Study On Flow Field Characteristics Of Stationary Downburst Under Three-dimensional Ideally Hill Model

Downburst is a type of extreme wind event,which brings in the local strong wind near the ground.In addition,it is commonly witnessed and recorded in the mountaineous regions.And the characteristics of downburst winds in the hilly area will be greatly affected by the topographical effects,such as the flow acceleration in the windward side and in the vicinity of the hill crest and flow separation and wake regions in the leewards side.And then,the wind flow fields are quite complex,which remain rarely investigated in the wind engineering community and the currment wind lode codes and standards lakc the corresponding specifications.Therefore,it is of great necessity to further our understanding of the downburst wind chracteristics under the hilly terrain conditions.In this paper,the downburst wind fields of ideally three-dimensional hills are studied based on the impinging jet model by combing the wind tunnel experiments and numerical simulations.The main contents of this paper include:(1)The state-of-the-art of research on the downburst wind characteristics is introduced,including downburst winds over flat terrain,topographic effects of the atmospheric boundary layer(ABL)flows and downburst winds.Moreover,the specifications on the speed-up effects in the current wind lode codes and standards are further compared.Accordingly,the research purpose,the main research contents and the research roadmap of this study are clarified.(2)The governing equations and turbulence model used in the CFD(Computational Fluid Dynamics)simulations of the downburst winds are introduced.And the impinging jet model based downburst simulatior adopted in this study is illucidated in details,including the velocity and pressure measurement systems.Also,the calibration of outflow wind speed for the downburst simulator is performed and the resuls are analyzed and discussed.(3)By establishing a three-dimensional CFD numerical model of impinging jet,the validation of CFD models is performed considering the different meshing strategies and turbulence model selection on the turbulence prediction of different velocity components,vertical average velocity wind profile,radial average velocity wind profile are studied.Compared with the experimental data of impinging jet device,it is found that the higher grid resolution under the same turbulence model has higher accuracy in the simulation of vertical average velocity wind profile and radial average velocity wind profile of downburst wind field.In addition,compared with other unsteady turbulence models,IDDES turbulence model has higher accuracy in simulating vertical average velocity wind profile,radial average velocity wind profile and turbulence intensity of velocity components in downburst wind field.It shows that IDDES model can calculate the flow field characteristics of downburst flow field while taking account of computational efficiency.(4)The results of impinging jet experiment are analyzed,including vertical and radial wind profiles under different experiment cases.Based on the results of large eddy simulation,a series of transient velocity characteristics from formation,development,diffusion and the distribution characteristics of instantaneous vorticity of downburst hill wind field are analyzed.Then the corresponding steady-state numerical simulation is carried out according to the experiment conditions,and the Reynolds irrelevance of downburst hill wind field is studied.Finally,the numerical simulation results and experimental results,including vertical wind profile,radial wind profile and wind pressure coefficient distribution,are compared to verify the validity of the numerical simulation.(5)Numerical simulation is used to study the speed-up ratio at the crest and foot of the leeward side of five hill model with the same height and five different gradients and five hill model with the same gradient and five different heights.The influence of the two parameters,the height and the gradient of the hill body,is analyzed.Based on the speed-up ratio "original algorithm",Considering the effect of height and gradient,The hill speed-up ratio formula is obtained by means of parameter fitting.