Doctor of Philosophy
Civil and Environmental Engineering
Dr. Girma Bitsuamlak
The study the flow structure of tornado-like vortices and their impact over engineering structures is important due to the extent of tornado induced fatalities and damages observed each year in North America and around the world. In the present study, a numerical modeling approach inspired by the WindEEE Dome and the modified version of Ward’s Tornado Simulator has been developed. Using a full-scale numerical simulator, tornadoes of different intensities have been simulated for different swirl ratio values to study flow structures in comparison with previous studies. The effect of topographic features on the tornado-like vortex has been investigated for the first time. A new approach to quantify the changes in the tornadic wind field “speed-up” due to the topography has been developed. Once a confidence level is achieved on tornado flow structure under flat terrain and with topographic features scenarios, the interaction of the tornado-like vortex with bluff bodies is modeled. For this purpose, both low- and high rise buildings are considered. More emphasis has been given to the low-rise building and hence it has been further investigated for stationary and translating tornado under sealed and opened conditions. The open condition represents breaching on the building envelope due to, for example, wind born debris. Selected experiments have been conducted at WindEEE Dome for validating the numerical model.
For the low-rise building, the overall pressure distribution along the wall surfaces of the body are dominated by the external tornado pressure field (suction) near the ground surface that develops due to the high angular momentum of the flow. For large tornado size with respect to the study buildings, the Cp on the entire building surface resemble the near ground suction irrespective of the shape. Thus, indicating that at the tornado center, the effect of the interaction of tornado with the bluff body is minimum. This is not the case for smaller tornadoes. The tornado building interaction effects (i.e. aerodynamics) start to be dominant as the tornado center is located far away from the study building. A comparison with ABL flow reveals that the Cp magnitudes and their distributions are quite different for tornado center and core radius locations. For offset position of tornado center with respect to the building, the tornado induced Cp value starts to resemble the ones from ABL-flow case. For opening case, ground suction dominates the overall pressure distribution for all the locations of tornado. As a result, irrespective of the number and location of openings, suction is higher both inside and outside the bluff-body at tornado center and it decreases as tornado moves away from the bluff-body (core radius and outside core regions). For the high-rise building, an additional vertical pressure variation, not seen in the shorter buildings, as the tornado vortex interacts with the building has been observed. The CFD study has been validated in comparison with WindEEE experiments. These studies are expected to contribute to addressing the lack of tornadic aerodynamic characteristics. In the long run this will contribute in enhancing the resiliency of the built environment and safety of our communities.
Nasir, Zoheb, "Numerical modeling of tornado-like vortex and its interaction with bluff- bodies" (2017). Electronic Thesis and Dissertation Repository. 4451.