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1. Differences in flow structures of tornado vortex and efficiency of different tornado chambers

   Verma, Sumit.; Selvam, Rathinam P. (January 2021, 6th AAWE workshop)

   This is a summary paper 

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2. Computer modelling of close-to-ground tornado wind-fields for different tornado widths

   Kashefizadeh, M.H; Verma, S; Selvam, R.P. (August 2019, Journal of wind engineering and industrial aerodynamics)

   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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3. Computer modelling of close-to-ground tornado wind-fields for different tornado widths

   Kashefizadeh, M.H; Verma, S; Selvam, R.P. (August 2019, Journal of wind engineering and industrial aerodynamics)

   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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4. The Influence of Turbulence Memory on Idealized Tornado Simulations

   https://doi.org/10.1175/MWR-D-20-0031.1  

   Wang, Aaron; Pan, Ying; Markowski, Paul M. (December 2020, Monthly Weather Review)

   null (Ed.)

   Abstract Surface friction contributes to tornado formation and maintenance by enhancing the convergence of angular momentum. The traditional lower boundary condition in atmospheric models typically assumes an instant equilibrium between the unresolved stress and the resolved shear. This assumption ignores the physics that turbulent motions are generated and dissipated at finite rates—in effect, turbulence has a memory through its lifetime. In this work, a modified lower boundary condition is proposed to account for the effect of turbulence memory. Specifically, when an air parcel moves along a curved trajectory, a normal surface-shear-stress component arises owing to turbulence memory. In the accompanying large-eddy simulation (LES) of idealized tornadoes, the normal surface-shear-stress component is a source of additional dynamic instability, which provides an extra pathway for the development of turbulent motions. The influence of turbulence memory on the intensity of quasi-steady-state tornadoes remains negligible as long as assumptions employed by the modified lower boundary condition hold over a relatively large fraction of the flow region of interest. However, tornadoes in a transient state may be especially sensitive to turbulence memory. Significance Statement Friction between the wind and the ground can influence atmospheric phenomena in important ways. For example, surface friction can be a significant source of rotation in some thunderstorms, and it can also help to intensify rotation when rotation is already present. Unfortunately, the representation of friction’s effects in atmospheric simulations is especially error-prone in phenomena characterized by rapid temporal evolution or strong spatial variations. Our work explores a new framework for representing friction to include the effect of the so-called turbulence memory. The approach is tested in idealized tornado simulations, but it may be applied to a wide range of atmospheric vortices. 

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5. Numerical Simulation of Tornado-Like Vortices generated in a Tornado Simulator Using Large Eddy Simulation

   Zhang, Hui; Tang, Zhuo; Zuo, Delong; James, Darryl (May 2022, 14th Americas Conference on Wind Engineering)