Design, construction and performance of a large tornado simulator for wind engineering applications

A laboratory tornado simulator was designed, constructed and tested to generate tornado-like vortices for the purpose of quantifying tornado-induced aerodynamic loads on civil engineering structures. This simulator generates a vortex that can translate along a ground plane to interact with models of structures on the ground. The simulator uses a “rotating forced downdraft” technique that loosely matches the rear flank downdraft (RFD) phenomenon often emphasized as important to tornadogenesis. The rotating forced downdraft is generated using a 1.83 m diameter fan and an annular duct suspended from an overhead crane that can translate along a 10.4 m ground plane. Measurements were conducted to quantify the simulator’s flow structures. An 18-hole pressure probe was used to traverse and map the vortex flow field. The flow structures of the vortices were validated by comparing with mobile Doppler radar observations of two major tornados. Results show that tornado vortex radii from 0.23 to 0.56 m can be generated. A wide range of maximum tangential velocities are also possible (from 6.9 to 14.5 m/s). The simulator generates a range of vortex structures from single-celled vortices to two-celled vortices with corresponding swirl ratios ranging from 0.08 to 1.14. Comparisons of general flow structure and normalized profiles of tangential velocity showed excellent match between simulator and radar data. The range of tornado sizes together with model structures of 1/100 to 1/500 scale will allow extensive examination of tornado-induced wind loads on terrestrial structures. The effects of a tornado’s size, translation speed and detailed flow structure on these loads can now be studied.