This thesis deals with the physical modeling of the initiation and propagation of upward positive leader discharges from grounded structures during lightning strikes. It includes the analysis of upward leaders initiated under the influence of the electric field produced by a dominant negative cloud charge and due to the combined action of a negative thundercloud and a descending downward stepped negative leader. Thus, a self-consistent model based on the physics of leader discharges is developed for the evaluation of the attachment of lightning flashes to any kind of grounded structure. The predictions of the model have been found to be in good agreement with the results of laboratory long air gap experiments and with classical and altitude rocket triggered lightning experiments. Due to the high application level and predictive power of the developed model, several contributions to the physical understanding of factors influencing the initiation and propagation of upward positive leaders during thunderstorms have been made. For instance, it has been found that the initiation of upward connecting leaders is strongly affected by the average velocity of the downward stepped leader. Similarly, it is shown that the switching voltage impulses used in the laboratory do not “fairly approximate” the electric fields produced by a descending downward leader, as claimed by supporters of Early Streamer Emission (ESE) devices. Furthermore, it is found that the space charge layer created by corona at ground level significantly increases the thundercloud electric fields required to initiate upward lightning leaders from tall objects. On the other hand, it is also shown that the upward leader velocity depends on the downward leader average velocity, the prospective return stroke current, the lateral distance of the downward leader channel and the ambient electric field. By implementing the model to the analysis of complex structures, it has been observed that the corners of actual buildings struck by lightning coincide rather well with the places characterized by low leader inception electric fields. Besides, it has been found that the leader inception zones of the corners of complex structures do not define symmetrical and circular regions as it is generally assumed.

Electrical discharges with a variety of different forms (streamers, glow corona, leaders, etc.) broadly exist in nature and in industrial applications. Under certain conditions, one electrical discharge can be transformed into another form. This thesis is aimed to develop and use numerical simulation models in order to provide a better physical understanding of two of such transitions, namely the glow-to-streamer and the streamer-to-leader transitions in air. In the first part, the thesis includes the two-dimensional simulation of the glow-to-streamer transition under a fast changing background electric field. The simulation is performed with a fluid model taking into account electrons. An efficient semi-Lagrangian algorithm is proposed to solve the convection-dominated continuity equations present in the model. The condition required for the glow-to-streamer transition is evaluated and discussed. In order to enable such simulations for configurations with large interelectrode gaps and long simulation times, an efficient simplified model for glow corona discharges and their transition into streamers is also proposed. The second part of the thesis is dedicated to investigate the dynamics of the streamer-to-leader transition in long air gaps at atmospheric pressure. The transition is studied with a one-dimensional thermo-hydrodynamic model and a detailed kinetic scheme for N2/O2/H2O mixtures. In order to evaluate the effect of humidity, the kinetic scheme includes the most important reactions with the H2O molecule and its derivatives. The analysis includes the simulation of the corresponding streamer bursts, dark periods and aborted leaders that may occur prior to the inception of a stable leader. The comparison between the proposed model and the widely-used model of Gallimberti is also presented. / Elektriska urladdningar av olika former (streamers (från engelska), glöd-korona, ledare, etc.) förekommer i stor utsträckning i naturen och i industriella applikationer. Under vissa förhållanden kan en elektrisk urladdning omvandlas till en annan form av elektrisk urladdning. Denna avhandling syftar till att utveckla och använda numeriska simuleringsmodeller för att ge en bättre fysikalisk förståelse av två sådana övergångar, nämligen glöd-till-streamer- och streamer-till-ledar-övergångar, i luft. I den första delen, avhandlas en tvådimensionell simulering av glöd-till-streamer-övergången med ett hastigt föränderligt elektriskt fält i bakgrunden. Simuleringen utförs med en flödesmodell som tar hänsyn till elektronerna. En effektiv semi-Lagrangesk algoritm föreslås för att lösa de konvektionsdominerade kontinuitetsekvationerna i modellen. Vidare utvärderas och diskuteras förutsättningarna för glöd-till-streamer-övergången. För att möjliggöra sådana simuleringar i konfigurationer med stora elektrodavstånd och långa simuleringstider, föreslås också en effektiv och förenklad modell för glöd-korona-urladdningar samt deras övergång till streamers. Den andra delen av avhandlingen är tillägnad att undersöka dynamiken i streamer-till-ledar-övergångar över långa avstånd i luft, under atmosfäriskt tryck. Övergången studeras med en endimensionell termohydrodynamisk modell och en detaljerad kinetisk modell för blandningar av N2/O2/H2O. För att utvärdera effekten av luftfuktighet, innefattar den kinetiska modellen de viktigaste reaktionerna med H2O-molekylen och dess derivat. Analysen innefattar simuleringen av motsvarande streamer-kedjor, mörka perioder och avbrutna ledare som kan förekomma före starten av en stabil ledare. En jämförelse mellan den föreslagna modellen och den allmänt använda modellen av Gallimberti presenteras också. / <p>QC 20170418</p>