Pattern recognition in tornado-like vortices at different swirl ratios

There has been a rise in tornado occurrences in the past few decades, and with it a surge of interest to better understand their behaviors. However, little is currently known of the underlying physics of tornado flow. In a new paper, Karami et al. investigate the structure and behavior of tornado-like vortices at different swirl ratios using a dynamic principal component analysis technique.

The authors found the flow structure of a vortex is strongly correlated with its swirl ratio. With increasing swirl ratio, the vortex undergoes a number of physical changes. Its radius and maximum tangential velocity both increase, and the flow evolves from laminar to highly turbulent. If the swirl ratio continues to rise, the vortex begins to break down, and a recirculation bubble emerges overhead. Eventually, a spiral forms around the recirculation bubble, and as the swirl ratio further increases, the single spiral becomes a double spiral.

To conduct the analysis, the researchers created physical vortices with different swirl ratios. After calibrating the data to remove random wandering motion of the vortices, the authors applied a dynamic principal component analysis method to study the time evolution of the dominant flow features in the tornado vortex, known as coherent structures. This revealed large-scale phenomena typically hidden behind turbulence. The researchers also conducted simulations and found the results agreed with experiment.

Though the vortex models used are a simplification of real-life tornado behavior, they can help understand the dominant physical mechanisms governing tornado dynamics. Author Mohammad Karami noted that the methods reported are not limited to tornado-like vortices and are applicable in the physical interpretation of any turbulent vortex flows experiencing random wandering or breakdown phenomena.

Source: “Coherent structures in tornado-like vortices,” by M. Karami, H. Hangan, L. Carassale, and H. Peerhossaini, Physics of Fluids (2019). The article can be accessed at https://doi.org/10.1063/1.5111530 .