Littérature scientifique sur le sujet « Equazioni differenzi »

Questo lavoro ha indagato il senso di appartenenza degli infermieri all'azienda ospedaliera, tenendo conto del genere, del reparto di afferenza e degli anni di permanenza nella struttura. Ulteriore obiettivo č stato quello di indagare la relazione tra il senso di appartenenza di questi operatori sanitari e altre variabili psicosociali, quali le percezioni di efficacia collettiva nella comunicazione in ambito ospedaliero, l'autoefficacia percepita nella comunicazione con il paziente, l'autoefficacia sociale e il burnout. Č stato somministrato un questionario self-report a 840 infermieri (47% maschi e 53% femmine) che lavorano in aziende ospedaliere del Centro e del Sud Italia. I risultati hanno evidenziato livelli piuttosto elevati di senso di appartenenza alla struttura da parte degli infermieri (con differenze per anzianitŕ di servizio e reparto di afferenza) e bassi livelli di burnout. Č stato testato un modello di equazioni strutturali che ha evidenziato come il senso di appartenenza, insieme alle percezioni di autoefficacia comunicativa, risulti un predittore negativo di tutte le dimensioni del burnout. Sono risultati predittori del senso di appartenenza all'azienda ospedaliera le percezioni di efficacia collettiva e l'anzianitŕ di servizio nello stesso ospedale.

Dopo un'introduzione generale sul fenomeno della regolarizzazione attraverso rumore in un contesto degenere, la prima parte di questa tesi si concentra nello stabilire le stime di Schauder, un strumento analitico utile per dimostrare anche il carattere ben posto di equazioni differenziali stocastiche (EDS), per due classi di equazioni di Kolmogorov sotto una condizione di tipo Hörmander debole, i cui coefficienti giacciono in opportuni spazi di Hölder anisotropi con multi-indici di regolarità. La prima classe considera un sistema non lineare controllato da un operatore simmetrico ⍺-stabile che agisce solo su alcune componenti. Il nostro metodo di dimostrazione si basa su un approccio perturbativo basato su espansioni della parametrice progressiva tramite formule di tipo Duhamel. A causa delle scarse proprietà regolarizzanti date dal contesto degenere, sfruttiamo anche alcuni controlli sulle norme di Besov, per trattare la perturbazione non lineare. Come estensione del primo modello, presentiamo anche delle stime di Schauder associate a un operatore di Ornstein-Uhlenbeck degenere guidato da una classe più ampia di operatori di tipo quasi-stabile, come quello stabile relativistico o quello di Lamperti. La dimostrazione di questo risultato si basa invece su un'analisi precisa del comportamento del semigruppo di Markov corrispondente tra spazi di Hölder anisotropici e alcune tecniche di interpolazione. Sfruttando un approccio della parametrice retrograda, la seconda parte di questa tesi cerca di stabilire il carattere ben posto in senso debole per una catena degenere di EDS guidate dalla stessa classe di processi quasi-stabili, sotto le assunzioni di regolarità di Hölder minime per i coefficienti. Come corollario del nostro metodo, presentiamo anche stime di tipo Krylov di interesse indipendente per il processo canonico sottostante. Infine, sottolineiamo attraverso opportuni controesempi che esiste effettivamente una soglia (quasi) ottimale sugli esponenti di regolarità che garantiscono il carattere ben posto debole per l'EDS. In relazione ad alcune applicazioni meccaniche per delle dinamiche cinetiche con attrito, concludiamo studiando la stabilità delle perturbazioni del secondo ordine per operatori degeneri di Kolmogorov nelle norme Lp e Hölder.
After a general introduction about the regularization by noise phenomenon in the degenerate setting, the first part of this thesis focuses at establishing the Schauder estimates, a useful analytical tool to prove also the well-posedness of stochastic differential equations (SDEs), for two different classes of Kolmogorov equations under a weak Hörmander-like condition, whose coefficients lie in suitable anisotropic Hölder spaces with multi-indices of regularity. The first class considers a nonlinear system controlled by a symmetric ⍺-stable operator acting only on some components. Our method of proof relies on a perturbative approach based on forward parametrix expansions through Duhamel-type formulas. Due to the low regularizing properties given by the degenerate setting, we also exploit some controls on Besov norms, in order to deal with the non-linear perturbation. As an extension of the first one, we also present Schauder estimates associated with a degenerate Ornstein-Uhlenbeck operator driven by a larger class of ⍺-stable-like operators, like the relativistic or the Lamperti stable one. The proof of this result relies instead on a precise analysis of the behaviour of the associated Markov semigroup between anisotropic Hölder spaces and some interpolation techniques. Exploiting a backward parametrix approach, the second part of this thesis aims at establishing the well-posedness in a weak sense of a degenerate chain of SDEs driven by the same class of ⍺-stable-like processes, under the assumptions of the minimal Hölder regularity on the coefficients. As a by-product of our method, we also present Krylov-type estimates of independent interest for the associated canonical process. Finally, we emphasize through suitable counter-examples that there exists indeed an (almost) sharp threshold on the regularity exponents ensuring the weak well-posedness for the SDE. In connection with some mechanical applications for kinetic dynamics with friction, we conclude by investigating the stability of second-order perturbations for degenerate Kolmogorov operators in Lp and Hölder norms.

This thesis deals with numerical techniques for the computation of modes in electromagnetic structures with arbitrary geometry. The approach proposed in this work is based on the Finite Difference (FD) and Vector Finite Difference (VFD), which are applied to rectangular, circular, elliptical geometries, and to combination of them. The FD is applied using a 2D cartesian, polar and elliptical grid in the waveguide section. A suitable Taylor expansion of the mode function allows, either for scalar and for vector FD, to take exactly into account the boundary condition. To prevent the raising of spurious modes, the VFD approximation results in a constrained eigenvalue problem, that has been solved using a decomposition method. All approaches presented have been validated comparing the results to the analytical modes of rectangular and circular waveguide, and to known data for the elliptic case. The standard calculation of the waveguide modes using FD requires the use of two different grids, namely one for TE modes and the other for TM modes, due to the different boundary condition. It has been shown that a single grid can be used for all modes, thus allowing an effective mode-matching solution. The FD approach has been extended to waveguides (and apertures) with irregular boundaries, and therefore non-regular discretization grids. It has been shown that a suitable FD approximation of the Laplace operator is still possible. A ridged-waveguide, with trapezoidal ridges, and a rounded-ended waveguide have been considered in detail.

Despite significant efforts have been directed toward reducing waste generation and encouraging alternative waste management strategies, landfills still remain the main option for Municipal Solid Waste (MSW) disposal in many countries. Hence, landfills and related impacts on the surroundings are still current issues throughout the world. Actually, the major concerns are related to the potential emissions of leachate and landfill gas into the environment, that pose a threat to public health, surface and groundwater pollution, soil contamination and global warming effects. To ensure environmental protection and enhance landfill sustainability, modern sanitary landfills are equipped with several engineered systems with different functions. For instance, the installation of containment systems, such as bottom liner and multi-layers capping systems, is aimed at reducing leachate seepage and water infiltration into the landfill body as well as gas migration, while eventually mitigating methane emissions through the placement of active oxidation layers (biocovers). Leachate collection and removal systems are designed to minimize water head forming on the bottom section of the landfill and consequent seepages through the liner system. Finally, gas extraction and utilization systems, allow to recover energy from landfill gas while reducing explosion and fire risks associated with methane accumulation, even though much depends on gas collection efficiency achieved in the field (range: 60-90% Spokas et al., 2006; Huitric and Kong, 2006). Hence, impacts on the surrounding environment caused by the polluting substances released from the deposited waste through liquid and gas emissions can be potentially mitigated by a proper design of technical barriers and collection/extraction systems at the landfill site. Nevertheless, the long-term performance of containment systems to limit the landfill emissions is highly uncertain and is strongly dependent on site-specific conditions such as climate, vegetative covers, containment systems, leachate quality and applied stress. Furthermore, the design and operation of leachate collection and treatment systems, of landfill gas extraction and utilization projects, as well as the assessment of appropriate methane reduction strategies (biocovers), require reliable emission forecasts for the assessment of system feasibility and to ensure environmental compliance. To this end, landfill simulation models can represent an useful supporting tool for a better design of leachate/gas collection and treatment systems and can provide valuable information for the evaluation of best options for containment systems depending on their performances under the site-specific conditions. The capability in predicting future emissions levels at a landfill site can also be improved by combining simulation models with field observations at full-scale landfills and/or with experimental studies resembling landfill conditions. Indeed, this kind of data may allow to identify the main parameters and processes governing leachate and gas generation and can provide useful information for model refinement. In view of such need, the present research study was initially addressed to develop a new landfill screening model that, based on simplified mathematical and empirical equations, provides quantitative estimation of leachate and gas production over time, taking into account for site-specific conditions, waste properties and main landfill characteristics and processes. In order to evaluate the applicability of the developed model and the accuracy of emissions forecast, several simulations on four full-scale landfills, currently in operative management stage, were carried out. The results of these case studies showed a good correspondence of leachate estimations with monthly trend observed in the field and revealed that the reliability of model predictions is strongly influenced by the quality of input data. In particular, the initial waste moisture content and the waste compression index, which are usually data not available from a standard characterisation, were identified as the key unknown parameters affecting leachate production. Furthermore, the applicability of the model to closed landfills was evaluated by simulating different alternative capping systems and by comparing the results with those returned by the Hydrological Evaluation of Landfill Performance (HELP), which is the most worldwide used model for comparative analysis of composite liner systems. Despite the simplified approach of the developed model, simulated values of infiltration and leakage rates through the analysed cover systems were in line with those of HELP. However, it should be highlighted that the developed model provides an assessment of leachate and biogas production only from a quantitative point of view. The leachate and biogas composition was indeed not included in the forecast model, as strongly linked to the type of waste that makes the prediction in a screening phase poorly representative of what could be expected in the field. Hence, for a qualitative analysis of leachate and gas emissions over time, a laboratory methodology including different type of lab-scale tests was applied to a particular waste material. Specifically, the research was focused on mechanically biologically treated (MBT) wastes which, after the introduction of the European Landfill Directive 1999/31/EC (European Commission, 1999) that imposes member states to dispose of in landfills only wastes that have been preliminary subjected to treatment, are becoming the main flow waste landfilled in new Italian facilities. However, due to the relatively recent introduction of the MBT plants within the waste management system, very few data on leachate and gas emissions from MBT waste in landfills are available and, hence, the current knowledge mainly results from laboratory studies. Nevertheless, the assessment of the leaching characteristics of MBT materials and the evaluation of how the environmental conditions may affect the heavy metals mobility are still poorly investigated in literature. To gain deeper insight on the fundamental mechanisms governing the constituents release from MBT wastes, several leaching experiments were performed on MBT samples collected from an Italian MBT plant and the experimental results were modelled to obtain information on the long-term leachate emissions. Namely, a combination of experimental leaching tests were performed on fully-characterized MBT waste samples and the effect of different parameters, mainly pH and liquid to solid ratio (L/S,) on the compounds release was investigated by combining pH static-batch test, pH dependent tests and dynamic up-flow column percolation experiments. The obtained results showed that, even though MBT wastes were characterized by relatively high heavy metals content, only a limited amount was actually soluble and thus bioavailable. Furthermore, the information provided by the different tests highlighted the existence of a strong linear correlation between the release pattern of dissolved organic carbon (DOC) and several metals (Co, Cr, Cu, Ni, V, Zn), suggesting that complexation to DOC is the leaching controlling mechanism of these elements. Thus, combining the results of batch and up-flow column percolation tests, partition coefficients between DOC and metals concentration were derived. These data, coupled with a simplified screening model for DOC release, allowed to get a very good prediction of metal release during the experiments and may provide useful indications for the evaluation of long-term emissions from this type of waste in a landfill disposal scenario. In order to complete the study on the MBT waste environmental behaviour, gas emissions from MBT waste were examined by performing different anaerobic tests. The main purpose of this study was to evaluate the potential gas generation capacity of wastes and to assess possible implications on gas generation resulting from the different environmental conditions expected in the field. To this end, anaerobic batch tests were performed at a wide range of water contents (26-43 %w/w up to 75 %w/w on wet weight) and temperatures (from 20-25 °C up to 55 °C) in order to simulate different landfill management options (dry tomb or bioreactor landfills). In nearly all test conditions, a quite long lag-phase was observed (several months) due to the inhibition effects resulting from high concentrations of volatile fatty acids (VFAs) and ammonia that highlighted a poor stability degree of the analysed material. Furthermore, experimental results showed that the initial waste water content is the key factor limiting the anaerobic biological process. Indeed, when the waste moisture was lower than 32 %w/w the methanogenic microbial activity was completely inhibited. Overall, the obtained results indicated that the operative conditions drastically affect the gas generation from MBT waste, in terms of both gas yield and generation rate. This suggests that particular caution should be paid when using the results of lab-scale tests for the evaluation of long-term behaviour expected in the field, where the boundary conditions change continuously and vary significantly depending on the climate, the landfill operative management strategies in place (e.g. leachate recirculation, waste disposal methods), the hydraulic characteristics of buried waste, the presence and type of temporary and final cover systems.