Date of Graduation

5-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Civil Engineering

Advisor/Mentor

Selvam, R. Panneer

Committee Member

Coffman, Richard A.

Second Committee Member

Couvillion, Rick J.

Third Committee Member

Heymsfield, Ernest

Keywords

Applied sciences; Computational fluid dynamics; Sheltering effect; Tornado; Vortex; Wind engineering

Abstract

A three dimensional computational fluid dynamics (CFD) model was utilized to investigate tornado-like vortex interactions with wide man-made structures. The tornado-like wind profile was approximated using Rankine vortex model. By utilizing the CFD model, it was explained why tornadoes exhibit less damage on leeward side of large structures. During the preliminary stage of this study, a perpendicular vortex-prism interaction was analyzed. The prism height and the length were equal to the vortex core radius. The prism was also 12 times wider than the vortex core radius. During the vortex-prism interaction, the near-ground portion of the vortex was blocked by the leading face of the prism. To proceed with the travel, the primary vortex had to introduce a new low-level vortex behind the prism, which mitigated maximum flow speeds on the prism's leeward side. Various visualization techniques were employed to understand and quantitatively study the vortex sheltering effect. It was shown that the vortex flow speeds are reduced by more than 30% in a region of length equal to 6 times the prism height. The sheltering effect was also investigated for different prism sizes. It was demonstrated that the thinner the prism is, the more it disrupts the near-ground strength of the translating vortex. Following these findings, the tornado sheltering performance of a wide wall was studied. During the vortex-wall interaction a 20 m high wall was able to reduce the maximum tornado-like wind speeds by 30%, on a distance of 102 m behind the wall. The magnitude of the wind speed reduction was found to be dependent on the wall width and the wall height, relatively to the vortex core radius. The sheltering efficiency of the wall also changes depending on the tornado-like vortex impact angle on the wall. The new findings arising in this study can be applied for designing tornado-safe structures and areas.

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