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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

Hangan, Horia M.

Abstract

The increased devastation associated with tornadic flow-fields in terms of human lives or destruction of properties and their subsequent economic losses highlights the significance of understanding the tornado vortex structure, particularly near the ground where the majority of this destruction happens. While the body of research in this area is increasing there is a lack of resolution associated with physical near-surface measurements in tornado-like vortices (TLVs) and the effects of translation, surface roughness, and surface vortex dynamics are not yet well understood.

In order to address these gaps, high spatial and temporal surface pressure measurements were carried out at WindEEE Dome to explore the characteristics of stationary and translating tornado-like vortices (TLV) for a wide range of swirl ratios ( S=0.21 to 1.03). The translational speed of the TLV and the surface roughness were varied to examine their effects on tornado ground pressures, wandering, and vortex structure. It was found that wandering is more pronounced at low swirl ratios and has a substantial effect on the peak pressure magnitude for stationary TLV (error percentage ≤ 35%). A new method for removing wandering was proposed which is applicable for a wide range of swirl ratios. For translating TLV, the near-surface part lagged behind the top of the vortex, resulting in a tilt of the tornado vertical axis at higher translating speeds. Also, a veering motion of the tornado base towards the left was observed. Wandering was less pronounced for higher translation speeds. Increasing the surface roughness caused an analogous effect as lowering the swirl ratio.

While surface pressure exploration of tornado-like vortices paved the way for understanding their complex structure near the ground, applying the acquired knowledge on buildings is intended to serve for the interpretation of tornado-induced pressures on buildings as a superposition of pressure deficit and aerodynamic effects. Thus, induced internal and external pressures on two generic low-rise buildings were quantified to assess the impacts of TLVs on low-rise buildings. The internal pressures were examined under different opening configurations, building orientations, building sizes, and offsets. It was found that a dominant opening on the roof induced the highest negative peak internal pressure. Increasing the building offset beyond the tornado core region caused a drastic reduction of the peak internal pressures by two-thirds. The peak internal pressures were higher for the smaller of the two buildings investigated herein. On the other hand, external pressure measurements showed that the building’s roof, leeward wall, and left side wall experienced the maximum uplift at the core radius location. The comparison with the recently modified ASCE 7-16 code showed that roof corner pressures are safely predicted for all the cases while the mid-roof zone and the middle of the walls experienced higher suctions exceeding the recommended wind loads in ASCE 7-16.

Summary for Lay Audience

Tornadoes can cause enormous destruction to life and properties. The aftermath of these fierce storms has a great impact on the economy. Thus, it is crucial to understand their structure and their associated damage near the ground where most of the structures lie.

In this study, high-resolution pressure measurements on the ground were carried out to better characterize the tornado vortex effect on the ground surface. Multiple translational speeds of tornadoes and surface ground roughness were examined in a large-scale tornado simulator. While understanding the near-surface tornado vortex structure is essential, the study of their effect on buildings will provide a full insight into their probable damage. Hence, internal and external pressure measurements were performed on two low-rise building structures under translating tornado flow-field. The internal pressures associated with the natural leak in the buildings or due to sudden breakage of windows or doors play a significant role by either mitigating or intensifying the overall tornado wind loading on structures. Herein, the internal pressures were examined under different opening configurations, building orientations, building sizes, and offsets. Also, the study of the induced external pressures due to tornadic flow revealed that the building code needs more improvements regarding their recommended wind loadings on buildings.

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