Numerical Simulation Of Moving Tornado Impacting On High-rise Building

Tornado is potential of severe damage to structures and has been one of important research objects in wind resistance & disaster prevention engineering. With the deteriorating of global environment in recent years, large cities are attacked by tornado occasionally. The researches of tornado effects on high-rise buildings began to receive concerns. Previous works most focuse on lab scale and qusistaic loads, few are focused on dynamic behavior of moving tornado and its impacting wind loads on full scale high rise buildings, and especially specific requirements and methods of structure safety design with respect to tornado attack are lacked in present code. It is necessary to begin investigations on such subject theoretically and experimentally. In this thesis, the wind model of moving tornado is established and the unsteady process of a moving tornado impacting against high rise building is numerically simulated with large eddy simulation (LES), then the wind flow characteristics as well as dynamic wind loads of tornado striking against large civil structures are analyzed. The results of experimental scale are then compared with those of full scale cases, the mechanism of flow field as well as wind loads difference between them are analyzed. It is concluded that (1) There exist technical difficulties of maintaining stability of tornado in applying both dynamic moving and rotating boundary conditions for numerical simulation of tornado vortex. Setting a driven zone with a predefined tornado swirling intensity in upper region of tornado wind field is an effective and efficient method to overcome this difficulty. (2) The wind loads of tornado striking on high-rise building are observed to be related with size of target building. For small size case, the loads are characterized by dual peak profiles, and impacting effects and unsteadiness are relatively small, while for large size case, the loads are characterized by multi-peak profiles with strong unsteadiness and large impacting effects. In process of tornado striking on relatively large size high-rise building, the main vortex is observed to be broken into several vortices, then a complicated interaction and coupling among vortices as well as wake flow of building is observed. No similar mechanical phenomenon has ever been reported in open literatures of tornado investigations. (3) The comparison of impacting wind loads on high-rise building by tornado for full scale case and experimental scale case manifests both similarity and scale effects. The similarity is observed in wind loads profiles and history when the ratio of radius of maximum rotating wind velocity v.s. model scale is similar, while the scale effects is observed in difference of peak loads and its corresponding positions. (4) The full scale simulations reveal that:the wind loads response of tornado impacting is related with cross configuration of building when other conditions such that building height and tornado scale etc. are same. Generally the building with rectangle cross section has largest peak loads, which is changed with attack angle of moving tornado.