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|Title:||Physical modelling of the lightning leaders interaction with tall grounded objects and its validation with observations||Authors:||Chan, Ming Kit||Degree:||Ph.D.||Issue Date:||2018||Abstract:||With the advance in smart society, more and more tall grounded objects equipped with high-tech facilities that are sensitive to lightning have been built around the world. Such tall objects include high-rise buildings, communication towers, power transmission line towers, wind turbines etc. When a thundercloud is growing over a tall earthed object, more and more charges accumulate in the cloud, causing the ambient electric field strength below the charged cloud base increases. If the electric field enhancement at the surface of the object reaches a certain critical value, an electrical breakdown process will occur there. This breakdown process may lead an upward leader starting from tip of the object and propagating towards the charged cloud, i.e. an upward lightning discharge is initiated. Since the upward lightning discharge is often with a large peak current and a long duration of continuous current, it may damage the object itself as well as the facilities and lives inside it. Most upward lightning from grounded objects are negative discharges initiated by upward positive leader (UPL). An UPL can be either self-initiated from a tall grounded object or triggered by other discharge, such as a negative downward leader (DNL) approaching the object. Although there are many observations of UPLs, statistical analysis is not enough to reveal the physical process of UPLs. Modeling of UPLs initiation and propagation is an effective way to reveal the mechanism of UPLs. Therefore, this study aimed at the modeling study of initiation of upward lightning from a tall grounded object for lightning protection analysis. Major works done in this study are summarized as following. i. A brief review of the classification of different upward lightning was done. The review included: 1) up-to-date observation results of lightning attachment process; 2) physical characteristics of both positive and negative leaders from both observations and theoretical modeling; 3) properties of the corona space charge layer near the ground during a complete lightning process; and 4) observed optical and electromagnetic features of self-triggered and other-triggered UPLs. (see Chapter 2).
ii. A macroscopic physical model that can simulate an upward leader self-initiated from a tall earthed object under various conditions was developed. Major assumptions and concepts in the model included: 1) a three-zone leader channel structure is defined; 2) the first leader segment is created when the local electric field enhancement on the top of a grounded object reaches the critical breakdown electric field in a certain range; 3) the leader speed is subjected to the conservation of energy and mass inside the streamer-to-leader transition zone around the leader head; 4) a steady leader requires the leader initial speed (energy) should be larger than the minimum (critical) speed observed for leaders in both field and lab experiments (≈10 4 m/s), which corresponding to a critical corona sheath radius hence a critical corona sheath charge ahead of the leader; 5) the leader charge distribution is calculated by charge simulation method (CSM); 6) the leader ceases when its channel electric field is larger than the ambient electric field. Our calculation shows that the estimated critical corona sheath charge is height dependent. By fine-tuning the energy loss factor, this estimated critical value at ground-level can match well with experimental results. (see Chapter 3). iii. The above model was applied to an UPL self-initiated from a tall structure under various thunderclouds with and without corona space charge layer effect. Based on the leader initiation criteria, the critical corona sheath length and minimum corona charge as a function of the leader initiation height and the minimum leader initiation height as a function of the thundercloud condition for UPL are estimated and discussed. Simulation results show that the estimated minimum leader initiation height is higher when the electric field due to the corona space charge layer near the ground is considered. Evolutions of the speed, charge distribution, current, electric field, conductance and conductivity, and channel size of UPL under different thundercloud conditions are also obtained and discussed. Our simulation results have been compared to other existing models. Results are quite similar. In addition, the model is tested with two sets of experiment data. Using a similar initial condition, the increasing trend of our proposed core radius equation can match well with the experimental result. (see Chapter 4). iv. The model was further modified and used to study an UPL triggered by a DNL from a tall grounded object under different circumstances. With the modified model, general properties of an DNL, as well as the spatial and temporal electric field profile caused by the DNL are evaluated and discussed. By taking into account the vertical electric field profile due to the both the thundercloud and the DNL, multiple UPLs triggered from different heights of grounded objects are simulated and discussed. For validation, two case studies of lightning attachment (UPL connecting to DNL) reported in observations are modelled with the model and promising results are obtained. Our model can also estimate other leaders' physical parameters and the ground electric field change among different horizonal distances. (see Chapter 5).
|Subjects:||Hong Kong Polytechnic University -- Dissertations
|Pages:||xi, 119 pages : color illustrations|
|Appears in Collections:||Thesis|
View full-text via https://theses.lib.polyu.edu.hk/handle/200/9361
Citations as of May 28, 2023
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