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Tangential velocity (Vt) of tornadoes is the major parameter that causes building damage. In-field tornado measurements are less reliable at less than 20 m above ground level (AGL). Laboratory tornado simulators suggest that swirl ratio (S) and radius (ro) are the major tornado parameters that influence the Vt. However, due to scaling problems, the laboratory simulators also report the Vt at greater than 20 m AGL. Well-refined computational fluid dynamics (CFD) models can evaluate the Vt at less than 10 m AGL. However, the CFD models are limited to ro = 1.0 km, and the effect of ro on Vt is not investigated. The aim of this study is to investigate the maximum Vt for different ro close to ground. Simulation results show that increasing ro decreases the maximum Vt with respect to Vro. Moreover, by increasing ro, the corresponding elevation of occurrence of maximum Vt (zmax) will increase. However, for all tornado radii, the zmax is between 20 m and 64 m AGL. In addition, results show that for all ro, the radial Vt profile has two peaks at z < 10 m AGL due to strong shear force close to the ground and at higher elevation the profile transits to Rankine Combined Vortex Model (RCVM).
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Differences in flow structures of tornado vortex and efficiency of different tornado chambers
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Tangential velocity (Vt) of tornadoes is the major parameter that causes building damage. In-field tornado measurements are less reliable at less than 20 m above ground level (AGL). Laboratory tornado simulators suggest that swirl ratio (S) and radius (ro) are the major tornado parameters that influence the Vt. However, due to scaling problems, the laboratory simulators also report the Vt at greater than 20 m AGL. Well-refined computational fluid dynamics (CFD) models can evaluate the Vt at less than 10 m AGL. However, the CFD models are limited to ro = 1.0 km, and the effect of ro on Vt is not investigated. The aim of this study is to investigate the maximum Vt for different ro close to ground. Simulation results show that increasing ro decreases the maximum Vt with respect to Vro. Moreover, by increasing ro, the corresponding elevation of occurrence of maximum Vt (zmax) will increase. However, for all tornado radii, the zmax is between 20 m and 64 m AGL. In addition, results show that for all ro, the radial Vt profile has two peaks at z < 10 m AGL due to strong shear force close to the ground and at higher elevation the profile transits to Rankine Combined Vortex Model (RCVM).more » « less
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Validation of CFD tornado wind field with experimental or field measurements is limited to comparison of tangential velocity profile at certain elevations above the ground level and few studies are based on comparison of pressure profile. However, important tornado vortex features such as touchdown swirl ratio (ST), core radius (rc), maximum tangential velocity (Vtmax), elevation of maximum tangential velocity (zc) and pressure distribution over a range of varying swirl ratios which strongly influences tornado forces on a building have not been accounted for validation of tornado wind field. In this study, important tornado vortex features are identified and validated with experimental measurements; the important tornado features obtained from the CFD model are found to be in reasonable agreement with experimental measurements. Besides, tornado chambers with different geometrical features (such as different outlet size and location and total heights) are used in different works of literature; however, the effect of variation of those key geometrical features on tornado wind field is not very well understood yet. So, in this work, the size of outlet and total height are systematically varied to study the effect on important tornado vortex parameters. Results indicate that reducing outlet diameter in a tornado chamber increases ST, Vtmax and zc and decreases rc. Similarly, increasing total height of tornado chamber decreases ST, Vtmax and rc whereas zc remains nearly constant. Overall, it is found that variation of outlet diameter has a stronger effect on tornado wind field than the variation in total height of tornado chamber.more » « less
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Abstract Tornado risk, as determined by the occurrence of atmospheric conditions that support tornado incidence, has exhibited robust spatial trends in the United States Southern Plains and Mid-South during recent decades. The consequences of these risk changes have not been fully explored, especially in conjunction with growing societal vulnerability. Herein, we assess how changes in risk and vulnerability over the last 40 years have collectively and individually altered tornado-housing impact potential. Results indicate that escalating vulnerability and exposure have outweighed the effects of spatially changing risk. However, the combination of increasing risk and exposure has led to a threefold increase in Mid-South housing exposure since 1980. Though Southern Plains tornado risk has decreased since 1980, amplifying exposure has led to more than a 50% increase in mean annual tornado-housing impact potential across the region. Stakeholders should use these findings to develop more holistic mitigation and resilience-building strategies that consider a dynamically changing tornado disaster landscape.more » « less
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