Wind Flow Characteristics of Multivortex Tornadoes

A multivortex tornado refers to a tornado that contains two or more small subvortices in the wind field. Due to the presence of multiple vortices, this type of tornado is likely to be more dangerous and destructive than single-vortex tornadoes. To understand the action of the multivortex tornado on civil structures, the wind flow characteristics are investigated and compared with those of single-vortex tornadoes, by using computational fluid dynamics (CFD) simulations. The results show that the inner flow structure of a multivortex tornado is completely different from that of a single-vortex tornado. First, a multivortex tornado possesses more than one subvortex in the domain around the core radius of the main vortex, and each subvortex flows together with the main vortex while rotating around its own center. Second, the wind flow of a multivortex tornado is more turbulent than a single-vortex tornado, which may lead to significant dynamic responses in some types of civil structures. Third, the maximum negative pressure occurs at the center of each subvortex instead of the center of the main vortex, which means that the largest negative pressure and highest wind speed occur at the same location. This unique feature in the multivortex tornado leads to different worst loading scenarios from single-vortex tornadoes and the worst-case scenario might be the combination of high tangential velocity and high negative pressure around the core radius. Fourth, for a multivortex tornado, the difference between instantaneous values and space-averaged values of parameters is remarkable. Thus, the space-averaged values should be carefully used for determining design tornadic wind loads for civil structures.