Key Parameters Of Tornado Wind Field And Their Effects Of Tornado-induced Wind Loads On Structures

As a kind of small-scale air vortices,tornadoes are always accompanied by strong three-dimensional wind velocities,pressure drops in the vortex center and rapid movements.In recent years,tornado-induced disasters have occurred frequently at home and abroad,resulting in the serious destruction of a large number of building structures.There are plenty of uncertainties in the temporal and spatial conditions in the evolution of tornadoes,which impedes the systematic studies about tornadoes by means of on-site measurements.Therefore,it is of great theoretical significance and engineering value to carry out tornado simulations to clarify the characteristics of tornado wind fields and tornado-induced wind loads.In this study,the computational fluid dynamics method was applied to conduct a series of refined simulations on tornado flow fields and wind loads,supplemented by comparisons with wind tunnel experiment results.In order to clarify the effects of key parameters on tornado wind fields,highprecision numerical simulations were carried out for the effects of swirl ratio,Reynolds number,and inflow aspect ratio.On the one hand,based on two kinds of typical mechanisms of physical tornado simulations,on the consideration of the effect of the rotating downdraft at the inflow boundary of the tornado,the effect of swirl ratio on tornado vortex configurations,statistic characteristics of tornado wind fields and nondimensional flow-field parameters,were mainly analyzed.The quantity conversion relationships among swirl ratios based on different definitions were also extracted.On the other hand,the effects of Reynolds number and aspect ratio on tornado vortex configurations,statistic characteristics of tornado wind fields,and non-dimensional flow-field parameters were clarified,respectively.In addition,the different tornado vortex configurations were mapped according to the combined strengths of the swirl ratio and Reynolds number.Aiming at the phenomena of vortex wandering in the actual observations and simulations of tornadoes,the flow field snapshots were sampled based on numerical simulations.The amplitudes of the vortex wondering of tornadoes under different vortex configurations were analyzed,and the two-dimensional probability distributions of instantaneous vortex centers were established.In order to clarify the effect of the vortex wandering on the statistical characteristics of tornado flow fields,the vortex recentering technique and statistical analysis were carried out in turn on the flow field snapshots influenced by the vortex wandering,compared with the statistical analysis of the uncorrected flow field snapshots.Based on the flow field mode decomposition technique,the vortex structures at different scales were identified on the basis of conditional averaging and decomposition and reconstruction of the transient flow field.Based on the mode distribution,it is qualitatively analyzed that the main reason for the tornado vortex wandering is the disturbance of the lateral and axial unstable velocities.Aiming at the similarity requirements for Reynolds number in the simulations of tornado-induced wind loads,based on the effect of Reynolds number on tornado wind fields,taking two typical kinds of columnar structures(cube,cylinder)as research objects,considering the cases with the different magnitudes of Reynolds number,the distributions of tornado wind fields around the columnar structures were analyzed,and the transient behaviors and statistical properties of the tornado-induced aerodynamics forces of columnar structures were studied.The effect of Reynolds number of tornadoinduced wind loads of columnar structures was therefore specified.By splitting tornado-induced wind loads of columnar structures,the comparison between tornadoinduced wind loads and straight-line wind loads of columnar structures was achieved.Aiming at the moving features of the actual tornadoes,referring to the mechanism of the physical simulation of a moving tornado,numerical simulations of a moving tornado were realized by using the sliding mesh method.Taking the moving speed and the swirl ratio as control parameters,the effects of moving speed on the threedimensional wind velocities and vortex radii of tornadoes under different vortex configurations were clarified.On this basis,taking the low-rise building as the research object,on the conditions of moving tornadoes with different moving speeds and vortex configurations,the evolution of vortex configuration in the process of moving tornado attacking the low-rise building was depicted,and the characteristics of moving tornado loads on the low-rise building were mainly explored,which was also compared with the study results based on physical tornado simulations.For the deeper understanding of the characteristics of probability statistics and extreme value of tornado-induced wind loads on structures,taking low-rise buildings with openings as research objects,based on the physical tornado simulation method,the statistical characteristics and probability distributions of wind pressures on the inner and outer surfaces of buildings were explored.Using the extreme value calculation methods based on the zero-value crossing theory and the classical extreme value theory,the extreme values of the internal and external wind pressures were estimated.Results show that the external pressures show strong non-Gaussian properties,while the internal pressures present Gaussian distributions.The effects of the opening parameters on the internal pressures were related to the radial position where the building is located.