Numerical study on flow fields of tornado-like vortices using the LES turbulence model

Flow fields of tornado-like vortices generated by a numerical tornado simulator have been investigated using the LES turbulence model for two typical swirl ratios. The core radii of simulated vortices with swirl ratios of 0.31 and 0.65 showed favorable agreement with visualized vortices by a laboratory tornado simulator. Mean velocity fields were examined to obtain detailed corner flow patterns. It was found that an one-cell type vortex with a central upward flow appears for the case of low swirl ratio and vertical velocities show peaks at the center of the vortex, while a two-cell type vortex with a central downward flow emerges for the case of high swirl ratio and the maximum tangential velocity appears near ground. The formations of one-cell and two-cell type vortices were investigated by examining the axisymmetric time averaged Navier-Stokes equations. The vertical pressure gradient generates vertical velocities at the center of vortex in the one-cell type vortex, whereas the centrifugal force balances with the radial pressure gradient and the vertical advection term of the radial velocity in case of the two-cell type vortex.