Date of Graduation

5-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Civil Engineering

Advisor/Mentor

Selvam, R. Panneer

Committee Member

Hale, W. Micah

Second Committee Member

Couvillion, Rick J.

Third Committee Member

Heymsfield, Ernest

Keywords

Applied sciences; Terrain sheltering; Tornado damage; Tornado field investigation; Tornado forces; Tornado path; Tornado-terrain interaction

Abstract

Tornado forces on structures have been a research focus for the past decades, and some comparisons have been made to distinguish between straight boundary layer (SBL) wind and tornado wind forces on specific structures. However, very little attention has been paid to terrain effects on tornado damage and path. Available damage investigation data for four tornado locations (Joplin-2011, Tuscaloosa-2011, Parish-2011 and Mayflower-2014) is utilized to explore terrain effects on tornado damage and path. Google Earth, site visits and aerial images are employed to study the influence of terrain on an extent of damage and path variation. Additionally, a three dimensional computer model is developed by employing computational fluid dynamics (CFD) to study terrain (hills) effects on tornado path deviation and tornado forces reduction in the sheltered region.

From the study, it is concluded that there is a significant effect of hills on tornado damage. Much damage is observed on the windward side of a hill comparing to its leeward side. When the tornado crosses the investigated hills, the hills provide sheltered zone on its leeward side. Furthermore, measurements of the sheltered zone on the leeward side of the investigated hills show that the sheltered zone length is about five times the hill height (5H) along tornado travelling path. After that, a modified version of the computer model presented in Selvam and Millet (2002) is utilized to simulate tornado-terrain interaction, and then the model is validated for further investigation of terrain effects on tornado path deviation and forces. The Navier–Stokes equations are approximated by the finite elements method (FEM), and the numerical domain is discretized using a terrain following coordinate system. It is shown that the ratio of tangential velocity to translational velocity (Vθ/Vt) significantly affects the tornado deviation shape when a tornado interacts with the investigated 2D hill. The deviation shape changes from straight line to double

curvature shape as the ratio (Vθ/Vt) increases. The UA numerical results for tornado path deviation shape are comparable to field data (single and no curvature) for (Vθ/Vt) < 4. The UA Numerical results for (Vϴ/Vt) >4 are comparable to wind tunnel data (Vθ/Vt ≈45) in which the deviation is double curvature. Therefore, the computer model is considered for further investigation. Finally, the computer model is utilized to measure tornado forces on a sheltered dome building. Rounded and triangular hill profiles are considered with varied heights, and it is concluded that the hills reduce tornado forces and velocities on their leeward side. The computer model outcome is that tornado forces applied on a structure are reduced by more than 70% when the structure is located within six times the hill height (6H) on the leeward side.

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