Numerical study of the structure and dynamics of a tornado at the sub-critical vortex breakdown stage

A tornado at the sub-critical vortex breakdown was studied in detail using large eddy simulations. The averaged flow fields and the fluctuations were examined. Correlations of the fluctuating parameters were also studied, and a special turbulence structure was found. To clarify this turbulence structure, the Reynolds averaged force balances in two directions were studied, in which the sub-terms of the turbulence forces were paid more attention. The turbulence kinetic energy budget was examined to understand the balance between turbulence production, transportation, dissipation, and advection, among others. The time histories of wind velocities at some representative points were recorded, and three types of motions were then observed. The strength of these motions was furthered examined through spectrum analysis to study the change in them at different heights. Finally, the flow field visualization was performed to reveal the mechanism underlying these motions and explain some interesting phenomena found in the present study.