Estudos numéricos em eletrohidrodinâmica

Orientador: Marcos Akira D'ÁvilaTese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia MecânicaResumo: A eletrohidrodinâmica (EHD) descreve o movimento do fluido induzido por tensões elétricas. Sob o efeito de um campo elétrico, as moléculas de fluido podem polarizar e uma migração de íons carregados ou cargas livres através do fluido é induzida. Estes fenômenos dão origem a forças elétricas que atuam sobre a superfície do fluido colocando-a em movimento até que a tensão superficial e as tensões viscosas proporcionem o equilíbrio necessário. Aplicações como atomização de líquidos, transferência de calor e massa, dispersão de polímeros e tecnologias microfluídicas fizeram com que a eletrohidrodinâmica fosse extensivamente estudada ao longo dos anos no intuito de compreender as respostas de sistemas de fluidos submetidos a campos elétricos e de desenvolver novos processos. No entanto, a natureza complexa dos processos EHD limita as explorações tanto experimentais como de desenvolvimento. Portanto, para obter resultados mais rápidos e com custos menores, são frequentemente utilizados estudos envolvendo modelagem e simulações numéricas. Neste trabalho, utilizando um solver eletrohidrodinâmico baseado no modelo leaky dielectric, analisamos dois problemas diferentes relacionados à EHD. O primeiro consiste em investigar o efeito da viscoelasticidade na deformação e quebra de gotículas inseridas em um campo elétrico uniforme. Este é um dos primeiros e mais fundamentais problemas em EHD, porém nunca foi avaliado através de simulações numéricas utilizando fluidos não-Newtonianos. Assim, com os resultados aqui apresentados, pretendemos elucidar alguns dos principais aspectos da deformação viscoelástica em problemas de EHD. O segundo caso é um problema aplicado tem atraído um crescente interesse nos últimos anos. Consiste na deformação de lentes líquidas pela aplicação de um campo elétrico. Por meio de simulações numéricas, investigamos a influência do formato do eletrodo na deformação da lente e analisamos seu desempenho usando uma plataforma de design óptico. Esta abordagem nunca foi feita antes e sugere uma nova visão sobre os sistemas micro-ópticos adaptáveisAbstract: Electrohydrodynamics (EHD) describes the fluid motion induced by electric stresses. Under the effect of an electric field the fluid molecules may get polarized and a migration of charged ions or free charges through the fluid is induced. These phenomena give rise to electric forces that act on the fluid surface putting it into motion until the surface tension and viscous stresses provide the necessary balance. Applications such as liquid atomization, heat and mass transfer, polymer dispersion and microfluidic technologies have made electrohydrodynamics to be extensively studied over the years in order to understand the responses of fluids systems subjected to electric fields and to develop new processes. However, the complex nature of EHD processes limits both experimental and development explorations. Therefore, in order to obtain faster results and at lower costs, studies involving modeling and numerical simulations are frequently used. In this work, using an EHD solver based on the leaky dielectric model, we analyze two different problems related to electrohydrodynamics. The first one consists on the investigation of the effect of viscoelasticity on the deformation and breakup of droplets inserted in a uniform electric field. This is one of the first and most fundamental problems in EHD. However it has never been evaluated through numerical simulations using non-Newtonian fluids. Thus, with the results presented here we aim to elucidate some of the main aspects of the viscoelastic deformation in EHD problems. The second case is an applied problem that has drawn increasing interest in the past few years. It consists in the deformation of liquid lenses by the application of an electric field. By means of numerical simulations we investigate the influence of the electrode shape on the lens deformation and we analyze its performance using an optical design platform. This approach has never been done before and it suggests a new insight into the adaptive micro-optical systemsDoutoradoMateriais e Processos de FabricaçãoDoutor em Engenharia Mecânica233361/2014-6CNP

Electrohydrodynamic flow simulation of newtonian and viscoelastic fluids

Orientador: Marcos Akira d'ÁvilaDissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia MecânicaResumo: A eletrohidrodinâmica trata basicamente dos efeitos de um campo elétrico em meios contínuos. Um fluido sob o efeito de um campo elétrico tende a se deformar devido a uma força elétrica que age sobre ele. Essa força elétrica é consequência de alguns fatores que têm como base as próprias propriedades do fluido. Entre elas estão à condutividade elétrica (capacidade do fluido de ionizar-se ou de conduzir corrente elétrica) e a permissividade elétrica (capacidade do fluido de polarizar-se). Diversos processos que utilizam efeitos elétricos em fluidos foram desenvolvidos nas últimas décadas. O fato de estes processos envolverem uma alta complexidade de parâmetros faz com que seja consumido bastante tempo e material durante a fase de testes. Por esse motivo, as simulações numéricas passaram a ser uma boa alternativa para otimizar tais processos, além de aumentar o conhecimento sobre eles. Nesse contexto, no presente trabalho foi implementado um código numérico (solver) no pacote de CFD OpenFOAM baseado no modelo para fluidos pouco condutores (leaky dieletric model) no intuito de descrever escoamentos eletrohidrodinâmicos. Entre eles estão o efeito de um campo elétrico em uma gota condutora e o afinamento de um jato na saída de um tubo capilar. Eventualmente, em processos eletrohidrodinâmicos, são utilizados polímeros. Entretanto, não há na literatura muitos estudos sobre a simulação de escoamentos eletrohidrodinâmicos utilizando fluidos viscoelásticos. Assim, um segundo código foi implementado, baseado em um código já existente. Este é capaz de simular efeitos elétricos em fluidos viscoelásticos, utilizando a equação constitutiva de Giesekus como modelo viscoelástico. Os resultados obtidos para a deformação da gota condutora foram comparados com resultados analíticos para fluidos newtonianos e com observações experimentais para fluidos viscoelásticos. Para o caso do jato na saída de um tubo capilar, ambos os tipos de fluidos (newtonianos e viscoelásticos) foram comparados com resultados experimentais e teóricosAbstract: Electrohydrodynamics deals basically on the effects of an electric field on a continuum media. A fluid under the effect of an electric field tends to deform due to an electric force that acts on it. This electric force is a consequence of some factors which are based on the fluid properties, including the electric conductivity (ability of the fluid to ionize or to conduct electrical current) and the permittivity (ability of the fluid to polarize). Many processes using electrical effects in fluids have been developed in recent decades. The fact that these processes involve a high complexity of parameters, it consumes time and materials during the test phase. For that reason, the numerical simulations start being a good alternative to optimize such processes, and also to increase the knowledge about them. In this context, on the present work was implemented a solver on the open CFD software OpenFOAM, based on the leaky dielectric model, in order to describe electrohydrodynamic flows. Among them, are the effect of an electric field on a conducting droplet and the thinning of a jet on the exit of a nozzle. Eventually, in electrohydrodynamic processes, polymers are used. However, there aren't many studies about electrohydrodynamic flow simulation using viscoelastic fluids. So, a second solver was made, based on another existing solver. This second solver is able to simulate electric effects on viscoelastc fluids, using the Giesekus model as a constitutive equation. The results of the deformation of a conducting droplet were compared to analytical results, for Newtonian fluids, and with experimental observations, for viscoelastic ones. As for the jet, both types of fluids, Newtonians and viscoelastics, were compared to experimental and theoretical resultsMestradoMateriais e Processos de FabricaçãoMestre em Engenharia Mecânic

Roles of packing fraction, microscopic friction and projectile spin in cratering by impact

From small seeds falling from trees to asteroids colliding with planets and moons, the impact of projectiles onto granular targets occurs in nature at different scales. In this paper, we investigate open questions in the mechanics of granular cratering, in particular the forces acting on the projectile, and the roles of granular packing, grain-grain friction and projectile spin. For that, we carried out DEM (discrete element method) computations of the impact of solid projectiles on a cohesionless granular medium, where we varied the projectile and grain properties (diameter, density, friction and packing fraction) for different available energies (within relatively small values). We found that a denser region forms below the projectile, pushing it back and causing its rebound by the end of its motion, and that solid friction affects considerably the crater morphology. Besides, we show that the penetration length increases with the initial spin of the projectile, and that differences in initial packing fractions can engender the diversity of scaling laws found in the literature. Finally, we propose an ad hoc scaling that collapsed our data for the penetration length and can perhaps unify existing correlations. Our results provide new insights into the formation of craters in granular matter.Comment: Accepted manuscript for Physical Review E, 105, 034903, (2023), DOI: 10.1103/PhysRevE.107.044901, https://doi.org/10.1103/PhysRevE.107.04490

Impact craters formed by spinning granular projectiles

Craters formed by the impact of agglomerated materials are commonly observed in nature, such as asteroids colliding with planets and moons. In this paper, we investigate how the projectile spin and cohesion lead to different crater shapes. For that, we carried out DEM (discrete element method) computations of spinning granular projectiles impacting onto cohesionless grains, for different bonding stresses, initial spins and initial heights. We found that, as the bonding stresses decrease and the initial spin increases, the projectile's grains spread farther from the collision point, and, in consequence, the crater shape becomes flatter, with peaks around the rim and in the center of craters. Our results shed light on the dispersion of the projectile's material and the different shapes of craters found on Earth and other planetary environments.Comment: Accepted manuscript for Physical Review E, 108, 054904, (2023

Guidelines for the use and interpretation of assays for monitoring autophagy (4th edition)

In 2008, we published the first set of guidelines for standardizing research in autophagy. Since then, this topic has received increasing attention, and many scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Thus, it is important to formulate on a regular basis updated guidelines for monitoring autophagy in different organisms. Despite numerous reviews, there continues to be confusion regarding acceptable methods to evaluate autophagy, especially in multicellular eukaryotes. Here, we present a set of guidelines for investigators to select and interpret methods to examine autophagy and related processes, and for reviewers to provide realistic and reasonable critiques of reports that are focused on these processes. These guidelines are not meant to be a dogmatic set of rules, because the appropriateness of any assay largely depends on the question being asked and the system being used. Moreover, no individual assay is perfect for every situation, calling for the use of multiple techniques to properly monitor autophagy in each experimental setting. Finally, several core components of the autophagy machinery have been implicated in distinct autophagic processes (canonical and noncanonical autophagy), implying that genetic approaches to block autophagy should rely on targeting two or more autophagy-related genes that ideally participate in distinct steps of the pathway. Along similar lines, because multiple proteins involved in autophagy also regulate other cellular pathways including apoptosis, not all of them can be used as a specific marker for bona fide autophagic responses. Here, we critically discuss current methods of assessing autophagy and the information they can, or cannot, provide. Our ultimate goal is to encourage intellectual and technical innovation in the field

Guidelines for the use and interpretation of assays for monitoring autophagy (4th edition)

International audienceIn 2008, we published the first set of guidelines for standardizing research in autophagy. Since then, this topic has received increasing attention, and many scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Thus, it is important to formulate on a regular basis updated guidelines for monitoring autophagy in different organisms. Despite numerous reviews, there continues to be confusion regarding acceptable methods to evaluate autophagy, especially in multicellular eukaryotes. Here, we present a set of guidelines for investigators to select and interpret methods to examine autophagy and related processes, and for reviewers to provide realistic and reasonable critiques of reports that are focused on these processes. These guidelines are not meant to be a dogmatic set of rules, because the appropriateness of any assay largely depends on the question being asked and the system being used. Moreover, no individual assay is perfect for every situation, calling for the use of multiple techniques to properly monitor autophagy in each experimental setting. Finally, several core components of the autophagy machinery have been implicated in distinct autophagic processes (canonical and noncanonical autophagy), implying that genetic approaches to block autophagy should rely on targeting two or more autophagy-related genes that ideally participate in distinct steps of the pathway. Along similar lines, because multiple proteins involved in autophagy also regulate other cellular pathways including apoptosis, not all of them can be used as a specific marker for bona fide autophagic responses. Here, we critically discuss current methods of assessing autophagy and the information they can, or cannot, provide. Our ultimate goal is to encourage intellectual and technical innovation in the field