Academic literature on the topic 'Fog layer'

Clouds are powerful computer centers that provide computing and storage facilities that can be remotely accessed. The flexibility and cost-efficiency offered by clouds have made them very popular for business and web applications. The use of clouds is now being extended to safety-critical applications such as factories. However, cloud services do not provide time predictability which creates a hassle for such time-sensitive applications. Moreover, delays in the data communication between clouds and the devices the clouds control are unpredictable. Therefore, to increase predictability an intermediate layer between devices and the cloud is introduced. This layer, the Fog layer, aims to provide computational resources closer to the edge of the network. However, the fog computing paradigm relies on resource-constrained nodes, creating new potential challenges in resource management, scalability, and reliability. Solutions such as lightweight virtualization technologies can be leveraged for solving the dichotomy between performance and reliability in fog computing. In this context, container-based virtualization is a key technology providing lightweight virtualization for cloud computing that can be applied in fog computing as well. Such container-based technologies provide fault tolerance mechanisms that improve the reliability and availability of application execution.  By the study of a robotic use-case, we have realized that persistent data storage for stateful applications at the fog layer is particularly important. In addition, we identified the need to enhance the current container orchestration solution to fit fog applications executing in container-based architectures. In this thesis, we identify open challenges in achieving dependable fog platforms. Among these, we focus particularly on scalable, lightweight virtualization, auto-recovery, and re-integration solutions after failures in fog applications and nodes. We implement a testbed to deploy our use-case on a container-based fog platform and investigate the fulfillment of key dependability requirements. We enhance the architecture and identify the lack of persistent storage for stateful applications as an important impediment for the execution of control applications. We propose a solution for persistent fault-tolerant storage at the fog layer, which dissociates storage from applications to reduce application load and separates the concern of distributed storage. Our solution includes a replicated data structure supported by a consensus protocol that ensures distributed data consistency and fault tolerance in case of node failures. Finally, we use the UPPAAL verification tool to model and verify the fault tolerance and consistency of our solution.

The uncertainty in the determination of the momentum and scalar fluxes remains one of the main obstacles to accurate numerical forecasts in low to moderate wind conditions. For example, latent heat fluxes computed from data using direct covariance and bulk aerodynamic methods show that there is good agreement in unstable conditions when the latent heat flux values are generally positive. However, the agreement is relatively poor in stable conditions, particularly when the moisture flux is directed downward. If the direct covariance measurements are indeed accurate, then they clearly indicate that the bulk aerodynamic formula overestimate the downward moisture flux in stable conditions. As a result, comparisons of the Dalton number for unstable and stable conditions indicate a marked difference in value between the two stability regimes. Investigations done for this thesis used data taken primarily at the Air-Sea Interaction Tower (ASIT) during the Coupled Boundary Layers and Air-Sea Transfer (CBLAST) Experiment 2003 from the 20-27 August 2003. Other data from the shore based Martha's Vineyard Coastal Observatory (MVCO) and moored buoys in the vicinity of the ASIT were also incorporated. During this eight day period, the boundary layer was often characterized by light winds, a stably stratified surface layer and a swell dominated wave field. Additionally, the advection of warm moist air over cooler water resulted in fog formation and a downward flux of moisture on at least three occasions. Therefore, a primary objective of this thesis is to present a case study to investigate the cause of this shortcoming in the bulk formula under these conditions by examining the physical processes that are unique to these boundary layers. Particular attention will be paid to the behavior of the Dalton number in a stable marine atmospheric boundary layer under foggy conditions using insights derived from the study of fog formation and current flux parameterization methods.

La cellulosa è il polimero naturale più abbondante sulla terra, una risorsa rinnovabile che ogni anno viene prodotta in miliardi di tonnellate da molti organismi vegetali. Per questa ragione, su di essa si sta concentrando una crescente attenzione nell’ipotesi di una sua sempre maggiore applicazione nei più diversi campi. Quello del food packaging, che è ancora fortemente dipendente da materiali di sintesi e provenienti da risorse non rinnovabili, è particolarmente interessato ad un suo più ampio impiego, anche con il fine di aumentare la sostenibilità dei suoi prodotti e di ridurne l’impatto ambientale. Con questa tesi di dottorato si è inteso mettere a fuoco le potenzialità di impiego della nano cellulosa (cellulose nanocrystals, CNs), sperimentare la produzione e valutare le proprietà di alcune lacche a base di CNs, destinate a ricoprire convenzionali materiali flessibili per il confezionamento alimentare. La tesi si compone di quattro parti distinte. Nella prima parte si è inteso rappresentare lo stato dell’arte delle conoscenze e delle applicazioni della nanocellulosa, attraverso un ampio lavoro di documentazione bibliografica. Dapprima si è voluto mettere a fuoco quanto noto sulla struttura e la classificazione delle varie forme di nanocellulosa che è oggi possibile produrre e, a proposito della cellulosa nanocristallina in particolare, si è fatto il punto sulle tecniche di preparazione, la morfologia e le principali applicazioni. Da questo lavoro di documentazione sono emerse le notevoli proprietà di barriera ai gas ed a potenziali migranti, le eccellenti proprietà meccaniche (resistenza alla rottura, massima elongazione tensile, modulo di Young) e le interessanti caratteristiche termiche (transizione vetrosa, punto di fusione e di decomposizione) della CNs da sola ed in combinazione con altri materiali. Nella seconda sezione della tesi, al fine di comprendere meglio la struttura e la morfologia dei nanocristalli di cellulosa ottenuti attraverso un processo di idrolisi acida di linter di cotone, sono state utilizzate diverse tecniche analitiche avanzate, sia per la caratterizzazione qualitativa che quantitativa. E’ stato così possibile ottenere informazioni precise sulle dimensioni dei nano cristalli, il rapporto di forma, la solubilità e numerose altre loro importanti proprietà. In particolare le tecniche di TEM, SEM, e AFM sono apparse come le più adatte per osservare la morfologia dei cristalli, studiare le caratteristiche e la rugosità delle superfici trattate con lacche a base di CNs. Si è inoltre indagato sulla distribuzione delle dimensioni dei cristalli ottenuti e, grazie all’uso di FTIR, XPS e NMR, sulla natura dei gruppi funzionali disponibili e sulle loro interazioni. La terza parte della tesi è dedicata ad uno studio delle proprietà di alcuni differenti film, largamente impiegati per il food packaging (PET, OPP, OPA e cellophane), rivestiti con uno strato sottile di CNs. In particolare, è statomesso a punto il processo di laccatura e sono state misurate le proprietà ottiche (la trasparenza, l’opacità e le proprietà anti-fog), il coefficiente di frizione statico e dinamico, le energie superficiali e gli angoli di contatto, le proprietà di barriera all’ossigeno ed al vapor d’acqua. Da questo lavoro è emerso come sia effettivamente possibile rivestire di uno strato sottile (intorno ad un micron di spessore), omogeneo e continuo, film plastici differenti e che attraverso questo processo di laccatura, si riduce significativamente il coefficiente di frizione, si incrementano le proprietà anti-fog, si aumenta decisamente la barriera all’ossigeno, senza pregiudicare la trasparenza dei film di supporto. La prospettiva molto concreta è quella di costituire, in un modestissimo spessore, un coating multifunzionale con spiccate caratteristiche di sostenibilità e di sicurezza alimentare. L’ultima sezione della tesi è dedicata al lavoro fatto per sperimentare la possibile applicazione di una tecnica di rivestimento molto moderna (layer-by-layer coating, LbL) che sfrutta la formazione di legami elettrostatici tra biopolimeri caricati diversamente. In particolare si è dimostrata la possibilità di costituire lacche di un composito ottenuto mediante la sovrapposizione alternata di sottilissimi strati (da 6 a circa 30 nm) di chitosano e cellulosa nanocristallina. Il diverso pH delle soluzioni in cui vengono dispersi i due biopolimeri determina un diverso grado di ionizzazione delle cariche, rispettivamente positive del chitosano e negative della cellulosa, e di conseguenza diversi spessori e proporzioni relative dei due biopolimeri nel coating composito che si realizza. Ciò, evidentemente, permette di modulare in un ampio intervallo di valori, la permeabilità del film ricoperto. Il coefficiente di permeabilità del composito giunge a valori pari 0.02 cm3 µm m-2 24h-1 kPa-1, molto simili a quelli espressi da copolimeri a base di EVOH, in condizioni anidre. I vantaggi di un simile rivestimento sono comunque fondamentalmente legati alla sicurezza e non tossicità dei biopolimeri impiegati, dalla loro sostenibilità e dall’ampio grado di libertà disponibile nel modulare le caratteristiche finali di barriera, secondo le esigenze del prodotto da confezionare. Lo strato di lacca LbL così costituita rappresenta, in definitiva, una barriera all’ossigeno particolarmente promettente negli impieghi reali più critici anche per la concreta possibilità di realizzarla convenientemente su oggetti tridimensionali come bottiglie, vassoi e altri imballaggi finiti. In conclusione, le ricerche condotte rappresentano una base di partenza molto promettente per un’innovazione di sostenibilità e di prestazioni nel campo dell’imballaggio flessibile e meritano ulteriori approfondimenti ed applicazioni.<br>Being cellulose the most abundant natural polymer in biosphere, more and more attention has been paid on its new functionalities, sustainability, and renewability. Meanwhile, food packaging materials is one of the largest products we are using in daily life, but most of conventional materials are still oil-based due to their low cost and good performances. Therefore, in order to improve the sustainability and renewability of food packaging materials, this PhD dissertation focuses on development new nano-material (cellulose nanocrystals, CNs) for food packaging and includes mainly four sections. In the first section of this PhD dissertation, we reviewed the progress in knowledge on nano-cellulose first and then, specifically, on CNs. In this section the structure and classifications of various nano-cellulose preparations are included, as well as the preparation, the morphologies, and applications of CNs. In CNs applications, we reviewed that it exhibits excellent barrier, mechanical, and thermal properties itself or combined with other polymers. Particularly, the barrier properties refer to oxygen, water vapor, and migration barrier; mechanical properties are related with tensile strength, Young’s modulus, and strain percentage; the thermal properties include glass transition and melting or decomposition temperature, heat flow, and thermal mechanical parameters. In the second section of this PhD dissertation, to better understand the structure and status of CNs itself or in other polymers, we have used different powerful analytical tools for qualification and quantification. Firstly, we have obtained the relatively precise dimensions of CNs and observe its redispersability in different solvents, mainly water solutions. In the following, we could gain the information of the CNs status in other polymers in order to interpret the final performance efficiently. Finally, we preliminarily concluded that TEM, SEM, and AFM are suitable tools for observing individual crystals, estimating the roughness, and learning the morphology in different scale, respectively. As for the size distribution, functional groups, and interactions between the atoms of CNs, the particle size distributor, FTIR, XPS, and NMR are used for determinations, respectively. In the third section of this PhD dissertation, we have systematically investigated the properties of conventional films coated with CNs. In particular, we have analyzed their optical properties (transparency and haze), mechanical properties (static and dynamic coefficient of friction), anti-fog (contact angle and surface energy) and barrier properties (oxygen and water vapor transmission rates). In doing this, we have demonstrated that CNs coatings mainly lead to a reduction of friction, a premium feature for industrial applications, and that their influence on the optical properties of the packaging is not significant. Excellent anti-fog property guarantees customers more conveniently to evaluate the product inside the packages easily. At last but not the least, CNs coatings dramatically improve not only the oxygen barrier properties of conventional flexible food packaging, but also lead to a certain reduction in the water vapor transmission rate. The perspective use of CNs as multi-functional coatings favors a reduction of the required thickness for plastic films, towards a more environmentally-friendly and sustainable approach to packaging. In the last section of this PhD dissertation, we demonstrated the use of chitosan (CS)/CNs nanocomposites realized by layer-by-layer (LbL) self-assembly as oxygen barrier under different pH combinations. The oxygen permeability coefficient of CS/CNs nanocomposites is as low as 0.02 cm3 µm m-2 24h-1 kPa-1, close to EVOH co-polymers, under dry conditions. Meanwhile, we consider that CNs has no potential risks for human beings and the renewable origin of the carbohydrate polymers as significant added values that justify a deeper investigation. Finally, it deserves to be underlined also the chance of finely tuning the oxygen permeability by means of the pH values and the sharp control of the thickness associated with this process. Therefore, based on the advantages outlined above, the LbL CS/CNs nanocomposite represents a promising oxygen barrier component in transparent flexible packaging materials and semi rigid tridimensional objects (bottles, trays, boxes and etc.). Based on our researches, we conclude that CNs leads to very promising applications in food packaging field and deserves to be further investigated in the future.

Thesis (S.M.)--Joint Program in Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Ocean Engineering; and the Woods Hole Oceanographic Institution), 2005.<br>Includes bibliographical references (leaves 70-72).<br>The uncertainty in the determination of the momentum and scalar fluxes remains one of the main obstacles to accurate numerical forecasts in low to moderate wind conditions. For example, latent heat fluxes computed from data using direct covariance and bulk aerodynamic methods show that there is good agreement in unstable conditions when the latent heat flux values are generally positive. However, the agreement is relatively poor in stable conditions, particularly when the moisture flux is directed downward. If the direct covariance measurements are indeed accurate, then they clearly indicate that the bulk aerodynamic formula overestimate the downward moisture flux in stable conditions. As a result, comparisons of the Dalton number for unstable and stable conditions indicate a marked difference in value between the two stability regimes. Investigations done for this thesis used data taken primarily at the Air-Sea Interaction Tower (ASIT) during the Coupled Boundary Layers and Air-Sea Transfer (CBLAST) Experiment 2003 from the 20-27 August 2003. Other data from the shore based Martha's Vineyard Coastal Observatory (MVCO) and moored buoys in the vicinity of the ASIT were also incorporated.<br>(cont.) During this eight day period, the boundary layer was often characterized by light winds, a stably stratified surface layer and a swell dominated wave field. Additionally, the advection of warm moist air over cooler water resulted in fog formation and a downward flux of moisture on at least three occasions. Therefore, a primary objective of this thesis is to present a case study to investigate the cause of this shortcoming in the bulk formula under these conditions by examining the physical processes that are unique to these boundary layers. Particular attention will be paid to the behavior of the Dalton number in a stable marine atmospheric boundary layer under foggy conditions using insights derived from the study of fog formation and current flux parameterization methods.<br>by Robert Farrington Crofoot.<br>S.M.

Le brouillard est un phénomène météorologique très difficile à prévoir, même à très courte échéance, en raison de sa grande variabilité spatiale et temporelle qui est due à des interactions complexes entre divers processus physiques. Dans cette thèse, les caractéristiques météorologiques locales et les processus synoptiques favorables aux brouillards sur la région du Grand Casablanca (Maroc) sont examinés à l'aide des observations horaires aux deux stations météorologiques permanentes de cette région côtière. Un algorithme de classification objectif est développé et utilisé pour classer les événements en des types de brouillard les plus rencontrés. Cette étude climatologique a mis en évidence que le brouillard a le plus souvent un caractère localisé et que le type d'advection-rayonnement est le plus fréquent sur la région, suivi des types d'affaissement de stratus et de rayonnement. Quand le brouillard intéresse simultanément les deux stations, la probabilité d'observer deux types différents est assez élevée. Les processus advectifs liés à la circulation de brise de mer au cours de l'après-midi, suivis de ceux radiatifs en début de nuit jouent un rôle important dans la formation du brouillard sur la région. Des simulations numériques à l'aide du modèle Méso-NH sont réalisées. Ces simulations ont confirmé que les processus physiques impliqués dans le cycle de vie du brouillard peuvent être différents selon la nature géographique de la surface. Elles ont aussi mis en évidence que la prévision numérique du brouillard en zone côtière est sensible à la température de la surface de la mer, à la topographie locale, et à l'occupation du sol. De plus, la prévision du brouillard côtier dépend fortement de la capacité du modèle à reproduire correctement la circulation de brise au cours de l'après-midi et les processus radiatifs en début de nuit. Les simulations systématiques des cas de brouillard de l'hiver 2013-2014 a montré la capacité du modèle Méso- NH à reproduire l'occurrence du brouillard avec néanmoins un taux relativement élevé de fausses alarmes, en particulier à la station côtière<br>The prediction of fog remains a challenge due to its time and space variability and to the complex interaction between the numerous physical processes influencing its life cycle. During the first stage of this thesis, the local meteorological and synoptic characteristics of fog occurrence over the Grand Casablanca region (Morocco) are investigated. To achieve this, hourly surface meteorological observations, at two synoptic stations of this coastal region, are used. An objective fog-type classification has been developed in this work and used to discriminate the fog events into the well known types. This fog climatology points out that the fog is often localized and that it is predominantly of advection-radiation type, followed by fog resulting from cloud base lowering and radiation fog. Besides, two different fog types can occur when fog simultaneously concerns the two stations. The advective processes associated with sea breeze circulation during the daylight, followed by the radiative processes often leads to fog formation over this coastal region. Numerical simulations are performed later using the meso-scale non-hydrostatic model Meso-NH. These simulations confirm that the physical processes, governing the life cycle of fog, can be different according to the physiographic features of the area. Moreover, the numerical prediction of coastal fog over heterogeneous area is very sensitive to sea surface temperature, land local topography and land cover. It also depends on the model's ability to reproduce the sea breeze circulation during the daylight followed by the radiative processes early in the night. The systematic numerical simulations of the fog events that occurred during the winter 2013-2014 indicate the Meso-NH's ability to well capture the fog occurrence with a relatively high false alarm rate, particularly over the coastal station

Le travail presente est centre sur la situation ou l'effet radiatif des aerosols modifie la structure de la couche limite atmospherique. Trois situations sont etudiees: le cas des brumes seches, le cas des aerosols urbains et la cas du brouillard. La simulation numerique et l'approche experimentale conduisent au developpement ou la mise au point de programmes de calcul du transfert radiatif