Influence of swirl ratio on the structure and dynamics of tornado-like vortices

We investigated the flow dynamics of tornado-like vortices, examining the influence of swirl ratio, S, defined as the ratio of tangential to radial momentum at the vortex base, on their structural characteristics. Using a combination of particle image velocimetry (PIV) in a custom-built simulator and large-eddy simulations (LES), we analyzed vortex flows at swirl ratios of S = 4.66 , 1.25, and 0.33. The results demonstrate that vortex flow characteristics strongly depend on S, with improved agreement between experimental and numerical data when employing flow-based swirl ratio definitions. Vortex wandering was quantified in experiments, and corrections were applied to refine tangential and radial velocity profiles. At S = 0.33 and 1.25 in experiments and S = 1.25 and 4.66 in simulations, the vortex transitioned from a single-celled to a double-celled structure, with further evolution into multi-celled vortices at the highest swirl ratio, substantially modifying circulation patterns. Proper orthogonal decomposition (POD) characterized the coherent structures governing vortex dynamics and their dependence on swirl ratio, revealing distinct physical features associated with each vortex regime.