation (MEng (Mechanical Engineering))--University of Pretoria, 2007. / Mechanical and Aeronautical Engineering / unrestricted

Quadratic curvature terms are commonly introduced in the action as first-order corrections of General Relativity, and, in this thesis, we investigated their impact on the most simple isolated objects, that are the static and spherically symmetric ones. Most of the work has been done in the context of Stelle's theory of gravity, in which the most general quadratic contractions of curvature tensors are added to the action of General Relativity without a cosmological constant. We studied this theory's possible static, spherically symmetric and asymptotically flat solutions with both analytical approximations and numerical methods. We found black holes with Schwarzschild and non-Schwarzschild nature, naked singularities which can have either an attractive or repulsive gravitational potential in the origin, non-symmetric wormholes which connects an asymptotically flat spacetime with an asymptotically singular one, and non-vacuum solutions modeled by perfect fluids with different equations of state. We described the general geometrical properties of these solutions and linked these short-scale behaviors to the values of the parameters which characterize the gravitational field at large distances. We studied linear perturbations of these solutions, finding that most are unstable, and presented a first attempt to picture the parameter space of stable solutions. We also studied the Thermodynamics of black holes and described their evaporation process: we found that either evaporation leads black holes to unstable configurations, or the predictions of quadratic gravity are unphysical. We also considered the possibility of generalizing Stelle's theory by removing the dependence from the only mass-scale present by including a new dynamical scalar field, making the theory scale invariant. Having a more complex theory, we did not investigate exotic solutions but limited ourselves to the impact of the new additional degrees of freedom on known analytical solutions. It was already known that in a cosmological setting this theory admits a transition between two de Sitter configurations; we analyzed the same problem in the context of static and spherically symmetric solutions and found a transition between two Schwarzschild-de Sitter configurations. In order to do that, we studied both linear perturbations and the semiclassical approximation of the path integral formulation of Euclidean quantum gravity. At last, we tried to extract some phenomenological signatures of the exotic solutions. In particular, we investigated the shadow of an object on background free-falling light, and a possible way of determining the behavior close to the origin using mass measurements that rely on different physical processes. We show that, whenever these measurements are applied to the case of compact stars, in principle it could be possible to distinguish solutions where different equations of state describe the fluid.

Peak profiles in X-ray Diffraction (XRD) patterns from nanocrystalline materials are affected by static and dynamic disorder which is specific of the size and shape of the nanocrystalline domains. Owing to their intrinsic differences, the two types of disorder can be separated, providing independent information from the modelling of the XRD patterns. In the present thesis a model for the static strain created by the nanoparticle surface is proposed. The model is built within the frame of the Whole Powder Pattern Modelling (WPPM) approach for XRD line profile analysis, developed at the University of Trento in the past 20 years. The WPPM approach is decribed in details. Based on a complex Fourier Transform of the diffraction profiles, the model leads to general equations to be used with the WPPM approach to represent the distorted atomic configuration with respect to the reference bulk one. The model was also implemented in TOPAS, a commercial and very popular software, developing a specific macro allowing a larger community of users to benefit of this new opportunity of studying nanocrystalline materials. The thesis work also extended to a more traditional and general description of strain broadening of XRD peak profiles, involving invariant forms under the Laue group symmetry operations of the material under study. As for the dynamic strain, the fundamentals of the Thermal Diffuse Scattering (TDS) contribution to the peak profiles are reviewed. Starting from the original work of B.E. Warren, the theory is generalized to account for surface effects, leading to a particular model developed recently at the University of Trento. This model was thoroughly reviewed and corrected. To test the model a parallel computer code in C was written, exploiting Molecular Dynamics simulations for obtaining reliable and independent estimates of static and dynamic disorder in nanocrystals.

The mechanics of impacts is not yet well understood due to the complexity of materials behaviour under extreme stress and strain conditions and is thus of challenge for fundamental research, as well as relevant in several areas of applied sciences and engineering. The involved complex contact and strain-rate dependent phenomena include geometrical and materials non-linearities, such as wave and fracture propagation, plasticity, buckling, and friction. The theoretical description of such non-linearities has reached a level of advance maturity only singularly, but when coupled -due to the severe mathematical complexity- remains limited. Moreover, related experimental tests are difficult and expensive, and usually not able to quantify and discriminate between the phenomena involved. In this scenario, computational simulation emerges as a fundamental and complementary tool for the investigation of such otherwise intractable problems. The aim of this PhD research was the development and use of computational models to investigate the behaviour of materials and structures undergoing simultaneously extreme contact stresses and strain-rates, and at different size and time scales. We focused on basic concepts not yet understood, studying both engineering and bio-inspired solutions. In particular, the developed models were applied to the analysis and optimization of macroscopic composite and of 2D-materials-based multilayer armours, to the buckling-governed behaviour of aerographite tetrapods and of the related networks, and to the crushing behaviour under compression of modified honeycomb structures. As validation of the used approaches, numerical-experimental-analytical comparisons are also proposed for each case.

La crisi finanziaria esplosa nel 2008 è degenerata in un’aspra e duratura ondata recessiva, che ha colpito l’economia reale a livello internazionale. La presente tesi si propone di contribuire alla letteratura econometrica sull’ultima crisi focalizzandosi su due temi ampiamente dibattuti: vincoli finanziari e produttività totale dei fattori. Il manifatturiero italiano si presenta come scenario preferenziale per condurre l’analisi, tenuto conto della struttura produttiva frammentata e della dipendenza delle imprese dal debito bancario. Si cercherà dapprima di analizzare l’impatto dei vincoli finanziari sulle dinamiche manifatturiere durante la crisi, per poi passare a trattare i temi della rigidità finanziaria e degli effetti di contagio che si sono verificati tra le imprese attraverso il canale del credito commerciale, quali determinanti della probabilità di insolvenza. Infine, il tema delle agglomerazioni tra imprese, di tipo geografico e settoriale, sarà analizzato quale fattore cruciale per la formazione di spillover di produttività, insieme alla capacità innovativa del territorio. I risultati confermano il carattere di pervasività dell’ultima recessione, che alterando le dinamiche manifatturiere ha contribuito ad esacerbare gli episodi di insolvenza. Il possedere una base produttiva clusterizzata ed eterogenea può tuttavia rappresentare ancora un punto di forza, persino all’interno di un contesto operativo plasmato dalla crisi. / The financial crisis that erupted in 2008 translated into harsh recessionary effects at an international level, that were passed on to the real economy. A solid recovery is still lagging behind. The dissertation contributes to the econometric literature on the great recession by focusing attention on two debated topics: financing constraints and total factor productivity (TFP). The fragmented and strongly bank-dependent Italian production base is a preferred environment to conduct the analysis. The role played by financing constraints as amplifiers of manufacturing dynamics is firstly investigated. As a second step, financial rigidity of firms and contagion effects that occurred via trade credit interconnections are considered, and jointly modelled as core determinants of distress likelihoods by resorting to spatial econometric techniques. In the last section, geographical and sectoral clustering phenomena are spatially analyzed in order to investigate knowledge spillovers at the micro level. Results highlight the pervasive nature of the last crisis. The harshness of the recessionary effects fostered a change in manufacturing equilibria and caused the proliferation of distress episodes. Nevertheless, a clustered production base still represents a driver for the formation of positive externalities.

La tesi si colloca nel filone di letteratura dell' Acceleratore Finanziario, che si è sviluppato a partire dagli anni 90. Il lavoro prende le mosse dai modelli di Kiyotaki e Moore (KM,1997, 2002) e di Greenwald e Stiglitz (GS 1993, 2003). L'obiettivo è quello di tentare di rispondere ad alcuni quesiti che sorgono spontaneamente dallo studio di questa tipologia di modelli, inquadrando le idee proposte in una modellistica teorica adeguata. La tesi si compone di 7 capitoli. Nei primi due capitoli viene presentato e discusso, sia in chiave microeconomica che in chiave macroeconomica, il framework proposto da KM. Nel capitolo 3 si introduce il problema della bancarotta e si esplorano gli effetti di eventuali bancarotte sui mercati dei beni e sui mercati finanziari. Nel capitolo 4 viene presentato e discusso un articolo di Cordoba e Ripoll (2004a) nel quale gli autori sviluppano un modello à la KM introducendo il ruolo della moneta tramite l'approccio del Cash In Advance (CIA) constraint . Nel quinto capitolo viene presentato e discusso un contributo originale che introduce il ruolo della moneta nel modello di KM tramite l'approccio della Moneta nella Funzione di Utilità. Il modello è relativamente semplice possono emergere equilibri multipli e permette di esplorare gli effetti della politica monetaria sulle variabili macroeconomiche. Nel capitolo 6 si presenta un modello a generazioni sovrapposte à la Diamond-Samuelson utilizzando il framework di KM. In tale contesto la moneta ha essenzialmente il ruolo di riserva di valore (permette di incrementare il consumo e il bequest da vecchi), mentre il bequest rappresenta una risorsa a disposizione del giovane. In fine il modello presentato nel capitolo 7 è un esempio di un modello macroeconomico microfondato con vincoli finanziari e agenti eterogenei à la Greenwald-Stiglitz. Viene presentata una procedura di aggregazione che permette di ottenere le variabili macroeconomiche tenendo in considerazione il comportamento individuale degli agenti. Il modello può essere studiato tramite delle simulazioni in una struttura Agent-Based. / The dissertation could be traced back to the so called Financial Accelerator literature, that has been developed during the 90's. In particular it is essentially an attempt to adapt, modify or even subvert the basic framework proposed by Kiyotaki and Moore (KM,1997, 2002) and by Greenwald and Stiglitz (GS 1993, 2003) in order to provide answers to some questions that naturally arise from the study of these types of models. The thesis is divided into 7 chapters. Chapters 1 and 2 present and discuss the benchmark model, i.e. the framework put forward by KM, at the micro level (optimization problems of the different types of agents, market equilibrium) and at the macro level (laws of motion of macro state variables) respectively. In chapter 3 we study the conditions under which bankruptcy can occur and we explore the consequences of actual bankruptcies in terms of disruption of financial and goods markets. In chapter 4 we present and discuss a paper by Cordoba and Ripoll in which the role of money in a framework à la KM is introduced by means of the Cash In Advance (CIA) constraint approach. In chapter 5 we follow the Money In the Utility function (MIU) approach to introduce money in the original KM framework. The model seems very promising because is relatively simple, there can be multiple equilibria and the effect of a monetary injection can be explored in a straightforward way. In chapter 6 we model a KM economy in an OLG setting à la Diamond-Samuelson in which money plays basically the role of a store of value, which allows to increase consumption and bequest when old and bequest plays the role of internal resources for the young. The model presented in chapter 7 is an example of a microfounded macroeconomic model with financing constraints and heterogeneous agents of the Greenwald-Stiglitz type. An aggregation procedure is proposed in order to go from the individual to the aggregate variable. The model can be explored by means of simulations in an Agent-Based setting.

Questo lavoro studia la distribuzione della dimensione d'impresa sulla base di due datasets. Il primo è l'indagine micro1 di istat, che include tutte le imprese manifatturiere con più di 20 addetti sopravvissute dal 1989 al 1997. Il secondo è il file Cerved riguardante l'universo delle imprese del settore meccanico (atecodk29), dal 1997 al 2002. Lo scopo generale della tesi è quello di espolare la possibilità di trovare nuove regolarità empiriche riguardanti la distribuzione della dimensione d'impresa, sulla base della passata evidenza empirica che attesta la (in)capacità di Lognormale e Pareto di modellare in modo soddisfacente la dimensione d'impresa nell'intero arco dimensionale. Vengono per questo proposti due modelli mai utilizzati prima. Gli stessi vengono poi convalidati su differenti variabili dimensionali e a diversi livelli di aggregazione. La tesi cerca anche di esplicitare al meglio le implicazioni economiche dei modelli parametrici di distribuzione adottati secondo diversi punti di vista. / The present work studies the firm size distribution of Italian manufacturing industries on the basis of two datasets. The first is the Micro1 survey carried out by ISTAT, which recorded all manufacturing firms with 20 employees and more surviving from 1989 to 1997. The second is the Cerved file regarding all firms of the mechanical sector (DK29) from 1997 to 2002. The general aim of this research is to explore the possibility to find new empirical regularities in the size distribution of firms, building on the relevant past evidence about the (in) capacity of the Lognormal and Pareto distribution of satisfactorily modelling the whole size range. Two unused statistical models are proposed and validated on different size proxies and at different levels of data aggregation. The thesis also addresses the economic implications of parametric models of firm size distribution in different aspects.

Obiettivo di questo lavoro è studiare i fattori socio-economici responsabili del cambiamento nella distribuzione del reddito dovuto a un cambiamento nel contesto politico di riferimento, in Vietnam durante il periodo delle riforme. La metodologia adottata analizza i cambiamenti nella distribuzione del reddito sia a livello micro che a livello macro. A livello micro, l'analisi indaga sulle caratteristiche individuali e familiari da cui dipende il livello e la distribuzione della spesa. E' possibile inoltre valutare gli effetti diretti di cambiamenti nel quadro politico di riferimento. Il livello macro di analisi consente di individuare le caratteristiche strutturali della disuguaglianza nella distribuzione del reddito personale e di isolare anche gli effetti indiretti delle politiche. Gli strumenti analitici selezionati in questo studio sono un modello supply-driven, rappresentato da un modello di microsimulazione e un modello demand-driven, costituito dalla Matrice di Contabilità Sociale. In particolare, il modello di microsimulazione ha consentito la derivazione di una distribuzione controfattuale e la disaggregazione della variazione della disuguaglianza in Vietnam in: effetto di prezzo, effetto di una variazione della componente non osservata dei salari, effetto dovuto a cambiamenti nelle scelte occupazioni e effetti dovuti a cambiamenti nella popolazione. Utilizzando una nuova metodologia di scomposizione ad un livello microscopico dei moltiplicatori derivati dalla SAM, è stato possibile derivare e isolare tutti gli effetti diretti e indiretti di uno shock esogeno sulla distribuzione personale del reddito. / The aim of this work is to investigate the socio-economic factors that affect in income distribution changes caused by changes in the policy framework in Vietnam during the period of reforms. The adopted methodology analyzes policy induced changes in income distribution both at the micro and the macro level. At the micro level, the analysis of inequality can help identifying the socio-economic factors affecting the level of household expenditure and its distribution and evaluating direct effects of policies. The macro level identifies the structural characteristics of inequality and evaluates also the indirect effects of policies on the personal income distribution. The two analytical tools have been selected have been a supply driven model represented by the microsimulation model and a demand driven model, constituted by the Social Accounting Matrix. The microsimulation model allowed deriving a counterfactual distribution of income and disaggregating change in the Vietnamese income inequality into four effects: price effect, effect of a change in the unobservable component of wages, occupational choice effect and population effect. Using a new technique of decomposition of SAM-based multipliers in 'microscopic' detail, the macro model allowed deriving all the direct and indirect effects of an exogenous shock to personal income distribution.