Flow Characteristics And Mathematical Modeling Of Downburst Winds Over The Idealized Three-dimensional Hill

When downburst impact on the ground,the air flow will spread along the ground,which will produce devastating strong wind,causing serious wind damage to engineering structures,including buildings and transmission line towers.At the same time,downburst occurs frequently in hillous areas such as southwest China.Due to the hill terrain effect,the velocityup effect appears at the top of the hill,which further aggravates the disaster of downburst.At present,the hill effect of boundary layer wind field is studied deeply and systematically,and the wind field model for engineering structure design is formed,but the hill speedup ratio model of downburst is lack.Therefore,it is urgent to study the characteristics and modeling of downburst wind field considering terrain effect.Based on the physical experiment,numerical simulation and analytical method of impinging jet and wall jet model,the characteristics of ideal three-dimensional hill wind field are studied systematically and deeply,and the hill speed up ratio model of downburst for engineering structure design is established.The main research work of this paper is as follows:(1)By analyzing the characteristics of horizontal velocity,maximum velocity attenuation and turbulent flow field,the fully developed region under the wall jet is explained.Different hill models are placed in the fully developed area,and the hill wind field characteristics of the wall jet and its variation with hill parameters are analyzed.The experimental results are similar to those of the impinging jet.The vertical height corresponding to the maximum velocity is selected to establish the relationship between impinging jet and wall jet,and the downburst wind field of the two models is compared,which shows that the data of impinging jet model and wall jet model are consistent.(2)Different turbulence models are selected to compare the transient flow characteristics of the flat wind field and the hill wind field,and reveal the evolution mechanism of the turbulent flow structure of the hill wind field under downburst.Compared with the experimental results of impinging jet model,quantitative and qualitative analysis are carried out to verify the effectiveness of the multi fidelity turbulence model,LES(Large eddy simulation)model and Realizable k-? model have higher accuracy.Pod(Proper orthogonal decomposition)is used to quantitatively analyze the flow characteristics around the hill wake.The first mode dominates the acceleration effect.Finally,the hill models with multiple slopes and radial positions are selected to carry out systematic and regular research.(3)Based on the model of downburst flat wind field and hill wind field,the method of deducing hill velocity is proposed,which provides reference for building structure design.According to the shape function of the flat wind field,the variation of the maximum velocity and the nonlinear development of the boundary layer,combined with the experimental data,the flat wind field of downburst in the whole basin is reproduced.According to the change of hill acceleration ratio with slope,radial position and vertical height,the hill acceleration ratio model of downburst is constructed and verified.The predicted value is in good agreement with the simulated value in the windward area,and it still needs to be further improved in the leeward area.The method of deriving hill velocity from hill model parameters is proposed by combining the model of flat wind field and hill acceleration ratio.