Simulation Tests And Models Of The Selection Process In Negative Cloud-to-Ground Lightning Flash

Lightning flash is a frequent natural disaster, and it causes massive casualties and property losses every year. Therefore, lightning protection is the focus in fields of energy, traffic, aerospace, construction, and et al. Model of negative cloud-to-ground lightning (CG) is the major tool to assist to design and improve lightning protection devices. However, defects in previous CG models lead to incorrect assessment of the performance of lightning protection devices. Therefore, it is extremely urgent to improve the previous CG models.In order to propose a novel improved CG model, which can simulate the tortuous and branched development of downward leader and the physical process of upward leader, a series of studies on the physics mechanism of negative stepped leaders, the charge distributions in downward leaders, the stochastic development model of downward leader, the characteristics and model of positive upward leader were carried out.Base on the energy balance equation in negative streamers and the potential distortion method for calculating the space charge, an equation describing the temperature variation in the streamer-leader transition zone during transformation processes was present. The background electric field in the transition zone was proposed to be the major influencing factor for the development of stepped leader. And the critical background electric field for the development of the stepped leader in natural lightning was proposed by extrapolation method according to the experiment results of negative long air gap discharges.The total charge and charge distribution in downward leader were studies by employing the charge simulation method. A relationship between the total charge and the return stroke current was established, and the distribution law of the charge was obtained. The influence of branched leader on the total charge and charge distribution in downward leader was studied. By introducing a charge factor, a novel calculation method of the charge distribution in branched downward leader was raised.Taking the background electric field as the core parameter for deciding the development direction of stepped leaders, a three-dimensional stochastic model of downward leader was then present. After analyzing the geometric features of simulated lightning channel, the relationships among the fractal dimension, the branched factor, and the charge factor were established. And according to the observational fractal dimension of natural lightning channels, the reasonable values of branched factor and charge factor were chosen. The three-dimensional stochastic model of downward leader was verified by observational results of natural lightning in the aspects of the fractal dimension, the total charge, and the ground electric field.Base on the equivalence of electric field in temporal and spatial (T-S) distribution, a novel simulation test method of positive upward leaders was proposed. After investigating the T-S distribution of electric field in natural lightning, it was found that designing a suitable electrode and choosing a reasonable applied voltage can ensure the equivalence of the simulation test. A series of simulation test on the positive upward leader was carried out, and the critical charge of the stem-leader transition, the shielding effect of space charge on the inception of the second corona, and the charge density of leader were investigated.Considering the initial leader’s propagation and the influence of space charges on electric field, a novel inception model of upward leader including the stem-leader transition and the inception of second corona was proposed. Combining the calculation method of leader current and the relationship between leader velocity and leader current obtained from the artificial triggering lightning experiments, upward leader simulation tests, and physical mechanism, a development model of upward leader was established. This model was verified by simulation tests and observations of natural lightning.Combining the stochastic model of negative downward stepped leaders and the model of positive upward leader by taking the electric field as an intermediary, a simulation model of negative CG flash was established. Using the model, the performances of lightning rods, including the lightning drawing range, the striking distance, and the protection radius on the ground, were investigated and compared with those in the electro-geometrical model (EGM) and the polygon method. And the striking distance in the EGM was found to just reflect the general case. Therefore, objects protected by lightning rods designed using EGM is still possible to be struck. The polygon method is only suitable to assess the performance of lightning rod with a low height, and the assessment for a high lightning rod will be over optimistic. The phenomenon of side striking often happens with a lower lightning current and higher lightning rod. The side development of downward leader is the major reason for the shielding failure of high lightning rods.