Study On Wind Load Of Structures Under Different Tornado Attacking Orientations

Unlike conventional wind, tornado is a kind of small-scale rotation with a high speed, which can cause extreme damages. Due to the low probability of the occurrence of tornado, the building load design code and standard have not yet contained anti-tornado design requirements currently in China. However, if the consequences of the damages of important facilities (such as nuclear power plants) are especially serious, the impact of the tornado attacking must be considered in order to ensure sufficient resistance ability. Tornado wind velocity and pressure distribution of tornado wind field are uneven, which causes great velocity gradient and pressure gradient. The wind field orientation has a great influence on the wind load of the structure suffered in the tornado. In this paper, computational fluid dynamics (CFD) numerical simulation technique was used to study the relationship between the wind load on structures and different positions of the structure subjected to tornado with different orientation angles. Considering these two factors, the study of the most unfavorable conditions for the structure suffered in tornado wind field is necessary, which can provide a reference for the anti-tornado design of the structures.A tornado numerical model was established by using the CFD numerical simulation technology and an open test tornado simulator was tried too. The results showed the good agreement between numerical tornado tangential velocity and Rankine vortex. What’s more, the test velocity distribution was obtained, which indicated that the measurement and numerical results were different inside the vortex core, but the result was consistent outside the vortex core. The experimental work showed certain feasibility for a tornado. Finally, the impacts of numerical control parameters of tornado simulation were discussed. The results showed that maximum tangential velocity decreased with the increase of swirl ratio, and the corresponding core radius increased gradually. As for the same swirl ratio, the core radius increased when height increased. That matched funnel-shaped tornado and validated the numerical simulation results. Besides, it was difficult to form a good vortex for a lower Reynolds number.Based on tornado numerical model, the surface wind load changes on two different roof slope (0°、30°) for different tornado attacking orientations was discussed. Firstly, the structure was placed at different radial positions of tornado wind field with different orientation angles. The wind pressure distribution under different conditions, and the maximum wind pressure and average pressure with the variation of the orientations of structures were discussed. Then, the impact of wind load changes were explained by slope changes of structures, which indicated the presence of roof slope results in uneven pressure distributions. But, the wind pressure distributions on the facade approached consistency. Finally, the maximum wind pressure and the average wind pressure of the two structures roofing were compared.Based on the results of the numerical simulations, two roofs were partitioned considering orientation effects. Firstly, monitoring points were arranged in vertical and horizontal grid of the roofs in order to know the trend of wind pressure distributions. The partition sizes were determined based on partition principles and greater volatility point of the pressure, and then, four kinds of wind load coefficients of each partition under four orientation angles were calculated. Then, according to the symmetry characteristic, partition shape coefficients for roofs at each angle were given. The value of partition coefficients of gable roof was larger than that of flat roof. Finally, according to the uncertain movement path of the tornado, the recommended values were given for the two roofs shape load coefficients of partitions, which provided a reference for simplify application of engineering design.