538 research outputs found

    Smooth solutions for the dyadic model

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    We consider the dyadic model, which is a toy model to test issues of well-posedness and blow-up for the Navier-Stokes and Euler equations. We prove well-posedness of positive solutions of the viscous problem in the relevant scaling range which corresponds to Navier-Stokes. Likewise we prove well-posedness for the inviscid problem (in a suitable regularity class) when the parameter corresponds to the strongest transport effect of the non-linearity

    Global regularity for a slightly supercritical hyperdissipative Navier-Stokes system

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    We prove global existence of smooth solutions for a slightly supercritical hyperdissipative Navier--Stokes under the optimal condition on the correction to the dissipation. This proves a conjecture formulated by Tao [Tao2009]

    Global regularity for a logarithmically supercritical hyperdissipative dyadic equation

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    We prove global existence of smooth solutions for a slightly supercritical dyadic model. We consider a generalized version of the dyadic model introduced by Katz-Pavlovic [KatPav2004] and add a viscosity term with critical exponent and a supercritical correction. This model catches for the dyadic a conjecture that for Navier-Stokes equations was formulated by Tao [Tao2009

    Numerical simulation of premixed combustion flows: a comparative study

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    In this work four different commercial and research CFD codes have been compared for the simulation of two combustion test cases. The aim was to get an overview of the capabilities of these different tools to simulate combustion flows in premixed regimes. Codes tested were Fluent, CFX, StarCD and Tanit. Three combustion models have been applied, namely the Eddy Break Up, the Eddy Dissipation Model and the Turbulent Flame Closure, the turbulence model used being the standard k-epsilon. Numerical results have been found to fairly fit experiments and helped to show some drawbacks of combustion models. In its theoretically correct range of applicability the TFC model has been found to give the better agreement with experiments

    Characterization, modeling and reliability of RF MEMS Switches and Photovoltaic Silicon Solar Cells

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    The main goal of this thesis is the failure and reliability investigation of RF-MEMS switches and photovoltaic solar cells. For technical developer people the reliability issue is often consider a secondary problem in electronic devices since it is not considered an important factor in the production chain. This concept is changing is the last years because reliability studies are considered an important technological step to improve the production process. This fact is confirmed by the investments that companies adopt to test their products. In the particular case of this thesis, we can easily mention the solar cell production line where the cells are subjected to reliability tests that extrapolate the efficiency and the fill factor in order to study the performances and to consequently improve the production process. Concerning RF MEMS Wireless communication systems for space applications require electronic components with a high level of reliability, a low power consumption and they should be as small as possible in order to be better integrated in satellites. Radio Frequency Micro Electro Mechanical System (RF-MEMS) can be considered one of the best candidates to comply with previous requirements and, under certain conditions, they can completely replace an entire solid-state circuit. RF-MEM devices in general are characterized by a good miniaturization, an easily integration in a standard solid-state circuit, an almost zero power consumption, a good RF linearity and a high quality factor Q. Concerning RF-MEMS switches RF performances, they exhibit a very low insertion loss, lower than 0.1 dBm up to 60 GHz and, at the same time, a good isolation, more than 20 dBm. From an electrical and mechanical point of view the power consumption of these switches is close to zero because of an “on-state” current around pA and they are almost unaffected by high level of acceleration or deceleration because of their mass that is extremely small. The possibility to integrate the production of these devices in the standard foundry silicon processes and their integration with mature semiconductor technology are a great advantage for their spread making possible to produce them in an easy and cheap way. Over the last 10 years important developments on MEMS switches have been done all over the world. As a matter of fact, these switches are quite attractive since they combine excellent RF performances and low power consumption of mechanical switches with the small size and low weight of semiconductor devices. However, the appearance of MEMS switches on the market has been hindered by the need for specific packaging as well as by reliability issues. Reliability is a major issue for any satellite since it is almost impossible to envisage any repair work once the spacecraft has been launched. Hence, reliability is a key driver when designing any RF equipment. If we consider a RF-MEMS switch, we have to guarantee that his electromechanical performances will be the same after an intensive usage in harsh environment, for instance after millions or billions of cycles and after the exposure to different kind of radiations. In case of their application in a redundancy scheme, they have to be completely operative even after a long period of activity or inactivity. The aim of this thesis is to perform an electrical characterization and several reliability tests on different kind of RF-MEMS switches in order to analyze which are the weaknesses and the strengths of this new technology. Electrical characterizations have been done using two different measurement systems. The first, based on a vector network analyzer and a power supply, has been used to test the RF performances of the devices and to extract the actuation and deactuation voltages. The second set up, based on the internal RF signal generator of the VNA, an 8-GHz digital signal oscilloscope and a profilometer (polytec MSA 500), has been used to characterize the electrical performances like actuation time, release delay and dynamic performances. Cycling stress, one of the most common test used to understand the robustness of this kind of devices, has been performed on different topologies of switch in order to better understand how some parameters of the RF MEMS switch, such as the shape of the beams or the actuation voltage, impact on the reliability of the device. Furthermore, the influence of continuous actuation stress on the reliability of dielectric-less switches has been investigated, comparing different designs and studying the variation of the main electrical parameters induced by the stress and the successive recovery phase. Concerning PV solar cells A solar cell, or photovoltaic cell, is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect. The operation of a photovoltaic (PV) cell requires 3 basic attributes: (i) the absorption of light, generating either electron-hole pairs or excitons, (ii) the separation of charge carriers of opposite types and (iii) the extraction of those carriers to an external circuit. Over the last decades, many research groups have tried to improve the conversion processes in order to increase the efficiency of solar cells and to reduce the parasitic effects that limit the energy conversion. This has generated a real challenge to the best conversion efficiency. The average efficiency of multicrystalline silicon solar cells at the beginning of 2014 was about 16% but in research labs different solar cells have exceeded the 20% with records over 24%. The continuous growth of the solar cells efficiency has been achieved thanks to the reliability study of the single cells and to the degradation analysis of the real photovoltaic systems. These studies have revealed the critical points of PV solar cells and have led to a constant improvement of the production processes. The aim of this thesis is the study of the reliability problems related to a single solar cell and to a string of solar cells subjected to different illumination conditions. Different characterization procedures have been developed in order to study the failure mechanisms and to study the weaknesses and the strengths of the technology. Four types of measurement set-ups have been utilized: (i) the first system is able to extract the IV curves in dark and light conditions. This simple measurement procedure has to be opportunely calibrated in order to obtain right results in term of efficiency and fill factor. (ii) The second system extracts the thermographic image of a single solar cell. It can be used to analyze hot spot and other failure mechanisms in the silicon structure. (iii) The third system extracts the electroluminescence and the photoluminescence of a single solar cell. It is able to extract and analyze the defects in the crystalline structure of the materials. (iv) The fourth is the LOANA system: a commercial tool able to extract the External Quantum Efficiency and the Internal Quantum Efficiency with the measurement of the reflectance. All these characterization procedures have been utilized to study the evolution of the failure mechanisms when a single solar cell is subjected to reverse biasing stresses. The study of the catastrophic degradation of solar cells submitted to reverse current stress is of crucial importance since the failure can lead to the rapid increase of the temperature with a consequent risk of fire and to the breaking of the entire PV system. This particular situation can occur when the PV system is not uniformly illuminated and the solar cells of the system present not uniform shunt resistance. Additional studies have been performed in the modelization of a solar cell with the two-diode model. The study and modeling of solar cells allow to obtain right results in term of efficiency and fill factor extrapolation. Moreover, the modelization allows the study of string of solar cells working in particular conditions in which the illumination level is not uniform in a whole panel. The simulations allow to predict the dangerous situations and to design appropriate prevention systems

    lDDT: a local superposition-free score for comparing protein structures and models using distance difference tests

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    Motivation: The assessment of protein structure prediction techniques requires objective criteria to measure the similarity between a computational model and the experimentally determined reference structure. Conventional similarity measures based on a global superposition of carbon α atoms are strongly influenced by domain motions and do not assess the accuracy of local atomic details in the model. Results: The Local Distance Difference Test (lDDT) is a superposition-free score that evaluates local distance differences of all atoms in a model, including validation of stereochemical plausibility. The reference can be a single structure, or an ensemble of equivalent structures. We demonstrate that lDDT is well suited to assess local model quality, even in the presence of domain movements, while maintaining good correlation with global measures. These properties make lDDT a robust tool for the automated assessment of structure prediction servers without manual intervention. Availability and implementation: Source code, binaries for Linux and MacOSX, and an interactive web server are available at http://swissmodel.expasy.org/lddt Contact: [email protected] Supplementary information: Supplementary data are available at Bioinformatics onlin

    Latent Space Regularization for Unsupervised Domain Adaptation in Semantic Segmentation

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    Deep convolutional neural networks for semantic segmentation achieve outstanding accuracy, however they also have a couple of major drawbacks: first, they do not generalize well to distributions slightly different from the one of the training data; second, they require a huge amount of labeled data for their optimization. In this paper, we introduce feature-level space-shaping regularization strategies to reduce the domain discrepancy in semantic segmentation. In particular, for this purpose we jointly enforce a clustering objective, a perpendicularity constraint and a norm alignment goal on the feature vectors corresponding to source and target samples. Additionally, we propose a novel measure able to capture the relative efficacy of an adaptation strategy compared to supervised training. We verify the effectiveness of such methods in the autonomous driving setting achieving state-of-the-art results in multiple synthetic-to-real road scenes benchmarks.Comment: Accepted at CVPR-WAD 2021, 11 pages, 7 figures, 1 table

    Strong existence and uniqueness of the stationary distribution for a stochastic inviscid dyadic model

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    We consider an inviscid stochastically forced dyadic model, where the additive noise acts only on the first component. We prove that a strong solution for this problem exists and is unique by means of uniform energy estimates. Moreover, we exploit these results to establish strong existence and uniqueness of the stationary distribution.Comment: 13 page

    The Holocene Evolution of the Volturno Coastal Plain (Northern Campania, Southern Italy): Implications for the Understanding of Subsidence Patterns

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    In the Mediterranean area, several alluvial coastal plains, developed after the Holocene transgression, are affected by subsidence. The Volturno alluvial‐coastal plain, along the eastern Tyrrhenian Sea (southern Italy) is characterized by subsidence rates determined through InSAR data analysis and ranging between 0 and <−20 mm/yr in an area of about 750 kmq across the Volturno River. Inside this area, the pattern of subsidence shows sites with apparently anomalous localized subsidence. To understand the driving mechanisms of this process, a lithostratigraphic reconstruction was provided focusing on the spatial distribution of the horizons considered weak by a geotechnical point of view; then, the subsidence map was overlain spatially with geological data in a Geographic Information System (GIS) environment. The spatial analysis highlighted the major ground deformation occurring within the outer boundary of the incised paleo‐valley, corresponding to the Holocene alluvial/transitional filling that overlies a compaction‐free Pleistocene basement. Inside this general trend, differential compaction was detected corresponding to the thick occurrence of clay and peat deposits, suggesting that the subsidence rate registered in the plain are due in part to the consolidation of primary settlements of soft and compressible soils that characterize the subsoil of these areas, and in large part to the secondary consolidation settlements
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