DNS on growth of a vertical vortex in convection due to external forces

We can find various types of vertical vortices in a convective boundary layer such as tornado and dust devil, which often cause some damage to human life activity. These vortices are formed by a rotating and an upward flow inside the super cell or convective performance in the atmosphere. According to the past studies for dust devil, vertical vortices are numerically simulated under the situation of natural convection driven by a strong heat supply from the ground surface. But due to the homogeneous computational condition, pairs of counter-rotating vortices appear at many locations and a prominent vortex hardly exists long. In order to investigate the formation process and the physical mechanism of a strong tornado-like vertical vortex in convection, we perform three-dimensional direct numerical simulations (DNS) with or without two types of external forces which cause the anticlockwise rotation of flow field and the upward flow at the center of the computational domain to make vertical vortices stronger by vorticity concentration and vortex stretching. There is no mean flow at the beginning of computation and constant heat flux is imposed on the ground surface. An inversion layer is settled on the top of the boundary layer to suppress the development of convection. Computational results of the case without external force show that the convection cells are made and many strong vertical vortices are formed near the cell boundary where upward flow exists (Figure 1). According to more detailed analysis by 3D visualization, we can see that the upward-flow region which forms the cell boundary makes a pair of horizontal vortices along itself, and the vortices are tilted up by plumes with local strong upward flow and change to vertical vortices (Figure 2). Also, it can be observed that the stretching or the merging of vortices makes vertical vortices grow, as its result stronger vortices survive a longer time, although the vortices are rather weaker than real dust devils. To investigate characteristics of the prominent vortex, we also present the result of the simulation with external force, comparing with that without external force. Figure 3 depicts that several vertical vortex tubes with anticlockwise rotation are interacted with each other and a strong tornado-like vortex is generated. This result would be expected to lead to deeper understanding of the structure of the tornadoes.