Study On Downburst And Its Effect On High-Rise Building

With the aggravation of the global warming,the downburst as the representative of the strong convective weather gradually increased,making its research become one of the hottest issues in the field of international wind engineering.The downburst is a kind of airflow process that is developed by the strong sinking airflow in the thunderstorm that impacts the ground and accelerates.As the profiles of downburst and atmospheric boundary layer are different,and the downburst will produce strong wind load at a very low distance from the ground,it often brings damages to the structures.In Chinese Load Code for the design of building structures,the design wind load is based on ordinary atmospheric boundary layer wind field,downburst is not taken into account.So the study on the downburst is extremely important.In this paper,a three-dimensional model of the downburst is established by using the impinging jet model.The steady numerical simulation of the wind field of the downburst is carried out by CFD numerical method.The focus of downburst wind field was analyzed.The downburst wind speed reaches the maximum near the ground,and then decreases rapidly as the height increases,leading to huge difference from atmospheric boundary layer.There is a positive pressure within the circle with a radius of 1D on the wall below the torrent jet tube,and the pressure coefficient at the center is close to 1.The different jet velocities have little effect on the dimensionless wind speed profile,and the different jet height H has a certain influence on the dimensionless wind speed profile.The development of the boundary layer of the downburst is a non-linear change.The downburst wind profiles are different in different radial positions,bringing different wind loads on the high-rise building model.The CFD method was used to analyze the wind loads in different radial locations.The SST k-ω turbulence model was used to simulate the steady-state numerical simulations,the model was placed in different radial positions,respectively,r=0D,r= 1D,r=1.5D,r=2D.Analysis of flow field around model in different radial locations:In the radial position r=0D,we can see that the sinking airflow collides vertically on the top of the structure,after striking the top surface the air flow spreads around it,and the air flow around the four sides of the model forms a closed cylinder.In the radial positions r=1D,r=1.5D,r=2D,there is an airflow stagnation point in the lower part of the windward surface of the model.In the upper part of the side surface of the model,the flow separates at the front edge then attaches at the rear end.In the lower part of the side surface of the model,the flow separates at the front edge but does not attach at the rear end.Analysis of wind pressure coefficient distribution on model surfaces in different radial locations:When the model is located at r=0D,all surfaces of the model are subjected to greater positive pressure.In this position,the building structure should be designed with sufficient compressive strength.In the radial positions r=1D,r =1.5D,r=2D,with the increase of radial position r,the peak wind pressure coefficient becomes smaller,and the wind pressure coefficient around this area becomes no longer full.The flow mainly produce suction at the side and back surfaces of the model,and in the upper part of the side surface,the front end suction is greater than the back end.In general,the radial positions have little effect on the pressure coefficient distribution at side surface and back surface.