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Wind and Structures
  Volume 11, Number 4, July 2008 , pages 291-302

Swirl ratio effects on tornado vortices in relation to the Fujita scale
H. Hangan and J-D. Kim

    Three-dimensional engineering simulations of momentum-driven tornado-like vortices are conducted to investigate the flow dynamics dependency on swirl ratio and the possible relation with real tornado Fujita scales. Numerical results are benchmarked against the laboratory experimental results of Baker (1981) for a fixed swirl ratio: S = 0.28. The simulations are then extended for higher swirl ratios up to S = 2 and the variation of the velocity and pressure flow fields are observed. The flow evolves from the formation of a laminar vortex at low swirl ratio to turbulent vortex breakdown, followed by the vortex touch down at higher swirls. The high swirl ratios results are further matched with full scale data from the Spencer, South Dakota F4 tornado of May 30, 1998 (Sarkar, et al. 2005) and approximate velocity and length scales are determined.
Key Words
    tornado-like vortices; swirl ratio; Fujita scale.
Alan G. Davenport Wind Engineering Group, The Boundary Layer Wind Tunnel Laboratory, The University of Western Ontario, Faculty of Engineering, London, Ontario, Canada

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