Modelling of Low-Current Gas Discharges by Means of Stationary Solvers

Este trabalho È dedicado ‡ modelizaÁ„o da igniÁ„o de descargas autÛnomas de baixa corrente, e de caraterÌsticas tens„o-corrente mÈdias de descargas de coroa. Foi desenvolvida uma abordagem para simulaÁ„o de descargas quasi-estacion·rias de baixa corrente, sendo aplic·vel para descargas DC e tambÈm de baixa frequÍncia. Esta abordagem colmata a lacuna entre os mÈtodos modernos de modelizaÁ„o numÈrica e as abordagens de engenharia, e foi implementada como parte de um modelo numÈrico que simule as descargas referidas em ar de alta press„o. O modelo inclui equaÁıes de conservaÁ„o e transporte de espÈcies carregadas e a equaÁ„o de Poisson. Utiliza-se um modelo cinÈtico ímÌnimoíde processos quÌmicos no plasma, em descargas de baixa corrente em ar de alta press„o, que tem em conta os eletrıes, uma espÈcie eÖcaz de iıes positivos, e os iıes negativos s OThis work is dedicated to modelling of ignition of self-sustaining low-current discharges and time-averaged characteristics of corona discharges. An approach for simulation of low-current quasi-stationary discharges has been developed, being applicable not only to DC but also to low-frequency discharges. Such approach bridges the gap between modern methods of numerical modelling and engineering approaches, and it has been implemented as a part of a numerical model of low-current quasi-stationary discharges in high-pressure air, The model comprises equations of conservation and transport of charged species and the Poisson equation. A íminimalí kinetic model of plasmachemical processes in low-current discharges in high-pressure air is used, which takes into account electrons, an e§ective species of positive ions, and negative ions The implementation of the numerical model is based on the use of stationary solvers, which o§er important advantages in simulations of quasi-stationary discharges. The developed model has been validated against experimental data over a wide range of conditions, and has been applied to several problems of high scientiÖc and technological interest. Firstly, the inception of discharges in high-pressure air has been investigated over a wide range of conditions.The method developed can be used as a predictor of the hold-o§ voltage in high-voltage devices, as well as to evaluate the sensitivity of the ignition voltage with respect to di§erent factors (the kinetic scheme used, the photoionization, and the secondary electron emission from the cathode). The second problem that has been studied was the e§ect that anode surface protrusions can have on pre-breakdown discharges in high-pressure air, which can be considered as an indirect evidence of the possible existence of microprotrusions on the electrode surface. The third problem concerned investigation of characteristics of DC corona discharges in a wide range of currents and gap widths. It is shown that computed inception voltages are in good agreement with experimental data. Anode microprotrusions with heights of the order of 50 m yield qualitative agreement with the experiment in all the cases. It is also shown that the calculated current-voltage characteristics are in good agreement with the measured data, and that stationary solvers allow decoupling of physical and numerical stability and reduction of computation time

Controlo de acesso ao meio em comunicações veiculares de tempo-real

Despite several preventive measures, the number of roadway accidents is still very high, being considered even a problem of public health by some entities. This thesis has as global purpose of contributing to the reduction of that number of accidents, and consequent fatalities, by using safety-related applications that use communication among vehicles. In particular, the primary goal is guaranteeing that communication between users in vehicular environments is done with appropriate time bounds to transfer safety-critical information. In detail, it is studied how to manage the scheduling of message’s transmissions (medium access control - MAC), in order to define precisely who will communicate and when is the appropriate instant. The preferable situation where a communication infrastructure is present with full coverage (RSUs) is also studied, from which medium access control is defined precisely, and vehicles (OBUs) become aware of medium utilization. Also, sporadic situations (e.g., absence of RSUs) are studied in which the communication network is “ad hoc” and solely formed by the current vehicles. It is used the recently WAVE / IEEE 802.11p standard, specific for vehicular communications, and it is proposed a TDMA based solution, with appropriate coordination between RSUs in order to effectively disseminate a critical safety event. It is taken into account two different ways of choosing the instant for the initial broadcast, and both cases are compared. In case there is no infrastructure available, methods are derived to minimize communication medium access collisions, and to maximize the available bandwidth. The results reflect the total end-to-end delay, and show that adequate times are attained, and meet with the requisites for the type of applications being considered. Also, enhancements are obtained when using the alternate choice for the initial broadcast instant.Apesar de diversas medidas preventivas, o número de acidentes rodoviários continua a ser muito elevado, sendo mesmo considerado uma questão de saúde pública por algumas entidades. Esta tese tem como objetivo geral contribuir para a redução desse número de acidentes, e consequentes fatalidades, através da utilização de aplicações de segurança que envolvem comunicação entre veículos. Em particular, o objetivo principal é garantir que a comunicação entre utentes, em ambientes veiculares, seja efetuada com limites temporais apropriados à transferência de informações críticas. De forma mais detalhada, é estudada a gestão do escalonamento das transmissões (controlo de acesso ao meio – MAC) que irá definir quem vai comunicar e quando o pode fazer. São estudadas situações (desejadas) onde há uma infra-estrutura de comunicações com cobertura integral (RSUs), a partir da qual se faz a coordenação do acesso ao meio pelos veículos (OBUs), e situações (esporádicas, por ausência de RSU) em que a rede de comunicação é “ad hoc” e apenas constituída pelos veículos presentes. Utiliza-se a recente norma WAVE / IEEE 802.11p, específica para comunicações veiculares, e propõe-se uma solução baseada em TDMA, com coordenação apropriada entre RSUs para disseminação efetiva de um evento crítico de segurança. A escolha do instante para o broadcast inicial do evento de segurança também é tida em conta, e são comparados dois casos distintos. No caso da ausência de infraestrutura, derivam-se métodos para minimizar colisões no acesso ao meio de comunicação, e maximizar a largura de banda disponível. Os resultados refletem o atraso total end-to-end, mostrando tempos apropriados para os requisitos das aplicações em causa, e evidenciando melhorias aquando da escolha alternativa para o instante do broadcast inicial.Programa Doutoral em Engenharia Eletrotécnic

Evaluation of a quality improvement intervention to reduce anastomotic leak following right colectomy (EAGLE): pragmatic, batched stepped-wedge, cluster-randomized trial in 64 countries

Background Anastomotic leak affects 8 per cent of patients after right colectomy with a 10-fold increased risk of postoperative death. The EAGLE study aimed to develop and test whether an international, standardized quality improvement intervention could reduce anastomotic leaks. Methods The internationally intended protocol, iteratively co-developed by a multistage Delphi process, comprised an online educational module introducing risk stratification, an intraoperative checklist, and harmonized surgical techniques. Clusters (hospital teams) were randomized to one of three arms with varied sequences of intervention/data collection by a derived stepped-wedge batch design (at least 18 hospital teams per batch). Patients were blinded to the study allocation. Low- and middle-income country enrolment was encouraged. The primary outcome (assessed by intention to treat) was anastomotic leak rate, and subgroup analyses by module completion (at least 80 per cent of surgeons, high engagement; less than 50 per cent, low engagement) were preplanned. Results A total 355 hospital teams registered, with 332 from 64 countries (39.2 per cent low and middle income) included in the final analysis. The online modules were completed by half of the surgeons (2143 of 4411). The primary analysis included 3039 of the 3268 patients recruited (206 patients had no anastomosis and 23 were lost to follow-up), with anastomotic leaks arising before and after the intervention in 10.1 and 9.6 per cent respectively (adjusted OR 0.87, 95 per cent c.i. 0.59 to 1.30; P = 0.498). The proportion of surgeons completing the educational modules was an influence: the leak rate decreased from 12.2 per cent (61 of 500) before intervention to 5.1 per cent (24 of 473) after intervention in high-engagement centres (adjusted OR 0.36, 0.20 to 0.64; P < 0.001), but this was not observed in low-engagement hospitals (8.3 per cent (59 of 714) and 13.8 per cent (61 of 443) respectively; adjusted OR 2.09, 1.31 to 3.31). Conclusion Completion of globally available digital training by engaged teams can alter anastomotic leak rates. Registration number: NCT04270721 (http://www.clinicaltrials.gov)