Numerical Simulation Study On Inception Process Of Upwards Positive Leader At The Tip Of The Buildings

The inception of the upward leader is the first condition that grounded tall buildings are struck by lightning.Generally,The tip of the buildings is also the inception of positive upward leader and the position of self-consistent propagation by a lightning stroke.Therefore,inception process of upward leader is also a key of studying the connection between lightning and ground objects.On the other hand, the inception process of the upward leader is important to predict damage of tall buildings or towers in the process of thunderstorms.In view of the existing problems of the previous models,this article first considers the physical process from the inception of leader,obtains the initial corona charge of the building tip by the potential distortion method,calculates streamer-positive leader translation time and the initial radius of the cone-shape stem according to the positive leader thermodynamic model of Gallimberti and the energy balance equation.Finally,this article establishes a more reasonable inception model of positive leader at the tip of tall buildings on the ground ,considering the shielding effect of space charge, determining the inception condition of the second corona,and this article employs the relationship between the radius and streamer length of cone-shape stem,which proves that the stem will have priority to transit into a leader as the current injects. streamer-positive leader transition time decreases with the increase of corona charge, at the same time, the larger radius of stem needs more time for the transition of stem-leader.In the process of streamer-positive leader translation , the conversion time is affected by a variety of factors.In this article , the thermodynamic section of the streamer-positive leader translation process at the tip of the ground building is discussed.compared with SLIM, and this article discusses the parameters with ignoring the transformation of vibration energy to translation energy. and analyzes the influence of environmental factors (such as the temperature, the atmospheric pressure and the absolute humidity of air) surrounding the location of the corner of the grounded structures in streamer-positive leader translation process. The results show :when the corona charge is less, the corresponding streamer-positive leader transition time is longer; The lower temperature, the larger humidity and the greater atmosphere pressure,leads to require a more the amount of minus corona charge in the streamer-positive leader transition process and a less streamer-positive leader transition time, and among of the environmental factors, there is the greatest influence of humidity.Based on inception model of positive upward leader, this article has calculated attractive radius in several conditions of different current and different structure height. The relationship among attractive radius, current and structure height are fitted according to the results. And we have compared the calculation results in the equivalent radius of the complex grounded structures based on our model with SLIM and others. At the same time, this article considers the influence of environmental factors (the temperature,the atmospheric pressure and the absolute humidity of air) on attractive radius of the corner of grounded structures. The results have been found: The equivalent radius based our model are is very close to SLIM with lower structure height, the equivalent radius still exist differences with higher structure height, but both of the results are greater than EGM. Atmospheric pressure and the absolute humidity of air have a certain impact on attract radius, however, the influence of the temperature is not significant.