Date of Graduation

8-2018

Document Type

Thesis

Degree Name

Master of Science in Civil Engineering (MSCE)

Degree Level

Graduate

Department

Civil Engineering

Advisor/Mentor

R. Panneer Selvam

Committee Member

W. Micah Hale

Second Committee Member

Ernie Heymsfield

Keywords

CFD, Tangential Velocity, Tornado, Tornado Simulator, Tornado Wind-field, Wind Engineering

Abstract

Tornadoes induce different wind forces on buildings than straight-line (SL) winds. The tangential velocity (Vθ) of tornados is the main parameter that causes damage to the buildings. In-field tornado measurements cannot evaluate the tornado’s Vθ at less than 20m above ground level (AGL). The laboratory tornado simulators suggest that the Swirl ratio (S) and the radius (ro) are the most influential factors affecting Vθ. However, due to scaling problems, laboratory simulators cannot report the Vθ for elevations less than 10m AGL. Well refined computational fluid dynamics (CFD) models can evaluate the Vθ at less than 10m AGL. However, the CFD models are limited to tornado radius ro=1.0km whereas observation of actual tornados by National Weather Service (NWS) shows that significant tornados in USA have width in the range of 0.7km to 2.3km. Thus, effect of ro on the Vθ is not investigated. Therefore, the aim of this study is to investigate the maximum Vθ (Vθ,max) for different tornado radii at elevations above and below 10m AGL. Simulation results show that by increasing the ro, the S parameter producing the Vθ,max will increase accordingly. In addition, results show that by increasing ro, the Vθ,max gradually reduces with respect to reference radial velocity Vr∞. In this respect, for 0.7km≤ ro ≤2.3km the Vθ,max is in the range of 6.5Vr∞ to 3.0Vr∞. Moreover, by increasing ro, the elevation of occurrence (zmax) of the Vθ,max will increase; However for all tornado radii, the zmax is always between 21m to 64m AGL. In addition, simulations show that for ro≤1.6km the radial Vθ profiles above 10m of the ground resemble the Rankine Combined Vortex Model (RCVM) flows, whereas at less than 10m of the ground the profile has two peaks for S greater than the touchdown S. Similarly, for ro≥1.8km the radial Vθ profiles below and above z=10m have two peaks for the S greater than the touchdown S.

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